Development of the Chinese Space-Based Radiometric Benchmark Mission LIBRA

Article Development of the Chinese Space-Based Radiometric Benchmark Mission LIBRA

Peng Zhang 1,2 , Naimeng Lu 1,2, Chuanrong Li 3,*, Lei Ding 4, Xiaobing Zheng 5, Xuejun Zhang 6, Xiuqing Hu 1,2, Xin Ye 6, Lingling Ma 3, Na Xu 1,2, Lin Chen 1,2 and Johannes Schmetz 7

1 Key Laboratory of Radiometric Calibration and Validation for Environmental Satellite, China Meteorological Administration, Beijing 100081, China; [email protected] (P.Z.); [email protected] (N.L.); [email protected] (X.H.); [email protected] (N.X.); [email protected] (L.C.) 2 National Satellite Meteorological Center, China Meteorological Administration, Beijing 100081, China 3 Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100094, China; [email protected] 4 Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China; [email protected] 5 Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China; [email protected] 6 Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China; [email protected] (X.Z.); [email protected] (X.Y.) 7 Retired former Chief Scientist of Eumetsat, Eumetsat Allee 1, D-64295 Darmstadt, Germany; [email protected] * Correspondence: [email protected]; Tel.: +86-13701390352

Received: 26 May 2020; Accepted: 6 July 2020; Published: 8 July 2020

Abstract: Climate observations and their applications require measurements with high stability and low uncertainty in order to detect and assess climate variability and trends. The difficulty with space-based observations is that it is generally not possible to trace them to standard calibration references when in orbit. In order to overcome this problem, it has been proposed to deploy space-based radiometric reference systems which intercalibrate measurements from multiple satellite platforms. Such reference systems have been strongly recommended by international expert teams. This paper describes the Chinese Space-based Radiometric Benchmark (CSRB) project which has been under development since 2014. The goal of CSRB is to launch a reference-type satellite named LIBRA in around 2025. We present the roadmap for CSRB as well as requirements and specifications for LIBRA. Key technologies of the system include miniature phase-change cells providing fixed-temperature points, a cryogenic absolute radiometer, and a spontaneous parametric down-conversion detector. LIBRA will offer measurements with SI traceability for the outgoing radiation from the Earth and the incoming radiation from the Sun with high spectral resolution. The system will be realized with four payloads, i.e., the Infrared Spectrometer (IRS), the Earth-Moon Imaging Spectrometer (EMIS), the Total Solar Irradiance (TSI), and the Solar spectral Irradiance Traceable to Quantum benchmark (SITQ). An on-orbit mode for radiometric calibration traceability and a balloon-based demonstration system for LIBRA are introduced as well in the last part of this paper. As a complementary project to the Climate Absolute Radiance and Refractivity Observatory (CLARREO) and the Traceable Radiometry Underpinning Terrestrial- and Helio- Studies (TRUTHS), LIBRA is expected to join the Earth observation satellite constellation and intends to contribute to space-based climate studies via publicly available data.

Keywords: earth observation; climate observation; radiometric metrology; calibration benchmark; SI traceability

Remote Sens. 2020, 12, 2179; doi:10.3390/rs12142179 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 2179 2 of 17

1. Introduction The role of satellites in Earth observation has increased substantially over the past sixty years since the first meteorological satellite TIROS 1 was launched in 1960. With satellite observations, we are able to probe the Earth system (atmosphere, ocean, and land surface) at increasingly high temporal, spatial, and spectral resolution [1]. However, the capability for climate monitoring from space is insufficient [2–4]. Even though satellite observed radiances today generally have a radiometric precision of 0.2 K or better, radiances assimilated in numerical weather prediction (NWP) models still need to be corrected for significant biases. For climate monitoring, current measurements are not yet adequate because small trends of the order of a tenth of a degree need to be measured with high confidence [5,6]. It is essential for climate monitoring and desirable for NWP that measurements are basically bias-free [7]. Without accurate, high quality observations on relevant time and space scales, climate science applications and services will be limited [8]. To anchor satellite-based climate observation, several reference-type missions against an absolute standard are being considered by several space agencies [9]. The Traceable Radiometry Underpinning Terrestrial- and Helio-Studies (TRUTHS [10] has been accepted by the ESA (European Space Agency) as a climate mission with an unprecedented SI-traceable accuracy. The Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission has been underway by the National Aeronautics and Space Administration (NASA) since the mid-2000s. CLARREO also aims to provide accurate and SI-traceable observations, allowing the assessment of decadal changes of climate [11]. More recently, a CLARREO Pathfinder mission with only the reflected solar spectrometer is being built for the International Space Station (ISS), with launch tentatively planned for 2023. The proposed CLARREO and TRUTHS missions would create a climate observing system measuring both the incoming and reflected solar energy with a 10-fold increase in data accuracy compared to existing systems. They would provide spectrally resolved thermal infrared and reflected solar radiation with high absolute accuracy. In addition, such systems will serve as on-orbit references for a wide range of visible and infrared Earth observation sensors in LEO and GEO orbits. This approach has already been realized by the Global Space-based Intercalibration System (GSICS) using the most stable current instruments as references [12]. In the recent Vision 2040 document of the WMO Integrated Observing System (WIGOS), it is presumed that operational meteorological satellite systems will remain the key elements of a space-based climate observing system capable of unambiguously monitoring indicators of changes in the Earth’s climate [13]. Satellite agencies are therefore encouraged to develop new satellite instruments with climate applications in mind. The proposed reference systems would go a long way in that direction. This will also enable the production of much improved Essential Climate Variables (ECVs) in accordance with established key requirements for climate monitoring [14]. Realizing the importance of reference-type missions for improving climate science and for harmonizing global satellite observations, an expert team on Earth observation and navigation of Ministry of Science and Technology (MOST) proposed the concept of the Chinese Space-based Radiometric Benchmark (CSRB) in 2006. The CSRB project was approved and initially funded by MOST in 2014. To date, Phase A of the CSRB project, completed in 2018, resolved the fundament problems of building the SI-traceable calibrator for the thermal infrared band and reflected solar band. The ultimate goal of the CSRB project is to build a flight model of the Chinese radiometric benchmark satellite, named ‘LIBRA’, for launch during 2022–2025. The roadmap of the CSRB project is presented in Section2, and the requirements and specifications for LIBRA follow in Section3. The payloads and key technologies are briefly described in Section4. The on-orbit operations for radiometric calibration traceability and the balloon-based demonstration experiments are introduced in Sections5 and6, respectively. Remote Sens. 2020, 12, 2179 3 of 17

2. CSRB Project in China China has established a comprehensive constellation network for space-based Earth observations with a perspective over 50 years. Currently, there are multiple satellite series operating in orbit, including the Fengyun (FY) meteorological satellites since 1988, the Ziyuan (ZY) Earth resources satellitesRemote since Sens. 1999, 2019, the11, x HaiyangFOR PEER REVIEW (HY) ocean satellites since 2002, and the Huanjing (HJ)3 of environment17 and disaster2. CSRB monitoring Project in satellitesChina since 2008. More recently, a new Earth observation system with high spatial, temporal,China and has spectral established resolution, a comprehensive named constellation ‘Gaofeng’ (GF), network was for developed space-based and Earth has operated since 2013,observation achievings with all-weather, a perspective all-day,over 50 years and. Currently, global coverage there are multiple observation satellite capability series operat anding providing operationalin orbit applications, including andthe Fengyun services (FY) in the meteorological fields of agriculture, satellites since disaster, 1988, the resource, Ziyuan (ZY) and Earth environmental studies [15resources]. satellites since 1999, the Haiyang (HY) ocean satellites since 2002, and the Huanjing (HJ) environment and disaster monitoring satellites since 2008. More recently, a new Earth observation In thesystem early with stages high ofspatial, China’s temporal Earth, and observation spectral resolution satellite, named programs, ‘Gaofeng there’ (GF) were, was developed hardly any onboard calibrationand systems. has operated On-orbit since 2013 radiometric, achieving accuracyall-weather, was all-day estimated, and global by coverage pre-launch observation calibrations and later correctedcapability by and vicarious providing calibrations operational applications using the Chineseand services Radiometric in the fields of Calibration agriculture, disaster, Site (CRCS) in the Dunhuangresource Gobi, and Desert environment for reflectedal studies solar [15]. bands and Qinghai Lake for thermal infrared bands [16]. In the early stages of China’s Earth observation satellite programs, there were hardly any Onboardonboard calibration calibration systems system weres. On- realizedorbit radiometric with theaccuracy advent was estimated of the second by pre-launch generation calibration Fengyuns polar orbitingand meteorological later corrected by satellite vicarious (FY-3A) calibrations in using the middle the Chinese of theRadiometric 2000s [Calibration17]. Best Site practice (CRCS) guidelines for pre-launchin the Dunhuang characterization Gobi Desert and for reflected calibration solar bands for passive and Qinghai optical Lake for remote thermal sensing infrared instrumentsbands are strictly followed[16]. Onboard to achieve calibration adequate systems absolutewere realized accuracy with the and advent stability of the second of the generation instruments Fengyun in orbit [18–20]. polar orbiting meteorological satellite (FY-3A) in the middle of the 2000s [17]. Best practice guidelines To providefora pre space-based-launch characterization radiometric and reference calibration for forEarth passive observations optical remote from sensing multiple instruments satellite are platforms and in orderstrictly to followed respond to toachieve requirements adequate absolute by the acc Globaluracy and Climate stability Observing of the instruments System in orbit (GCOS), [18– the CSRB concept was20]. T proposedo provide a space by an-based expert radiometric team onreference Earth for observation Earth observations and navigationfrom multiple of satellite the Ministry of Science andplatforms Technology and in order in to 2006 respon [21d]. to Therequirements project by has the three Global phases: Climate Observing Phase A, System extended (GCOS) from, 2014 to the CSRB concept was proposed by an expert team on Earth observation and navigation of the 2018, withMinistry the goal of Science to develop and Technology the SI-traceable in 2006 [21] calibrator. The project for has the three thermal phases: infraredPhase A, extended band (IR) and the reflectivefrom solar 2014 band to 2018, (RSB). with Phasethe goal B,to fromdevelop 2018 the SI to-traceable 2022, has calibrator the objective for the thermal to develop infrared band an engineering model of( theIR) and reference the reflective instruments. solar bandDuring (RSB). Phase Phase B, from C, from 2018 2022to 2022, to has 2025, the aobjective flight modelto develop ready an for launch will be developed.engineering model The roadmap of the reference of the instruments. CSRB project During is Phase shown C, from in Figure 2022 to1 .2025, a flight model ready for launch will be developed. The roadmap of the CSRB project is shown in Figure 1.

Figure 1. The roadmap of the Chinese Space-based Radiometric Benchmark (CSRB) project. EOS is the acronym for Earth Observation System, DCC for Deep Convective Cloud, PICS for Pseudo Invariant Calibration Sites, and FCDR for Fundamental Climate Data Record. Remote Sens. 2020, 12, 2179 4 of 17

To date, Phase A has been completed. The prototypes, including an absolute radiance standard IR calibrator based on ITS-90 miniature phase-change temperature points, a RSB self-correction absolute calibrator based on Spontaneous Parametric Down-Conversion (SPDC) principles with eight spectral bands spanning 450–1000 nm, and a cavity-type absolute cryogenic radiometer (ACR) with 20 K operational temperature, were built together with the corresponding radiometric scale transfer chains. In this exploratory phase, an uncertainty better than 0.15 K and 0.3% was achieved for all of the three benchmark calibrators. Based on these promising results, Phase B of the CSRB project was started in 2018 and the engineering model is ongoing.

3. Prototype Model of LIBRA

3.1. Specification of LIBRA The LIBRA mission consists of one satellite carrying four payloads: an Infrared Spectrometer (IRS) with hyperspectral resolution, an Earth-Moon Imaging Spectrometer (EMIS) measuring in reflected solar radiation, a Total Solar Irradiance (TSI) instrument, and a Solar spectral Irradiance monitoring instrument Traceable to Quantum benchmark (SITQ). A GPS/Beidou Globle Navigation Satellite System Radio Occultation (GPS/BD GNSS RO) will be an optional instrument depending on the spacecraft bus capability. The detailed payload specifications of the LIBRA prototype model and the key technologies used are listed in Table1. The IRS will be positioned on the spacecraft bus deck for nominal nadir views. Using an internal mirror, the IRS will enable nadir, off-nadir, internal calibration, and deep-space (zenith) observations. The EMIS and TSI will be co-boresighted and mounted to a two-axis gimbal to enable nadir (nominal operations) and off-nadir lunar and solar observations. The spacecraft bus should provide an independent means for acquiring necessary position and attitude information.

Table 1. Detailed payloads speciﬁcations of the LIBRA prototype model.

Instrument Name Payload Requirements Key Technology 1 Spectral range: 600–2700 cm− 1 Spectral resolution: 0.5 cm− IFOV: 24 mrad Miniature ﬁxed-temperature IRS Sensitivity: 0.1 K@270 K phase-change cells Emissivity of BB: 0.999 ≥ Measurement uncertainty: 0.15 K (k = 2) Spectral range: 380–2350 nm, Spectral resolution: 10 nm, Spectral precision: 0.5 nm, Space Cryogenic Absolute EMIS Spatial resolution: 100 m, Radiometer Coverage: 50 km, Measurement uncertainty: 1% (k = 2) Spectral range: 0.2–35 µm, Space Cryogenic Absolute TSI Measurement uncertainty: 0.05% (k = 2) Radiometer Long-term stability:0.005% Spectral range: 380–2500 nm, Spectral resolution: 3 nm (380–1000 nm), 8 nm (1000–2500 nm) Spontaneous Parametric SITQ Spectral precision: 0.1–0.3 nm, Down-Conversion Self-calibration uncertainty: 0.2%, Measurement uncertainty: 0.35% (k = 2)

The LIBRA experimental observatory has been designed for an operational lifetime of ﬁve years with consumables for eight years. The follow-up operational satellites will be designed for a long-term mission (20 years or more), with satellites in orbit overlapping in time. The spacecraft operations system will be designed to minimize the cost and risk of operations, including minimizing the number Remote Sens. 2020, 12, 2179 5 of 17 of operators required to safely command and control the spacecraft, and maximizing use of spacecraft and ground system fault detection, reporting and protection tools. Other potential platforms considered for the LIBRA mission include the Chinese Space Station (CSS) as an instrument platform and independent small satellite missions with only one SI instrument each. Lagrangian orbit locations are also being investigated for complementary observations.

3.2. Comparison with CLARREO and TRUTHS As space-based climate and calibration observatories, the proposed CLARREO and TRUTHS missions have important potential contributions to make both directly through well-calibrated measurements and indirectly through facilitating intercalibration of the data from other platforms. By using advanced technologies, such as phase-change points, a cryogenic absolute radiometer, and spontaneous parametric down-conversion (SPDC), LIBRA will provide measurements with SI traceability for both the IR and the reﬂected solar components. The main characteristics of LIBRA, CLARREO, and TRUTHS are summarized in Table2. More details of the LIBRA technologies will be introduced in the next section.

Table 2. Comparison among radiometric benchmark satellite.

Satellite Libra Clarreo Truths Instrument IR RS TS SS IR RS RS TS SS Type * Spectral 600–2700 380–2350 0.2–35 380–2500 200–2000 320–2300 380–2300 0.2–35 320–2450 1 1 Coverage cm− nm µm nm cm− nm nm µm nm Spectral 0.5 cm 1 10 nm – 3~8 nm 0.5 cm 1 8 nm 5~10 nm – 1~10 nm Resolution − − Measurement 0.15 K 1% 0.05% 0.35% 0.065 K 0.3% 0.1% 0.02% 0.2% Uncertainty (k = 2) (k = 2) (k = 2) (k = 2) (k = 2) (k = 2) (k = 2) (k = 2) (k = 2) SI-traceability Yes Yes Yes Yes Yes Yes Yes Yes Yes * In the line ‘Instrument type’, IR represents the instrument to measure the spectrally resolved infrared radiance, RS represents the instrument to measure the spectrally resolved reﬂectance of solar radiation, TS represents the instrument to measure the total solar irradiance, and SS represents the instrument to measure the spectrally resolved solar irradiance.

4. Instrument Design and Key Technologies

4.1. EMIS and TSI Make Use of the Space Cryogenic Absolute Radiometer (SCAR) In order to improve the measurement accuracy and long-term stability of the reflective solar band and the total solar irradiance observations, it is important to develop a new onboard calibration system with high accuracy. The LIBRA will provide a calibrated reflected solar spectrum and total solar irradiance based on the Space Cryogenic Absolute Radiometer (SCAR) [22], instead of a solar diffuser, standard lamps, vicarious calibration methods, and ground-based calibration techniques. The prototype payload under development since 2018 is illustrated in Figure2. Essentially, the SCAR is an electrical substitution radiometer operating at 20 K [23]. The detector of SCAR is a blackbody cavity with super high absorptance. Incident light induces a temperature increase in the blackbody cavity by absorption. The power of the incident light can be obtained by precisely measuring the electrical power applied in order to maintain constant temperature, as the incident light is duty cycled and its power is replaced by the electrical power. The heating effects of the incident light and electrical heater are nearly equivalent at 20 K, reducing non-equivalence corrections. Remote Sens. 2019, 11, x FOR PEER REVIEW 6 of 17

4. Instrument Design and Key Technologies

4.1. EMIS and TSI Make Use of the Space Cryogenic Absolute Radiometer (SCAR) In order to improve the measurement accuracy and long-term stability of the reflective solar band and the total solar irradiance observations, it is important to develop a new onboard calibration system with high accuracy. The LIBRA will provide a calibrated reflected solar spectrum and total solar irradiance based on the Space Cryogenic Absolute Radiometer (SCAR) [22], instead of a solar diffuser, standard lamps, vicarious calibration methods, and ground-based calibration techniques. The prototype payload under development since 2018 is illustrated in Figure 2. Essentially, the SCAR is an electrical substitution radiometer operating at 20 K [23]. The detector of SCAR is a blackbody cavity with super high absorptance. Incident light induces a temperature increase in the blackbody cavity by absorption. The power of the incident light can be obtained by precisely measuring the electrical power applied in order to maintain constant temperature, as the incident light is duty cycled and its power is replaced by the electrical power. The heating effects of the incident light and electrical heater are nearly equivalent at 20 K, reducing non-equivalence corrections. Through the onboard calibration system, the radiometric scale of the reflected solar band and total solar irradiance can be traced to SCAR. The accuracy of the space cryogenic radiometric measurement, the in-orbit self-calibration, and the high spectral resolution radiance observation are the critical parts to the final performance of SCAR. The experimental prototype of SCAR has been in development since 2015. The Stryn-type Pulse Tube Cryocooler (SPTC) is used to obtain the 20 K working temperature. The SPTC is optimized to provide the refrigerating capacity of 350 mW at 20 K for space applications. The measurement uncertainty of the SCAR prototype is 0.029%, according toRemote uncertainty Sens. 2020 ,analyses.12, 2179 The measurement uncertainty was verified by an indirect comparison with6 of 17 the ground-based cryogenic radiometer of the National Institute of Metrology (NIM) [24,25].

Stryn-type Pulse TSI Tube Cryocooler SCAR

EMIS laser diodes

Mirror

Tungsten halogen lamp BTC Transfer Diffuser Radiometer

Figure 2. The design chart of Earth-Moon Imaging Spectrometer (EMIS) and Total Solar Irradiance (TSI). Figure 2. The design chart of Earth-Moon Imaging Spectrometer (EMIS) and Total Solar Irradiance (TSI)Through. the onboard calibration system, the radiometric scale of the reflected solar band and total solar irradiance can be traced to SCAR. The accuracy of the space cryogenic radiometric measurement, 4.1.1.the in-orbit Earth-Moon self-calibration, Imaging Spectrometer and the high spectral resolution radiance observation are the critical parts to theReflected final performance solar spectrum of SCAR. radiance The experimentalis measured by prototype the Earth of-Moon SCAR Imaging has been Spectrometer in development (EMIS). since The2015. optical The Stryn-typedesign of the Pulse EMIS Tube consists Cryocooler of a telescope (SPTC) and is used a hyperspectral to obtain the imaging 20 K working spectrometer. temperature. The telescopeThe SPTC utilizes is optimized a four to-mirror provide anastigmat the refrigerating (4MA) capacity to eliminate of 350 mWaberration at 20 Ks for. The space hyperspectral applications. imagingThe measurement spectrometer uncertainty is based on of an the Offner SCAR design prototype. A prism is 0.029%, is used according as the dispersion to uncertainty element analyses.. The telescopeThe measurement uses the image uncertainty space wastelecentric verified structure by an indirect to achieve comparison matching with conditions the ground-based with the cryogenicpupil of theradiometer spectrometer of the, which National uses Institute a telecentric of Metrology design. By (NIM) setting [24 ,baffles25]. at the intermediate image plane, the influence of stray light is reduced. The EMIS makes a trade-off between the spectral and spatial 4.1.1. Earth-Moon Imaging Spectrometer resolution, with spectral and spatial sampling better than 10 nm and 100 m, respectively. The swath widthReflected is about 5 solar0 km spectrumat nadir from radiance a 600 iskm measured orbit. The by EMIS the Earth-Moon selects lens parameters, Imaging Spectrometer such as thickness, (EMIS). curvature,The optical etc design., to achieve of the lower EMIS dispersion consists of non a telescope-linearities and and a better hyperspectral spectral performance. imaging spectrometer. The telescope utilizes a four-mirror anastigmat (4 MA) to eliminate aberrations. The hyperspectral imaging spectrometer is based on an Offner design. A prism is used as the dispersion element. The telescope uses the image space telecentric structure to achieve matching conditions with the pupil of the spectrometer, which uses a telecentric design. By setting baffles at the intermediate image plane, the influence of stray light is reduced. The EMIS makes a trade-off between the spectral and spatial resolution, with spectral and spatial sampling better than 10 nm and 100 m, respectively. The swath width is about 50 km at nadir from a 600 km orbit. The EMIS selects lens parameters, such as thickness, curvature, etc., to achieve lower dispersion non-linearities and better spectral performance. The radiometric scale of the EMIS can be traced to SCAR. The measurement accuracy and long-term stability of the EMIS can be improved by the onboard hyperspectral calibration shown in Figure3 and consists of the SCAR and Benchmark Transfer Chain (BTC). The SCAR will realize a long-term stable and highly accurate radiation measurement. The light power benchmark of SCAR is converted to a radiance benchmark by the Transfer Radiometer (TR) and multiple laser diodes. The multi-spectral calibration of the EMIS is realized by observing the reflected light of a diffuser. Based on the hyperspectral curve reconstruction technique, the EMIS is full-spectral calibrated by the TR and the tungsten halogen lamp inside an integrating sphere. In order to ascertain linearity over the large dynamic range, the Sun is used as the light source, with solar attenuations provided by attenuators. Remote Sens. 2019, 11, x FOR PEER REVIEW 7 of 17

The radiometric scale of the EMIS can be traced to SCAR. The measurement accuracy and long- term stability of the EMIS can be improved by the onboard hyperspectral calibration shown in Figure 3 and consists of the SCAR and Benchmark Transfer Chain (BTC). The SCAR will realize a long-term stable and highly accurate radiation measurement. The light power benchmark of SCAR is converted to a radiance benchmark by the Transfer Radiometer (TR) and multiple laser diodes. The multi- spectral calibration of the EMIS is realized by observing the reflected light of a diffuser. Based on the hyperspectral curve reconstruction technique, the EMIS is full-spectral calibrated by the TR and the tungsten halogen lamp inside an integrating sphere. In order to ascertain linearity over the large Remote Sens. 2020, 12, 2179 7 of 17 dynamic range, the Sun is used as the light source, with solar attenuations provided by attenuators.

SI Transfer Mirror Mirror radiometer

SCAR BTC

EMIS

Integral sphere light source

Len Attenuator diffuser Sun Laser Laser

Figure 3. Schematic diagram of hyperspectral calibration on satellite. Figure 3. Schematic diagram of hyperspectral calibration on satellite. 4.1.2. Total Solar Irradiance 4.1.2. Total Solar Irradiance Total Solar Irradiance (TSI) is a kind of electrical substitution radiometer; the detector is alternately exposedTotal to aSolar radiant Irradiance energy source(TSI) is (solar a kind irradiance) of electrical andthen, substitution to known radiometer; internal electrical the detector heating. is Thealternately temperature exposed increase to a radiant due to the energy absorbed source solar (solar irradiance irradiance) is then and compared then, to known to the same internal temperature electrical increaseheating. due The to temperature a precisely measuredincrease due equivalent to the absorbed electrical solar power. irradiance In order is to then improve compared the measurement to the same uncertainty,temperature the increase detector due thermal to a precisely compensation measured of TSI equivalent is investigated. electrical The thermal power. conductionIn order to structure improve isthe optimized measurement by the uncertainty, blackbody cavitythe detector design. thermal Based com on thepensation application of TSI of is a investigated. temperature The bridge, thermal the temperatureconduction driftstructure of the is heat optimized sink is compensated by the blackbody to improve cavity measurement design. Based repeatability. on the application of a temperatureThe TSI bridge, calibration the temperature on the satellite drift isof designedthe heat sink to improveis compensated the long-term to improve stability measurement of TSI measurements,repeatability. as shown in Figure4. The SCAR is also the benchmark of TSI calibration onboard the satellite.The The TSI SCAR calibration and TSI on are the installed satellite on is the designed precision to Sun improve tracker. the The long Sun- Sensorterm stability (SS) is used of TSI to obtainmeasurements, the position as ofshown the Sun. in Figure Apertures 4. The suppress SCAR is stray also lightthe benchmark in the TSI measurements. of TSI calibration The onboard irradiance the scalesatellite. of the The TSI SCAR is traced and TSI to the are SCAR installed via on simultaneous the precision solar Sun observations. tracker. The Sun The Sensor system (SS) deviation is used isto correctedobtain the by position the real-time of the Sun. correction Apertures based suppress on a super stray stable light voltage in the TSI reference. measurement The TSIs. calibrationThe irradiance on Remotethescale satellite Sens.of the 2019 improvesTSI, 11 ,is x FORtraced the PEER absoluteto REVIEW the SCAR measurement via simultaneous accuracy solar and long-termobservations. stability. The system deviation8 of 17 is corrected by the real-time correction based on a super stable voltage reference. The TSI calibration on the satellite improves the absolute measurementAp accuracyertures and long-term stability.

TSI

SI Sun SCAR

SS Sun tracker Total Solar Irradiance

Figure 4. Schematic diagram of Total Solar Irradiance (TSI) calibration on satellite. Figure 4. Schematic diagram of Total Solar Irradiance (TSI) calibration on satellite. 4.2. Solar Spectral Irradiance Traceable to Quantum Benchmark (SITQ) 4.2. Solar Spectral Irradiance Traceable to Quantum Benchmark (SITQ) We propose an approach to trace the solar spectral irradiance to Planck’s constant and photon countWe number propose by an means approach of SPDC to trace (Spontaneous the solar spectral Parametric irradiance Down-Conversion), to Planck’s constant a quantum and photon optical counteﬀect occurringnumber by in means a non-linear of SPDC crystal (Spontaneous pumped Parametric by a laser beamDown [-26Conversi]. In anon), SPDC a quantum process, optical a one pumpeffect occurring photon decays in a non into-linear a pair crystal of photons, pumped usually by a laser called beam correlated [26]. In an or SPDC entangled process, photons, a one pump since photon decays into a pair of photons, usually called correlated or entangled photons, since they have definite relations between their frequencies, directions and polarizations. SPDC satisfies energy conservation 1/p = 1/1 + 1/2 and momentum conservation kp = k1 + k2, where s are photon wavelengths, ks are wave vectors, and the subscript p, 1 and 2 denote pumping and correlated photons, respectively. Detector calibration with SPDC is intrinsically absolute and independent of traceability to external radiometric standards. Theoretically, the accuracy of measured quantum efficiency is solely determined by how accurately the photon number is counted. This distinctive feature makes SPDC a promising candidate for an SI primary standard [27], as well as a space benchmark. With known quantum efficiency , the photon detector can measure photon rate with N = M/, or flux with  = Nh, or irradiance with E = Nh/A, where A is the area of receiving aperture of the system. This forms the physical basis for tracing solar irradiance measurements to Planck’s constant and the photon rate. Figure 5 shows the concept of a solar spectral irradiance radiometer working in the visible to shortwave infrared spectrum and incorporating internal SPDC calibration. The radiometer has two working modes, i.e., a calibration mode for traceability to SI and an observation mode for solar irradiance measurement.

Solar radiation aperture Monochromator Avalanche / Analog 1 photodiodes Correlated photons Rotating Middle by SPDC mirror optics Monochromator Avalanche / Analog  photodiodes 2

Figure 5. Concept for solar spectral irradiance measurement with Spontaneous Parametric Down- Conversion (SPDC) calibration.

In the calibration mode, correlated photons are directed by a rotating mirror into the middle optics, and then, are spectrally separated by two monochromators, before finally reaching avalanche photodiodes. The middle optics suppress residual pump photons and the monochromators ensure the wavelength correlation 1/p = 1/1 + 1/2. The calibration mode gives the quantum efficiency or absolute responsivity of the whole system and naturally accounts for any degradation of components of the system. This approach is especially useful as a reference or benchmark in space, and it appears advantageous over the endeavor to maintain a ‘highly stable’ onboard standard in space.

Remote Sens. 2019, 11, x FOR PEER REVIEW 8 of 17

Apertures

TSI

SI Sun SCAR

SS Sun tracker Total Solar Irradiance

Figure 4. Schematic diagram of Total Solar Irradiance (TSI) calibration on satellite.

4.2. Solar Spectral Irradiance Traceable to Quantum Benchmark (SITQ) We propose an approach to trace the solar spectral irradiance to Planck’s constant and photon

Remotecount Sens.number2020, 12by, 2179means of SPDC (Spontaneous Parametric Down-Conversion), a quantum optical8 of 17 effect occurring in a non-linear crystal pumped by a laser beam [26]. In an SPDC process, a one pump photon decays into a pair of photons, usually called correlated or entangled photons, since they have definitethey have relations definite between relations the betweenir frequencies, their frequencies, directions directions and polarizations. and polarizations. SPDC satisfies SPDC satisfies energy conservationenergy conservation 1/p = 1/ 1/λ1 p+= 1/1/λ2 1and+ 1 / momentumλ2 and momentum conservation conservation kp = k1 k+ pk=2, k where1 + k2 , wheres are photonλs are photonwavelengths, wavelengths, ks are k waves are wave vectors, vectors, and the and subscript the subscript p, 1p , and 1 and 2 denote 2 denote pumping pumping and and correlated correlated photons, respectively. Detector calibration calibration with with SPDC SPDC is intrinsically is intrinsically absolute absolute and independent and independent of traceability of traceability to external to externalradiometric radiometric standards. standards. Theoretically, Theoretically, the accuracy the of accuracy measured of quantummeasured e ffiquantumciency is efficiency solely determined is solely determinedby how accurately by how the accurately photon numberthe photon is counted. number is This counted. distinctive This featuredistinctive makes feature SPDC makes a promising SPDC a promisingcandidate forcandidate an SI primary for an SI standard primary [ 27standard], as well [27] as, as a space well as benchmark. a space benchmark. With known With quantum known quantumefficiency efficiencyη, the photon , the detector photon detector can measure can measure photon photon rate with rateN with= M /Nη, = or M flux/, or with flux Φwith= Nh  ν=, Nhor irradiance, or irradiance with withE = Nh E =ν /NhA, where/A, whereA is theA is area the area of receiving of receiving aperture aperture of the of system.the system. This This forms forms the physicalthe physical basis basis for tracingfor tracing solar solar irradiance irradiance measurements measurements to Planck’s to Planck’s constant constant and and the the photon photon rate. rate. Figure 55 showsshows thethe conceptconcept ofof aa solarsolar spectralspectral irradianceirradiance radiometerradiometer workingworking inin thethe visiblevisible toto shortwave infrared spectrum and incorporating internal SPDC calibration. The radiometer has two working modes, i.e., a calibration mode for traceability to SI and an observation mode for solar irradiance measurement.measurement.

Solar radiation aperture Monochromator Avalanche / Analog 1 photodiodes Correlated photons Rotating Middle by SPDC mirror optics Monochromator Avalanche / Analog  photodiodes 2

FigureFigure 5.5. ConceptConcept for forsolar solar spectral spectral irradiance irradiance measurement measurement with Spontaneous with Spontaneous Parametric Parametric Down- Down-Conversion (SPDC) calibration.Conversion (SPDC) calibration.

In the calibration mode, correlated photons are are directed by a rotating mirror into the middle optics, and then, are spectrally separated by two monochromators, before finally finally reaching avalanche photodiodes. The The middle optics suppress residual pump photons and the monochromators ensure the wavelength correlation 1/λ = 1/λ + 1/λ . The calibration mode gives the quantum efficiency or the wavelength correlation 1/pp = 1/11 + 1/22. The calibration mode gives the quantum efficiency or absolute responsivity of the whole system and naturally accounts for any degradation of components of the system. This This approach approach is especially useful as a reference or benchmark in space, and it appears advantageous over the endeavor to maintain a ‘highly stable’ onboard standardstandard inin space.space. In the observation mode, solar radiation passes an entrance aperture and propagates along the same optical path before it is received by analogue photodiodes. The prototype uses analogue photodiodes, instead of single photon detectors, in the observation mode, since currently, no compact single photon detector is commercially available beyond 1700 nm, and solar photon rate is about five orders of magnitude higher than the maximum count rate of single photon detectors below 1700 nm under our current configuration. The absolute responsivities of the analogue photodiodes are determined in calibration mode by receiving a photon flux measured by avalanche photodiodes. Photon flux beyond 1700 nm is measured at their correlated wavelength below 1700 nm, given N1 = N2 at a pair of correlated wavelengths. The solar irradiance will be obtained as E = Nhν/A with the known area A of entrance aperture.

4.3. IRS Based on Miniature Fixed-Temperature Phase-Change Cells Aiming at a high precision traceability of measurements with the space reference for the infrared spectrum, an infrared sounder with hyperspectral resolution traceable to international units will be designed. Fourier infrared spectrum detection technology and miniature phase-change cells as temperature standard technology are adopted for the IRS. Schematic diagram of the optical system Remote Sens. 2019, 11, x FOR PEER REVIEW 9 of 17

In the observation mode, solar radiation passes an entrance aperture and propagates along the same optical path before it is received by analogue photodiodes. The prototype uses analogue photodiodes, instead of single photon detectors, in the observation mode, since currently, no compact single photon detector is commercially available beyond 1700 nm, and solar photon rate is about five orders of magnitude higher than the maximum count rate of single photon detectors below 1700 nm under our current configuration. The absolute responsivities of the analogue photodiodes are determined in calibration mode by receiving a photon flux measured by avalanche photodiodes. Photon flux beyond 1700 nm is measured at their correlated wavelength below 1700 nm, given N1 = N2 at a pair of correlated wavelengths. The solar irradiance will be obtained as E = Nh/A with the known area A of entrance aperture.

4.3. IRS Based on Miniature Fixed-Temperature Phase-Change Cells Aiming at a high precision traceability of measurements with the space reference for the infrared spectrumRemote Sens., an2020 infrared, 12, 2179 sounder with hyperspectral resolution traceable to international units will9 of be 17 designed. Fourier infrared spectrum detection technology and miniature phase-change cells as temperature standard technology are adopted for the IRS. Schematic diagram of the optical system forfor the the IRS IRS is isdrawn drawn in Figure in Figure 6. Accurate6. Accurate blackbody blackbody temperatures temperatures (fixed (ﬁxed points) points) are based are on based ‘phase on- change‘phase-change cells’ technology. cells’ technology.

FigureFigure 6. 6. SchematicSchematic diagram diagram of of the the optical optical system system for for the the I Infrarednfrared Spectrometer Spectrometer ( (IRS).IRS). Accurate Accurate blackbodyblackbody temperatures temperatures (fixed (ﬁxed points) points) are are based based on on ‘ ‘phase-changephase-change cells cells’’ technology technology..

TraceabilityTraceability chain chain of of the the IRS IRS is is shown shown in in Figure Figure 77.. Three Three on on-orbit-orbit a absolutebsolute r radianceadiance IR calibrators realizerealize on on-orbit-orbit self self-calibration-calibration of of the the cavity cavity blackbody. blackbody. A Ann on on-orbit-orbit temperature temperature scale scale from from 270 270 to to 350350 K K is is establish establisheded using using ITS ITS-90-90 miniature miniature phase phase-change-change cells cells traceable traceable to to ITS ITS-90-90 with with an an uncertainty uncertainty ofof better better than than 10 10 mK mK (k (k == 2)2) [28]. [28]. Based Based on on high high precision precision temperature temperature control control and and a a transfer transfer method, method, the blackbody temperature is traceable to the SI standard. The blackbody emissivity is measured based on the controlled environment radiation method and is transferred to another blackbody using the laser. The infrared interferometer realizes the high spectral resolution measurement over the infrared spectrum. The high sensitivity response over the wide spectral band is achieved by using a small-array detector. The high stability performance is realized by system temperature control technology using multiple temperature zones. High accuracy spectral calibration is achieved by referring to a standard spectral line source (high frequency infrared laser and gas absorption line). Remote Sens. 2019, 11, x FOR PEER REVIEW 10 of 17 the blackbody temperature is traceable to the SI standard. The blackbody emissivity is measured based on the controlled environment radiation method and is transferred to another blackbody using the laser. The infrared interferometer realizes the high spectral resolution measurement over the infrared spectrum. The high sensitivity response over the wide spectral band is achieved by using a small- array detector. The high stability performance is realized by system temperature control technology using multiple temperature zones. High accuracy spectral calibration is achieved by referring to a Remote Sens. 2020, 12, 2179 10 of 17 standard spectral line source (high frequency infrared laser and gas absorption line).

Figure 7. Traceability chainchain ofof thethe InfraredInfrared SpectrometerSpectrometer (IRS).(IRS). 5. On-Orbit Mode to Support Intercalibration with Radiometric Traceability 5. On-Orbit Mode to Support Intercalibration with Radiometric Traceability In the calibration mode, LIBRA will be considered as the reference satellite to intercalibrate a In the calibration mode, LIBRA will be considered as the reference satellite to intercalibrate a target satellite with radiometric traceability. Intercalibration and the radiometric transfer from LIBRA target satellite with radiometric traceability. Intercalibration and the radiometric transfer from LIBRA to other satellites require the measurements from each spacecraft are taken along similar lines of sight, to other satellites require the measurements from each spacecraft are taken along similar lines of sight, and within a few minutes of each other [29]. Similar techniques are currently used for intercalibration and within a few minutes of each other [29]. Similar techniques are currently used for intercalibration of satellite sensors within GSICS, an international effort to improve the consistency and accuracy of of satellite sensors within GSICS, an international effort to improve the consistency and accuracy of satellite intercalibration [12]. Examples of GSICS Products to support intercalibration with radiometric satellite intercalibration [12]. Examples of GSICS Products to support intercalibration with traceability are listed in Table3. LIBRA would serve the international community by intercalibrating radiometric traceability are listed in Table 3. LIBRA would serve the international community by other Earth-observing instruments. The following sections describe the possible standard transfer intercalibrating other Earth-observing instruments. The following sections describe the possible methods from the LIBRA reference instruments. standard transfer methods from the LIBRA reference instruments. Table 3. Products to support intercalibration with radiometric traceability. Table 3. Products to support intercalibration with radiometric traceability. Instruments Products Intercalibration Method Example Instruments Products Intercalibration Method Example Quasi-synchronous intercalibration [16] Quasi-synchronous Spectrally-resolved infrared [16] IRS LEO-LEOintercalibration SNO [30,31] IRS Spectrally-resolvedradiance infrared radiance GEO-LEOLEO SNO-LEO SNO [32,33[30] ,31] GEO-LEO SNO [32,33] Quasi-synchronous intercalibration [34] Quasi-synchronous SpectralSpectrally-resolvedly-resolved reflectance of solar [34] EMIS reflectance of solar radiation LEO-LEOintercalibration SNO [35,36] radiation GEO-LEOLEO SNO-LEO SNO [37] [35,36] EMIS Selected DCC reflectance DCC [38,39] Selected PICS reflectance PICS [40] Selected Lunar reflectance Lunar [41,42] Remote Sens. 2019, 11, x FOR PEER REVIEW 11 of 17

GEO-LEO SNO [37] Selected DCC reflectance DCC [38,39] Selected PICS reflectance PICS [40] Selected Lunar reflectance Lunar [41,42]

5.1. Quasi-Synchronous Intercalibration Transfer Mode by Orbital Maneuver LIBRA will be operated such that its sub-satellite track is close to and overlaps with the track of the satellite to be intercalibrated (as seen in Figure 8). The intercalibration is carried out in near real- time in the nadir zone. Calibration biases due to differences in viewing angles and sub-satellite tracks have to meet matchingRemote Sens. constraints2020, 12, 2179 between LIBRA and the target satellite to be calibrated. There is no need to11 carry of 17 out attitude maneuvers because collocated observations can be obtained over longer time periods. In this mode, altitude and inclination adjustments for LIBRA are needed to conduct an intercalibration5.1. Quasi-Synchronous and to Intercalibrationprovide match Transfer-ups for Mode intercalibration by Orbital Maneuver over a longer period. Because of the substantialLIBRA fuel will consumptions be operated suchrequired that to its perform sub-satellite the orbit track maneuvers, is close to the and quasi overlaps-synchronous with the mode track willof the only satellite be used to bewhen intercalibrated required by (asthe seentarget in satellite. Figure8 ).The The frequency intercalibration of such isintercalibrations carried out in near will dependreal-time on in the the available nadir zone. fuel of the satellite platform.

Current stage quasi-synchronous observation A ft Targeted sat. 1 er ma Reference ne (h1,i1) uv sat. (h2,i2) er

Man euver Next stage quasi-synchronous Adjus t h, i Reference observation sat. (h3,i3) Targeted sat. 2 (h4,i4) Substar bias N 0~15km

}

Substar bias } 0~15km

i1 i Ea 2 rth i ra 3 diu s R i4 e Equator O

FigureFigure 8. 8. QuasiQuasi-synchronous-synchronous intercalibration intercalibration transfer transfer mode mode by by orbital orbital maneuver maneuvers.s.

5.2. SimultaneousCalibration Nadir biases Overpass due to di(SNO)ﬀerences Cross in Intercalibration viewing angles Transfer and sub-satellite Mode (GEO-LEO tracks or haveLEO-LEO) to meet matching constraints between LIBRA and the target satellite to be calibrated. There is no need to carry out attitudeWith the maneuvers reference satellite because LIBRA collocated and observationsthe targeted satellite can be obtained flying in overdifferent longer orbit timeal planes, periods. the crossingIn thisarea mode,of the ground altitude tracks and inclinationis chosen as adjustments the intercalibration for LIBRA area. are Differences needed toin observation conduct an timeintercalibrations and viewing and angle to providehave to meet match-ups certain forrequirements. intercalibration over a longer period. Because of the substantialAs depicted fuel in consumptionsFigure 9, LIBRA required flies into to the perform intercalibration the orbit maneuvers, area at time the tC1quasi-synchronous and point C1, and exitsmode the will area only at be time used tC3 when and point required C3. byWhen the targetLIBRA satellite. observeS Thes with frequencyin the intercalibration of such intercalibrations area, the attitudewill depend is adjusted on the availablesuch that fuelLIBRA of theand satellite the targeted platform. satellite view the same area on the ground. The observation time differences Δt = tC − tF shall be small enough to meet transfer accuracy requirements. 5.2. Simultaneous Nadir Overpass (SNO) Cross Intercalibration Transfer Mode (GEO-LEO or LEO-LEO)

With the reference satellite LIBRA and the targeted satellite flying in different orbital planes, the crossing area of the ground tracks is chosen as the intercalibration area. Differences in observation times and viewing angle have to meet certain requirements. As depicted in Figure9, LIBRA flies into the intercalibration area at time t C1 and point C1, and exits the area at time tC3 and point C3. When LIBRA observes within the intercalibration area, the attitude is adjusted such that LIBRA and the targeted satellite view the same area on the ground. The observation time differences ∆t = t t shall be small enough to meet transfer accuracy requirements. C − F RemoteRemote Sens.Sens.2020 2019,,12 11,, 2179 x FOR PEER REVIEW 1212 ofof 1717 Remote Sens. 2019, 11, x FOR PEER REVIEW 12 of 17

FigureFigure 9. Simultaneous 9. Simultaneous Nadir Nadir Observation Observation (SNO) (SNO intercalibration) intercalibration transfer transfer mode mode (Geostationary (Geostationary earth Figure 9. Simultaneous Nadir Observation (SNO) intercalibration transfer mode (Geostationary orbit (GEO)-Low earth orbitearth (LEO)orbit (GEO) or LEO-LEO).-Low earth orbit (LEO) or LEO-LEO). earth orbit (GEO)-Low earth orbit (LEO) or LEO-LEO). 5.3.5.3. UsingUsing LunarLunar ObservationsObservations forfor IntercalibrationIntercalibration 5.3. Using Lunar Observations for Intercalibration ObservationsObservations ofof thethe MoonMoon provideprovide aa viableviable methodmethod forfor thethe on-orbiton-orbit crosscross calibrationcalibration ofof EarthEarth remoteremoteObservations sensingsensing instruments. of the Moon Therefore Therefore, provide, m a monthly onthlyviable lunarmethod lunar observations observations for the on -areorbit are major majorcross components calibration components of of the ofEarth theon- on-orbitremoteorbit calibration sensing calibration instruments. strategies strategies of Therefore ofLIBRA LIBRA and, and m otheronthly other instruments. instruments.lunar observations Taking Taking theare the Mmajor Moonoon componentsas as a a calibration calibration of the source, on- theorbitthe reference referencecalibration and and strategies target target satellite ofsatellite LIBRA observe observand the othere same the instruments. lunar same phase lunar Taking which phase the is which realized Moon isas by realizeda attitudecalibration by maneuvers attitudesource, eachthemaneuver reference months each (as and shown month target in (as Figuresatellite shown 10 in). observ Figure Then,e thethe10). intercalibration sameThen, the lunar intercalibration phase can bewhich performed can is berealized performed using by a standard attitude using a lunarmaneuverstandard radiation lunars each model,radiation month such (as model shown as RObotic, such in Figure as Lunar RObotic 10). Observatory Then, Lunar the Observatory intercalibration (ROLO) [43 (ROLO]. can) [43]be performed. using a standard lunar radiation model, such as RObotic Lunar Observatory (ROLO) [43].

FigureFigure 10.10. LunarLunar observationobservation intercalibrationintercalibration transfertransfer mode.mode. Figure 10. Lunar observation intercalibration transfer mode. 5.4. Using Vicarious Reference Targets for Intercalibration 5.4. Using Vicarious Reference Targets for Intercalibration 5.4. UsingThe reference Vicarious satelliteReference can Targets also observefor Intercalibration stable targets such as Pseudo Invariant Calibration Sites (PICSs)The and reference Deep Convective satellite can Clouds also observe (DCC) stable over atargets long period such as of Pseudo time, thereby Invariant collecting Calibration multiple Sites The reference satellite can also observe stable targets such as Pseudo Invariant Calibration Sites angle(PICSs) observational and Deep Convective data. Figure Clouds 11 shows (DCC) the over schematic a long diagramperiod of oftime, intercalibration thereby collecting transfer multiple mode (PICSs)angle observation and Deep Convectiveal data. Figure Clouds 11 shows (DCC) the over schematic a long period diagram of time,of intercalibration thereby collecting transfer multiple mode using vicarious reference targets. When the target satellite ﬂies over those reference sites or DCC, the angleusing observationvicarious referenceal data. targets.Figure 11W henshows the the target schematic satellite diagram flies over of th iosentercalibration reference sites transfer or DCC mode, the co-located observations will be stored to monitor the long-term stability of instrument calibration [29]. usingco-located vicarious observations reference will targets. be stored When to the monitor target the satellite long- termflies overstability those of reference instrument sites calibration or DCC, [29].the co-located observations will be stored to monitor the long-term stability of instrument calibration [29].

Remote Sens. 2020, 12, 2179 13 of 17 Remote Sens. 2019, 11, x FOR PEER REVIEW 13 of 17

Reference satellite Target satellite 2 Target satellite1

Reference Sites

Figure 11. Intercalibration transfer mode using vicarious reference targets.targets. 6. Balloon-Borne Demonstration System 6. Balloon-Borne Demonstration System When radiometric benchmark transfer methods are developed, an issue is the validation of the When radiometric benchmark transfer methods are developed, an issue is the validation of the full chain of the space-based radiometric benchmark transfer. This problem is a primary motivation full chain of the space-based radiometric benchmark transfer. This problem is a primary motivation for the development of a balloon-borne demonstration system. There are three notable benefits in for the development of a balloon-borne demonstration system. There are three notable benefits in using this aerostat: First, the flight altitude of the aerostat can reach at least 18 km, above most of the using this aerostat: First, the flight altitude of the aerostat can reach at least 18 km, above most of the atmosphere that contributes to the radiances observed from satellite. Second, there is a quasi-zero atmosphere that contributes to the radiances observed from satellite. Second, there is a quasi-zero wind speed layer at the height range of 18–25 km in middle-latitude areas, such as over China, at the wind speed layer at the height range of 18–25 km in middle-latitude areas, such as over China, at the end of spring and in summer, so it is easier to have longer observations with the aerostat. Third, the end of spring and in summer, so it is easier to have longer observations with the aerostat. Third, the onboard sensors can be recovered after a flight, so that high accuracy calibrations of sensors are feasible onboard sensors can be recovered after a flight, so that high accuracy calibrations of sensors are before and after an aerostat flight. feasible before and after an aerostat flight. Considering the state of the art of aerostats, a high altitude balloon was chosen as the demonstration Considering the state of the art of aerostats, a high altitude balloon was chosen as the platform because it provides capacity for heavy payloads. However, the motion of the aerostat by demonstration platform because it provides capacity for heavy payloads. However, the motion of the the wind will affect spatial and temporal data matching between the aerostat and satellite. Therefore, aerostat by the wind will affect spatial and temporal data matching between the aerostat and satellite. before launching an aerostat flight mission, historical wind field data and the previous two days’ Therefore, before launching an aerostat flight mission, historical wind field data and the previous radiosonde data will be collected to anticipate the balloon flight path, in order to choose the proper two days’ radiosonde data will be collected to anticipate the balloon flight path, in order to choose flight time and plan for an optimized flight path. During the flight, the flight control unit of the the proper flight time and plan for an optimized flight path. During the flight, the flight control unit balloon can update the flight path, based on real-time wind fields, and precisely forecast the short-term of the balloon can update the flight path, based on real-time wind fields, and precisely forecast the trajectory and determine fall point of the payload cabin. Due to atmospheric friction, the payload cabin short-term trajectory and determine fall point of the payload cabin. Due to atmospheric friction, the will rotate around the linkage cable. Therefore, azimuth of the payload is controlled with a flywheel payload cabin will rotate around the linkage cable. Therefore, azimuth of the payload is controlled alleviating balloon rotation. Precise calculation of sensor position and attitude will be realized through with a flywheel alleviating balloon rotation. Precise calculation of sensor position and attitude will the onboard high accuracy position and orientation system (POS). be realized through the onboard high accuracy position and orientation system (POS). Currently, there are two reference sensors considered for the balloon test flights. The first Currently, there are two reference sensors considered for the balloon test flights. The first is a is a simulator of the space-based benchmark spectroradiometer, covering a spectral range from simulator of the space-based benchmark spectroradiometer, covering a spectral range from 400 to 400 to 2500 nm with a spectral resolution of 3.5 nm@(400~1000 nm), 10 nm@(1000~1700 nm), 2500 nm with a spectral resolution of 3.5 nm@(400~1000 nm), 10 nm@(1000~1700 nm), 12 12 nm@(1700~2500 nm) and a maximum FOV of 10 . The second is a thermal sensor covering nm@(1700~2500 nm) and a maximum FOV of 10°. The ◦second is a thermal sensor covering the same the same spectral range and a similar spectral resolution as the future space-based MIR-TIR benchmark spectral range and a similar spectral resolution as the future space-based MIR-TIR benchmark sensor. sensor. Two blackbodies (BB), i.e., a high temperature (30 C) BB and low temperature (10 C) BB, are Two blackbodies (BB), i.e., a high temperature (30 °C ) BB◦ and low temperature (10 °C ) BB◦ , are used used to perform onboard calibration. The sensors are installed in a special chamber with high shock to perform onboard calibration. The sensors are installed in a special chamber with high shock resistance and they are operated with active temperature control to maintain 20 C. resistance and they are operated with active temperature control to maintain 20◦ °C . At least two stratospheric balloon-based test flights will be performed in the demonstration At least two stratospheric balloon-based test flights will be performed in the demonstration phase. During each flight, there will be several flight phases of the aerostat: (i) launch, (ii) ascend, phase. During each flight, there will be several flight phases of the aerostat: (i) launch, (ii) ascend, (iii) observation, and (iv) descend (Figure 12). The total duration time is about 8 h, ensuring that there are

Remote Sens. 2020, 12, 2179 14 of 17

Remote Sens. 2019, 11, x FOR PEER REVIEW 14 of 17 (iii) observation, and (iv) descend (Figure 12). The total duration time is about 8 h, ensuring that there atare least at least 6 h 6for h forobservation observations.s. Considering Considering the the narrow narrow swath swath of of balloon balloon-based-based reference reference sensors, sensors, the the balloonballoon needs to observeobserve uniformuniform targetstargets in in order order to to alleviate alleviate inﬂuences influences of of di ﬀdifferenterent resolutions resolutions of theof theto be to calibratedbe calibrated satellite satellite instruments. instruments The. The Golmud Golmud area area (94.9 (94.9◦E,°E, 36.436.4◦°N)N) andand Qinghai Lake (9 (99.49.4°◦E,E, 36.36.33°◦N)N) are are considered considered as as candidate candidate calibration calibration sites. sites. In In the the experiment, experiment, the the Chinese Chinese high high resolution resolution GFGF series series and and the the ZY ZY series series satellite satellites,s, the the FY FY series series satellites, satellites, as as well well as as Landsat Landsat-8,-8, Sentinel Sentinel-2,-2, and and NPP NPP willwill be considered as satellites for which intercalibration will be demonstrated.

Figure 12. Sketch of ﬂight test of the balloon-based demonstration system.

7. Conclusions Figure 12. Sketch of flight test of the balloon-based demonstration system. Accurate, long-term, consistent data are essential to climate science. Such data need to be calibrated 7. Conclusions to an absolute standard. The nature of satellite observations is the key to obtain global data of the earth andAccurate, atmospheric long system.-term, However, consistent so data far, are calibration essential directly to climate traceable science. to a Such reference data has need not to been be calibratedpossible for to satellitean absolute observations. standard. The The nature principle of satellite of referring observations measurements is the key to anto obtain absolute global standard data ofis the not e new,arth and as it atmospher has beenic realized system. for However, in situ observationsso far, calibration which directly are traceable traceable to to standardsa reference with has notwell been quantified possible uncertainties. for satellite observations. This paper describes The principle a novel of space-based referring measurements system, providing to an absolute a similar standardtraceability is not and new, accuracy as it forhas space-based been realized radiance for in measurements.situ observations The which realization are traceable of the LIBRA to standard systems withwill enable well quanti climatefied monitoring uncertainties. with muchThis paper higher describes sensitivity a tonovel trend space detection-based and system provide, providing benefits in a similarscience traceability and economic and terms accuracy [44]. for space-based radiance measurements. The realization of the LIBRALIBRA system has will important enable climate potential monitoring contributions with much to make higher to futuresensitivity climate to tre observingnd detection systems, and provideboth directly benefits through in science well-calibrated and economic measurements terms [44]. and indirectly through facilitating intercalibration of theLIBRA data from has important other platforms. potential LIBRA contributions will measure to make solar reflectedto future and climate infrared observing emitted system high spectrals, both directlyresolution through benchmark well-calibrated radiances, measurements and provide accurate,and indirectly credible, through and testedfacilitating climate intercalib records.ration These of themeasurements data from other will platforms. be used to LIBRA detect will climate measure change solar trends reflected and test,and validate,infrared emitted and improve high spectral climate resolutionprediction benchmark models. LIBRA radiances, can be and used provide to calibrate accurate, other credible, solar and and infrared tested space-borne climate records. sensors These and measurementsthereby improve will climate be used monitoring to detect climate accuracy change for a widetrends range and test, of measurements validate, and acrossimprove the clim globalate predictionspace-based models. observing LIBRA system. can be used to calibrate other solar and infrared space-borne sensors and therebyAs improve a complementary climate monitoring project to CLARREOaccuracy for and a wide TRUTHS, range LIBRAof measurements is expected across to join the an global Earth spaceobserving-based satellite observing constellation system. and intends to contribute to space-based climate studies via publicly availableAs a complementary data. Intercalibration project ofto dataCLARREO from space-basedand TRUTHS, observations LIBRA is expected falls under to join the an auspices Earth observof theing international satellite constellation CEOS Working and intends Group to oncontribute Calibration to space and-based Validation climate (WGCV) studies via and publicly GSICS. availableTherefore, data. intensive Intercalibration cooperation of with data the from WGCV space and-based GSICS observations is highly recommended falls under the during auspices instrument of the internationaldevelopment CEOS and data Working utilization Group of theon Calibration LIBRA project. and Validation (WGCV) and GSICS. Therefore, intensive cooperation with the WGCV and GSICS is highly recommended during instrument development and data utilization of the LIBRA project.

Remote Sens. 2020, 12, 2179 15 of 17

Author Contributions: P.Z., N.L. and C.L. proposed the framework of the manuscript and supervised the progress. L.D., X.Z. (Xiaobing Zheng), X.Z. (Xuejun Zhang), X.H., X.Y., L.M. contributed to the designing of reference instruments, intercalibration modes, and Balloon-borne demonstration system. N.X., L.C., and J.S. aided in the manuscript revision. P.Z. prepared and finalised the manuscript. All authors contributed to the writing and commented on the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This work has been supported by the National Key Research and Development Program of China (2018YFB0504900; 2018YFB0504800), and the Bureau of International Co-operation Chinese Academy of Sciences (181811KYSB20160040). Acknowledgments: The authors do appreciate the discussion and comments from Kenneth Holmlund, Tim J. Hewison of EUMETSAT, Mitch Goldberg of NOAA, Nigel Fox of NPL, Bruce A. Wielicki of NASA, Greg Kopp of University of Colorado, and Wenjian Zhang of WMO. The authors are grateful to Wei Zhu and Wei Liang of Shanghai Institute of Satellite Engineering for their work concerning the orbit simulation and demonstration of Chinese LIBRA satellite. Conflicts of Interest: The authors declare that they have no conflict of interest.

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