The Navy Geosat Mission: an Overview

DONALD R. McCONATHY and CHARLES C. KILGUS

THE NAVY GEOSAT MISSION: AN OVERVIEW

The limited GEOS and Seas at data sets have proven the satellite radar altimeter to be a versatile and powerful tool for the remote sensing of the oceans. Altimetry data support Navy requirements in the areas of geodesy, the operational measurement of fronts and eddies, winds, waves, and ice topography. The Navy GEOSAT Mission is working to satisfy these requirements by producing a dense, global altim­ etric database. This article gives an overview of radar altimetry, the Navy GEOSA T Mission, and ongo­ ing applications of the data.

BACKGROUND 24

The GEOSAT I radar altimeter measures the distance 20 between the satellite orbit and the subsatellite point on the ocean urface with a precision of a few centimeters. en E16 Since the shape of the orbit can be measured indepen­ Q) / dently, the altimeter gives a precise measurement of the E :- 12 ( shape of the ocean's surface along a line under the sat­ .r:. Tonga OJ i ellite. 'w . trench .r:. 8 In the absence of di turbing forces, the ocean flows Q) / u under the influence of gravity so that its surface con­ -t 4 forms to the shape of the earth's geopotential field. The ::l t{' cu altimeter then directly measures the marine geoid. The Q) process is disturbed both by rapidly changing oceano­ (f) 0 graphic features (e.g., rings and eddies) and slowly vary­ ing components of ocean circulation (e.g., major currents - 4 such as the Gulf Stream). A long-term average of altim­ eter data reduces the impact of the "noise" introduced - 8 20 10 o -10 - 20 -30 by these oceanographic features and gives a "mean sur­ Latitude (degrees) face" that is a good approximation of the marine geoid in many areas (Fig. 1). The direction of the normal to Figure 1-The geoid feature in the ocean topography across the Tonga Trench as measured by the GEOSAT altimeter (see the mean surface, the "local vertical," is an important the article by Cheney in this issue). term in navigation models. The need for precise naviga­ tion has made geodesy the most important application of radar altimeter data. 150.---.-----,,----,,-----.----~------, Oceanographic features can be recovered from the al­ timeter data by subtracting the long-term mean surface (Fig. 2). Recent research has indicated that the time-de­ pendent mesoscale (50- to 300-kilometer) features (e.g., front, rings, and eddies) that can be sensed by the altim­ eter have a strong impact on underwater acoustic propa­ gation. The operational generation of these environmen­ tal products is a maturing application of radar altimeter data. Finally, the significant wave height and reflection co­ efficient of the ocean surface can be determined by mea­ suring the leading-edge slope and amplitude of the re­ flected radar pulse. Ground processing allows the surface Surface depression wind speed to be derived from the reflection coefficient. - 60~--~----~----~----~----~------~ 20 10 o -10 - 20 -30 D. R. McConathy was, until recently, the GEOSAT Project Manag­ Latitude (degrees) er in SPA WAR, Department of the avy, Washington, DC 20362 . Figure 2-Mesoscale oceanographic features in the differ­ C. C. Kilgus is the GEOSA T Program Manager, Space Department, ence of two repeat-orbit passes over the Tonga Trench (see APl, laurel, MD 20707. the article by Cheney in this issue).

170 Johns H opkins APL Technica l Diges c, Volume 8, Number 2 (1987) ALTIMETER MISSIONS c:: 100.0 .2 C/l The in-orbit measurement precision of the altimeter .(3 - has improved from the I-meter precision accomplished Earth Observing System for late 1993. peatedly justifies this expensive space-qualified sensor. THE GEOSAT PRIMARY MISSION The improvement in the altimeter's ability to measure the marine geoid coincided with a growing Navy require­ GEOSA T's primary mission is to provide the dense global grid of altimeter data required to improve the de­ ment for more accurate geodesy data. Increased accuracy requirements for Advanced Submarine Launched Ballis­ termination of the earth's gravitational field. The second­ tic Missile systems have resulted in an extensive effort ary mission is to detect mesoscale oceanographic features to understand and reduce a variety of error sources. As in a timely manner. these error sources are reduced, the geodetic/ geophysical The mission uses a single instrument, the radar altime­ error contributors become an increasingly large percent­ ter, with a 3.5-centimeter precision at a 2-meter signifi­ age of the total system error budget. A reduction of the cant wave height. Dual-frequency Doppler tracking al­ error budget can be realized by extensive ship and and lows precise satellite-orbit determination. Data are stored satellite surveys to achieve the necessary improvement on board the spacecraft for approximately 12 hours and to the earth gravitational models. However, a high-den­ are then transmitted to the Satellite Tracking Facility at sity intermediate- and low-frequency gravity database is APL. The GEOSA T ground station preprocesses the required worldwide that is impractical to obtain by ship data for distribution to a variety of users. Companion survey. As a consequence of the early loss of Seasat, articles elsewhere in this issue describe the systems com­ GEOSAT was conceived within the Navy in late 1978 ponents: the radar altimeter (MacArthur et a1.), the to produce a detailed worldwide geodetic database by spacecraft (Frain et al.), and the ground system (Jones 1985-something no other proposed system could do. et al.). The primary mission of GEOSAT was therefore to The satellite was launched on March 12, 1985, into provide a homogeneous high-density intermediate- and an 800-kilometer altitude, 108-degree inclination orbit long-wavelength gravity database over the global ocean. that generated 3-day near-repeat ground tracks. At the completion of the 18-month-long primary mission on The miss ion directly supports September 30, 1986, the average spacing of the ground I. Improved map compensations for geoid height, track grid was 4 kilometers. The altimeter had then ac­ vertical deflections, and in-flight gravity; cumulated 270 million observations of sea level along 2. Definition of specific geophysical/geologic areas 200 million kilometers of the world's oceans. Having for more detailed ship survey follow-up; completed the primary mission, GEOSA T was maneu­ 3. Detection of the possible bathymetric hazards to vered into a 17-day exact repeat orbit optimized for submerged navigation (an existing Defense Map­ oceanography to begin the GEOSAT Exact Repeat Mis­ ping Agency requirement). sion. Data from this mission are helping to satisfy the The mission was to be satisfied by three 6-month data need for measurements of oceanographic features that sets with an equatorial satellite subpoint separation of are critical to Navy operations. approximately 10 kilometers.

Johns H opkins APL Technical Digest. \' olu l11t' 8. Number :2 (198 7) 171 McConathy, Kilgus - The Navy GEOSA T M ission: A n O vervie w

Partial justification for the GEOSAT mission resulted ted via S-band telemetry to the Satellite Tracking Facility from the cost benefit to be realized. The entire GEOSAT at APL (Fig. 4). After demodulation and bit synchroni­ program (i.e. , altimeter, spacecraft, launch, and 3-year zation, the encrypted data are stored by analog tape re­ mission operations) is projected to cost approximately corders. Post pass , the archive tape is read and the data $60 million. Using GEOSAT data as a reconnaissance are frame synchronized and stored on computer disk for survey upon which future ship-survey track spacing could later processing. be planned was estimated in 1980 to save 16 ship-years No single-point-failure redundancy is incorporated in of survey time at a cost of $60 million. Additionally, in the telemetry and command chains at the APL facility. GEOSAT will detect 98 percent of the bathymetric haz­ This allows system operation using the single ground sta­ ards to submerged navigation during a 6-month opera­ tion without loss of data or command opportunity dur­ tion. (Ocean Survey Program ships can provide only 49 ing the entire 18-month geodesy mission. percent of the data in major operating areas because of Precise orbital ephemeris for the user is computed by track spacing.) It is estimated that the ship survey time the Naval Surface Weapons Center, Dahlgren, Va., using saved is 30 ship-years at a cost of $280 million (1980 dol­ Doppler beacon tracking data obtained by the Defense lars). GEOSAT has already shown an extraordinary cost Mapping Agency TRANET Network. Navy Space Sur­ benefit to the Navy. veillance Activity (radar track) elements are used by the During the early GEOSAT planning stages, it became APL facility for alerts and antenna pointing. obvious that other scientific disciplines could benefit Three primary data products are provided by the APL from altimeter measurements. Seasat data analysis was facilit y for user applications: showing an important relationship between altitude and major oceanographic features (e.g., current boundaries 1. Sensor Data Record- a classified computer-com­ and mesoscale eddies). In February 1982, the Navy estab­ patible tape containing measured altitude, signifi­ lished a secondary GEOSAT mission to support tactical cant wave height, wind speed, and automatic gain mesoscale oceanography, which was not to interfere with control, plus corrections for satellite and instru­ the geodesy mission during the first 18 months of oper­ ment errors. After generation, the tapes are vali­ ation. In October 1986, the satellite was maneuvered into dated for independent quality control; during the a 17 -day exact repeat orbit designed to optimize the mea­ primary mission period, they were delivered to the surement of tactical oceanography. The Exact Repeat Naval Surface Weapons Center within two weeks Mission is to continue for as long as the satellite operates. of data receipt. GEOSA T Primary Mission Data Flow 2. Naval Ocean Research and Development Activity Radar altimeter data are stored on board the space­ (NORDA) Data Record-a classified data record craft for approximately 12 hours and are then transmit- (with contents similar to the Sensor Data Record)

Defense Mapping Agency ~Doppler Doppler tracking ~ Geodesy users: Naval Sensor Geophysical Defense Mapping Surface Data Data Agency; Naval Weapons f-+- ---. Record Record Oceanographic Center Office S-band . ~ telemetry

APL NORDA Naval Winds/waves Fleet Satellite Data Record Ocean surface topography Numerical Research and Tracking Data line Oceanographic l Facility Development Center Activity

I Ice index t

Waveform Naval Research Naval Polar Data Research , data Oceanography Record Laboratory Center

Figure 4-GEOSAT primary mission data flow.

172 Johns H opkins A PL Technica l Digesl , Volume 8, N um ber 2 (1987) McConathy, Kilgus - The Navy GEOSA T Mission: An Overview generated in near-real time and transmitted to gasso Sea, a feature that does not show up in the infrared NORDA over an encrypted data line. data. Articles by Lybanon and Crout and by Mitchell 3. Waveform Data Record- an unclassified compu­ et al. in this issue describe the oceanographic processing ter-compatible tape containing the raw waveform at NORDA with examples of the products that are gen­ samples from the altimeter. The tape is given to erated. the Naval Research Laboratory for ice studies. GEOSAT Wind and Wave Research GEOSA T Geodesy Determination The altimeter directly measures significant wave height The Geophysical Data Record, the primary data set by measuring the range extent of the ocean surface. Wind for geodesy determination, is produced at the Naval Sur­ speed, however, must be inferred from the altimeter's face Weapons Center. First, additional quality control, measurement of the backscatter (reflection) coefficient of including land/ ice separation, is performed on raw Sen­ the surface. The development of the best algorithm to sor Data Record data. The precise ephemeris is then fit­ determine wind speed has been an active research goal ted to the data, and records with corrections for tides at the Naval Research Laboratory and at APL. As de­ and the environment are added. The article by Smith et scribed by Shuhy et al. and Dobson et al. elsewhere in ai. elsewhere in this issue describes the production of the this issue, a direct comparison between altimeter- and Geophysical Data Record. buoy-measured winds is difficult, with errors inherent in The resulting classified tapes are given to the Naval the buoy instruments and with both spatial and temporal Oceanographic Office and to the Defense Mapping sampling errors masking the differences between candi­ Agency Aerospace Center for final geodesy processing. date algorithms. The dense 18-month grid of 3-centimeter precision-altim­ GEOSAT Oceanographic Research etry data has dramatically improved the Department of The Northwest Atlantic Regional Energetics Experi­ Defense global geoid. (See the articles by Van Hee and ment Project. The GEOSAT data are supporting this by Sailor and LeSchack elsewhere in this issue for a dis­ Navy research, development, test, and evaluation project cussion of the quality of the data and the final geoid at NORDA. The project combines GEOSAT altimetry determination.) data; infrared data; deep (800-meter) airborne expend­ GEOSAT Tactical Oceanography able bathythermography surveys; deployed inverted echo In 1981 , the Navy began the GEOSAT Ocean Appli­ sounder and bottom pressure-gauge arrays; and eddy-re­ cations Program to develop a semioperational tactical solving, primitive-equation numerical modeling to study oceanography capability for Fleet support. The objective the dynamics and energetics of mesoscale fluctuations was to provide a Sensor Data Record field in near-real in the Gulf Stream near the New England Seamount time to NORDA. A 9600-baud encrypted telecommuni­ Chain. In addition to providing excellent surface truth cations circuit was installed between the SEL 32 com­ data for GEOSAT, the project will, for the first time, puters at APL and NORDA to support this data require­ attempt to correlate surface topography signature and ment. Processing software to produce a NORDA Data measured subsurface features to provide modeling tech­ Record (essentially a nonquality-controlled Sensor Data niques for oceanographic analyses. The article by Mitch­ Record) at APL was developed, as well as the communi­ ell et al. in this issue describes the project. cations software to support the IBM X.25 circuit proto­ NOAA Research at APL. A program sponsored and col at both APL and NORDA. Data have been transmit­ funded by the NOAA National Geodetic Survey entitled ted routinely over this line since launch. Global Sea Level Variability from GEOSAT Altimetry At NORDA, wind and wave data are extracted from was approved by the Navy and implemented in late 1985. the NORDA Data Record and forwarded in near-real Under the direction of R. E. Cheney and B. C. Douglas, time to the Fleet Numerical Oceanographic Center, Mon­ a classified computer facility was established at APL terey, Calif., for inclusion in numerical analysis models. within existing GEOSAT security guidelines. An access Twice weekly a mesoscale analysis of a section of the agreement was signed by APL, NOAA, and the Navy, northwest Atlantic Ocean is produced and sent to the wherein processed Sensor Data Record tapes and precise Fleet Numerical Oceanographic Center and the Naval ephemeris data are given to NOAA along with environ­ Eastern Oceanography Center in Norfolk, Va. Finally, mental corrections from the Fleet Numerical Oceano­ an ice-edge depiction for the Arctic and Antarctic areas graphic Center. A Geophysical Data Record is produced is provided each day to the loint Ice Forecast Center by NOAA, and both crossover differences and sea height located at the Naval Polar Oceanography Center in Suit­ variability fields are created. Under the agreement, rel­ land, Md. evant research papers are cleared by the Navy for open The results of analyzing GEOSA T data for tactical publication. A validation procedure is being developed oceanographic applications have been encouraging. An to make possible the release of crossover-difference data extremely good correlation between altitude variations tapes from the 18-month geodesy mission. The article and significant oceanographic features has been verified by Cheney et al. in this issue describes this NOAA re­ using National Oceanic and Atmospheric Administration search effort. (NOAA) and Geostationary Operational Environmental Regional Topography Determination. As antisub­ Satellite infrared imagery. Of particular significance is marine warfare grows more sophisticated, it has become the altimeter signature of cold-water eddies in the Sar- critical that the operational commander have an accurate

Johns H opkins APL Technical Diges(, Volume 8, Number 2 (/987) 173 McConathy, Kilgus - The Navy GEOSA T Mission: An Overview map of local mesoscale oceanography. A. R. Robinson Ocean Coverage in the and hi s colleagues at Harvard have demonstrated that Exact Repeat Mission Orbit regional models of mesoscale features can be installed The GEOSAT satellite was moved into a 17-day exact on small computers suitable for implementation on ma­ repeat orbit with an equatorial ground-track separation jor ships. The model accepts input from several environ­ of 164 kilometers. The transfer began at the completion mental data sources including surface temperature maps, of the primary geodesy mission on September 30, 1986, available expendable bathythermography data, the to­ and the system was declared operational in the Exact pography generated directly by the radar altimeter, etc. Repeat Mission by the Navy on November 8, 1986. The Navy Tactical Environmental Support System, now In the Gulf Stream, the ystem provides a grid with in its early deployment phase, will implement this con­ llO-kilometer spacing. Along that ground track, the al­ cept on major surface ships. timeter gives global, all-weather measurements of the sur­ The NOAA Geophysical Data Record produced at face-elevation change caused by mesoscale oceanogra­ APL supports a joint regional topography research effort phy. At these latitudes, however, a single altimeter in by Harvard University, NOAA, and APL as described orbit produces a grid too widely spaced to allow the mea­ in articles by CaIman, Dobson et aL, Robinson and Wal­ surement of mesoscale features using altimeter data stad, and Cheney et al. elsewhere in this iss ue. alone. In the operational system, the altimeter data are NASA Waveform Modeling and Ice Research. An correlated at NORDA with satellite infrared measure­ adaptive tracker in the altimeter measures range, leading­ ments that map ocean-surface temperature in cloud-free edge slope, and automatic gain control level by process­ areas. Fortuitously, the satellite ground tracks converge, ing the radar return. Both the measured parameters and and altimeter coverage improves at higher latitudes (Fig. the raw radar-return waveform are telemetered to the S) where cloud cover is a severe problem for infrared ground. measurements. Three altimeters in orbit provide a stand­ The data are being used at NASA to provide better alone measurement system with satisfactory coverage at models of the shape of the radar pulse reflected from all latitudes. the ocean surface, thus allowing the design of a more The ascending nodes of the exact repeat orbit are ap­ accurate tracker for future altimeters. proximately at 1.0S + n(l.47SrE longitude, where The radar signals over ice are more complex than n = °to 244. The period of the satellite will be increased returns over the open ocean. The surface topography approximately once a month to compensate for drag and of ice sheets can be measured by analyzing the raw re­ to maintain the nodes within 1 kilometer of the exact turn waveforms to correct the range values for leads and repeat nodes. Three orbit maintenance burns were per­ lags in the altimeter tracking circuit. NASA research is formed between November 8, 1986, and April 30, 1987, described in the article by Hayne and Hancock III and to maintain the repeat within 600 meters (1 a). The exact by Zwally et al. in this issue. repeat nodes were chosen to follow the tracks generated THE GEOSAT EXACT REPEAT MISSION by the Seasat spacecraft. Since the Seasat altimeter data were released freely, Exact Repeat Mission data can be Under the guidance of J. L. Mitchell at NORDA, distributed without the security restrictions that applied planning was initiated in early 1983 for an Exact Repeat to the geodesy mission. The orbit maneuver and repeat Mission with an orbit optimized for the operational mea­ orbit parameters are described in the article by Born et surement of fronts and eddies. The accuracy of measur­ al. elsewhere in this issue. ing mesoscale topography at NORDA during the 18- month geodetic mission is limited when the marine geoid is subtracted from the altimeter data to produce meso­ Ascending orbits: i = 108.044, p = 6037.560 scale dynamic topography. This technique is computa­ tionally simple, but errors in knowledge of the global -5co geoid directly contaminate the oceanographic measure­ C. ments. Placing the satellite in an exactly repeating orbit +-'c Advanced Q) 120 u very-high- allows long-term averaging to give an accurate local co resolution :0'- mean surface (along the tracks) that can be subtracted co ~ radiometer o Q) required from the data to greatly reduce this source of error. +-'0) Q) E 80 The Navy Requirement uc _0 co '- for Tactical Oceanography +-'~ . ~~ For the Exact Repeat Mission, GEOSAT is producing '0 +-' Ul 40 operational measurements of oceanographic features 2 (fronts and eddies) that are critical to strategic and tactic­ 0 Stand L al decisions on a continuous, timely, all-weather, global en alone ocean basis. This supports both the operational require­ 0 ment for environmental data (OR-WOS27-0S) and the 0 20 40 60 80 Joint Chiefs of Staff, validated, Commander-in-Chief Nort h latitude (degrees) Atlantic Fleet Required Operational Capability for Figure 5-Altimeter ground-track spacing versus altitude for Fronts and Eddies (MJCS 204-83). 1, 2, or 3 altimeters in orbit.

174 Johns H opkins APL Technical Digesl, Volume 8, Number 2 (/987) McConathy, Kilgus - The Navy GEOSA T Mission: An Overview

Predicted "Sound wall" features, Acoustic flags, sound velocity propagation featu re locations profiles loss

APL NORDA Infrared Future Future (AVHRR) NORDA shipboard Raw Time In-situ alt tag Orbit Environment (XBT, etc.)

Predictive ocean model

Climatology

Functions:

Instrument Geographic Geoid/mean Merge Merge data Model corrections, location, surface multisensor into a acoustic time tagging, tropospheric/ removal data physical model propagation gap filling ionospheric corrections

Legend: XBT - Expendable bathythermography AVHRR - Advanced very-high­ CZCS - Coastal zone color scanner resolution radiometer Figure 6-Exact Repeat Mission processing flow.

Exact Repeat Mission NOAA has assumed responsibility for generating the un­ Data Products and Orbit Determination classified data sets. A copy of each Sensor Data Record The operational generation and transmission of near­ is transmitted to a NOAA processing facility in Rock­ real-time data to NORDA assumes primary importance ville, Md. The raw data are merged with the ephemeris in the Exact Repeat Mission. Altimeter and timing data data supplied by the Navy Astronautics Group and cor­ are merged and an orderly data set is created at APL rection fields are added for tides, wet and dry tropo­ (Fig. 6). Atmospheric corrections, orbit merge, and mean sphere, and ionosphere to yield Geophysical Data Rec­ surface removal at NORDA produce records of sea-sur­ ords. Completed records are sent to the NOAA National face heights along the altimeter tracks. The data are Environmental Satellite Data and Information Services merged with other satellite and in-situ data to produce in Washington, D.C., where they are made available to operational maps of feature locations that directly indi­ the public. Detailed information on the data format and cate regions of anomalous sonar propagation. The data content can be obtained from R. E. Cheney, Mail Code are also used to initialize and update developmental N/ CG 11, National Ocean Service, NOAA, Rockville, ocean models that predict the future evolution of oceano­ Md. 20852. graphic features and that map underwater sound velocity fields. The ultimate goal of the system is the produc­ NOTE tion of global environmental data needed to drive range­ 1GEOSA T is a research and development program under the Chief of Naval dependent sonar models. Operations Satellite Di vision (OP-943), wit h management and scientific guid­ ance given by the Space and Naval Warfare Systems Command, Office of Na­ Orbit parameters for the operational system are gener­ val Research, Naval Surface Weapons Center, Naval Oceanographic Office, ated by the Navy Astronautics Group using a GEM-lO aval Ocean Research and Development Activity, avy Astronautics Group, gravity model and Doppler tracking data from the four aval Research Laboratory, and Defense Mapping Agency. Transit OPNET tracking stations. Orbital data are given to APL for orbit maintenance, to NORDA for geoloca­ ACKNOWLEDGMENT - We wish to recognize the APL professional team tion of the altimeter data, and to NOAA for inclusion for the successful design, fabrication, and test of the altimeter, spacecraft, and ground system; the APL Satellite Tracking Facility team and the Ford Aerospace on the Geophysical Data Record tapes. Co. Mission Operations Team for 18 months of fla wless systems operation; and, The distribution network was extended for the Exact in particular, the Navy and Defense Mapping Agency individuals who created and sponsored the program: CDR T. M. Piwowar (OP-943D), C. Martin (DMA), Repeat Mission to provide data to unclassified users. Un­ R. G. Joiner (ONR) , CAPT N. Slezak (Code 63X), CAPT R. J . Munn der an agreement reached with the Navy and with APL, (OP-943D), W. J. Senus (OM A), and CDR C. Tomajczyk (OP-21) ..

Johns Hopkins APL Technical Digest, Volume 8, Number 2 (1987) 175