The ESA Permanent Facility for Altimetry Calibration: Monitoring

remote sensing

Article The ESA Permanent Facility for Altimetry Calibration: Monitoring Performance of Radar Altimeters for Sentinel-3A, Sentinel-3B and Jason-3 Using Transponder and Sea-Surface Calibrations with FRM Standards

Stelios Mertikas 1,*, Achilleas Tripolitsiotis 2, Craig Donlon 3, Constantin Mavrocordatos 3, Pierre Féménias 4, Franck Borde 3, Xenophon Frantzis 1, Costas Kokolakis 2, Thierry Guinle 5, George Vergos 6 , Ilias N. Tziavos 6 and Robert Cullen 3

1 Technical University of Crete, Geodesy and Geomatics Engineering Laboratory, GR 73100 Chania, Greece; [email protected] 2 Space Geomatica P.C., Xanthoudidou 10A, GR 73132 Chania, Greece; [email protected] (A.T.); [email protected] (C.K.) 3 European Space Agency/European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, AZ 2201 Noordwijk, The Netherlands; [email protected] (C.D.); [email protected] (C.M.); [email protected] (F.B.); [email protected] (R.C.) 4 European Space Agency/European Space Research Institute (ESA/ESRIN), Largo Galileo Galilei, I 00044 Frascati, Italy; [email protected] 5 Centre National d’ Etudes Spatiales (CNES), CEDEX 31401 Toulouse, France; [email protected] 6 Aristotle University of Thessaloniki, School of Geodesy and Surveying, University Box 440, 54124 Thessaloniki, Greece; [email protected] (G.V.); [email protected] (I.N.T.) * Correspondence: [email protected]; Tel.: +30-28-2103-7629

Received: 6 July 2020; Accepted: 14 August 2020; Published: 16 August 2020

Abstract: This work presents the latest calibration results for the Copernicus Sentinel-3A and -3B and the Jason-3 radar altimeters as determined by the Permanent Facility for Altimetry Calibration (PFAC) in west Crete, Greece. Radar altimeters are used to provide operational measurements for sea surface height, signiﬁcant wave height and wind speed over oceans. To maintain Fiducial Reference Measurement (FRM) status, the stability and quality of altimetry products need to be continuously monitored throughout the operational phase of each altimeter. External and independent calibration and validation facilities provide an objective assessment of the altimeter’s performance by comparing satellite observations with ground-truth and in-situ measurements and infrastructures. Three independent methods are employed in the PFAC: Range calibration using a transponder, sea-surface calibration relying upon sea-surface Cal/Val sites, and crossover analysis. Procedures to determine FRM uncertainties for Cal/Val results have been demonstrated for each calibration. Biases for Sentinel-3A Passes No. 14, 278 and 335, Sentinel-3B Passes No. 14, 71 and 335, as well as for Jason-3 Passes No. 18 and No. 109 are given. Diverse calibration results by various techniques, infrastructure and settings are presented. Finally, upgrades to the PFAC in support of the Copernicus Sentinel-6 ‘Michael Freilich’, due to launch in November 2020, are summarized.

Keywords: satellite altimetry; Copernicus Sentinel-3; calibration; ﬁducial reference measurements; transponder; sea surface; uncertainty analysis; Crete

Remote Sens. 2020, 12, 2642; doi:10.3390/rs12162642 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 2642 2 of 39

1. Introduction Earth observation is the gathering of information about planet Earth’s physical, chemical and biological systems via remote sensing technologies. It is meant to observe our environment, to collect, store and analyze Earth data in order to furnish products for effective decisions on potential risks and vulnerabilities of our planet and its ecosystems. Satellite altimetry falls in that category of Earth observation. International missions have been measuring geocentric sea level as of 1992 (TOPEX/Poseidon and Jason [1]), with an accuracy, at present, of 1–2 cm. Altimetry measurements coupled with other data from various sea-level observing ± systems (i.e., tide gauges) have established a long-term record of sea level on global scale. The trend and acceleration of sea level have been found to be +3.35 0.40 mm/yr and 0.12 0.07 mm/yr2, ± ± respectively, for 1992–2017 [2]. In addition, other acceleration rates of 0.08 0.008 mm/yr2 [3] and ± 0.084 0.025 mm/yr2 [4] have been reported. ± Sea level rise threatens coasts, and certainly people, through flooding, coastline erosion and submergence, salinization of soil, contamination of underground water, destruction of shoreline defenses, etc. [5–7]. More than 190 million people are at present vulnerable to sea level rise as a result of coastal flooding even under an optimistic scenario of climate change (increase in global warming by 1.0 ◦C and in sea level by +0.24 m over the period 2046–2065) [8]. Europe, and in particular the European Space Agency (ESA) together with the European Union and Eumetsat, has implemented a large, ambitious and sustained space-based Earth Observation Program, with Copernicus being one of its main components. Copernicus (http://www.copernicus.eu/) is a system for monitoring the Earth in support of European policy. It includes Earth Observation satellites (notably the Sentinel series developed by ESA), ground-based measurements, and services to processes data to provide users with reliable and up-to-date information through a set of Copernicus operational services related to environmental and security issues. These include:

Copernicus Marine Environmental Monitoring Service (CMEMS (http://marine.copernicus.eu)); • Copernicus Land Monitoring Service (CLMS (http://land.copernicus.eu/)); • Copernicus Atmospheric Monitoring Service (CAMS (https://atmosphere.copernicus.eu/)); • Copernicus Emergency Management Service (CEMS) (http://emergency.copernicus.eu/); • Copernicus Climate Change Service (C3S) (http://climate.copernicus.eu). • Copernicus services provide critical information to support a wide range of applications, including environment protection, management of urban areas, regional and local planning, agriculture, forestry, fisheries, health, transport, climate change, sustainable development, civil protection and tourism. Copernicus satellite missions are designed to provide ‘upstream’ inputs to all Copernicus Services as systematic measurements of Earth’s oceans, land, ice and atmosphere to monitor and understand large-scale global dynamics. The primary users of Copernicus services are policymakers and public authorities that need information to develop environmental legislation and policies or to take critical decisions in the event of an emergency, such as a natural disaster or a humanitarian crisis. The Copernicus programme is coordinated and managed by the European Commission. The development of the observation infrastructure is performed under the aegis of the European Space Agency for the space component and of the European Environment Agency and the Member States for a separate, but important, in-situ measurement component. Copernicus is a programme mandated to make all data products available to subscribing scientists, policy makers, entrepreneurs and the citizens in a full, free, and open access policy. The space element of Copernicus comprises a family of satellites called “Sentinels”, yet exploits data coming from other Earth Observation satellites developed by other agencies. Of particular note are Sentinel-3 and Sentinel-6 that carry a Ku- and C-band radar altimeter and other microwave instruments, designed to observe the topography of the ocean surface, in support of operational oceanography Remote Sens. 2020, 12, 2642 3 of 39 and monitoring changes in sea level, a key indicator of climate change. These missions also provide information to derive ocean currents, wind speed, ocean heat content and wave height, contributing to maritime safety. Copernicus has implemented a sustained operational Earth Observation System for the next 20 years with Sentinel satellites, now in place with multiple satellites in each family. For example, Sentinel-3 operates in a sun-synchronous orbit at an altitude of ~800 km, while Sentinel-6 continues the legacy of the TOPEX/Poseidon and Jason series of missions flying at an altitude of ~1300 km at an inclined orbit of 66 degrees—referred to as the ‘altimeter reference orbit’. The Senitinel-6 satellites have been developed by the ESA together with NASA, EUMETSAT, NOAA, and the European Commission with the support of CNES. Still, Jason-3 currently provides operational altimeter measurements following preceding satellite missions occupying the same ground track (including TOPEX/Poseidon, Jason-1 and OSTM/Jason-2). A specific set of design and performance requirements is associated with each separate satellite mission to address their primary mission objectives and performance needs. In addition to the Sentinel-3, Sentinel-6 and Jason-3 missions, a number of additional international satellite altimeters either on orbit or shortly to be launched (e.g., CryoSat-2, IceSat-2, SWOT, HY-2C and Quanlan), implement diverse measuring technology (interferometry, wide swath and different frequencies), and will contribute to the monitoring of geocentric sea level. Small differences between successive satellite missions of the same design may be anticipated. If fundamental changes are implemented using new measurement approaches such as synthetic aperture radar (SAR) altimetry, different supporting radiometer channels, or even different choices of microwave radiometer channels, great care must be taken to ensure that such changes do not introduce artificial artefacts into the multi-mission altimetry time series. Furthermore, since range calibration of a satellite altimeter can only be derived effectively once in orbit, a dedicated program of ground-based validation and verification is required. All satellite altimeters are required to rely upon reference standards and calibration/validation (Cal/Val) infrastructures on the ground to understand relative and absolute differences due to design choices or in-flight degradation of specific components (e.g., antenna, electronics). To maintain an understanding of in-flight performance and maintain the quality of satellite altimetry products, each mission must be regularly monitored using independent data in an objective and absolute sense [9]. This obligation has been implemented using the strategy of Fiducial Reference Measurements (FRM) [9,10] for satellite observations and is supported by the European Space Agency for all its Sentinels. The FRM strategy addresses the need, at first, for Cal/Val facilities to report their results along with a realistic, exhaustive and trustworthy uncertainty budget. It promotes evaluations for the uncertainty of altimetry observations but also to trace uncertainties for water level observations to undisputable standards. It thus aims at achieving reliable, long-term and consistent satellite Earth observations and products, via reliable calibration and standards. It provides a procedure-based service to support the long-term calibration and validation of satellite multi-mission altimetry, and provides independent FRM-class data for use in harmonization of long-term multi-satellite observation of the Earth system, and comprehensive and integrated comparisons and assessments [11]. The Gavdos/Crete facilities in west Crete, Greece have been providing altimetry Cal/Val services since 2004. In 2015, the European Space Agency declared Gavdos facilities as the ESA Permanent Facility for Altimetry Calibration (PFAC) in support of Copernicus altimetry and ESA CryoSat mission calibration and validation activities. Over the past 5 years, the facility has pioneered FRM standards and procedures for altimetry calibration products. The PFAC Cal/Val infrastructure consists of one transponder Cal/Val site for external range calibration that is complemented by three permanent sea-level-surface Cal/Val sites. A second transponder (providing range and sigma-0 retrieval functionality) is being developed for operational redundancy in support of the Copernicus Sentinel-6 mission. The infrastructure that is permanently installed and maintained at the PFAC is Remote Sens. 2020, 12, 2642 4 of 39 used routinely for multi-mission altimeter calibration activities and additional relative calibration at crossover locations. The PFAC is designed to allow assessment of different Cal/Val techniques (i.e., sea-surface heights versus transponder) and to perform calibration activities using satellite measurements taken only a fewRemote seconds Sens. 2020 apart, 12, x along FOR PEER the same REVIEW orbit. For example, the Sentinel-6 mission will fly 30 s in time apart4 of 39 from Jason-3 along the same nominal ground track for a 12-month tandem flight designed to maintain thecross-comparison stability of the mean by diverse sea level and trend redundant time series. Cal/Val The te abilitychniques to perform at different cross-comparison settings (at 1050 by diverse m, 160 andm altitude redundant and Calat sea/Val level, techniques for example) at diff iserent required settings to establish (at 1050 m,confidence 160 m altitude in the calibration and at sea results level, forthat example) are derived. is required to establish confidence in the calibration results that are derived. TheThe revisit revisit cycle cycle of of Sentinel-3A Sentinel-3A andand -3B-3B is is every every 27 27 days days and and that that of of Jason-3 Jason-3/Sentinel-6/Sentinel-6 isis every every 1010 days, days, with with ascending ascending and descendingand descending orbits crossing orbits overcrossing PFAC over Cal/Val PFAC transponder Cal/Val infrastructure. transponder Thisinfrastructure. allows the PFACThis allows to support the PFAC annually to support about 50 annually calibrations about for 50 each calibra Sentinel-3Ations for each and Sentinel-3BSentinel-3A satelliteand Sentinel-3B and more satellite than 70 calibrationsand more than per year70 calibrations for Jason-3 (andper yea subsequentlyr for Jason-3 Sentinel-6). (and subsequently It can be seenSentinel-6). from Figure It can1 thatbe seen the PFACfrom Figure calibrates 1 that three the Sentinel-3A PFAC calibrates Passes three (No. Sentinel-3A 14, No. 278, Passes and No. (No. 335), 14, threeNo. 278, Sentinel-3B and No. Passes 335), three (No. Sentinel-3B 14, No. 71, andPasses No. (No. 335) 14, and No. two 71, Jason-3 and No. Passes 335) and (No. two 18 andJason-3 No. Pa 109).sses

(a) (b)

Figure 1. Cont. Remote Sens. 2020, 12, 2642 5 of 39 Remote Sens. 2020, 12, x FOR PEER REVIEW 5 of 39

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Figure 1. TheThe ground ground tracks of Sentinel-3A, Sentinel-3 Sentinel-3B,B, Jason-3, HY-2B, HY-2B, SARAL/AltiKa SARAL/AltiKa over the Permanent Facility forfor AltimeterAltimeter CalibrationCalibration in in west west Crete, Crete, Greece Greece (a ().a). The The established established Cal Cal/Val/Val sites sites in inGavdos Gavdos and and Crete Crete (b );(b the); the star star denotes denotes another another coastal coastal tide tide gauge gauge site site of of “SUG1” “SUG1” to to be be established established in in2020. 2020. The The CDN1 CDN1 transponder transponder Cal/Val Cal/Val on the on mountains the mountains of mainland of mainland Crete (c,d Crete) and the(c,d sea-surface) and the sea-surfaceCal/Val sites Cal/Val (GVD7 sites and GVD8)(GVD7 atand the GVD8) Gavdos at harborthe Gavdos (e,f). harbor (e,f).

This work presents the latest calibration results for Sentinel-3A, Sentinel-3B and Jason-3 as determined atat the the PFAC, PFAC, making making use ofuse three of independentthree independent Cal/Val techniques:Cal/Val techniques: transponder, transponder, sea-surface sea-surfaceand crossover and analysis. crossover A short analysis. description A ofshort the PFACdescription infrastructure of the and PFAC instrumentation infrastructure is given and in instrumentationSection2, together is withgiven a summaryin Section of2, thetogether three calibrationwith a summary methods of employed.the three calibration The procedures methods to employed.ensure the CalThe/Val procedures measurements to ensure (compliant the withCal/Val the FRMmeasurements standards), and(compliant the uncertainty with the analysis FRM standards),for the sea-surface and the and uncertainty transponder analysis calibrations, for the are sea-surface described inand Section transponder3. Section calibrations,4 presents theare describedabsolute Cal in/Val Section results 3. for Section each satellite 4 presents (i.e., Sentinel-3A, the absolute Sentinel-3B Cal/Val andresults Jason-3), for each along satellite with relative (i.e., Sentinel-3A,calibration results Sentinel-3B of Sentinel-3A and Jason-3), and Sentinel-3B along with with relative respect tocalibration Jason-3 at results crossover of locationsSentinel-3A over and the Sentinel-3Bsea surface. Sectionwith respect5 discusses to Jason-3 single-mission at crossover results locations and evaluates over the performance sea surface. using Section each Cal5 discusses/Val site. single-missionInter-validation results of calibration and evaluates results obtained performance by diverse using Cal each/Val methodsCal/Val issite. also Inter-validation reported. Section of6 calibrationsummarizes results calibration obtained results by and diverse the performance Cal/Val methods of Sentinel-3A, is also reported. Sentinel-3B Section and Jason-3 6 summarizes obtained calibrationby the PFAC. results Finally, and future the plansperformance for the calibrationof Sentinel-3A, of the Sentinel-3B upcoming Sentinel-6and Jason-3 are alsoobtained discussed. by the PFAC. Finally, future plans for the calibration of the upcoming Sentinel-6 are also discussed. 2. Infrastructure, Instrumentation and Calibration Methods 2. Infrastructure,This section Instrumenta presents thetion infrastructure and Calibration and Methods instrumentation of the Permanent Facility for AltimetryThis section Calibration presents in west the Crete, infrastructure Greece. The and principle instrumentation of operations of the for eachPermanent calibration Facility method for (i.e.,Altimetry transponder, Calibration sea-surface, in west crossoverCrete, Greece. analysis) The principle employed of along operations with the for parametrization each calibration of method each is (i.e.,also brieﬂytransponder, described. sea-surface, crossover analysis) employed along with the parametrization of each is also briefly described. 2.1. PFAC Infrastructure and Instrumentation 2.1. PFACThe location Infrastructure and setting and Instrumentation of the Gavdos /Crete Permanent Facility for Altimetry Calibration, in the center of the eastern Mediterranean, oﬀers a unique location for the present generation of multi-mission The location and setting of the Gavdos/Crete Permanent Facility for Altimetry Calibration, in altimetry calibration (Figure1). the center of the eastern Mediterranean, offers a unique location for the present generation of multi-mission altimetry calibration (Figure 1).

Remote Sens. 2020, 12, 2642 6 of 39

The PFAC consists of three permanent coastal Cal/Val sites (GVD8 at Gavdos island, CRS1 and RDK1 at the south and west coast of Crete, respectively, Figure1b) and one transponder Cal /Val site on a mountainous area in the mainland of Crete (Figure1a). A detailed presentation of these Cal /Val sites has been provided in [12]. Table1 presents an overview of each Cal /Val site in terms of the altimeters supported, the calibration method employed and its main instrumentation.

Table 1. Summary of existing (PFAC) Cal/Val sites along with the main instrumentation at each Cal/Val site.

Site Cal/Val Method Mission/Pass Instrumentation

S3A P14 Ku-band range transponder • S3B P335 Microwave radiometer CDN1 Transponder • JA3 P18 Two GNSS * receivers • CryoSat-2 Two meteo stations • JA3 P18 Five tide gauges (radar, • JA3 P109 acoustic, pressure) GVD8 Sea-Surface S3A P335 Two GNSS receivers • S3A P14 Two meteo stations • JA3 P109 Two tide gauges (radar) RDK1 Sea-Surface • S3B P71 One GNSS receiver • S3A P278 Two tide gauges (radar, pressure) • CRS1 Sea-Surface S3B P14 One GNSS receiver • HY-2B P280 One meteo station • * Global Navigation Satellite System.

These instruments are accessed and controlled remotely. Their ﬁeld observations are transmitted to, via satellite and/or GPRS communications link, and directly archived at the PFAC Operations Control Center at the Technical University of Crete, Greece. Details of the data processing techniques and analysis of raw measurements are given in Section3. Two more calibration sites are currently under development: A sea-surface-elevation Cal/Val site at the south coast of Crete (SUG1, Figure1) and a second transponder Cal /Val site (GVD1) on Gavdos island to support Sentinel-6 Cal/Val activities. A summary of the new GVD1 site is provided in Section6.

2.2. Calibration and Validation Methods Independent calibration and validation activities are required to assess the end-to-end performance characteristics of satellite altimeter measurements. Cal/Val activities monitor the measurement uncertainty, stability and behavior, providing independent knowledge of measurement quality. External techniques that regularly support post-launch calibration of satellite altimeters include: (1) sea-surface Cal/Val, (2) transponder Cal/Val, (3) backscatter coeﬃcient Cal/Val, (4) crossover analysis, (5) tandem phase analysis, and (6) comparison to tide gauge networks. The latter is an indirect method, which compares the altimeter-derived sea surface heights with measurements obtained by a global network of tide gauges [13]. Details on PFAC-employed Cal/Val techniques can be found in [14]. A short description of the remaining four techniques and their implementation in the PFAC is given in the following sections.

2.2.1. Sea-Surface Cal/Val Satellite altimeters are designed to measure the sea (or water) surface height (SSH) above a reference ellipsoid. The sea-surface calibration method compares the sea surface height, as measured by a satellite Remote Sens. 2020, 12, 2642 7 of 39 altimeter at a certain location and time in open water, to sea surface heights measured by reference ground measurements. A number of corrections need to be applied to the measured satellite range to account for propagation errors caused by the atmosphere (ionosphere, wet and dry troposphere), the sea state bias due to surface roughness impacting the radar measurement, and geophysical corrections including solid earth tide, ocean tides, etc. Ideally, the bias of an altimeter-derived sea-surface height is determined as the diﬀerence between the satellite and the ground-truth observations implemented at the same location and time. Ground-truth observations are provided by a combination of water level sensors, absolute positioning receivers, meteorological and oceanographic sensors, and models of the geoid, tides, mean dynamic topography, etc. At the PFAC, the land mass of Gavdos and Crete contaminates the altimeter’s signal, leading to invalid sea-surface measurements close to the coast (about 7.5 km). To overcome this, the Cal/Val site observations have to be transferred to the open sea at about 20 km, with about 3000 m depth, far from the coast where valid satellite measurements exist. This introduces an uncertainty in the approach compared to the ideal case where truly contemporaneous and co-located measurements are available.

2.2.2. Transponder Cal/Val The sea-surface calibration method compares a derived geophysical parameter (sea-surface height or sea-surface anomaly) at a close-by location with ocean truth measurements regularly made on the coast. Nonetheless, the primary measurement of radar altimeter is the propagation time (converted to range if multiplied by speed of light) of a signal pulse from the altimeter antenna to the Earth’s surface and back to the satellite. A transponder provides absolute calibration of these direct and fundamental range observations made by the altimeter if the location of the transponder and its internal path delay are precisely and accurately known. An active microwave transponder receives the altimeter’s signal when a satellite flies over it (which takes about 4 s), amplifies it with minimum distortion and sends it back to the satellite where it is recorded. At first, the received signal is distinctly separated from other surrounding reflections (clutter) of the ground surface as it had been powerfully amplified by the transponder. Then, at explicitly the same time, the geocentric positions of the altimeter in orbit and the transponder on the ground are determined. The theoretical distance between these two instruments in space is subsequently computed. Finally, after extensive processing, the observed range by the altimeter is compared against the theoretical range of the transmitting satellite point in orbit and the center of the transponder. During the overpass period, a time series of the differences between the observed range and the theoretical range provides an estimate of the altimeter range, if short time scale atmospheric components of the troposphere are sufficiently known. The PFAC has been providing transponder calibration services for Jason-2, Jason-3, CryoSat-2, Sentinel-3A, and Sentinel-3B from September 2015 as part of the ESA FRM4ALT project. After each calibration, the echo of the transponder response is recorded by the altimeter and two recent such examples are shown in Figure2. Transponder calibrations presuppose that the Agency operating the satellite has programmed the altimeter to observe within the measuring window of the CDN1 Cal/Val site on the mountains at the elevation of 1050 m. Remote Sens. 2020, 12, x FOR PEER REVIEW 7 of 39 troposphere), the sea state bias due to surface roughness impacting the radar measurement, and geophysical corrections including solid earth tide, ocean tides, etc. Ideally, the bias of an altimeter-derived sea-surface height is determined as the difference between the satellite and the ground-truth observations implemented at the same location and time. Ground-truth observations are provided by a combination of water level sensors, absolute positioning receivers, meteorological and oceanographic sensors, and models of the geoid, tides, mean dynamic topography, etc. At the PFAC, the land mass of Gavdos and Crete contaminates the altimeter’s signal, leading to invalid sea-surface measurements close to the coast (about 7.5 km). To overcome this, the Cal/Val site observations have to be transferred to the open sea at about 20 km, with about 3000m depth, far from the coast where valid satellite measurements exist. This introduces an uncertainty in the approach compared to the ideal case where truly contemporaneous and co-located measurements are available.

Figure 2. Transponder responses as captured by Sentinel-3A (left) and CryoSat-2 (right, Credit: FigureESA), Marco 2. Transponder Fornari) altimetersresponses duringas captured the transponder by Sentinel-3A calibrations (left) andcarried CryoSat-2 out (right, on 3-May-2020 Credit: ESA), and Marco20-Sept-2019, Fornari) respectively. altimeters during the transponder calibrations carried out on 3-May-2020 and 20-Sept-2019, respectively. 2.2.3. Backscatter Coeﬃcient Cal/Val

In addition to the signal propagation time, a radar altimeter measures the power level of these radar signals [15]. The relation between the amplitude of the transmitted and received echo signals provides information about the characteristics of the imaged surface on the ground. This is expressed as the back-scatter coefficient (sigma-naught, or sigma-0). The sigma0 coefficient is used to retrieve wind speed, and empirical models for the sea state bias required to determine sea surface height. Calibration of the sigma-naught coefficient can be obtained using either active or passive targets with known and stable radar cross-section, as they are easily detected within the altimeter backscatter records. Previously, a sigma-naught transponder was developed by the European Space Agency and used to calibrate the ENVISAT RA-2 altimeter [16]. This old transponder has been recently (2019) refurbished and is expected to support Sentinel-3 sigma-naught calibrations in 2020 [17]. In support of Copernicus Sentinel-6, a new range and sigma-naught transponder is currently under development and is expected to be installed at the PFAC. The new processing technique of fully focused SAR altimetry [18] may also provide sigma-naught calibration with passive corner reflectors [17] and this will also be investigated at the PFAC. Details on these future plans are given in Section6.

2.2.4. Crossover Analysis This technique aims at providing relative calibration between two or more contemporaneous satellite altimeters [19]. The analysis usually takes place on a global scale and at locations where the ascending and descending ground-tracks of altimeters intersect. A crossing event occurs when the time difference of the intersecting passes of two (or more) missions is commonly less than two days [20]. Under the tenuous assumption that the variability and signature of sea level has not changed significantly in the two-day lag between measurements, the sea surface height at the crossover location, as determined by the two missions, can be compared. A significant advantage of this relative performance approach is that it does not require ground measurements and can be performed throughout the missions’ lifetime and on a global scale [19]. At the PFAC, the crossover analysis has been modified and applied to retrieve the relative bias of Sentinel-3A Pass No. 14 and Jason-3 Pass No. 109 about 20 km south of Gavdos (Figure3). Remote Sens. 2020, 12, x FOR PEER REVIEW 8 of 39

2.2.3. Backscatter Coefficient Cal/Val In addition to the signal propagation time, a radar altimeter measures the power level of these radar signals [15]. The relation between the amplitude of the transmitted and received echo signals provides information about the characteristics of the imaged surface on the ground. This is expressed as the back-scatter coefficient (sigma-naught, or sigma-0). The sigma0 coefficient is used to retrieve wind speed, and empirical models for the sea state bias required to determine sea surface height. Calibration of the sigma-naught coefficient can be obtained using either active or passive targets with known and stable radar cross-section, as they are easily detected within the altimeter backscatter records. Previously, a sigma-naught transponder was developed by the European Space Agency and used to calibrate the ENVISAT RA-2 altimeter [16]. This old transponder has been recently (2019) refurbished and is expected to support Sentinel-3 sigma-naught calibrations in 2020 [17]. In support of Copernicus Sentinel-6, a new range and sigma-naught transponder is currently under development and is expected to be installed at the PFAC. The new processing technique of fully focused SAR altimetry [18] may also provide sigma-naught calibration with passive corner reflectors [17] and this will also be investigated at the PFAC. Details on these future plans are given in Section 6.

2.2.4. Crossover Analysis This technique aims at providing relative calibration between two or more contemporaneous satellite altimeters [19]. The analysis usually takes place on a global scale and at locations where the ascending and descending ground-tracks of altimeters intersect. A crossing event occurs when the time difference of the intersecting passes of two (or more) missions is commonly less than two days [20]. Under the tenuous assumption that the variability and signature of sea level has not changed significantly in the two-day lag between measurements, the sea surface height at the crossover location, as determined by the two missions, can be compared. A significant advantage of this relative performance approach is that it does not require ground measurements and can be performed throughout the missions’ lifetime and on a global scale [19]. Remote Sens. 2020, 12, 2642 9 of 39 At the PFAC, the crossover analysis has been modified and applied to retrieve the relative bias of Sentinel-3A Pass No.14 and Jason-3 Pass No. 109 about 20 km south of Gavdos (Figure 3).

Figure 3. An intersection of the Sentinel-3A Pass No. 14 and Jason Pass No. 109 occurs about 20 km south of Gavdos,Gavdos, Crete,Crete, Greece. Greece. This This location location is is used used to to perform perform crossover crossover analysis analysis and and determine determine the therelative relative bias bias between between the twothe two altimeters. altimeters. 2.2.5. Tandem Phase Cal/Val 2.2.5. Tandem Phase Cal/Val The ideal scenario to characterize the relative bias between two altimeters is to make them observe the same location of the Earth at the same time or a few seconds apart [21]. This is accomplished during a tandem phase, which many new satellite missions incorporate into their regular operations. During this phase, the newly launched satellite is placed on the same orbital track as a reference orbit. The two satellites fly in unison on the same orbit but separated only some seconds apart. For example, Sentinel-3B flew in tandem [22] with Sentinel-3A for four months (6-June-2018 to 16-October-2018) before being placed on its nominal orbit. Similarly, Jason-3 was flying in tandem with Jason-2 with a temporal distance of 80 s for six months (12-Febuary-2016 to 2-October-2016). In [12], the PFAC-derived calibration results of the Jason-2/Jason-3 tandem phase have been presented. The present plan for the future Sentinel-6 will implement a 12-month tandem phase with Jason-3, where the two satellites will fly 30 s apart [23]. This work (see Section4) presents the relative calibration results from the Sentinel-3 A /B tandem phase. The following Section presents the PFAC’s in-situ measurements, the models employed, and the processing strategies followed for the implementation of the aforementioned calibration methodologies for Sentinel-3A, Sentinel-3B and Jason-3 altimeters for absolute and relative calibration.

3. The PFAC Measurements and the FRM Uncertainty Analysis In-situ measurements used for external calibration and validation of Earth observation have to follow and be traceable to internationally agreed reference standards. In addition, the uncertainty analysis of these measurements has to accompany the respective Cal/Val results. This uncertainty should be available to allow altimetry operators to evaluate the credibility and suitability of the Cal/Val results for efficient monitoring of the altimeter’s performance. A strategy for assuring a uniform and absolute standardization for calibrating satellite altimeters is given in [24]. The main uncertainty constituents for sea-surface and transponder Cal/Val techniques have been identified in [9] and quantified in [14]. Accurate determination of the absolute geodetic coordinates for a Cal/Val site constitutes one of the key uncertainty components in both sea-surface and transponder calibration. Particularly, the height component is of utmost importance as it controls both the sea-surface height at the sea-surface Cal/Val site and the distance between the transponder and altimeter phase center. The following section presents the strategy followed at the PFAC to determine the absolute coordinates of the Cal/Val sites Remote Sens. 2020, 12, 2642 10 of 39 and their uncertainty. Later, the remaining constituents of uncertainty contributing to each calibration method are examined separately.

3.1. Absolute Coordinates Constituent In the PFAC, a network of continuously operating GNSS stations distributed in western Crete and Gavdos (Figure4) provides absolute geodetic coordinates and velocities for reference points at each RemoteCal/Val Sens. site. 2020, 12, x FOR PEER REVIEW 10 of 39

Figure 4. Location of the geodetic GNSS network that provides absolute coordinates of the PFAC Figure 4. Location of the geodetic GNSS network that provides absolute coordinates of the PFAC Cal/Val sites (left, map of west Crete). Example photos from the main sea-surface Cal/Val facility in Cal/Val sites (left, map of west Crete). Example photos from the main sea-surface Cal/Val facility in Gavdos and the CDN1 transponder Cal/Val site on the mountains of Crete. Operations of two GNSS Gavdos and the CDN1 transponder Cal/Val site on the mountains of Crete. Operations of two GNSS receivers at each Cal/Val site ensures redundancy and reliability of observations. receivers at each Cal/Val site ensures redundancy and reliability of observations. According to FRM requirements, redundancy of the scientific equipment has to be ensured. According to FRM requirements, redundancy of the scientific equipment has to be ensured. This implies that at least two instruments should operate side by side to measure the same parameter This implies that at least two instruments should operate side by side to measure the same to ensure that any potential errors and drifts in the operations are identified. For example, Table2 parameter to ensure that any potential errors and drifts in the operations are identified. For example, presents the coordinates and velocities for the two GNSS receivers operating at the CDN1 transponder Table 2 presents the coordinates and velocities for the two GNSS receivers operating at the CDN1 Cal/Val site (CDN0 and CDN2) and the two operating at the Gavdos sea-surface facility (GVD7 and transponder Cal/Val site (CDN0 and CDN2) and the two operating at the Gavdos sea-surface facility GVD8). All sites have been processed using the GAMIT scientific software, which applies relative (GVD7 and GVD8). All sites have been processed using the GAMIT scientific software, which positioning techniques [25]. The reference epoch of these coordinates and velocities is 2013.5 and the applies relative positioning techniques [25]. The reference epoch of these coordinates and velocities geodetic frame used is the ITRF 2014. is 2013.5 and the geodetic frame used is the ITRF 2014. In order to assess the values for the ellipsoidal height for each pair of the GNSS receivers (i.e., the CDN0 with the CDN2 and the GVD7 with the GVD8) their antenna reference points have to be Table 2. The International Terrestrial Reference Frame 2014 (ITRF2014) coordinates and velocities of geodetically tied together but also connected to other reference control points in the vicinity. This task two pairs of GNSS stations operating at the CDN1 transponder (CDN0 and CDN2) and Gavdos is achieved through precise leveling that provides height differences with uncertainty of the order of sea-surface (GVD7 and GVD8) Cal/Val sites. The time span and uncertainties in the form of a 1 mm. These levelling surveys are carried out on a semi-annual basis as well as during a major event ± weighted root mean square error of each component are also given. (i.e., GNSS antenna replacement, earthquake, etc.). The derived uncertainty of GNSS processing is Ellipsoi N_veloc E_veloc UP_velo givenSite/Time in Table 2. The finalLat [degree] value for uncertaintiesLon [degree] in the determination of absolute coordinates for the d Height ity ity city referenceSpan benchmarks on each Cal/Val site are used as input in the processing chain to derive the FRM constituent uncertainty. [m] [m yr-1] [m yr-1] [m yr-1] CDN0 N35°20’16.02437 Ε23°46’46.85480 1049.518 0.0076 −0.0126 0.0002 [2014.49– 5’’ 9’’ 6 ±0.5 ±0.3 mm ±1.0 mm 2019.99] ±3.1 mm ±2.1 mm ±8.0 mm mm CDN2 N35°20’16.29110 Ε23°46’46.82911 1050.406 0.0070 −0.0127 −0.0025 [2016.40– 9’’ 2’’ 8 ±0.5 ±0.4 mm ±2.0 mm 2019.99] ±1.8 mm ±2.2 mm ±8.5 mm mm GVD7 N34°50’52.74456 Ε24°7’11.205643 0.0083 20.1685 −0.0137 −0.0004 [2009.37– 8’’ ’’ ±0.2 ±6.0 mm ±0.1 mm ±0.5 mm 2019.55] ±1.6 mm ±2.2 mm mm GVD8 N34°50’52.61221 Ε24°7’11.399214 22.2760 −0.0143 0.0078 0.0004 [2010.36– 1’’ ’’ ±6.4 mm ±0.2 mm ±0.2 ±0.5 mm

Remote Sens. 2020, 12, 2642 11 of 39

Table 2. The International Terrestrial Reference Frame 2014 (ITRF2014) coordinates and velocities of two pairs of GNSS stations operating at the CDN1 transponder (CDN0 and CDN2) and Gavdos sea-surface (GVD7 and GVD8) Cal/Val sites. The time span and uncertainties in the form of a weighted root mean square error of each component are also given. Remote Sens. 2020, 12, x FOR PEER REVIEW 11 of 39

Ellipsoid N_velocity E_velocity UP_velocity Site/Time Span Lat [degree] Lon [degree] 1 1 1 2019.99] ±1.8 mm ±2.2 mm Height [m] [m yr− ] [m mm yr− ] [m yr− ] CDN0In order toN35 assess20 16.024375 the values E23for 46the46.854809 ellipsoidal he1049.5186ight for each pair0.0126 of the GNSS0.0076 receivers0.0002 (i.e., ◦ 0 00 ◦ 0 00 − [2014.49–2019.99]the CDN0 with the CDN23.1 mm and the GVD72.1 mmwith the GVD8)8.0 mm their antenna0.3 mm reference0.5 points mm have to1.0 be mm ± ± ± ± ± ± geodeticallyCDN2 tiedN35 together20 16.291109 but alsoE23 connected46 46.829112 to other1050.4068 reference control0.0127 points in0.0070 the vicinity. 0.0025This ◦ 0 00 ◦ 0 00 − − [2016.40–2019.99] 1.8 mm 2.2 mm 8.5 mm 0.4 mm 0.5 mm 2.0 mm task is achieved through± precise leveling± that provides± height differences± with± uncertainty ±of the GVD7 N34 50 52.744568 E24 7 11.205643 20.1685 0.0137 0.0083 0.0004 order of ±1 mm. These◦ 0 levelling00 surveys◦ 0 are carried00 out on a semi-annual− basis as well as during− a [2009.37–2019.55] 1.6 mm 2.2 mm 6.0 mm 0.1 mm 0.2 mm 0.5 mm major event (i.e., GNSS± antenna replacement,± earthquake,± etc.). The± derived uncertainty± of ±GNSS GVD8 N34 50 52.612211 E24 7 11.399214 22.2760 0.0143 0.0078 0.0004 processing is given◦ in0 Table 002. The ◦final0 value00 for uncertainties in− the determination of absolute [2010.36–2019.99] 1.8 mm 2.2 mm 6.4 mm 0.2 mm 0.2 mm 0.5 mm coordinates for the± reference benchmarks± on each Ca±l/Val site are± used as input± in the processing± chain to derive the FRM constituent uncertainty. GNSS Processing Constituent 3.1.1. GNSS Processing Constituent The positioning results presented in Table2 are those derived by relative positioning processing. Using theThe same positioning GNSS observations, results presented the alternative in Table technique2 are those of Precisederived Point by relative Positioning positioning (i.e., GIPSY scientificprocessing. software Using [26]) isthe also same applied GNSS to deriveobservations, solutions the for alternative the coordinates technique and of velocities Precise ofPoint the same GNSSPositioning stations. Processing(i.e., GIPSY scientific of the same software GNSS [26]) observations is also applied with to diderivefferent solutions processing for the strategies coordinates ensures and velocities of the same GNSS stations. Processing of the same GNSS observations with different validity of the derived absolute positioning values, and follows the FRM4ALT study recommendation processing strategies ensures validity of the derived absolute positioning values, and follows the to use multiple approaches to arrive at the same result [24]. The results of different processing strategies FRM4ALT study recommendation to use multiple approaches to arrive at the same result [24]. The are shownresults inof Figuredifferent5 forprocessing the GVD8 strategies Cal /Val are siteshown in Gavdos.in Figure 5 for the GVD8 Cal/Val site in Gavdos.

Figure 5. Dispersion values for the height component of the GVD8 GNSS station in the form of coordinate time series as derived by relative diﬀerential (upper image) and precise point positioning Figure 5. Dispersion values for the height component of the GVD8 GNSS station in the form of processing (lower image). The reference epoch is 2013.5 and solutions refer to the ITRF2014 reference coordinate time series as derived by relative differential (upper image) and precise point positioning frame. The gap observed in the period between 2016.5 and 2017.6 corresponds to construction works processing (lower image). The reference epoch is 2013.5 and solutions refer to the ITRF2014 reference carried out at the Gavdos harbor that forced temporary decommissioning of GVD8.

Remote Sens. 2020, 12, 2642 12 of 39

The coordinates and velocities of each GNSS station as determined by diverse GNSS processing techniques have been found to agree at the mm-level and sub-mm level, respectively (Table3).

Table 3. GNSS processing results (Cartesian coordinates and velocities) for the GVD8 station as derived by the relative diﬀerential and precise point positioning techniques.

GVD8 Station Relative Diﬀerential Precise Point Positioning Diﬀerence X [m] 4,782,603.4086 4,782,603.4080 0.6 mm Y [m] 2,141,348.9747 2,141,348.9748 0.1 mm Z [m] 3,624,048.9145 3,624,048.9142 0.3 mm

VX [m/yr] 0.0042 0.0035 0.7 mm/yr

VY [m/yr] 0.0105 0.0104 0.1 mm/yr V [m/yr] 0.0117 0.0117 0.0 mm/yr Z − − Time Span [yr] 2010.4973–2019.1712 2011.0060–2019.1743

In the following subsection, the uncertainty for the sea-surface and transponder Cal/Val, as implemented at the PFAC, is given along with the veriﬁcation that in-situ measurements are in compliance with the FRM4ALT standards.

3.2. Uncertainty Budget for Sea-Surface Calibration Apart from the uncertainty constituent in the determination of the absolute coordinates for the Cal/Val site, presented in Section 3.1, other major contributing sources of uncertainty in sea-surface calibration are: (1) determination of the sea-surface height at the Cal/Val location, (2) transfer of sea-surface height from the Cal/Val site to open sea, and (3) processing errors. Estimations of the uncertainties at the PFAC are explained using indicative examples.

3.2.1. Sea-Surface Height Constituent Measurements obtained by tide gauges are used to determine the sea surface height above a reference ellipsoid at the PFAC Cal/Val sites. According to FRM standards, at least two such instruments of diverse make and operating principle are to operate at each site. This redundant instrumentation permits eﬃcient monitoring of each instrument’s performance and inter-validation of their measurements. For example, in Gavdos Cal/Val, there are ﬁve continuously operating tide gauges: KVR3 (radar), KVR4 (acoustic), KVR5 (pressure), KVR6 (radar and operating in a stilling well) and KVR7 (radar) (Figure6). Remote Sens. 2020, 12, x FOR PEER REVIEW 13 of 39

Figure 6. The current set up of the “Karave” Cal/Val site in Gavdos harbor for sea-surface Figure 6. The current set up of the “Karave” Cal/Val site in Gavdos harbor for sea-surface calibrations.calibrations. Five tideFive gaugestide gauges (KVR3-radar, (KVR3-radar, KVR4-acoustic, KVR4-acoustic, KVR5-pressure, KVR5-pressure, KVR6-radar KVR6-radar in a stilling in a stilling well, andwell, KVR7-radar), and KVR7-radar), two GNSS two GNSS receivers receivers (GVD7 (GVD and7 GVD8)and GVD8) and oneand meteorologicalone meteorological station station comprise the maincomprise scientiﬁc the instrumentationmain scientific instrumentation of the Gavdos of the Cal Gavdos/Val site. Cal/Val site.

The overall performance of these tide gauges is monitored on a daily and/or monthly basis. A 24-hour recording of four of those tide gauges, on 14-Oct-2019, is given in Figure 7.

Figure 7. Single-day (14-October-2019) water level measurements of the KVR3 (radar), KVR4 (acoustic), KVR6 (radar in a stilling well) and KVR7 (radar) tide gauges operating at the Gavdos Cal/Val site.

A more detailed analysis of water level observations involves, among others, the evaluation of the Van de Casteele diagrams on a daily basis. The shape of the Van de Casteele diagram provides a qualitative illustration of the type of error involved in tide gauge measurements [27]. Simultaneous water level heights of a certain tide gauge are compared against a master reference tide gauge (Figure 8). The x-axis corresponds to the difference between the two tide gauge measurements over a full tidal cycle. The y-axis is the “true” sea-surface height as measured by the reference tide gauge. The shape of the diagram may be used to identify the source of any deviation between the measurements obtained by the reference and the tide gauge under investigation [28]. In Gavdos, the KVR3 radar tide gauge is considered to be the reference water level sensor. The Van de Casteele test is applied and the performance of the remaining tide gauges (i.e., KVR4, KVR6, KVR7) is monitored. Various sampling intervals exist for the different tide gauges. In order to apply the Van de Casteele diagram, the daily tide gauge records are averaged, when needed, to match a 6-min sampling interval. Figure 8 presents an indicative example of such a single day

Remote Sens. 2020, 12, x FOR PEER REVIEW 13 of 39

Figure 6. The current set up of the “Karave” Cal/Val site in Gavdos harbor for sea-surface calibrations. Five tide gauges (KVR3-radar, KVR4-acoustic, KVR5-pressure, KVR6-radar in a stilling Remote Sens. 2020, 12, 2642 13 of 39 well, and KVR7-radar), two GNSS receivers (GVD7 and GVD8) and one meteorological station comprise the main scientific instrumentation of the Gavdos Cal/Val site.

The overallThe overall performance performance of theseof these tide tide gauges gauges is monitoredis monitored on on a a daily daily and and/or/or monthly monthly basis.basis. A 24-h recording24-hour of recording four of those of four tide of gauges,those tide on gauges, 14-Oct-2019, on 14-Oct-2019, is given is in given Figure in Figure7. 7.

Figure 7. Single-day (14-October-2019) water level measurements of the KVR3 (radar), KVR4 (acoustic), Figure 7. Single-day (14-October-2019) water level measurements of the KVR3 (radar), KVR4 KVR6 (radar in a stilling well) and KVR7 (radar) tide gauges operating at the Gavdos Cal/Val site. (acoustic), KVR6 (radar in a stilling well) and KVR7 (radar) tide gauges operating at the Gavdos A moreCal/Val detailed site. analysis of water level observations involves, among others, the evaluation of the Van de Casteele diagrams on a daily basis. The shape of the Van de Casteele diagram provides a A more detailed analysis of water level observations involves, among others, the evaluation of qualitativethe Van illustration de Casteele ofdiagrams the type onof a daily error basis. involved The shape in tide of gaugethe Van measurements de Casteele diagram [27]. provides Simultaneous a waterqualitative level heights illustration of a certain of the tide type gauge of error are invo comparedlved in againsttide gauge a master measurements reference [27]. tide Simultaneous gauge (Figure 8). The x-axisRemotewater Sens. correspondslevel 2020 heights, 12, x FOR of to PEER a the certain REVIEW diﬀerence tide gauge between are compared the two tide against gauge a master measurements reference overtide 14gauge a of full 39 tidal cycle.Remote(Figure The Sens. y-axis 8). 2020 The is, 12x-axis the, x FOR “true” corresponds PEER sea-surface REVIEW to the height difference as measured between the by two the tide reference gauge measurements tide gauge. The 14over of shape 39 a of (14-October-2019) monitoring for the KVR4 and the KVR7 sensors. It can be seen that, for this the diagramfull tidal may cycle. be The used y-axis to identify is the “true” the sourcesea-surf oface any height deviation as measured between by the reference measurements tide gauge. obtained particular day, all tide gauge records agree with an uncertainty of the order of ±1cm. by the(14-October-2019)The reference shape of and the the monitoringdiagram tide gauge may for underbethe usedKVR4 investigation to and identi thefy KVR7 [the28 ].source sensors. of Itany can deviation be seen that,between for thisthe particularmeasurements day, obtainedall tide gauge by the records reference agree and with the an tide uncertainty gauge under of the investigation order of ±1cm. [28]. In Gavdos, the KVR3 radar tide gauge is considered to be the reference water level sensor. The Van de Casteele test is applied and the performance of the remaining tide gauges (i.e., KVR4, KVR6, KVR7) is monitored. Various sampling intervals exist for the different tide gauges. In order to apply the Van de Casteele diagram, the daily tide gauge records are averaged, when needed, to match a 6-min sampling interval. Figure 8 presents an indicative example of such a single day

Figure 8. The Van de Casteele diagrams applied for tide gauge records at the Gavdos Cal/Val site on 14 OctoberFigure 20198. The. TheVan radarde Casteele KVR3 diagrams sensor is applied considered for tide to begauge the referencerecords at againstthe Gavdos which Cal/Val the performance site on of the14Figure acoustic October 8. The KVR4 2019. Van (upper), deThe Casteele radar and KVR3diagrams radar KVR7sensor applied (lower)is cons for tideidered tide gauge gauges to berecordsis the evaluated. referenceat the Ga Each vdosagainst diagramCal/Val which site represents the on the seaperformance14 levelOctober (SSH, 2019. of ythe axis)The acoustic versusradar KVR4 KVR3 the (upper), di sensorﬀerence andis betweencons radaideredr KVR7 records to (lower) be ofthe thetide reference twogauges instruments againstis evaluated. which (∆ H,Each the x-axis). The samplingdiagramperformance represents rate of is the 6min. acousticthe sea levelKVR4 (SSH, (upper), y axis) and radaversusr KVR7 the differe (lower)nce tide between gauges records is evaluated. of the Eachtwo instrumentsdiagram represents (ΔH, x-axis). the sea The level sampling (SSH, rate y axis) is 6min. versus the difference between records of the two instruments (ΔH, x-axis). The sampling rate is 6min. Monitoring of the long-term performance of all tide gauges with an external and independent systemMonitoring without interruptingof the long-term their performance measurements of allposes tide agauges challenge with in an the external PFAC andoperations. independent Two approachessystem without are followed:interrupting In theirthe first, measurements the monthly poses means a challenge of each intide the gauge PFAC measurement operations. Two are monitoredapproaches to are verify followed: their overall In the performance. first, the mont Thehly second means uses of an each external tide referencegauge measurement instrument (i.e., are tidemonitored pole) for to verifysea-surface their overallheight performance.determination. The A selevelingcond uses survey an external is carried reference out to tieinstrument the tide pole(i.e., andtide tidepole) gauge for sea-surface reference points.height Thedetermination. Van de Casteele A leveling diagrams survey are isagain carried created, out to but tie using the tide the poletide poleand tideas reference. gauge reference This procedure points. The is carried Van de out Casteele at least diagrams once per yearare again and acreated, detailed but record using is thekept. tide If thepole difference as reference. between This procedure the tide pole is carried and one out of at the least tide once gauges per year exceeds and acertain detailed criteria record (i.e., is kept. 1 cm), If thenthe difference the external between validation the tideis repeated pole and within one of tw theo months. tide gauges If the exceeds difference certain continues criteria to (i.e.,exceed 1 cm), the criteriathen the set, external then the validation tide gauge is repeated is decommissioned within two andmonths. sent Ifto theexternal difference facilities continues for calibration. to exceed This the techniquecriteria set, is then also the used tide to gauge precisely is decommissioned estimate the electrical and sent center, to external i.e., the facilities measuring for calibration. reference point This (“zero”technique point) is also of theused tide to gaugeprecisely records. estimate the electrical center, i.e., the measuring reference point (“zero” point) of the tide gauge records. 3.2.2. Reference Surface Constituent 3.2.2.In Reference the PFAC, Surface the sea-surface Constituent height has to be transferred from the Cal/Val location in the harbor to openIn the sea PFAC, where the satellite sea-surface measurements height has areto be ma trande sferreduncontaminated from the Cal/Val by land location mass (about in the harbor20 km away).to open To sea accomplish where satellite this transfer, measurements the reference are ma geoidde uncontaminated and the mean dynamicby land masstopography (about (MDT)20 km haveaway). to Tobe accuratelyaccomplish known. this transfer, In the past,the reference terrestri al,geoid marine and and the airbornemean dynamic gravity topography surveys have (MDT) been have to be accurately known. In the past, terrestrial, marine and airborne gravity surveys have been

Remote Sens. 2020, 12, 2642 14 of 39

In Gavdos, the KVR3 radar tide gauge is considered to be the reference water level sensor. The Van de Casteele test is applied and the performance of the remaining tide gauges (i.e., KVR4, KVR6, KVR7) is monitored. Various sampling intervals exist for the different tide gauges. In order to apply the Van de Casteele diagram, the daily tide gauge records are averaged, when needed, to match a 6-min sampling interval. Figure8 presents an indicative example of such a single day (14-October-2019) monitoring for the KVR4 and the KVR7 sensors. It can be seen that, for this particular day, all tide gauge records agree with an uncertainty of the order of 1cm. ± Monitoring of the long-term performance of all tide gauges with an external and independent system without interrupting their measurements poses a challenge in the PFAC operations. Two approaches are followed: In the first, the monthly means of each tide gauge measurement are monitored to verify their overall performance. The second uses an external reference instrument (i.e., tide pole) for sea-surface height determination. A leveling survey is carried out to tie the tide pole and tide gauge reference points. The Van de Casteele diagrams are again created, but using the tide pole as reference. This procedure is carried out at least once per year and a detailed record is kept. If the difference between the tide pole and one of the tide gauges exceeds certain criteria (i.e., 1 cm), then the external validation is repeated within two months. If the difference continues to exceed the criteria set, then the tide gauge is decommissioned and sent to external facilities for calibration. This technique is also used to precisely estimate the electrical center, i.e., the measuring reference point (“zero” point) of the tide gauge records.

3.2.2. Reference Surface Constituent In the PFAC, the sea-surface height has to be transferred from the Cal/Val location in the harbor to open sea where satellite measurements are made uncontaminated by land mass (about 20 km away). To accomplish this transfer, the reference geoid and the mean dynamic topography (MDT) have to be accurately known. In the past, terrestrial, marine and airborne gravity surveys have been carried out in the vicinity of the PFAC Cal/Val sites. Regional geoid and MDT models have been developed and validated by dedicated ﬁeld campaigns using boats and buoys. Details on those reference surfaces are presented in [29]. In 2018, additional gravity ﬁeld measurements have been carried out at the PFAC Cal/Val sites and around the Gavdos island (Figure9). A CG-5 microgravity sensor with a resolution of 1 µGal and standard deviation less than 5 µGals has been used. Three control points with known absolute gravity values have been chosen as reference points to conduct this gravity survey. Absolute gravity measurements have been conducted in the past by the Hellenic Military Geographical Service and international research institutes. These absolute gravity points are located at Chania Airport (AIR), and Sfakia (SFK) and Paleochora (PAL) villages (Figure9). Absolute gravity values for various locations at the PFAC sites have been determined by relying upon those absolute reference points (AIR, PAL, SFK), and measuring relative gravity between them and the PFAC Cal/Val sites. These new gravity observations have been incorporated in the regional gravity database to improve the geoid accuracy. Remote Sens. 2020, 12, 2642 15 of 39 Remote Sens. 2020, 12, x FOR PEER REVIEW 15 of 38

Figure 9.9. Terrestrial gravitygravity measurementsmeasurements carried out in 2018 at variousvarious locations around the PFAC CalCal/Val/Val sitessites in in Crete Crete (top-left) (top-left) and and Gavdos Gavdos (top-right). (top-right). Locations Locations with knownwith known absolute absolute gravity gravity values arevalues marked are marked with a triangle.with a triangle. Gravity Gravity observations observat areions shown are at shown the PAL at the absolute PAL absolute gravity point gravity in southpoint Cretein south (bottom-left), Crete (bottom-left), the RDK1 the (bottom-middle) RDK1 (bottom-middle) and Karave and/Gavdos Karave/Gavdos (bottom-right) (bottom-right) Cal/Val sites. Cal/Val sites. 3.2.3. FRM Uncertainty Estimation for Sea-Surface Calibration 3.2.3.For FRM sea-surface Uncertainty calibration, Estimation the principalfor Sea-Surface sources Calibration of uncertainty arise from (1) absolute coordinates of theFor Cal /sea-surfaceVal site (Section calibration, 3.1), (2) thethe sea-surface principal heightssources (Section of uncertainty 3.2.1), and arise (3) the from reference (1) absolute surfaces tocoordinates transfer sea-surface of the Cal/Val heights site to(Section open sea3.1), (Section (2) the 3.2.2sea-surface). A complete heights list (Section and analysis 3.2.1), and of various (3) the constituentsreference surfaces of uncertainty to transfer involved sea-surface in sea-surface heights calibrationto open sea may (Section be found 3.2.2). in [A10 ].complete The uncertainty list and foranalysis each constituentof various constituents is classified intoof uncertainty two categories: involved Type in A sea-surface and Type B.calibration Their leading may dibeff erencefound in is that[10]. Type The uncertainty A uncertainties for each are evaluated constituent by statisticalis classified analysis into two of observations,categories: Type whereas A and Type Type B onesB. Their are estimatedleading difference by scientific is that judgment Type A based uncertainties on manufacturer’s are evaluated specs, by prior statistical experience, analysis handbooks, of observations, etc. [30]. whereasTable Type4 quantifies B ones the are uncertainty estimated for by the sea-surfacescientific judgment Cal/Val as implementedbased on manufacturer’s in the PFAC. It specs, also updates prior theexperience, uncertainty handbooks, budget analysis etc. [30]. that has been previously presented in [14,24]. The FRM standardized uncertainty at a 68% confidence level for the sea-surface calibration is calculated to be 31.91 mm. Table 4 quantifies the uncertainty for the sea-surface Cal/Val as implemented± in the PFAC. It also updates the uncertainty budget analysis that has been previously presented in [14,24]. The FRM standardized uncertainty at a 68% confidence level for the sea-surface calibration is calculated to be ± 31.91 mm.

Remote Sens. 2020, 12, 2642 16 of 39

Table 4. Uncertainty budget for the sea-surface calibration. The final standardized uncertainty (at 68% confidence level), described by a Root-Sum-Square value, has been computed as: q FRM (Uncertainty) = 31.91 mm = (0.10)2 + (3.50)2 + (2.00)2 + + (0.30)2 + (5.80)2 + (11.55)2. SSH ± ··· Standard Uncertainty Error Constituent Type Uncertainty Estimate (Confidence 68%) GNSS Height Repeatability 1 A 0.10 mm 0.10 mm ± GNSS Receiver 2 B 6.00 mm 3.50 mm ± GNSS Antenna Reference Point 1 B 2.00 mm 2.00 mm ± Water Level 1 B 1.30 mm 1.30 mm ± Tide-Gauge Zero Reference 1 A 0.15 mm 0.15 mm ± Tide Gauge Vertical Alignment 2 B 2.40 mm 1.39 mm ± Tide-Gauge Sensor Certificate 1 B 5.50 mm 5.50 mm ± Leveling Repeatability 1 A 0.11 mm 0.11 mm ± Monumentation 2 B 1.10 mm 0.64 mm ± Vertical Misalignment 2 B 1.00 mm 0.60 mm ± Leveling Observer 2 B 1.00 mm 0.60 mm ± Leveling Instrument/Method 2 B 1.00 mm 0.60 mm ± Tide Pole Reading During Calibration 2 B 1.00 mm 0.60 mm ± Geoid and Mean Dynamic Topography 1 B 30.00 mm 30.00 mm ± Processing and Approximations 2 B 0.50 mm 0.30 mm ± Geoid Slope/Offshore Transfer 2 B 10.00 mm 5.80 mm ± Unaccounted effects 2 B 20.00 mm 11.55 mm ± Root-Sum-Squared Uncertainty 31.91 mm ± 1 assuming that residuals are random and normally distributed, with no autocorrelation. 2 assuming that uncertainty constituents follow the uniform distribution.

3.3. Uncertainty Budget for Transponder Calibration Following a similar analysis, the FRM uncertainty for the transponder calibration has also been determined. Here, the atmospheric propagation delays of satellite signals and the transponder’s internal delay dominate the overall uncertainty [9].

3.3.1. Atmospheric Propagation Delay Constituent Determination of the signal delays caused by the atmosphere constitutes the most challenging task in transponder calibration. This is because the radiometer on-board the satellite does not operate properly over land. Thus, other means have to be figured out to correct signal propagation delays when the altimeter flies over a transponder land site. In the PFAC, this is carried out using GNSS-derived corrections for the atmospheric delays. The GNSS-derived delays refer to both the ionospheric and the zenith tropospheric (wet and dry component) delays at the transponder Cal/Val site. The ionosphere delays over the PFAC are regularly stable. They do not change rapidly and can be accurately determined via GNSS processing with an uncertainty of the order of 1 mm [24]. The dry (or hydrostatic) troposphere delay is also stable. It can ± also be accurately modeled using in-situ surface pressure measurements made by meteorological sensors. The wet component of the troposphere is dynamic, presents unstable variability rapidly changing over time, and cannot be easily modeled. In GNSS processing, this wet troposphere delay is estimated as the remaining part of the total delay minus the dry troposphere delay. Different GNSS processing methodologies generate diverse atmospheric propagation delays (Figure 10). Remote Sens. 2020, 12, x FOR PEER REVIEW 17 of 39

Determination of the signal delays caused by the atmosphere constitutes the most challenging task in transponder calibration. This is because the radiometer on-board the satellite does not operate properly over land. Thus, other means have to be figured out to correct signal propagation delays when the altimeter flies over a transponder land site. In the PFAC, this is carried out using GNSS-derived corrections for the atmospheric delays. The GNSS-derived delays refer to both the ionospheric and the zenith tropospheric (wet and dry component) delays at the transponder Cal/Val site. The ionosphere delays over the PFAC are regularly stable. They do not change rapidly and can be accurately determined via GNSS processing with an uncertainty of the order of ±1 mm [24]. The dry (or hydrostatic) troposphere delay is also stable. It can also be accurately modeled using in-situ surface pressure measurements made by meteorological sensors. The wet component of the troposphere is dynamic, presents unstable variability rapidly changing over time, and cannot be easily modeled. In GNSS processing, this wet troposphere delay is estimated as the remaining part of the total delay minus the dry troposphere Remote Sens. 2020, 12, 2642 17 of 39 delay. Different GNSS processing methodologies generate diverse atmospheric propagation delays (Figure 10).

Figure 10.10. The wet troposphere (up) and total tropospheretroposphere (down) delays coming out afterafter GNSSGNSS processingprocessing withwith precise precise point point positioning positioning (GIPSY) (GIPSY) and relativeand relative diﬀerential differential (GAMIT) (GAMIT) positioning positioning during Sentinel-3Aduring Sentinel-3A Pass No. Pass 14 transponderNo.14 transponder calibrations calibrations at the CDN1 at the CalCDN1/Val Cal/Val site. site.

The didifferenceﬀerence observed observed in in Figure Figure 10 10 may may be be attributed attributed to variousto various models models applied applied to determine to determine the drythe dry troposphere troposphere delay delay in theirin their derivation, derivation, the the means means implemented implemented to to estimate estimate surfacesurface pressurepressure (i.e.,(i.e.,Remote models models Sens. 2020 or or, in-situ12 in-situ, x FOR measurements), PEER measurements), REVIEW as well as wellas to the as tosampling the sampling rate used rate for GNSS used fordata GNSS processing18 data of 39 processing(Figure 11). (Figure 11).

Figure 11. Hourly zenith wet delays as estimated by GNSS processing using relative diﬀerential positioningFigure 11. (left)Hourly and zenith precise wet point delays positioning as estimated (right) by forGNSS the Sentinel-3Aprocessing using transponder relative calibration,differential onpositioning 27-October-2019 (left) and at theprecise CDN1 point transponder positioning Cal (rig/Valht) site. for the Sentinel-3A transponder calibration, on 27-October-2019 at the CDN1 transponder Cal/Val site. In order to be FRM-compliant, the PFAC should verify the GNSS-derived wet tropospheric delaysIn at order the timeto be of FRM-compliant, satellite overpass theusing PFAC an shou independentld verify the technique. GNSS-derived A ground wet tropospheric microwave radiometer,delays at the provided time of on satellite loan from overpass the European using Space an independent Agency, has beentechnique. installed A inground 2019 at microwave the CDN1 radiometer, provided on loan from the European Space Agency, has been installed in 2019 at the CDN1 transponder Cal/Val site to fulfill this FRM requirement (Figure 12). This instrument measures the brightness temperature of microwave radiation from the atmosphere and determines the respective contribution/attenuation from water vapor and liquid water. Preliminary results indicate an agreement of the wet troposphere delays from the radiometer and the GNSS-derived delays. At this preliminary stage, dispersion of wet delay is in the order of ±1–2 cm (maximum) (Figure 12). Research and field measurements are ongoing. As more data are built up, more robust and reliable results on the final wet troposphere estimation will be estimated.

Remote Sens. 2020, 12, x FOR PEER REVIEW 18 of 38 Remote Sens. 2020, 12, 2642 18 of 39 In order to be FRM-compliant, the PFAC should verify the GNSS-derived wet tropospheric delays at the time of satellite overpass using an independent technique. A ground microwave transponderradiometer, Calprovided/Val site on to loan fulfill from this the FRM European requirement Space (Figure Agency, 12 ).has This been instrument installed measuresin 2019 at the the brightnessCDN1 transponder temperature Cal/Val of microwave site to radiationfulfill this from FRM the atmosphererequirement and(Figure determines 12). This the respectiveinstrument contributionmeasures the/attenuation brightness from temperature water vapor of andmicrowave liquid water. radiation Preliminary from the results atmosphere indicate and an agreementdetermines ofthe the respective wet troposphere contribution/attenuation delays from the radiometer from wate andr thevapor GNSS-derived and liquid delays.water. AtPreliminary this preliminary results stage,indicate dispersion an agreement of wet delayof the is wet in thetroposphere order of 1–2delays cm from (maximum) the radiometer (Figure 12 and). Research the GNSS-derived and field ± measurementsdelays. At this are ongoing. preliminary As more stage, data dispersion are built up, of more wet robust delay and is reliable in the results order on the of final ±1–2 wet cm troposphere(maximum) estimation will be estimated.

FigureFigure 12. 12.The The microwave microwave radiometer radiometer operating operating at the at the CDN1 CD transponderN1 transponder Cal/ ValCal/Val site (upper site (upper image) image) and preliminaryand preliminary assessment assessment of GNSS-derived of GNSS-derived wet tropospheric wet tropospheric delays against delays radiometer’s against measurements radiometer’s (middle).measurements The lower (middle). picture The shows lower the picture setup of shows the radiometer the setup andof the the radiometer two GNSS stations.and the two GNSS stations. An alternative technique to determine wet tropospheric delay can be derived from measurements of theAn imaging alternative spectrometer technique ﬂown to aboard determine the Sentinel-3 wet tropospheric mission. It relies delay upon can the be Integrated derived Water from measurements of the imaging spectrometer flown aboard the Sentinel-3 mission. It relies upon the Integrated Water Vapor (𝐼𝑊𝑉) products derived from the Ocean and Land Color Instrument (OLCI). There is a direct relation between the zenith wet delay (𝛧𝑊𝐷) and the 𝐼𝑊𝑉, Equation (1). Remote Sens. 2020, 12, 2642 19 of 39

Vapor (IWV) products derived from the Ocean and Land Color Instrument (OLCI). There is a direct relation between the zenith wet delay (ZWD) and the IWV, Equation (1).    6   10  IWVOLCI = [F] ZWD =   ZWD (1)  C1  · ρ RV + C2 · · · Tm where: F is a temperature-dependent factor given in the right-hand equation, IWV is the atmospheric 2 water vapor content in (kg m− ), ρ is the mass density of water, RV the gas constant for water vapor in 1 1 2 1 1 (N M K− gr− ), C1 and C2 are refractivity coeﬃcients in (K hPa− ) and (K hPa− ), respectively, Tm is the weighted temperature of the atmosphere in (◦K) and ZWD is the zenith wet delay caused by the wet troposphere in (mm). The OLCI instrument of Sentinel-3 has a swath of 1270 km and thus permits global coverage within 2 days. OLCI is an optical instrument that captures images during daytime. The spatial resolution of these images is 300 m (at nadir). Both the SAR radar altimeter (SRAL) and OLCI are collocated but OLCI measurements are only available during periods of solar illumination. For example, Sentinel-3A SRAL measurements over the CDN1 transponder Cal/Val site are available at 20:00 UTC but no OLCI images are captured at this time of day (actually night). On the other hand, simultaneous measurements of Sentinel-3B SRAL and OLCI instruments are made over the CDN1 Cal/Val site at 08:49:15 UTC every 27 days. Therefore, it is possible to directly compare the OLCI-derived IWV value at the 300m pixel, when the CDN1 site is observed, against the Remoteone GNSS-derived Sens. 2020, 12, x FOR by thePEER ground REVIEW operating GNSS stations (Figure 13). 20 of 39

FigureFigure 13.13. ComparisonComparison of of the the Integrated Integrated Water Water Vapor Vapor as measured as measured by the Oceanby the Land Ocean Color Land Instrument Color Instrument(OLCI) instrument (OLCI) onboardinstrument Sentinel-3B, onboard and Sentin estimatedel-3B, and through estimated GAMIT th andrough GIPSY GAMIT GNSS and processing. GIPSY GNSSThe comparison processing. takes The place comparison at the exact takes time place of Sentinel-3B at the exact transponder time of Sentinel-3B calibrations attransponder the CDN1 calibrationsCal/Val site at and the uses CDN1 GNSS Cal/Val observations site and fromuses theGNSS CDN0 observations station. from the CDN0 station.

3.3.2. InFRM Figure Uncertainty 13, this Estimation comparison for is Transponder made for the Calibration CDN0 GNSS station using three independent techniques: relative diﬀerential, precise point positioning and OLCI imagery. In GAMIT, the IWV is computedSeveral as attempts a secondary have product been in made the GNSS in [14,24] processing. to estimate In GIPSY, the the uncertainty IWV value hasof beenthe results estimated in transponderusing Equation calibration. (1). In this Every present time, analysis, a new and only bette cloud-freer estimate conditions for some are uncertainty considered constituents in this OLCI is presentedinvestigation as whendifferent a Sentinel-3B elements transponderof the uncert calibrationainty budget takes are place. improved. The comparison, In fact, asa shownprimary in applicationFigure3, starts of aftera good the uncertainty Sentinel-3B satellitebudget reachedis to highlight its nominal the orbit.largest source of uncertainty in a measurementIt seems that, system, at the and moment, then target there arethat four aspect diﬀerent for further techniques improvement. for determining In this the manner, wet troposphere effort is givendelays to during the areas transponder that will deliver calibrations the maximum of the CDN1 improvement Cal/Val site in in performance, West Crete. Asfinally more converging observations to aare reliable accumulated, measure more of the reliable uncertainty results for will transponder be reported. calibrations. In the same vein, Table 5 updates previous estimations for the uncertainty of the transponder calibration at the CDN1 transponder Cal/Val site.

Table 5. Uncertainty budget for the transponder calibration. The final standardized uncertainty (at 68% confidence level), described as Root-Sum-Square value, has been computed as:

𝐹𝑅𝑀(𝑈𝑛𝑐𝑒𝑟𝑡𝑎𝑖𝑛𝑡𝑦) = ±34.46 𝑚𝑚 = (0.13) + (3.50) + (2.00) +⋯+(17.32) + (0.17) + (11.55)

Standard Typ Uncertainty Error Constituent Uncertainty e Estimate (Confidence 68%) GNSS Height1 A 0.13 mm ±0.13 mm GNSS Receiver2 B 6.00 mm ±3.50 mm GNSS Antenna Reference B 2.00 mm ±2.00 mm Point1 Measured Range2 B 3.00 mm ±1.73 mm Transponder Internal Delay1 B 30.00 mm ±15.00 mm Dry Tropospheric Delay2 B 2.00 mm ±1.15 mm Wet Tropospheric Delay2 B 14.00 mm ±8.08 mm Ionospheric Delay2 B 4.00 mm ±2.31 mm Geophysical Corrections2 B 20.00 mm ±11.55 mm Satellite Orbit Height2 B 30.00 mm ±17.32 mm Pseudo-Doppler Correction1 B 2.00 mm ±2.00 mm Leveling B 1.00 mm ±1.00 mm Instrument/Method1 Transponder Leveling1 A 0.50 mm ±0.16 mm

Remote Sens. 2020, 12, 2642 20 of 39

3.3.2. FRM Uncertainty Estimation for Transponder Calibration Several attempts have been made in [14,24] to estimate the uncertainty of the results in transponder calibration. Every time, a new and better estimate for some uncertainty constituents is presented as different elements of the uncertainty budget are improved. In fact, a primary application of a good uncertainty budget is to highlight the largest source of uncertainty in a measurement system, and then target that aspect for further improvement. In this manner, effort is given to the areas that will deliver the maximum improvement in performance, finally converging to a reliable measure of the uncertainty for transponder calibrations. In the same vein, Table5 updates previous estimations for the uncertainty of the transponder calibration at the CDN1 transponder Cal/Val site.

Table 5. Uncertainty budget for the transponder calibration. The final standardized uncertainty (at 68% confidence level), described as Root-Sum-Square value, has been computed as: q FRM (Uncertainty) = 34.46 mm = (0.13)2 + (3.50)2 + (2.00)2 + + (17.32)2 + (0.17)2 + (11.55)2. TRP ± ··· Standard Uncertainty Error Constituent Type Uncertainty Estimate (Confidence 68%) GNSS Height 1 A 0.13 mm 0.13 mm ± GNSS Receiver 2 B 6.00 mm 3.50 mm ± GNSS Antenna Reference Point 1 B 2.00 mm 2.00 mm ± Measured Range 2 B 3.00 mm 1.73 mm ± Transponder Internal Delay 1 B 30.00 mm 15.00 mm ± Dry Tropospheric Delay 2 B 2.00 mm 1.15 mm ± Wet Tropospheric Delay 2 B 14.00 mm 8.08 mm ± Ionospheric Delay 2 B 4.00 mm 2.31 mm ± Geophysical Corrections 2 B 20.00 mm 11.55 mm ± Satellite Orbit Height 2 B 30.00 mm 17.32 mm ± Pseudo-Doppler Correction 1 B 2.00 mm 2.00 mm ± Leveling Instrument/Method 1 B 1.00 mm 1.00 mm ± Transponder Leveling 1 A 0.50 mm 0.16 mm ± Processing and Approximations 2 B 30.00 mm 17.32 mm ± Orbit Interpolations 2 B 0.30 mm 0.17 mm ± Unaccounted effects 2 B 20.00 mm 11.55 mm ± Root-Sum-Squared Uncertainty 34.46 mm ± 1 Assuming that residuals are random and normally distributed, with no autocorrelation. 2 Assuming that constituent uncertainty follows a uniform distribution.

In this Section, the uncertainty of the sea-surface and transponder calibration and for the procedures followed at the PFAC has been derived, following the FRM4ALT standards.

4. PFAC Calibration Results This section presents the bias and uncertainty of the radar altimeters in Sentinel-3A, Sentinel-3B and Jason-3. First, absolute Cal/Val results are determined for each pass of the altimeters using the sea-surface and/or the transponder techniques. Then, relative calibration is carried out for the tandem phase of Sentinel-3A and Sentinel-3B, followed by crossover analysis for Sentinel-3A/Jason-3.

Processing and B 30.00 mm ±17.32 mm Approximations2 Orbit Interpolations2 B 0.30 mm ±0.17 mm Unaccounted effects2 B 20.00 mm ±11.55 mm Root-Sum-Squared ±34.46 mm Uncertainty 1Assuming that residuals are random and normally distributed, with no autocorrelation. 2 Assuming that constituent uncertainty follows a uniform distribution. In this Section, the uncertainty of the sea-surface and transponder calibration and for the procedures followed at the PFAC has been derived, following the FRM4ALT standards.

4.1.Sentinel-3A Three Sentinel-3A passes are calibrated at the PFAC (Figure 14): The ascending Pass No. 14 with sea-surface and transponder calibration at the Gavdos and the CDN1 Cal/Val sites, respectively, the descending Pass No. 335 also using the Gavdos Cal/Val site, and the descending Pass No. 278 with the CRS1 Cal/Val site. In Sections 3.1 and 3.2, the calibration processing of the in-situ ground measurements and models has been described. Satellite measurements also require some kind of rectification, as the altimeter and radiometer’s observations are contaminated by the land mass of Gavdos and Crete. Regions with valid satellite measurements have to be identified and selected as calibrating regions for comparison with in-situ Cal/Val measurements. This process is carried out for each satellite’s Remote Sens. 2020, 12, 2642 21 of 39 pass. Using the latest processing baseline for Sentinel-3A (version 2.61) that is common for all of its

Figure 14.14. TheThe Sentinel-3ASentinel-3A sea-surface sea-surface calibration calibration regions regions for for its Passits Pass No. No. 14, No.14, No. 278 and278 No.and 335No. over335 theover PFAC the PFAC Cal/Val Cal/Val facility facility in west in Crete, west Greece.Crete, Greec Sea-surfacee. Sea-surface calibration calibration is carried is outcarried at three out diatﬀ threeerent seadifferent regions sea based regions upon based the Gavdos,upon the CRS1 Gavdos, and RDK1CRS1 and Cal/ ValRDK1 sites. Cal/Val The CDN1 sites. transponderThe CDN1 transponder Cal/Val site isCal/Val also illustrated site is also with illustrated a red star. with a red star.

In Sections 3.1 and 3.2, the calibration processing of the in-situ ground measurements and models has been described. Satellite measurements also require some kind of rectification, as the altimeter and radiometer’s observations are contaminated by the land mass of Gavdos and Crete. Regions with valid satellite measurements have to be identified and selected as calibrating regions for comparison with in-situ Cal/Val measurements. This process is carried out for each satellite’s pass. Using the latest processing baseline for Sentinel-3A (version 2.61) that is common for all of its cycles, three sea-calibrating regions have been defined at the PFAC, as illustrated in Figure 14. For a successful transponder calibration, the satellite altimeter generally needs to operate in “open-loop” mode. This is necessary to adjust its range gate to operate over the transponder’s elevation and collect range measurements. The “open-loop” mode relies upon high-resolution Digital Elevation Models (DEM) stored in the altimeter or additionally with a direct commanding as in the Sentinel-6 case that additionally allows the fixing of the instrument gain given the transponder characteristics. Using this a-priori knowledge, the altimeter adjusts its internal receive window appropriately, and thus the signal amplified by the transponder can be received by the satellite altimeter. It is not the purpose of this work to explain the steps followed to process the waveforms for pulse-limited (e.g., Poseidon-3B in Jason-3) and delay-Doppler altimeters (i.e., SRAL/Sentinel-3A/B and Poseidon-4 Sentinel-6) to arrive at the value of range bias, as details can be found in [14,31]. Processing of Sentinel-3 waveforms is a complex procedure applied to about 200 radar bursts (of which about 30–40 bursts contain a useful transponder response over 1.5 to 2 s integration time) each of which contains 64 Ku-band pulses. Examples of altimeter waveforms that include the transponder’s echo have been presented in Figure2. The required parameters for transponder calibrations are obtained through Sentinel-3 telemetry and satellite calibration products [14].

4.1.1. Sentinel-3A Pass No. 14 Cal/Val Results The ascending Sentinel-3A Pass No. 14 approaches the Gavdos island from the south. Calibration over sea surface is carried out using the Gavdos Cal/Val facility. About 9 s later, on its ascending pass, the satellite reaches the CDN1 transponder Cal/Val site where it is also calibrated (Figure 15). The bias in sea-surface height and range for Sentinel-3A along its Pass No. 14 are given in Figure 16 for all available passes at the time of writing. Remote Sens. 2020, 12, x FOR PEER REVIEW 22 of 39

For a successful transponder calibration, the satellite altimeter generally needs to operate in “open-loop” mode. This is necessary to adjust its range gate to operate over the transponder’s elevation and collect range measurements. The “open-loop” mode relies upon high-resolution Digital Elevation Models (DEM) stored in the altimeter or additionally with a direct commanding as in the Sentinel-6 case that additionally allows the fixing of the instrument gain given the transponder characteristics. Using this a-priori knowledge, the altimeter adjusts its internal receive window appropriately, and thus the signal amplified by the transponder can be received by the satellite altimeter. It is not the purpose of this work to explain the steps followed to process the waveforms for pulse-limited (e.g., Poseidon-3B in Jason-3) and delay-Doppler altimeters (i.e., SRAL/Sentinel-3A/B and Poseidon-4 Sentinel-6) to arrive at the value of range bias, as details can be found in [31,14]. Processing of Sentinel-3 waveforms is a complex procedure applied to about 200 radar bursts (of which about 30–40 bursts contain a useful transponder response over 1.5 to 2 seconds integration time) each of which contains 64 Ku-band pulses. Examples of altimeter waveforms that include the transponder’s echo have been presented in Figure 2. The required parameters for transponder calibrations are obtained through Sentinel-3 telemetry and satellite calibration products [14].

4.1.1. Sentinel-3A Pass No. 14 Cal/Val Results Τhe ascending Sentinel-3A Pass No.14 approaches the Gavdos island from the south. Calibration over sea surface is carried out using the Gavdos Cal/Val facility. About 9 seconds later, on its ascending pass, the satellite reaches the CDN1 transponder Cal/Val site where it is also Remote Sens. 2020, 12, 2642 22 of 39 calibrated (Figure 15). The bias in sea-surface height and range for Sentinel-3A along its Pass No.14 are given in Figure 16 for all available passes at the time of writing.

Figure 15.15.The The Sentinel-3A Sentinel-3A Pass Pass No. No. 14 is 14 calibrated is calibrate usingd bothusing sea-surface both sea-surface and transponder and transponder calibration Remote Sens. 2020, 12, x FOR PEER REVIEW 23 of 39 techniquescalibration attechniques the Gavdos at the and Gavdos CDN1 Caland/Val CDN1 facilities, Cal/Val respectively. facilities, respectively.

FigureFigure 16. The16. The latest latest calibration calibration results results for for thethe ascendingascending Pass No. No. 1414 of ofSentinel-3A Sentinel-3A using using the the sea-surfacesea-surface (upper) (upper) and and transponder transponder (lower) (lower) calibrations. calibrations.

The sea-surfaceThe sea-surface bias bias for for Sentinel-3A Sentinel-3A Pass Pass No. No.14 14 is calculated as as −11.9811.98 mm, mm, whereas whereas the range the range − bias withbias thewith transponder the transponder is determined is determined as +4.90 as mm.+4.90 Thesemm. These results results indicate indicate that the that altimeter the altimeter measures largermeasures ranges than larger the ranges “ground than reference” the “ground by referenc+4.90 mme” by with +4.90 the mm present with settings.the present The settings. negative The value negative value of −11.98 mm in the SSH bias seems to support both Cal/Val results. The absolute of 11.98 mm in the SSH bias seems to support both Cal/Val results. The absolute diﬀerence between − difference between the two Cal/Val results is less than 1 cm, whereas the FRM uncertainty of each the twoindependent Cal/Val results technique is less is of than the 1order cm, of whereas ±3 cm. the FRM uncertainty of each independent technique is of the order of 3 cm. ± 4.1.2. Sentinel-3A Pass No. 278 and No. 335 Cal/Val Results Two sea-surface Cal/Val sites support the calibration of the descending Pass No.335 of Sentinel-3A (Figure 17). As the satellite descends, it meets first RDK1 Cal/Val where it is calibrated, and then about 6 seconds later it is again calibrated with the other CalVal site of Gavdos. The results obtained by these two Cal/Val sites agree at the sub-centimeter level. At this stage, the RDK1 Cal/Val results present somewhat larger dispersion compared to Gavdos Cal/Val. Larger scattering for the RDK1 Cal/Val may be caused by the employed calibrating region, covering a 35 km stretch (1000 km depth) of ocean water between Crete and Gavdos. That water stretch is shorter in length than that of the Gavdos south calibrating region (with 3000 m depth), and also fewer uncontaminated by land values are available for the satellite calibration (mountains of 2500 m height in the north of RDK1). Several updates and improvements of the marine geoid and the mean dynamic topography models in support of the RDK1 Cal/Val are currently ongoing.

Remote Sens. 2020, 12, 2642 23 of 39

4.1.2. Sentinel-3A Pass No. 278 and No. 335 Cal/Val Results Two sea-surface Cal/Val sites support the calibration of the descending Pass No.335 of Sentinel-3A (Figure 17). As the satellite descends, it meets ﬁrst RDK1 Cal/Val where it is calibrated, and then about 6 s later it is again calibrated with the other CalVal site of Gavdos. The results obtained by these two Cal/Val sites agree at the sub-centimeter level. At this stage, the RDK1 Cal/Val results present somewhat larger dispersion compared to Gavdos Cal/Val. Larger scattering for the RDK1 Cal/Val may be caused by the employed calibrating region, covering a 35 km stretch (1000 km depth) of ocean water between Crete and Gavdos. That water stretch is shorter in length than that of the Gavdos south calibrating region (with 3000 m depth), and also fewer uncontaminated by land values are available for the satellite calibration (mountains of 2500 m height in the north of RDK1). Several updates and improvements of the marine geoid and the mean dynamic topography models in support of the RDK1 CalRemote/Val Sens. are 2020 currently, 12, x FOR ongoing. PEER REVIEW 24 of 39

FigureFigure 17.17. The latest calibrationcalibration resultsresults forfor thethe descendingdescending PassPass No.No. 335 of Sentinel-3ASentinel-3A usingusing thethe sea-surfacesea-surface calibration site site located located south south of of Crete Crete site site of RDK1 of RDK1 (upper) (upper) and andthe Gavdos the Gavdos Cal/Val Cal sites/Val sites(lower). (lower).

InIn thethe southwest southwest tip tip of Crete,of Crete, the CRS1the CRS1 Cal/Val Cal/Val site provides site provides support forsupport the sea-surface for the sea-surface calibration ofcalibration the descending of the descending pass No. 278 pass of Sentinel-3A. No. 278 of Sentinel Figure 18-3A. presents Figure the18 presents latest results the latest for the results Sentinel-3A. for the The estimated value for the bias in the sea-surface heights is found to be 3.68 mm based on this CRS1 Sentinel-3A. The estimated value for the bias in the sea-surface heights− is found to be −3.68 mm Calbased/Val on site. this CRS1 Cal/Val site.

Figure 18. The latest calibration results for the descending Pass No. 278 of Sentinel-3A using the calibration methodology at the CRS1 Cal/Val site.

In summary, the bias in sea-surface height for Sentinel-3A has been determined to be on average −8.04 mm (±5.5 mm, ±32 mm FRM uncertainty) along its descending pass No.335 based on Gavdos and RDK1 Cal/Val sites, while it is found to be −3.68 mm (±4 mm, ±32 mm FRM uncertainty)

Remote Sens. 2020, 12, x FOR PEER REVIEW 24 of 39

Figure 17. The latest calibration results for the descending Pass No. 335 of Sentinel-3A using the sea-surface calibration site located south of Crete site of RDK1 (upper) and the Gavdos Cal/Val sites (lower).

In the southwest tip of Crete, the CRS1 Cal/Val site provides support for the sea-surface calibration of the descending pass No. 278 of Sentinel-3A. Figure 18 presents the latest results for the Remote Sens. 2020, 12, 2642 24 of 39 Sentinel-3A. The estimated value for the bias in the sea-surface heights is found to be −3.68 mm based on this CRS1 Cal/Val site.

FigureFigure 18. The 18. The latest latest calibration calibration results results for for the the descendingdescending Pass Pass No. No. 278 278 of ofSentinel-3A Sentinel-3A using using the the calibrationcalibration methodology methodology atthe at the CRS1 CRS1 Cal Cal/Val/Val site. site.

In summary,In summary, the bias the inbias sea-surface in sea-surface height height for Sentinel-3A for Sentinel-3A has beenhas been determined determined to beto onbe averageon 8.04average mm ( 5.5−8.04 mm, mm (±5.532 mm mm, FRM ±32 uncertainty)mm FRM uncertainty) along its al descendingong its descending pass No. pass 335 No.335 based based on Gavdos on − ± ± and RDK1Gavdos Cal and/Val RDK1 sites, Cal/Val while sites, it is foundwhile it tois befound3.68 to be mm −3.68 ( mm4 mm, (±4 mm,32 mm±32 mm FRM FRM uncertainty) uncertainty) along − ± ± another descending Pass of No. 278 and using the other CRS1 Cal/Val site. It appears that the bias of Sentinel-3A in sea-surface height, as monitored at the PFAC: Remote Sens. 2020, 12, x FOR PEER REVIEW 25 of 39 Displays a negative sign for all these calibrating passes; • along another descending Pass of No. 278 and using the other CRS1 Cal/Val site. It appears that the Shows an average bias of 6.82mm ( 32 mm FRM uncertainty) based on all three Cal/Val sites • bias of Sentinel-3A in sea-surface− height, ±as monitored at the PFAC: with• Displays descending a negative orbits; sign for all these calibrating passes; Has• no pass directional error in SSH bias as there is not signiﬁcant trend at all between ascending • Shows an average bias of −6.82mm (±32 mm FRM uncertainty) based on all three Cal/Val sites and descendingwith descending passes orbits; ( 11.98 mm for Pass No. 14 and 8.04 mm for Pass No. 335). − − • Has no pass directional error in SSH bias as there is not significant trend at all between 4.2. Sentinel-3Bascending Cal /andVal descending passes (−11.98 mm for Pass No.14 and −8.04 mm for Pass No.335). Three Sentinel-3B passes are calibrated at the PFAC. Calibration over sea surface is performed for 4.2. Sentinel-3B Cal/Val the ascending Passes No. 14 and No. 71 at the CRS1 and RDK1 Cal/Val sites, respectively, using the latest processingThree Sentinel-3B baseline for passes Sentinel-3B are calibrated (PB 1.33). at the The PFAC. implemented Calibration calibration over sea surface regions is performed for Sentinel-3B for the ascending Passes No. 14 and No. 71 at the CRS1 and RDK1 Cal/Val sites, respectively, using are shown in Figure 19. The results refer to the Operational Phase of Sentinel-3B, which is when the the latest processing baseline for Sentinel-3B (PB 1.33). The implemented calibration regions for satellite has been on its nominal orbit. Calibration results obtained during the Sentinel-3B Tandem Sentinel-3B are shown in Figure 19. The results refer to the Operational Phase of Sentinel-3B, which Phaseis are when given the separately satellite in has Section been 4.3 on . its nominal orbit. Calibration results obtained during the

FigureFigure 19. 19.The The Sentinel-3B Sentinel-3B calibrating regions regions over over sea surface sea surface for its forPass its No. Pass 14, No. No. 71 14,and No.No. 335 71 and No. 335in west in west Crete, Crete, Greece. Greece. Sea-surfac Sea-surfacee calibration calibration is carried is out carried relying out upon relying the CRS1 upon and the RDK1 CRS1 Cal/Val and RDK1 Cal/Valsites. sites. The TheCDN1 CDN1 transponder transponder Cal/Val Cal site/Val has site been has used been for usedthe calibration for the calibration of Sentinel-3B of Sentinel-3Bpass No.335 pass (descending). No. 335 (descending). 4.2.1. Sentinel-3B Transponder Cal/Val Results The range bias of Sentinel-3B has regularly been determined at the CDN1 Cal/Val using its descending Pass No. 335, and continuously after Cycle No. 21 (5-Febuary-2019). Figure 20 presents the results of this transponder calibration, covering its nominal satellite orbit. Remote Sens. 2020, 12, 2642 25 of 39

4.2.1. Sentinel-3B Transponder Cal/Val Results

RemoteThe Sens. range2020, 12 bias, x FOR of PEER Sentinel-3B REVIEW has regularly been determined at the CDN1 Cal/Val using26 of its39 descending Pass No. 335, and continuously after Cycle No. 21 (5-Febuary-2019). Figure 20 presents theRemote results Sens. 2020 of this, 12, transponderx FOR PEER REVIEW calibration, covering its nominal satellite orbit. 26 of 39

Figure 20. The latest calibration results of Sentinel-3B using its descending Pass No. 335 at the CDN1 transponder Cal/Val site. FigureFigure 20.20. TheThe latestlatest calibrationcalibration resultsresults ofof Sentinel-3BSentinel-3B usingusing itsits descendingdescending PassPass No.No. 335335 atat thethe CDN1CDN1 Thetranspondertransponder range bias CalCal/Val/ Valof Sentinel-3B site. site. is calculated to be B(S3B) = −3.46 mm (±3.50 mmm, ±35 mm FRM uncertainty). This value displays an opposite sign with respect to the previous Cal/Val results of The range bias of Sentinel-3B is calculated to be B(S3B) = 3.46 mm ( 3.50 mmm, 35 mm FRM Sentinel-3AThe range (Section bias of4.1.1, Sentinel-3B B(S3A) =is +4.90 calculated mm). toIn beboth B(S3B) satellites, =− −3.46 the mm absolute ±(±3.50 mmm,value of± ±35 their mm range FRM uncertainty). This value displays an opposite sign with respect to the previous Cal/Val results of biasuncertainty). appears toThis be valueabout displays±5 mm: anThis opposite means signthat withSentinel-3A respect andto the Sentinel-3B previous seemCal/Val to resultsmeasure of Sentinel-3A (Section 4.1.1, B(S3A) = +4.90 mm). In both satellites, the absolute value of their range bias altimetricSentinel-3A distances (Section of4.1.1, about B(S3A) 814 =km +4.90 as mm).accurately In both as satellites, ±5 mm thebased absolute on observations value of their of range this appears to be about 5 mm: This means that Sentinel-3A and Sentinel-3B seem to measure altimetric transponder.bias appears to be ±about ±5 mm: This means that Sentinel-3A and Sentinel-3B seem to measure distances of about 814 km as accurately as 5 mm based on observations of this transponder. altimetric distances of about 814 km as± accurately as ±5 mm based on observations of this transponder. 4.2.2. Sentinel-3B Sentinel-3B Cal/Val Cal/Val Resultsresults over Over sea Sea surface Surface 4.2.2.The Sentinel-3B CRS1CRS1 and and Cal/Val RDK1 RDK1 sea-surface results sea-surface over Cal sea /ValCal/Val surface sites provide sites provide ocean truthocean measurements truth measurements for the calibration for the calibrationof the ascending of the Sentinel-3B ascending PassSentinel-3B No. 14 andPass Pass No. No. 14 and 71, respectively.Pass No. 71, Figurerespectively. 21 presents Figure the 21 calibration presents The CRS1 and RDK1 sea-surface Cal/Val sites provide ocean truth measurements for the theresults calibration of Sentinel-3B results overof Sentinel-3B sea surface over as derived sea surface by these as derived two Cal by/Val these sites. two Cal/Val sites. calibration of the ascending Sentinel-3B Pass No. 14 and Pass No. 71, respectively. Figure 21 presents the calibration results of Sentinel-3B over sea surface as derived by these two Cal/Val sites.

Figure 21. Cont.

Remote Sens. 2020, 12, x FOR PEER REVIEW 26 of 39

Figure 20. The latest calibration results of Sentinel-3B using its descending Pass No. 335 at the CDN1 transponder Cal/Val site.

The range bias of Sentinel-3B is calculated to be B(S3B) = −3.46 mm (±3.50 mmm, ±35 mm FRM uncertainty). This value displays an opposite sign with respect to the previous Cal/Val results of Sentinel-3A (Section 4.1.1, B(S3A) = +4.90 mm). In both satellites, the absolute value of their range bias appears to be about ±5 mm: This means that Sentinel-3A and Sentinel-3B seem to measure altimetric distances of about 814 km as accurately as ±5 mm based on observations of this transponder.

4.2.2. Sentinel-3B Cal/Val results over sea surface The CRS1 and RDK1 sea-surface Cal/Val sites provide ocean truth measurements for the calibration of the ascending Sentinel-3B Pass No. 14 and Pass No. 71, respectively. Figure 21 presents the calibration results of Sentinel-3B over sea surface as derived by these two Cal/Val sites.

Remote Sens. 2020, 12, 2642 26 of 39

Remote Sens. 2020, 12, x FOR PEER REVIEW 27 of 39

Figure 21. The latest calibration results of Sentinel-3B for its ascending Pass No. 14 (upper) and No. 71Figure (lower), 21. Theusing latest sea-surface calibration calibration. results of Sentinel-3B for its ascending Pass No. 14 (upper) and No. 71 (lower), using sea-surface calibration. The bias of Sentinel-3B for the sea-surface height is +4.00 mm (±7.50 mm, ±32 mm FRM uncertainty)The bias based of Sentinel-3B on both ascending for the sea-surfacePass No. 14 height(B(S3B) is = ++4.554.00 mm, mm CRS1 ( 7.50 Cal/Val) mm, 32and mm Pass FRM No. ± ± 71uncertainty) (B(S3B) = based +4.85 onmm, both RDK1 ascending Cal/Val). Pass No.As expected, 14 (B(S3B) the= + 4.55sign mm, of the CRS1 sea-surface Cal/Val) and height Pass bias No. 71is (B(S3B)opposite= +to4.85 the mm transponder, RDK1 Cal /rangeVal). Asbias, expected, while theit signappears of the that sea-surface their magnitude height bias agrees is opposite at the to millimeterthe transponder level. range bias, while it appears that their magnitude agrees at the millimeter level.

4.3. Jason-3 Jason-3 Cal/Val Cal/Val Results Jason-3 occupiesoccupies aa speciﬁcspecific reference reference orbit orbit for for the the multi-mission multi-mission altimetry altimetry constellation. constellation. It ﬂiesIt flies on onexactly exactly the samethe same ground-track ground-track as its as predecessors its predecessors TOPEX TOPEX/Poseidon,/Poseidon, Jason-1 Jason-1 and Jason-2, and Jason-2, its repeat its repeatcycle is cycle 10 days, is 10 and days, its and measurements its measurements have provided have provided accurate accurate and consistent and consistent time-series time-series for nearly for nearlythree decades. three decades. In west In Crete, west theCrete, Gavdos the Gavdos Island is Is locatedland is underlocated a under crossover a crossover of Jason groundof Jason tracks ground of tracksPass No. of Pass 18 (descending) No.18 (descending) and Pass and No. Pass 109 (ascending)No. 109 (ascending) (Figure 22(Figure). 22).

Figure 22. The sea-surface calibrating regions for Jason-3Jason-3 along its Pass No.No. 109 and No. 018 over the PFAC CalCal/Val/Val facility.facility. Sea-surface calibration is ca carriedrried out at the Gavdos and at the RDK1 Cal/Val Cal/Val sites. The The CDN1 CDN1 transponder transponder Cal/Val Cal/Val site site has has also also been been used used for the for calibration the calibration of Jason-3 of Jason-3 Pass No.18 Pass (descending).No. 18 (descending).

4.3.1. Jason-3 Cal/Val Results (No.18) Range as well as sea-surface calibrations are carried out for every cycle of Jason-3 Pass No. 18 using the CDN1 and Gavdos Cal/Val sites, respectively. First, the satellite passes over the CDN1 transponder Cal/Val site on the west Crete mountains and about 9 seconds later it reaches the Gavdos island in the south. Figure 23 presents the latest results in range and sea-surface calibration for Jason-3 over its descending Pass No.18.

Remote Sens. 2020, 12, 2642 27 of 39

4.3.1. Jason-3 Cal/Val Results (No. 18) Range as well as sea-surface calibrations are carried out for every cycle of Jason-3 Pass No. 18 using the CDN1 and Gavdos Cal/Val sites, respectively. First, the satellite passes over the CDN1 transponder Cal/Val site on the west Crete mountains and about 9 s later it reaches the Gavdos island in the south. Figure 23 presents the latest results in range and sea-surface calibration for Jason-3 over itsRemote descending Sens. 2020, 12 Pass, x FOR No. PEER 18. REVIEW 28 of 39

Figure 23. The latest calibration results for Jason-3 alongalong its ascending passpass No.No. 18 with transponder (upper)(upper) andand sea-surfacesea-surface (lower)(lower) calibrations.calibrations.

The sea-surface bias of Jason-3 is determined to be B(JA3) = 4.34 mm, whereas the transponder The sea-surface bias of Jason-3 is determined to be B(JA3) = −4.34 mm, whereas the transponder range bias isis B(JA3)B(JA3) == +5.30 +5.30 mm. mm. As As already already described, described, the the sign sign of of the Jason-3 bias derived from the transponder andand the the sea-surface sea-surface calibrations calibrations is opposite. is opposite. Both calibrationsBoth calibrations result inresult the samein the absolute same valueabsolute of bias,value thus of bias, supporting thus supporting the reliability the reliability of the derived of the results. derived results. 4.3.2. Jason-3 Cal/Val Results (No. 109) 4.3.2. Jason-3 Cal/Val Results (No. 109) The ascending Pass No. 3 of Jason-3 has been calibrated over the sea surface using the Gavdos The ascending Pass No. 3 of Jason-3 has been calibrated over the sea surface using the Gavdos Cal/Val site. The latest calibration results for Jason-3, along the sea surface of Pass No. 109, with GDR-D Cal/Val site. The latest calibration results for Jason-3, along the sea surface of Pass No. 109, with data and for about 150 cycles, are given in Figure 24. GDR-D data and for about 150 cycles, are given in Figure 24.

Remote Sens. 2020, 12, x FOR PEER REVIEW 28 of 39

Figure 23. The latest calibration results for Jason-3 along its ascending pass No. 18 with transponder (upper) and sea-surface (lower) calibrations.

The sea-surface bias of Jason-3 is determined to be B(JA3) = −4.34 mm, whereas the transponder range bias is B(JA3) = +5.30 mm. As already described, the sign of the Jason-3 bias derived from the transponder and the sea-surface calibrations is opposite. Both calibrations result in the same absolute value of bias, thus supporting the reliability of the derived results.

4.3.2. Jason-3 Cal/Val Results (No. 109) The ascending Pass No. 3 of Jason-3 has been calibrated over the sea surface using the Gavdos Cal/Val site. The latest calibration results for Jason-3, along the sea surface of Pass No. 109, with Remote Sens. 2020, 12, 2642 28 of 39 GDR-D data and for about 150 cycles, are given in Figure 24.

Figure 24. The Jason-3 calibration results for the sea-surface height, based on ascending pass No. 109 and using the Gavdos Cal/Val. The results cover cycles 1–150 with Geophysical Data Records-version D (GDR-D) data.

To recap, Sections 4.1 and 4.2 presented the sea-surface and transponder calibration results per altimeter mission and with respect to the Permanent Facility for Altimetry Calibration in west Crete, Greece. Table6 summarizes the contribution of each PFAC Cal /Val site to the determination of the altimeter bias for Sentinel-3A, Sentinel-3B and Jason-3. Interpretation of these Cal/Val results per satellite follows at the end of Section4.

Table 6. Latest calibration results for satellite altimetry missions as determined by four operational Cal/Val sites that comprise the Permanent Facility for Altimetry Calibration in west Crete, Greece. The Gavdos, CRS1 and RDK1 Cal/Val sites are used for sea-surface calibration, whereas the CDN1 is used for range transponder calibration.

Cal/Val Site/ Sentinel-3A Sentinel-3B Jason-3 Satellite 11.98 mm (No. 14) 4.34 mm (No. 18) Gavdos − - − 10.08 mm (No. 335) 2.78 mm (No. 109) − − CRS1 3.68 mm (No. 278) +4.55 mm (No. 14) − RDK1 6.00 mm (No. 335) +4.85 mm (No. 71) − CDN1 Transponder +4.90 mm (No. 14) 3.46 mm (No. 335) +5.30 mm (No. 18) −

The Gavdos sea-surface Cal/Val site appears to produce consistent results per satellite altimeter. Two different passes in ascending and descending mode of Sentinel-3A and of Jason-3 are supported by the Gavdos Cal/Val facility. It seems that there is no significant difference for ascending and descending passes of Sentinel-3A and Jason-3. Moreover, the difference in bias results between transponder and sea-surface calibrations using the same orbit and with calibration taken place a few seconds apart, agree within 1 cm for Sentinel-3A (Pass No. 14, ascending) and for Jason-3 (Pass No. 18, descending). ± This setting clearly advocates the FRM requirements for having diverse, redundant and independent methodologies to assess performance of satellite altimeters.

4.4. Sentinel-3A -3B Cal/Val Results in Tandem Phase Sentinel-3B was ﬂying in tandem 30 s ahead of Sentinel-3A from 6-June-2018 to 17-October-2018 (Figure 25). Figure 26 presents the Cal/Val results for Sentinel-3A and Sentinel-3B during this tandem phase along sea surface Passes No. 14 (S3A), No. 278 and No. 335. Remote Sens. 2020, 12, 2642 29 of 39 Remote Sens. 2020, 12, x FOR PEER REVIEW 30 of 39

Remote Sens. 2020, 12, x FOR PEER REVIEW 30 of 39

Figure 25. Sentinel-3A and -3B flying (not to scale) in tandem from June to October 2018 over west Figure 25.25. Sentinel-3A and -3B ﬂyingflying (not to scale) in tandem from June to OctoberOctober 2018 over west Crete, Greece and the transponder (CDN1) and sea-surface (Gavdos) permanent Cal/Val sites. Crete, GreeceGreece and and the the transponder transponder (CDN1) (CDN1) and and sea-surface sea-surface (Gavdos) (Gavdos) permanent perm Calanent/Val Cal/Val sites. Similar sites. Similar situations occurred for S3A and S3B for Pass No. 278 and No. 335 over the CRS1 and Gavdos Similar situations occurred for S3A and S3B for Pass No. 278 and No. 335 over the CRS1 and Gavdos situationssea–surface occurred Cal/Val for S3Asites. and S3B for Pass No. 278 and No. 335 over the CRS1 and Gavdos sea–surface Calsea–surface/Val sites. Cal/Val sites.

Remote Sens. 2020, 12, x FOR PEER REVIEW 31 of 39

Figure 26. Sea-surface calibration results for Sentinel-3A and Sentinel-3B satellites during their Figure 26. Sea-surface calibration results for Sentinel-3A and Sentinel-3B satellites during their tandemtandem phase. phase.

Over the sea surface and during tandem, the bias difference of Sentinel-3B with respect to Sentinel-3A is calculated to be +20.66 mm (average). This implies that Sentinel-3B seems to measure sea-surface height by +2 cm higher than Sentinel-3A, at least during their tandem phase. Nonetheless, this result is based only on very few cycles and thus cannot be statistically significant but only indicative. During the tandem Sentinel-3A and Sentinel-3B phase, transponder calibrations were carried out at the CDN1 Cal/Val site (Figure 27). One of them was not successful, as Sentinel-3B’s altimeter was operating in Low Resolution Mode. Here also, it can be seen that Sentinel-3A seems to measure a slightly shorter range than Sentinel-3B in that short transponder Cal/Val analysis.

Figure 27. Transponder results for Sentinel-3A and Sentinel-3B satellites during their tandem phase. The bias results are given in mm to reveal slight differences between Sentinel-3A and Sentinel-3B.

Both results in sea surface and transponder analysis corroborate each other during the tandem phase of Sentinel-3A and Sentinel-3B. The analysis continues with crossover Cal/Val results.

4.5. Cal/Val Results at Crossovers The groundtracks of the Sentinel-3A and Jason-3 intersect at a point (Lat = 34.63893946, Long = 23.98522449) about 20 km south of Gavdos island (Figure 3). To compare sea-surface heights observed by these two satellites at that crossover location with some confidence, their flyover had to be separated by no more than 2 days. Between March 2016 and March 2020, this condition was fulfilled 19 times: 9 times the temporal separation of Sentinel-3A and Jason-3 was 1 day and 2 days (48-hours) for the remaining cases. For each pair of altimeters, linear regression was carried out on orbital data to accurately determine the geodetic coordinates of their crossover location at sea. Then, the exact SSH value at the crossover location for each satellite altimeter was computed, taking into consideration the geoid models and

Remote Sens. 2020, 12, x FOR PEER REVIEW 31 of 39

Figure 26. Sea-surface calibration results for Sentinel-3A and Sentinel-3B satellites during their Remotetandem Sens. 2020 phase., 12, 2642 30 of 39

Over the sea surface and during tandem, the bias difference of Sentinel-3B with respect to Over the sea surface and during tandem, the bias diﬀerence of Sentinel-3B with respect to Sentinel-3A is calculated to be +20.66 mm (average). This implies that Sentinel-3B seems to measure Sentinel-3A is calculated to be +20.66 mm (average). This implies that Sentinel-3B seems to measure sea-surface height by +2 cm higher than Sentinel-3A, at least during their tandem phase. sea-surface height by +2 cm higher than Sentinel-3A, at least during their tandem phase. Nonetheless, Nonetheless, this result is based only on very few cycles and thus cannot be statistically significant this result is based only on very few cycles and thus cannot be statistically signiﬁcant but only indicative. but only indicative. During the tandem Sentinel-3A and Sentinel-3B phase, transponder calibrations were carried out During the tandem Sentinel-3A and Sentinel-3B phase, transponder calibrations were carried at the CDN1 Cal/Val site (Figure 27). One of them was not successful, as Sentinel-3B’s altimeter was out at the CDN1 Cal/Val site (Figure 27). One of them was not successful, as Sentinel-3B’s altimeter operating in Low Resolution Mode. Here also, it can be seen that Sentinel-3A seems to measure a was operating in Low Resolution Mode. Here also, it can be seen that Sentinel-3A seems to measure slightly shorter range than Sentinel-3B in that short transponder Cal/Val analysis. a slightly shorter range than Sentinel-3B in that short transponder Cal/Val analysis.

FigureFigure 27. 27. TransponderTransponder results results for for Sentinel-3A Sentinel-3A and and Sentinel Sentinel-3B-3B satellites satellites during during their their tandem tandem phase. phase. TheThe bias bias results results are are given given in in mm mm to to reveal reveal slight slight differences diﬀerences betw betweeneen Sentinel-3A Sentinel-3A and and Sentinel-3B. Sentinel-3B.

BothBoth results results in in sea sea surface surface and and transponder transponder anal analysisysis corroborate corroborate each each other other during during the the tandem tandem phasephase of of Sentinel-3A Sentinel-3A and and Sentinel-3B. Sentinel-3B. The The analysis analysis continues continues with with crossover crossover Cal/Val Cal/Val results. 4.5. Cal/Val Results at Crossovers 4.5. Cal/Val Results at Crossovers The groundtracks of the Sentinel-3A and Jason-3 intersect at a point (Lat = 34.63893946, The groundtracks of the Sentinel-3A and Jason-3 intersect at a point (Lat = 34.63893946, Long = Long = 23.98522449) about 20 km south of Gavdos island (Figure3). To compare sea-surface heights 23.98522449) about 20 km south of Gavdos island (Figure 3). To compare sea-surface heights observed by these two satellites at that crossover location with some confidence, their flyover had to be observed by these two satellites at that crossover location with some confidence, their flyover had to separated by no more than 2 days. be separated by no more than 2 days. Between March 2016 and March 2020, this condition was fulfilled 19 times: 9 times the temporal Between March 2016 and March 2020, this condition was fulfilled 19 times: 9 times the temporal separation of Sentinel-3A and Jason-3 was 1 day and 2 days (48-h) for the remaining cases. For each separation of Sentinel-3A and Jason-3 was 1 day and 2 days (48-hours) for the remaining cases. For pair of altimeters, linear regression was carried out on orbital data to accurately determine the geodetic each pair of altimeters, linear regression was carried out on orbital data to accurately determine the coordinates of their crossover location at sea. Then, the exact SSH value at the crossover location for geodetic coordinates of their crossover location at sea. Then, the exact SSH value at the crossover each satellite altimeter was computed, taking into consideration the geoid models and the satellite location for each satellite altimeter was computed, taking into consideration the geoid models and direction in the region of crossover. Accordingly, 10 sampling points astride the crossover location have been used. These 20 altimeter observation points correspond to a distance of about 5 km along the satellite ground track. Figure 28 presents the outcome of this crossover analysis. It can be seen that the dispersion of the relative bias is of the order of 4 cm but the average bias has been calculated to be +1.7 cm. This implies ± that Sentinel-3A measures the SSH +1.7 cm higher compared to Jason-3. In addition, it seems that the uncertainty of this simplified and localized version of the multi-mission crossover analysis is similar to the FRM uncertainty (Tables4 and5) of the sea-surface and transponder calibrations. Note here that in crossover analysis no in-situ measurements were used. Remote Sens. 2020, 12, x FOR PEER REVIEW 32 of 39

the satellite direction in the region of crossover. Accordingly, 10 sampling points astride the crossover location have been used. These 20 altimeter observation points correspond to a distance of about 5 km along the satellite ground track. Figure 28 presents the outcome of this crossover analysis. It can be seen that the dispersion of the relative bias is of the order of ±4 cm but the average bias has been calculated to be +1.7 cm. This implies that Sentinel-3A measures the SSH +1.7 cm higher compared to Jason-3. In addition, it seems that the uncertainty of this simplified and localized version of the multi-mission crossover analysis is similar to the FRM uncertainty (Tables 4 and 5) of the sea-surface and transponder calibrations. Note here that in crossover analysis no in-situ measurements were Remoteused. Sens. 2020, 12, 2642 31 of 39

FigureFigure 28. 28.Relative Relative sea sea surface surface height height bias ofbias Sentinel-3A of Sent andinel-3A Jason-3 and altimeters Jason-3 overaltimeters their intersection over their point,intersection 20 km southpoint, of20 Gavdos.km south of Gavdos.

This section presented the latest calibration results for Sentinel-3A, Sentinel-3B and Jason-3 satellite This section presented the latest calibration results for Sentinel-3A, Sentinel-3B and Jason-3 altimeterssatellite altimeters as determined as determined by diverse by techniques diverse te atchniques the PFAC. at Tablethe PFAC.7 provides Table a concise7 provides summary a concise of thesesummary Cal/Val of these results. Cal/Val results. The results of this investigation can be summarized as follows: (1) the bias in sea-surface height determination by the satellite altimeters is within their specification of 1.5–2.0 cm [17], Table 7. Absolute and relative calibration results for Sentinel-3A, Sentinel-3B and Jason-3± based on (2) nosea-surface, significant transponder drift in altimeters’ techniques, performance as well as crossover has been analysis monitored, and S3A (3) confidenceand S3B tandem on calibration phase resultsanalysis has been at the improved Permanent as Facility diverse for and Altimetry independent Calibration. Cal All/Val values techniques are in mm. produce similar results, (4) the difference between sea-surface and transponder Cal/Val methodologies is within their FRM Sentinel-3A Sentinel-3B Jason-3 uncertainty, (5) Sentinel-3A measures sea-surface height higher than Jason-3, and (6) Sentinel-3B Nominal Orbit Productmeasures sea-surface PB2.61 height PB2.61 higher PB2.61 compared PB2.61 to Sentinel-3A. PB2.61 PB2.61 GRD-D GRD-D Cycles 2:57 2:52 1:56 20:34 20:38 21:35 1:150 1:150 Pass No. 14 No. 278 No.335 No.14 No.71 No.335 No.18 No.109 Range +5 mm - - - - −4.51 mm +5.60 mm - SSH(GVD,CRS1) −10 mm −12 mm −4 mm +4.55 mm +4.85 mm - −4.34 mm −2.78 mm SSH(RDK1) −6 mm Mean SSH −7.93 mm +4.70 mm −3.56 mm Tandem Phase Sentinel-3A Sentinel-3B S3B-S3A Offset Product PB2.61 PB2.61 Pass P14 P278 P335 P14 P278 P335 P14 P278 P335 SSH −1.62 mm −8.40 mm −14.96 mm +18.39 mm +2.40 mm +16.23 mm +20.01 mm +10.80 mm +31.19 mm Mean +20.66 mm Crossover Analysis Sentinel-3A Pass No. 14 Jason-3 Pass No.109 S3A-Jason-3 Offset Cycles 1–56 1–149 SSH +17 mm ± 49 mm

The results of this investigation can be summarized as follows: (1) the bias in sea-surface height determination by the satellite altimeters is within their specification of ±1.5–2.0 cm [17], (2) no

Remote Sens. 2020, 12, 2642 32 of 39

Table 7. Absolute and relative calibration results for Sentinel-3A, Sentinel-3B and Jason-3 based on sea-surface, transponder techniques, as well as crossover analysis and S3A and S3B tandem phase analysis at the Permanent Facility for Altimetry Calibration. All values are in mm.

Sentinel-3A Sentinel-3B Jason-3 Nominal Orbit Product PB2.61 PB2.61 PB2.61 PB2.61 PB2.61 PB2.61 GRD-D GRD-D Cycles 2:57 2:52 1:56 20:34 20:38 21:35 1:150 1:150 Pass No. 14 No. 278 No.335 No. 14 No.71 No.335 No. 18 No. 109 Range +5 mm - - - - 4.51 mm +5.60 mm - − SSH(GVD,CRS1) 10 mm 12 mm 4 mm − +4.55 mm +4.85 mm - 4.34 mm 2.78 mm SSH(RDK1) − − 6 mm − − − Mean SSH 7.93 mm +4.70 mm 3.56 mm − − Tandem Phase Sentinel-3A Sentinel-3B S3B-S3A Oﬀset Product PB2.61 PB2.61 Pass P14 P278 P335 P14 P278 P335 P14 P278 P335 SSH 1.62 mm 8.40 mm 14.96 mm +18.39 mm +2.40 mm +16.23 mm +20.01 mm +10.80 mm +31.19 mm − − − Mean +20.66 mm Crossover Analysis Sentinel-3A Pass No. 14 Jason-3 Pass No. 109 S3A-Jason-3 Oﬀset Cycles 1–56 1–149 SSH +17 mm 49 mm ± Remote Sens. 2020, 12, x FOR PEER REVIEW 33 of 39

significant drift in altimeters’ performance has been monitored, (3) confidence on calibration results has been improved as diverse and independent Cal/Val techniques produce similar results, (4) the Remotedifference Sens. 2020 between, 12, 2642 sea-surface and transponder Cal/Val methodologies is within their FRM33 of 39 uncertainty, (5) Sentinel-3A measures sea-surface height higher than Jason-3, and (6) Sentinel-3B measures sea-surface height higher compared to Sentinel-3A. 5. Discussion 5. Discussion This Section reviews calibration results of Section4 in perspective of other Cal /Val works. It also elaboratesThis upon Section future reviews research calibration and infrastructureresults of Section upgrading, 4 in perspective scheduled of other to takeCal/Val place works. at the It also PFAC. Moreelaborates speciﬁcally, upon future the Sentinel-3 research and Mission infrastructure Performance upgrading, Center scheduled (S3MPC) to has take recently place at assessedthe PFAC. the performanceMore specifically, of Sentinel-3A the Sentinel-3 and Sentinel-3B Mission Performance using independent Center processing(S3MPC) has of recently the CDN1 assessed transponder the dataperformance along with of Sentinel-3A sea-surface and calibration Sentinel-3B results using using independent the Corsica, processing France of the permanent CDN1 transponder calibration data along with sea-surface calibration results using the Corsica, France permanent calibration facilities [32]. These S3MPC calibration results are compared against the range and sea-surface facilities [32]. These S3MPC calibration results are compared against the range and sea-surface calibration results obtained at the PFAC. Furthermore, the on-going upgrading of the PFAC as a range calibration results obtained at the PFAC. Furthermore, the on-going upgrading of the PFAC as a and sigma-naught transponder facility is also presented. range and sigma-naught transponder facility is also presented. 5.1. Cross-Examination of Transponder Cal/Val Results 5.1. Cross-examination of Transponder Cal/Val Results The S3MPC publishes the Sentinel-3 altimeter Cyclic Performance Reports. These reports assess The S3MPC publishes the Sentinel-3 altimeter Cyclic Performance Reports. These reports assess the performance of the scientiﬁc instruments on-board Sentinel-3A and Sentinel-3B. They are available the performance of the scientific instruments on-board Sentinel-3A and Sentinel-3B. They are onlineavailable [33]. online The S3MPC [33]. The report S3MPC for report the Cycle for the No. Cycl 56e of No. the 56 SRAL of the altimeter SRAL altimeter of Sentinel-3A of Sentinel-3A contains Sentinel-3Acontains Sentinel-3A and Sentinel-3B and Sentinel range-3B bias range results bias using results the using CDN1 the CalCDN1/Val Cal/Val transponder. transponder. These These results, as publishedresults, as published by the S3MPC, by the have S3MP beenC, have determined been determined independently independently to the PFAC to the methodology PFAC methodology presented inpresented Section2. in Figure Section 29 2. illustrates Figure 29 theillustrates range the bias range results bias as results calculated as calculated by the by S3MPC the S3MPC and the and PFAC the forPFAC Sentinel-3A for Sentinel-3A Cycles 3–56Cycles and 3–56 Sentinel-3B and Sentinel-3B Cycles Cycles 11–35 11–35 as of as their of their launch launch dates: dates: 16-Feb-2016 16-Feb-2016 and 25-April-2018,and 25-April-2018, respectively. respectively.

Figure 29. Absolute range calibration results for Sentinel-3A Pass No. 14 as determined by the Sentinel-3 Mission Performance Center and the PFAC based upon the transponder at the CDN1 Cal/Val site. Remote Sens. 2020, 12, 2642 34 of 39

The range bias of Sentinel-3A and Sentinel-3B has been determined by the S3MPC to be +6.79 mm and 6.59 mm, respectively [29]. The same biases as presented under FRM methodology in this − investigation (Table7) are, respectively, +4.90 mm and 3.46 mm. Thus, there is an agreement of the − two transponder processing techniques at the 2 mm level. This is within the expected uncertainty of ± the diﬀerent and independent methods of data analysis used.

5.2. Cross-Examination of Sea-Surface Calibration Results The Corsica, France calibration facility calculated the sea-surface height bias of Sentinel-3A and Sentinel-3B as +16 mm (S3A) and +19 mm (S3B), respectively [29]. The mean values of all passes calibrated at the PFAC have been (Table7) 7.93 mm (S3A) and +4.70 mm (S3B), respectively. − The deviation (S3MPC minus PFAC) between the calibration results is of the order of +2.5 cm for Sentinel-3A and +1.5 cm for Sentinel-3B. This discrepancy is within the reported FRM uncertainty of the PFAC sea-surface calibration results ( 34.5 mm, Table5). ± There are several other reasons that may explain the discrepancy between PFAC and S3MPC such as: (1) the S3MPC Cal/Val results are based on processing baseline No. 2.33 to 2.45 while the PFAC applied the latest PB.2.61, (2) Sentinel-3A Cal/Val results from the S3MPC refer to cycles 1 to 47, but this work is applied till cycle 56, (3) the S3MPC Cal/Val results for Sentinel-3B cover four cycles during its tandem phase with Sentinel-3A and not under its nominal orbit as determined for these PFAC results, and (4) in contrast to transponder Cal/Val where S3MPC used the same data (CDN1 transponder) as the PFAC, sea-surface calibration is carried out at diﬀerent geographic locations, with diﬀerent instrumentation and processing methodologies.

5.3. A Second Transponder in Gavdos The European Space Agency FRM for the Sentinel-6 (FRM4S6) project is developing a prototype transponder to be installed and operated as part of the PFAC at Gavdos, called GVD1 Cal/Val, to ensure redundancy (FRM standard requirement) in transponder calibrations for Sentinel-6. This transponder is specifically responding to the need of Copernicus Sentinel-6. The second transponder is to calibrate not only the range but also the backscatter coefficient (sigma-naught, σ0) of radar altimetry. The backscatter coefficient (sigma-naught, σ0) is a parameter that is useful for all Earth observing radars. It controls the efficiency of imaging radars when land deformations are observed at the mm level. It is used in radar scatterometers to determine wind speed and direction on the ocean surface and moisture determination in soil. It is also applied to radar altimeters to provide accurate measurements of the sea level, wave heights and wind speed over the ocean. Some ocean and meteorological models make use of satellite altimeter-derived wind speed and significant wave height observations. Indeed, the longest and most complete record of global ocean sea state is derived from satellite altimetry. This transponder will be established at the GVD1 Cal/Val site under a triple crossover of Sentinel-6 (descending Pass No. 18, and ascending No. 109) and Sentinel-3A (descending Pass No.335) (Figure 30). Remote Sens. 2020, 12, 2642 35 of 39

Remote Sens. 2020, 12, x FOR PEER REVIEW 34 of 38

Figure 30. The location of the GVD1 transponder Cal/Val site in Gavdos island is under a crossover of the Figure 30. The location of the GVD1 transponder Cal/Val site in Gavdos island is under a crossover Sentinel-6 descending Pass No. 18, its ascending Pass No. 109 and the descending pass of Sentinel-3A of the Sentinel-6 descending Pass No.18, its ascending Pass No.109 and the descending pass of Pass No. 335. The land of GVD1 is property of the Technical University of Crete. A permanent Sentinel-3A Pass No. 335. The land of GVD1 is property of the Technical University of Crete. A GNSS station has been operating there since 2004 (GVD0) whereas a Doppler Orbitography and permanent GNSS station has been operating there since 2004 (GVD0) whereas a Doppler Radiopositioning Integrated by Satellite (DORIS) beacon (GAVB) also operated for almost a decade Orbitography and Radiopositioning Integrated by Satellite (DORIS) beacon (GAVB) also operated (start 27-September-2003, end 27-March-2014). for almost a decade (start 27-September-2003, end 27-March-2014). The future research direction of the PFAC is to provide two alternative, independent techniques to The future research direction of the PFAC is to provide two alternative, independent techniques perform range and sigma-naught calibration primarily for the upcoming Sentinel-6 [34]. Transponder to perform range and sigma-naught calibration primarily for the upcoming Sentinel-6 [34]. and sea-surface Cal/Val is already carried at the PFAC, whereas the GVD1 transponder Cal/Val is to Transponder and sea-surface Cal/Val is already carried at the PFAC, whereas the GVD1 transponder operate in late 2020 (Figure 31). Cal/Val is to operate in late 2020 (Figure 31).

Remote Sens. 2020, 12, 2642 36 of 39 Remote Sens. 2020, 12, x FOR PEER REVIEW 36 of 39

Figure 31. Flowchart of the way to calibrate Copernicus Sentinel-6 radar altimeter and its FigureAdvanced 31. Flowchart Microwave of Radiometer-Cthe way to calibrate (AMR-C) Copernicus microwave Sentinel-6 radiometer radar altimeter at the Permanent and its Advanced Facility Microwavefor Altimetry Radiometer-C Calibration. (AMR-C) microwave radiometer at the Permanent Facility for Altimetry Calibration. 6. Conclusions 6. Conclusions This work has provided a thorough assessment of the performance of Sentinel-3A, Sentinel-3B and Jason-3This work satellite has provided altimeters a thorou usinggh the assessment Permanent of Facility the performance for Altimetry of Sentinel-3A, Calibration Sentinel-3B of the ESA. andPerformance Jason-3 satellite is monitored altimeters thorough using land the transponder Permanent andFacility sea surfacefor Altimetry techniques Calibration at diﬀerent of the locations ESA. Performanceand settings, asis wellmonitored as crossover thorough analysis. land Caltransp/Valonder results and are accompaniedsea surface techniques by their uncertainty at different as locationscalculated and in the settings, context as of thewell new as ESAcrossover strategy analysis. of Fiducial Cal/Val Reference results Measurements. are accompanied by their uncertaintyThe performance as calculated of all in satellites the context appears of to the be withinnew ESA their strategy design requirements,of Fiducial exhibitingReference Measurements.altimetry biases of 1.5–2.0 cm. Figure 32 shows all results in a boxplot where all satellite altimeter ± biases,The as performance presented in Tableof all7, aresatellites compared. appears It appears to be within that transponder their design Cal requirements,/Val results inSentinel-3A exhibiting altimetryand Sentinel-3B biases areof ±1.5–2.0 less dispersed cm. Figure (inter 32 quantile shows rangeall results is smaller) in a boxplot than those where in Jason-3.all satellite This altimeter may be biases,attributed as topresented the SAR modein Table of Sentinel-3 7, are compared. on one hand, It appears but also tothat the transponder implementation Cal/Val of Level-0 results data in Sentinel-3Ain SAR calibration and Sentinel-3B instead of are Level-2 less dispersed data carried (inter out qu inantile Jason-3. range Evaluation is smaller) of than the transponderthose in Jason-3. and Thissea-surface may be calibration attributed methodologiesto the SAR mode indicates of Sentinel-3 that they on bothone hand, present but an also FRM to uncertainty the implementation of the order of Level-0of 3.0–3.5 data cm. in SAR calibration instead of Level-2 data carried out in Jason-3. Evaluation of the ± transponder and sea-surface calibration methodologies indicates that they both present an FRM uncertainty of the order of ±3.0–3.5 cm.

Remote Sens. 2020, 12, 2642 37 of 39 Remote Sens. 2020, 12, x FOR PEER REVIEW 37 of 39

FigureFigure 32. 32.The The Cal Cal/Val/Val results results for for Jason-3, Jason-3, Sentinel-3A Sentinel-3A and and Sentinel-3B Sentinel-3B in in box box plots. plots.

CalibrationCalibration results results derived derived by theseby these two calibrationtwo calibration techniques techniques as well as as well relative as relative calibration calibration given bygiven the crossover by the crossover analysis analysis all agree all to withinagree to 1within cm. Sentinel-3 ±1 cm. Sentinel-3 results are results in agreement are in agreement with results with ± givenresults by given other Calby other/Val groups, Cal/Val as groups, previously as previously described. described. AA major major upgrade upgrade of of the the PFAC PFAC is is currently currently under under way. way. The The target target missions missions for for this this altimetry altimetry CalCal/Val/Val service service would would be primarily be primarily the Copernicus the Copernicus Sentinel-6 andSentinel-6 Sentinel-3, and whileSentinel-3, other international while other missions,international such asmissions, Jason-3, such HY-2, as SWOT, Jason-3, CryoSat-2, HY-2, SWOT, etc., mayCryoSat-2, beneﬁt etc., as well. may benefit as well.

AuthorAuthor Contributions: Contributions:Conceptualization, Conceptualization, S.M., S.P.M, C.D. C. andD and C.M.; C.M.; Methodology, Methodology, S.M. S.P.M. and C.M.; and Software, C.M.; Software, X.F., C.K., X.F, G.V.;C.K., Validation: G.V.; Validation: X.F., I.N.T., X.F., and I.T., A.T.; and Investigation, A.T.; Investigation, S.M., C.M., S.P.M., C.D., C.M., F.B., C.D., and P.F.; F.B., Resources, and P.F.; Resources, C.M., X.F., C.K.,C.M., T.G. X.F., andC.K. A.T.; and Data A.T.; curation, Data X.F.,curation, C.K., andX.F., A.T.; C.K., Writing—original and A.T.; Writing—original draft preparation, draft S.M. preparation, and A.T.; Writing—reviewS.P.M. and A.T.; andWriting—review editing, S.M., C.D., and A.T.,editing, R.C.; S.P.M., Visualization, C.D., A.T., X.F.; R.C.; Supervision, Visualization, S.M., X.F.; P.F., Supervision, and C.D.; Project S.P.M., administration, P.F., and C.D.; S.M., P.F., and C.D.; Funding acquisition, S.M. All authors have read and agreed to the published version of Project administration, S.P.M., P.F., and C.D.; Funding acquisition, S.P.M. All authors have read and agreed to the manuscript. the published version of the manuscript. Funding: This research was funded by European Union and the European Space Agency grant numbers [4000122240Funding: /This17/I-BG research and 4000129892 was funded/20/ NLby/ FFEuropean/ab] and theUnion APC and was the funded European by Technical Space Agency University grant of Crete. numbers [4000122240/17/I-BG and 4000129892/20/NL/FF/ab] and the APC was funded by Technical University of Crete. Acknowledgments: We thank very much the three reviewers and the editor for their excellent and meticulous commentsAcknowledgments: and observations We thank made very on thismuch manuscript. the three reviewers We thank Dr.and Antoniothe editor Martellucci for their excellent of the European and meticulous Space Research and Technology Center of ESA in the Netherlands for all his help and support with the ESA radiometer. comments and observations made on this manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest. Conflicts of Interest: The authors declare no conflict of interest. References References 1. Wilson, W.S.; Lindstrom, E.J.; Apel, J.R. Satellite Oceanography, History, and Introductory Concepts. 1. Wilson, W.S.; Lindstrom, E.J.; Apel, J.R. Satellite Oceanography, History, and Introductory Concepts. 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