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NASA NTRS2009

GFO and JASON Altimeter Engineering Assessment Report. Update: GFO-Acceptance to End of Mission on October 22, 2008, JASON-Acceptance to September 29, 2008

The purpose of this document is to present and document GEOSAT Follow-On (GFO) performance analyses and results. This is the ninth Assessment Report since the initial report and is our final one. This report extends the performance assessment since acceptance on November 29, 2000 to the end of mission (EOM) on October 22, 2008. Since launch, February 10, 1998 to the EOM, we performed a variety of GFO performance studies; Appendix A provides an accumulative index of those studies. We began the inclusion of analyses of the JASON altimeter after the end of the Topographic Experiment (TOPEX) mission. Prior to this, JASON and TOPEX were compared during our assessment of the TOPEX altimeter. With the end of the TOPEX mission, we developed methods to report on JASON as it related to GFO. It should be noted the GFO altimeter, after operating for over 7 years, was power cycled off to on and on to off approximately 14 times a day for over 18 months in space with no failure. The GFO altimeter proved to be a remarkable instrument providing stable ocean surface measurements for nearly eight years. This report completes our GFO altimeter performance assessment.

Conger, A. M.↗
NASA NTRS1985

Design and benefits of a multibeam Earth Observing Radar

The oceanographic rationale is described for continuing to advance the state of the art in satellite radar altimetry, and the expected capabilities of a multibeam Earth Observing Radar are noted. At the end of this decade, there is the possibility that altimeters may be in orbit aboard the American TOPEX, the ESA ERS-1, and the French SPOT satellites at the same time. The TOPEX version will be the most precise altimeter yet built. Global ocean circulation will be measured by using TOPEX to monitor the elevation changes across the ocean basins due to oceanic currents. It will then be possible to monitor the 'mean' circulation patterns in the oceans. The multiple beams of EOR might be used to measure the curvature of the topographic surface. The use of curvature is especially beneficial because it is directly related to ocean circulation, which is a function only of the Laplacian of the topographic height field in a given area. With the EOR, that height field will be known and ocean circulation can be immediately computed.

Parsons, C. L.↗
NASA NTRS1994

Ionospheric calibration for single frequency altimeter measurements

This study is a preliminary analysis of the effectiveness (in terms of altimeter calibration accuracy) of various ionosphere models and the Global Positioning System (GPS) to calibrate single frequency altimeter height measurements for ionospheric path delay. In particular, the research focused on ingesting GPS Total Electron Content (TEC) data into the physical Parameterized Real-Time Ionospheric Specification Model (PRISM), which estimates the composition of the ionosphere using independent empirical and physical models and has the capability of adjusting to additional ionospheric measurements. Two types of GPS data were used to adjust the PRISM model: GPS receiver station data mapped from line-of-sight observations to the vertical at the point of interest and a grid map (generated at the Jet Propulsion Laboratory) of GPS derived TEC in a sun-fixed longitude frame. The adjusted PRISM TEC values, as well as predictions by the International Reference Ionosphere (IRI-90), a climatological (monthly mean) model of the ionosphere, were compared to TOPEX dual-frequency TEC measurements (considered as truth) for a number of TOPEX sub-satellite tracks. For a 13.6 GHz altimeter, a Total Electron Content (TEC) of 1 TECU 10(exp 16) electrons/sq m corresponds to approximately 0.218 centimeters of range delay. A maximum expected TEC (at solar maximum or during solar storms) of 10(exp 18) electrons/sq m will create 22 centimeters of range delay. Compared with the TOPEX data, the PRISM predictions were generally accurate within the TECU when the sub-satellite track of interest passed within 300 to 400 km of the GPS TEC data or when the track passed through a night-time ionosphere. If neither was the case, in particular if the track passed through a local noon ionosphere, the PRISM values differed by more than 10 TECU and by as much as 40 TECU. The IRI-90 model, with no current ability to unseat GPS data, predicted TEC to a slightly higher error of 12 TECU. The performance of PRISM is very promising for predicting TEC and will prove useful for calibrating single frequency altimeter height measurements for ionospheric path delay. When adjusted to the GPS line-of-sight data the PRISM URSI empirical model predicted TEC over a day's period to within a global error of 8.60 TECU rms during a nighttime ionosphere and 9.74 TECU rms during the day. When adjusted to the GPS derived TEC grid, the PRISM parametrized model predicted TEC to within an error of 8.47 TECU rms for a nighttime ionosphere and 12.83 TECU rms during the day. However, the grid cannot be considered globally due to the lack of sufficient numbers of GPS stations and large latitude gaps in GPS data. It is the opinion of the authors that using the PRISM model and adjusting to the global sun-fixed TEC grid regenerated with a localized weighted interpolation offers the best possibility of meeting the 10 TECU global rms (or 2 cm at 13.6 GHz) ionosphere range correction accuracy requirement of TOPEX/Poseidon and should be the subject of further study. However, it is clear that the anticipated requirement of 34 TECU global rms for TOPEX/Poseidon Follow-On (corresponding to the TOPEX/Poseidon performance) can not be met with any realizable combination of existing models and data assimilation schemes.

Schreiner, William S.↗
NASA NTRS1994

Accuracy assessment of the large-scale dynamic ocean topography from TOPEX/POSEIDON altimetry

The quality of TOPEX/POSEIDON determinations of the global scale dynamic ocean topography have been assessed by determining mean topography solutions for successive 10-day repeat cycles and by examining the temporal changes in the sea surface topography to identify known features. The assessment is based on the analysis of TOPEX altimeter data cycles 1 through 36. Important errors in the tide model used to correct the altimeter data have been identified. The errors were reduced significantly by use of a new tide model derived with the TOPEX/POSEIDON measurements. Maps of the global 1-year mean topography, produced using four of the most accurate of the marine geoid, show that the largest error in the dynamic ocean topography show expected features, such as the known annual hemispherical sea surface rise and fall and the seasonal variability due to monsoon influence in the Indian Ocean. Changes in the sequence of 10-day topography maps show the development and propagation of an equatorial Kelvin wave in the Pacific beginning in December 1992 with a propagation velocity of approximately 3 m/s. The observations are consistent with observed changes in the equatorial trade winds, and with tide gauge and other in situ observations of the strengthening of the El Nino. Comparison of TOPEX-determine sea surface height at points near oceanic tide gauges shows agreement at the 4 cm root-mean-square (RMS) level over the tropical Pacific. The results show that the TOPEX altimeter data set can be used to map the ocean surface with a temporal resolution of 10 days and an accuracy which is insonsistent with traditional in situ methods for the determination of sea level variations.

Tapley, B. D.↗
NASA NTRS1994

Correlated environmental corrections in TOPEX/POSEIDON, with a note on ionospheric accuracy

Estimates of the effectiveness of an altimetric correction, and interpretation of sea level variability as a response to atmospheric forcing, both depend upon assuming that residual errors in altimetric corrections are uncorrelated among themselves and with residual sea level, or knowing the correlations. Not surprisingly, many corrections are highly correlated since they involve atmospheric properties and the ocean surface's response to them. The full corrections (including their geographically varying time mean values), show correlations between electromagnetic bias (mostly the height of wind waves) and either atmospheric pressure or water vapor of -40%, and between atmospheric pressure and water vapor of 28%. In the more commonly used collinear differences (after removal of the geographically varying time mean), atmospheric pressure and wave height show a -30% correlation, atmospheric pressure and water vapor a -10% correlation, both pressure and water vapor a 7% correlation with residual sea level, and a bit surprisingly, ionospheric electron content and wave height a 15% correlation. Only the ocean tide is totally uncorrelated with other corrections or residual sea level. The effectiveness of three ionospheric corrections (TOPEX dual-frequency, a smoothed version of the TOPEX dual-frequency, and Doppler orbitography and radiopositioning integrated by satellite (DORIS) is also evaluated in terms of their reduction in variance of residual sea level. Smooth (90-200 km along-track) versions of the dual-frequency altimeter ionosphere perform best both globally and within 20 deg in latitude from the equator. The noise variance in the 1/s TOPEX inospheric samples is approximately (11 mm) squared, about the same as noise in the DORIS-based correction; however, the latter has its error over scales of order 10(exp 3) km. Within 20 deg of the equator, the DORIS-based correction adds (14 mm) squared to the residual sea level variance.

Zlotnicki, V.↗
NASA NTRS1994

Comparison of TOPEX/POSEIDON sigma(sub 0) and significant wave height distributions to Geosat

Monthly Ku band sigma(sub 0) and significant wave height (SWH) histograms from the NASA altimeter on the TOPEX/POSEIDON satellite are preseneted for January through June 1993 for three latitude bands between +/- 60 degrees. The data are compared to distributions from the Geosat mission for the same months in 1987-1989. Generally, the distributions agree quite well, although there are some seasonal/hemispherical differences. The sigma(sub 0) comparison reveals an overall bias between the two altimeters with the TOPEX sigma(sub 0) higher by about 0.7 dB, which is consistent with algorithm improvements for TOPEX. The SWH distributions show strong hemispherical/seasonal changes. The seasonal/hemispherical differences between TOPEX and Geosat are consistent for SWH and sigma(sub 0). The joint distribution of sigma(sub 0) and SWH is extremely stable friom month to month. The typical SWH is independent of sigma(sub 0) for sigma(sub 0) greater than 11.3 dB. The minimum SWH grows exponentially with wind speed. This joint distribution may be useful for understanding electromagnetic bias in altimeter measurements. Finally, altimeter data are compared to buoy values from 21 overflights of the NASA verification site near Pt. Conception, California. Wave heights agree well with an root mean square (RMS) difference of only 0.2 m. Altimeter sigma(sub 0) values are compared to buoy wind speeds. The results are consistent with the -0.7 dB sigma(sub 0) offset from the histogram comparisons.

Callahan, Philip S.↗
NASA NTRS1995

On-orbit measurement of TOPEX/Poseidon altimeter antenna pattern

The NASA and CNES altimeters on the TOPEX/Poseidon satellite share a 1.5-m antenna. Early data from the NASA altimeter suggested that the beam was broader than measured preflight. An altimeter transponder was modified to output a relative measurement of received power. The instrument is briefly described. The instrument was deployed on the TOPEX ground track at the coast near Los Angeles, California. Measurements from three of these deployments are presented to show the on-orbit antenna pattern. The measured pattern is effectively broader than preflight in the central region, particularly the part of the pattern which corresponds to the tail of the altimeter waveform where the attitude is determined. This result is consistent with the general shape of both the TOPEX and Poseidon waveforms. As the TOPEX corrections which depend on values from the tail of the waveform have been compensated for deviations from the preflight measurements, no appreciable effect on the final data is expected.

Callahan, Philip S.↗
NASA NTRS1997

Non-Gimbaled Antenna Pointing

The small satellite community has been interested in accessing fixed ground stations for means of space-to-ground transmissions, although a problem arises from the limited global coverage. There is a growing interest for using the Space Network (SN) or Tracking and Data Relay Satellites (TDRS) as the primary support for communications because of the coverage it provides. This thesis will address the potential for satellite access of the Space Network with a non-gimbaled antenna configuration and low-power, coded transmission. The non-gimbaled antenna and the TDRS satellites, TDRS-East, TDRS-West, and TDRS-Zone of Exclusion, were configured in an orbital analysis software package called Satellite Tool Kit to emulate the three-dimensional position of the satellites. The access potential, which is the average number of contacts per day and the average time per contact, were obtained through simulations run over a 30-day period to gain all the possible orientations. The orbital altitude was varied from 600 km through 1200 km with the results being a function of orbital inclination angles varying from 20 deg through 100 deg and pointing half-angles of I0 deg through 40 deg. To compare the validity of the simulations, Jet Propulsion Laboratory granted the use of the TOPEX satellite. The TOPEX satellite was configured to emulate a spin-stabilized antenna with its communications antenna stowed in the zenith-pointing direction. This mimicked the antenna pointing spin-stabilized satellite in the simulations. To make valid comparisons, the TOPEX orbital parameters were entered into Satellite Tool Kit and simulated over five test times provided by Jet Propulsion Laboratory.

Vigil, Jeannine S.↗
NASA NTRS2003

Collaborative Research: Atlantic Ocean Tropical/Subtropical Processes from Seasonal to Decadal Time Scales: Model/Data, Model/Model Comparison and Model/Data Synthesis Through Assimilation

The effort of this first year of research has been focused on the assimilation of TOPEX/Poseidon altimetric data into a primitive equation model of the Atlantic tropical/subtropical circulation. A reduced-rank, stationary Kalman filter has been constructed to assimilate the altimetric sea surface height anomaly (SHA) into the model. The goal is to assess how the inter-hemispheric transports between the Atlantic subtropics and tropics are affected by the assimilation and how the subsurface thermocline structure , and its variability ,is dynamically constrained by the SHA. The model is a reduced-gravity primitive equation GCM of the upper Atlantic Ocean between 30 S and 30 N. The assimilation scheme is an approximation to the extended Kalman filter in which the error covariances of the state estimates are calculated only in a reduced- dimension subspace. The subspace is defined by the leading empirical orthogonal functions calculated from an unconstrained model calculation. Both an identical twin experiment using simulated SHA observations and assimilation of the real TOPEX data were performed. Results from the twin experiments demonstrate the ability of the method to constrain the ocean circulation and the subsurface temperature structure. The impact on the subsurface temperature structure of TOPEX assimilation was assessed using data from expandable bathythermographs. This showed a substantial improvement in the estimated temperature variability only within 13 degrees in latitude around the equator. The impact of TOPEX SHA assimilation on zonally integrated meridional transport across different latitudes was also estimated. Again within 13 degrees from the equator both the mean amplitude and interannual variability of the surface and subsurface transports were significantly enhanced, while the transports were insensitive to the assimilation in the subtropics.

Malannotte-Rizzoli, Paola↗
NASA NTRS1993

A Look at a Day of Data from The TOPEX/POSEIDON GPS Receiver

Launched on 10 August, 1992, the Topex/Poseidon satellite is a joint project of NASA and the French space agency CNES. The purpose of this Earth orbiting altimetric satellite (for brevity, referred to as Topex) is to enable monitoring of the ocean topography to higher levels of accuracy than previously achieved. Essential to obtaining this greater level of accuracy is the use of improved navigational systems which can yield position estimates for Topex good to a few centimeters. One of the navigational systems utilized by Topex is the Global Positioning System (GPS) -- a network of navigational beacons that has become widely used in many areas beside spacecraft navigation...

Jee, J. Rodney↗
NASA NTRS2007

GEOSAT Follow-On (GFO) Altimeter Document Series, Volume 9. GFO and JASON Altimeter Engineering Assessment Report Update: GFO-Acceptance to December 18, 2006, JASON-Acceptance to December 24, 2006. Version 1: June 2007

The initial GFO Altimeter Engineering Assessment Report, March 2001 (NASA/TM-2001-209984/Ver.1/Vol.1) covered the GFO performance from Launch to Acceptance (10 February 1998 to 29 November 2000). The second of the series covered the performance from Acceptance to the end of Cycle 20 (29 November 2000 to 21 November 2001). The third of the series covered the performance from Acceptance to the end of Cycle 42 (29 November 2000 to 30 November 2002). The fourth of the series covered the performance from Acceptance to the end of Cycle 64 (29 November 2000 to 17 December 2003). The fifth of the series covered performance from Acceptance to the end of Cycle 86 (29 November 2000 to 17 December 2004). The sixth of the series covered performance from Acceptance to the end of Cycle 109 (29 November 2000 to 26 December 2005). In this year's GFO report, we have begun the inclusion of analyses of the JASON altimeter. In past years, JASON and TOPEX were compared during our assessment of the TOPEX altimeter; however, with the end of the TOPEX mission, we have developed methods to report on JASON as it relates to GFO. We see no change trend between the three altimeters and conclude all three are stable based on our cross comparison analyses.

Hayne, G. S.↗
NASA NTRS2008

GFO and JASON Altimeter Engineering Assessment Report. Update: GFO--Acceptance to December 27, 2007, JASON--Acceptance to December 26, 2007. Version 1: June 2008

The purpose of this document is to present and document GEOSAT Follow-On (GFO) performance analyses and results. This is the eighth Assessment Report since the initial report. This report extends the performance assessment since acceptance to 27 December 2007. Since launch, a variety of GFO performance studies have been performed: Appendix A provides an accumulative index of those studies. We began the inclusion of analyses of the JASON altimeter after the end of the Topographic Experiment (TOPEX) mission. Prior to this, JASON and TOPEX were compared during our assessment of theTOPEX altimeter. With the end of the TOPEX mission, we developed methods to report on JASON as it relates to GFO.

Conger, A. M.↗
NASA NTRS1980

Future plans for NASA's Oceanic Processes Program

NASA's Oceanic Processes Program has the objectives to develop a scientific basis for viewing the oceans from space and to demonstrate the utility of such an undertaking. There are currently four flight projects concerning the oceanic area. These projects are related to the National Oceanic Satellite System (NOSS), the Dynamic Ocean Topography Experiment (TOPEX), the Gravity Satellite (GRAVSAT), and a Synthetic Aperture Radar (SAR) mission. NOSS is being planned as a limited operational demonstration of viewing the oceans from space. It is to provide a capability to observe surface winds, waves, temperature, color, ice cover, and currents. It is scheduled for launch in 1986. TOPEX and GRAVSAT are being planned as research missions and are both scheduled for launch about 1986. The SAR mission is being planned to provide a spaceborne capability to observe the surface of the land and the oceans.

Wilson, W. S.↗
NASA NTRS1983

Oceanic mesoscale variability and general circulation from satellite altimetry: A status report

Progress on the applications of satellite altimetry from SEASAT and Geos-3 to the study of oceanic mesoscale variability and general circulation is reviewed. The major conclusion for the applications to mesoscale variability is that an optimally designed altimetric mission with a lifetime of several years will improve our knowledge of the global mesoscale variability to an extent unattainable by any other practical means. The proposed Topex mission will allow one to view the global oceanic variability in such a wide range of periods and wavelengths: from 20 days to 3 to 5 years; from 50 to 10,000 km. However, the goal of determining the general circulation cannot be achieved by a single altimetric mission, because a highly accurate geoid needs to be determined independently. The scenario of the combination of Topex with Gravsat, a gravity mission that will give accurate geoid information, will allow the global general circulation to be determined at scales as small as 100 km. Areas of research needing to be performed with existing altimeter data are also discussed.

Fu, L. L.↗
NASA NTRS1987

Geophysical parameters from the analysis of laser ranging to starlette

Starlette Satellite Laser Ranging (SLR) data were used, along with several other satellite data sets, for the solution of a preliminary gravity field model for TOPEX, PTGF1. A further improvement in the earth gravity model was accomplished using data collected by 12 satellites to solve another preliminary gravity model for TOPEX, designated PTGF2. The solution for the Earth Rotation Parameter (ERP) was derived from the analysis of SLR data to Starlette during the MERIT Campaign. Starlette orbits in 1976 and 1983 were analyzed for the mapping of the tidal response of the earth. Publications and conference presentations pertinent to research are listed.

Schutz, B. E.↗
NASA NTRS1989

The importance of batteries in unmanned missions

A brief overview of the current set of unmanned missions that are currently in operation and those planned for the next 5 to 10 years was presented. The important role of batteries in the development of successful missions was discussed. The workhorse battery, the NiCd, and its use throughout historical missions was explored. Its role on Observers and Landers was discussed, and the success those batteries had. The recent events in the issues of quality and life for NiCd batteries were discussed. These discussions focused on the current design activities for Mars Observer (MO), TOPEX, and Mariner Mark II. Separator selection and the need for uniform testing procedures to develop a quality battery product that can perform to mission requirements were discussed. The recent experiences with the GRO battery failure and the other events have led JPL to the decision to develop a testing program in conjunction with other NASA centers to determine the best separator material for these two missions. The decision for separator material and battery design must be made a year from now. Thus there will not be time to perform a total 2-year accelerated test. The JPL plan for coping with this compromise was discussed. A discussion of where NiH2 batteries are beginning to have an impact and the difficulties with their implementation were addressed. Such missions at Earth Observing System (EOS) and the NOAA series of satellites rely upon NiH2 for success but were not chosen for TOPEX or MO due to their large volume requirements.

Klein, John W.↗
NASA NTRS1991

Structural verification of an aged composite reflector

A structural verification program applied to qualifying two heritage composite antenna reflectors for flight on the TOPEX satellite is outlined. The verification requirements and an integrated analyses/test approach employed to meet these requirements are described. Structural analysis results and qualification vibration test data are presented and discussed. It was determined that degradation of the composite and bonding materials caused by long-term exposure to an uncontrolled environment had not severely impaired the integrity of the reflector structures. The reflectors were assessed to be structurally adequate for the intended TOPEX application.

Lou, Michael C.↗
NASA NTRS1991

Ocean topography experiment (TOPEX/Poseidon): Emergency support

The DSN (Deep Space Network) mission support requirements for the Ocean Topography Experiment (TOPEX/Poseidon) and the Global Positioning System (GPS) demonstration are summarized. The TOPEX mission consists of a single spacecraft which will be placed in a high earth circular orbit, with an altitude of 1334 km and a 63-deg inclination. The TOPEX mission objectives are outlined and the DSN support requirements are defined through the presentation of tables and narratives describing the spacecraft flight profile; DSN support coverage; frequency assignments; and tracking support responsibility.

Stiver, R. A.↗