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NASA NTRS1993

Prelaunch performance of the NASA altimeter for the TOPEX/POSEIDON project

Validation of in-orbit performance has demonstrated the ability of satellite radar altimetry to measure mesoscale absolute dynamic sea surface topography and to measure the variation in the general circulation on the largest spatial scales. Measurements of basin scale mean circulation, however, have been corrupted by system inaccuracies. The TOPEX/POSEIDON radar altimeter satellite applies recent advances in remote sensing instrumentation to reduce long wavelength measurement errors to dramatically lower levels. The TOPEX altimeter measures the range to the ocean surface with 2-cm precision and accuracy through the use of both Ku- and C-band radars, a high pulse repetition frequency, an agile tracker, and absolute internal height calibration. Dual pulse bandwidths for both frequencies make it possible to quickly acquire the surface and begin tracking after crossing the land/ocean boundary. This paper presents the altimeter requirements and the elements of the altimeter design that have resulted in meeting these requirements. Prelaunch test data, based on the use of a Radar Altimeter System Evaluator to simulate the backscatter from the ocean surface, are presented to demonstrate that the TOPEX altimeter will meet these requirements and provide the data necessary to the understanding of basin scale mean circulation.

Marth, Paul C.↗
NASA NTRS1993

Expected orbit determination performance for the TOPEX/Poseidon mission

Each of the components required for the computation of precise orbits for the TOPEX/Poseidon (T/P) spacecraft - gravity field modeling, nonconservative force modeling, and satellite tracking technologies - is examined. The research conducted in the Space Geodesy Branch at Goddard Space Flight Center in preparation for meeting the 13-cm radial orbit accuracy requirement for the T/P mission is outlined. New developments in modeling the earth's gravitational field and modeling the complex nonconservative forces acting on T/P are highlighted. The T/P error budget is reviewed, and a prelaunch assessment of the predicted orbit determination accuracies is summarized.

Nerem, R. S.↗
NASA NTRS1993

Global ocean tide models on the eve of Topex/Poseidon

Some existing global ocean tide models that can provide tide corrections to Topex/Poseidon altimeter data are described. Emphasis is given to the Schwiderski and Cartwright-Ray models, as these are the most comprehensive, highest resolution models, but other models that will soon appear are mentioned. Differences between models for M2 often exceed 10 cm over vast stretches of the ocean. Comparisons to 80 selected pelagic and island gauge measurements indicate the Schwiderski model is more accurate for the major solar tides, Cartwright-Ray for the major lunar tides. The adequacy of available tide models for studying basin-scale motions is probably marginal at best.

Ray, Richard D.↗
NASA NTRS1993

Electromagnetic bias in sea surface range measurements at frequencies of the TOPEX/POSEIDON satellite

Measurements of EM bias at the 13.6 GHz and 5.3 GHz operating frequencies of the NASA altimeter on the TOPEX/POSEIDON satellite in a series of 11 aircraft flights from January 17, 1991 through March 4, 1991, during the Surface Wave Dynamics Experiment, are reported. The data are consistent with an earlier set of airborne measurements and indicate that EM bias is slightly higher at 5.3 GHz than at 13.6 GHz, and that the magnitudes of both biases increase with increasing wind speed, as does their difference. With some exceptions, EM bias shows little variation over a mesoscale region on a given day or within 1 or 2 h, but it can change significantly over a 6-h period. Recent tower, airborne, and satellite measurements exhibit a consistency in the characteristics of the wind speed dependence but suggest that there may be a height dependence in the determinations, with the bias decreasing with increasing altitude.

Hevizi, Laszlo G.↗
NASA NTRS1993

On-orbit performance of the Extended Gyrocompass Controller for the TOPEX/Poseidon spacecraft

The paper describes the Earth Acquisition Mode Extended Gyrocompass Controller flown aboard the NASA-CNES TOPEX/Poseidon Attitude Determination and Control Subsystem spacecraft for providing highly accurate measurements of the surface elevations over all the ocean basins. Particular attention is given to the on-orbit performance of the Extended Gyrocompass Controller used during quaternion initialization and Update Filter convergence processing, both of these procedures requiring maintenance of the spacecraft attitude control and knowledge. The development of the Extended Gyrocompass control technique is reviewed, and the results of the on-orbit yaw attitude control and knowledge performance provided by this algorithm are presented.

Sanneman, P. A.↗
NASA NTRS1992

Topex/Poseidon: A United States/France mission. Oceanography from space: The oceans and climate

The TOPEX/POSEIDON space mission, sponsored by NASA and France's space agency, the Centre National d'Etudes Spatiales (CNES), will give new observations of the Earth from space to gain a quantitative understanding of the role of ocean currents in climate change. Rising atmospheric concentrations of carbon dioxide and other 'greenhouse gases' produced as a result of human activities could generate a global warming, followed by an associated rise in sea level. The satellite will use radar altimetry to measure sea-surface height and will be tracked by three independent systems to yield accurate topographic maps over the dimensions of entire ocean basins. The satellite data, together with the Tropical Ocean and Global Atmosphere (TOGA) program and the World Ocean Circulation Experiment (WOCE) measurements, will be analyzed by an international scientific team. By merging the satellite observations with TOGA and WOCE findings, the scientists will establish the extensive data base needed for the quantitative description and computer modeling of ocean circulation. The ocean models will eventually be coupled with atmospheric models to lay the foundation for predictions of global climate change.

Source record↗
NASA NTRS1994

TOPEX/POSEIDON operational orbit determination results using global positioning satellites

Results of operational orbit determination, performed as part of the TOPEX/POSEIDON (T/P) Global Positioning System (GPS) demonstration experiment, are presented in this article. Elements of this experiment include the GPS satellite constellation, the GPS demonstration receiver on board T/P, six ground GPS receivers, the GPS Data Handling Facility, and the GPS Data Processing Facility (GDPF). Carrier phase and P-code pseudorange measurements from up to 24 GPS satellites to the seven GPS receivers are processed simultaneously with the GDPF software MIRAGE to produce orbit solutions of T/P and the GPS satellites. Daily solutions yield subdecimeter radial accuracies compared to other GPS, LASER, and DORIS precision orbit solutions.

Guinn, J.↗
NASA NTRS1994

Compatibility of TOPEX/Poseidon trajectory propagation with JPL and GSFC/FDF operational software

Two independent trajectory software systems are used to perform the TOPEX/Poseidon operational orbit determination and propagation: the Goddard Trajectory Determination System (GTDS) at the GSF/FDF and the Double Precision Trajectory System (DPTRAJ) at JPL. GTDS is used for operational tracking and TDRS-based orbit determination. DPTRAJ is used for ephemeris generation necessary to conduct day-to-day mission operations. This paper describes the DPTRAJ/GTDS trajectory comparison analysis conducted jointly by JPL and GSFC to ensure the compatibility of these two independent trajectory software systems.

Salama, Ahmed↗
NASA NTRS1993

TOPEX/Poseidon mission early results

The joint U.S./France TOPEX/Poseidon mission launched on August 10, 1992 and aimed at optimizing the sea surface height measurements is described. Results of the first six months of the mission intended to calibrate and validate the missions measurements are presented. The verification results show that almost all measurements have exceeded performance requirements. The root-mean-square accuracy of the sea surface height measurement is estimated to be about 5.7 cm, which is significantly less than the specification of 13.4 cm.

Fu, Lee-Lueng↗
NASA NTRS1994

Telecommunications end-to-end systems monitoring on TOPEX/Poseidon: Tools and techniques

The TOPEX/Poseidon Project Satellite Performance Analysis Team's (SPAT) roles and responsibilities have grown to include functions that are typically performed by other teams on JPL Flight Projects. In particular, SPAT Telecommunication's role has expanded beyond the nominal function of monitoring, assessing, characterizing, and trending the spacecraft (S/C) RF/Telecom subsystem to one of End-to-End Information Systems (EEIS) monitoring. This has been accomplished by taking advantage of the spacecraft and ground data system structures and protocols. By processing both the received spacecraft telemetry minor frame ground generated CRC flags and NASCOM block poly error flags, bit error rates (BER) for each link segment can be determined. This provides the capability to characterize the separate link segments, determine science data recovery, and perform fault/anomaly detection and isolation. By monitoring and managing the links, TOPEX has successfully recovered approximately 99.9 percent of the science data with an integrity (BER) of better than 1 x 10(exp 8). This paper presents the algorithms used to process the above flags and the techniques used for EEIS monitoring.

Calanche, Bruno J.↗
NASA NTRS1994

An in situ evaluation of TOPEX/Poseidon altimetric measurements versus meaurements made by moorings and inverted echo sounders for sea surface height

The classical method of observing the sea surface height has been to make shipboard measurements of the vertical - density profile, and then calculating the surface height relative to a deeper reference surface. Two methods (a moored vertical string of instruments and an inverted echo sounder) were subsequently developed to obtain longer time in situ measurements. The first of these can be thought of as an extension of the discrete bottle hydrocast while the second integrates acoustically over the water column. One purpose of this note is to compare the result when coincidental observations are made by these two methods. This was done at two sites in the western tropical Pacific. Two inverted echo sounders were deployed alongside two enhanced TOGA-COARE moorings to be used in an in situ evaluation of TOPEX/Poseidon altimetric measurements of sea surface height. The mooring and inverted echo sounder data reproduced one another, at low frequency, with a correlation of 0.93 and 0.95 and the altimeter correlated with each of the above values ranging from 0.84 to 0.94. It is concluded that the altimetric measurements are statistically equivalent to the in situ measurements in the area of study.

Source record↗
NASA NTRS1994

First assessment of GPS-based reduced dynamic orbit determination on TOPEX/Poseidon

The reduced dynamic Global Positioning System (GPS) tracking technique has been applied for the first time as part of the GPS experiment on TOPEX/Poseidon. This technique employs local geometric position corrections to reduce orbit errors caused by the mismodeling of satellite forces. Results for a 29-day interval in early 1993 are evaluated through postfit residuals and formal errors, comparison with GPS and laser/DORIS dynamic solutions, comparisons on 6-hr overlaps of adjacent 30-hr data arcs, altimetry closure and crossover analysis. Reduced dynamic orbits yield slightly better crossover agreement than other techniques and appear to be accurate in altitude to about 3 cm RMS.

Yunck, T. P.↗
NASA NTRS1994

Observations of geographically correlated orbit errors for TOPEX/Poseidon using the global positioning system

We have compared Global Positioning System (GPS)-based dynamic and reduced-dynamic TOPEX/Poseidon orbits over three 10-day repeat cycles of the ground-track. The results suggest that the prelaunch joint gravity model (JGM-1) introduces geographically correlated errors (GCEs) which have a strong meridional dependence. The global distribution and magnitude of these GCEs are consistent with a prelaunch covariance analysis, with estimated and predicted global rms error statistics of 2.3 and 2.4 cm rms, respectively. Repeating the analysis with the post-launch joint gravity model (JGM-2) suggests that a portion of the meridional dependence observed in JGM-1 still remains, with global rms error of 1.2 cm.

Christensen, E. J.↗
NASA NTRS1994

Dynamic orbit determination using GPS measurements from TOPEX/POSEIDON

The GPAS data acquired by the TOPEX/POSEIDON (T/P) Demonstration Receiver (DR) have been used in a dynamic orbit determination, which was based on the description of the gravitational and nongravitational forces in the equations of motion. The GPS carrier phase data were processed in a double difference mode to remove clock errors, including the effects of Selective Availability. Simultaneous estimation of the T/P orbit and GPS orbits was performed using five 10-day cycles in the interval between December (1992) and April (1993). The resulting T/P orbits have been compared with the orbits determined from Satellite Laser Ranging, the French one-way Doppler tracking system, DORIS, and with the JPL reduced dynamic orbit determination strategies and force models with the GPS/DR to those used with SLR/DORIS, the radial component of the T/P orbit (based on JGM-2) was found to agree better than 30 mm (rms) and 35 mm with the JPL reduced dynamic orbit. An experiment gravity tuning was accomplished using four cycles of GPS/DR data. The resulting GPS./DR-orbits, determined by the dynamic technique with the experimental gravity field, are in better agreement with the JPL reduced dynamic orbits in both the radial component (21-25 mm) and altimeter crossover residuals than the JGM-2 orbits. (21-25 mm) and altimeter crossover residuals than the JGM-2 orbits.

Schultz, B. E.↗
NASA NTRS1994

TOPEX/POSEIDON mission overview

TOPEX/POSEIDON is the first space mission specifically designed and conducted for studying the circulation of the world's oceans. A state-of-the-art radar altimetry system is used to measure the precise height of sea level, from which information on the ocean circulation is obtained. The satellite, launched on August 10, 1992, has been making observations of the global oceans with unprecedented accuracy since late September 1992. To meet the stringent measurement accuracy required for ocean circulation studies, a number of innovative improvements have been made to the mission design, including the first dual-frequency space-borne radar altimeter capable of retrieving the ionospheric delay of the radar signal, a three-frequency microwave radiometer for retrieving the signal delay caused by the water vapor in the troposphere, an optimal model of the Earth's gravity field and multiple satellite tracking systems for precision orbit determination. Additionally, the satellite also carries two experimental instruments to demonstrate new technologies: a single-frequency solid-state altimeter for the technology of low-power, low-weight altimeter and a Global Positioning System receiver for continuous,precise satellite tracking. The performance of the mission's measurement system has been tested by numerous verification studies. The results indicate that the root-sum-square accuracy of a single-pass sea level measurement is 4.7 cm for the TOPEX system and 5.1 cm for the POSEIDON system; both are more than a factor of 2 better than the requirement of 13.7 cm. This global data set is being analyzed to improve understanding of the global ocean circulation as well as the ocean tides, geodesy, and geodynamics, and ocean wind and waves. The mission is designed to last for at least 3 years with a possible extension to 6 years. The multiyear global data set will go a long way toward understanding the ocean circulation and its variability in relation to climate change. A summary of the mission's systems and their performance as well as the mission's science team is presented.

Fu, Lee-Lueng↗
NASA NTRS1994

Gravity model development for TOPEX/POSEIDON: Joint gravity models 1 and 2

The TOPEX/POSEIDON (T/P) prelaunch Joint Gravity Model-1 (JGM-1) and the postlaunch JGM-2 Earth gravitational models have been developed to support precision orbit determination for T/P. Each of these models is complete to degree 70 in spherical harmonics and was computed from a combination of satellite tracking data, satellite altimetry, and surface gravimetry. While improved orbit determination accuracies for T/P have driven the improvements in the models, the models are general in application and also provide an improved geoid for oceanographic computations. The postlaunch model, JGM-2, which includes T/P satellite laser ranging (SLR) and Doppler orbitography and radiopositioning integrated by satellite (DORIS) tracking data, introduces radial orbit errors for T/P that are only 2 cm RMS with the commission errors of the marine geoid for terms to degree 70 being +/- 25 cm. Errors in modeling the nonconservative forces acting on T/P increase the total radial errors to only 3-4 cm root mean square (RMS), a result much better than premission goals. While the orbit accuracy goal for T/P has been far surpassed geoid errors still prevent the absolute determination of the ocean dynamic topography for wavelengths shorter than about 2500 km. Only a dedicated gravitational field satellite mission will likely provide the necessary improvement in the geoid.

Nerem, R. S.↗
NASA NTRS1994

Evaluation of the TOPEX/POSEIDON altimeter system over the Great Lakes

The TOPEX/POSEIDON altimeter measurment system is evaluated for the first 46 repeat cycles (September 23, 1992-December 23, 1992) using tracks over the Great Lakes. The temporal variations in lake level are removed from the altimeter measurements using in-situ lake level measurements, thus permitting the performance of the altimeter system to be assessed. For the NASA altimeter, the root-mean-square (RMS) scatter of the residuals is 3.95 cm using all the tracks over the lakes. However, some of the scatter in this result is probably due to lake tides or seiche, which can amount to a few centimeters amplitude near the ends of the lakes. When the seven best tracks are used, which cross the center of the lakes where tides/seiche effects are minimal, the RMS error is included to either 2.9 or 3.0 cm, depending on whether the Centre National d'Etudes Spatiales (CNES) or NASA orbit is used. This places an upper limit on the error budget of the altimeter system, excluding ocean tides and inverse barometer effect. There are several short-period variations in the residuals. The most pronounced is a 55-day period, with a 1-cm amplitude, which we believe is (at least in part) due to orbit error. When the model-derived wet tropospheric correction is substituted for the TOPEX microwave radiometer correction, the RMS error increases significantly, possibly resulting in an annual cycle of a few centimeters. Evaluation of the ionospheric correction indicates that the dual-frequency correction provides an average improvement of 0.85 cm over the Doppler orbitography and radiopositioning integrated by satellite (DORIS) correction. Although there are insufficient data to directly assess the CNES altimeter, the relative bias between the altimeters is estimated to be either -14.3 or -15.6 cm (NASA altimeter measuring short), depending on whether the DORIS or dual-frequency ionospheric correction is applied to the NASA altimeter.

Morris, Charles S.↗
NASA NTRS1994

A preliminary evaluation of ocean topography from the TOPEX/POSEIDON mission

We have analyzed 50 ten-day cycles of TOPEX/POSEIDON (T/P) altimeter data to evaluate the ocean dynamic topography and its temporal variations. We have employed data from both the U.S. and French altimeters along with the NASA precision orbits in this analysis. Errors in the diurnal and semidiurnal components of the Cartwright-Ray tide model have been significantly reduced using a correction developed from a combination of JGM-2 and OSU91A was employed, as well as a geoid model based solely on OSU91A. The long wavelengths of the comparisons to historical data, although geoid error still corrupts the dynamic topography for wavelengths shorter than 2500 km. The root mean square (RMS) variability is similar to previous results from Geosat, with bakground 'noise' approaching 3 cm RMS. The computed annual and semiannual variations are also similar to previous Geosat results, although the hemispheric distribution of the annual heating cycle is much better presented in the T/P results. They also compare reasonably well with the Levitus hydrographic compilation in the northern hemisphere, although the T/P variations generally have larger amplitudes. Ten-day average maps of variations in sea level compare well with simulations measurements at ocean tide gauges, with RMS differences of less than 4 cm and correlations greater than 0.6 for most of the island gauges. Time-longitude plots of these sea level variations at different latitudes in the Pacific clearly show the presence of equatorial Kelvin waves and Rossby waves, with the wave speeds agreeing well with theoretical and observed values. Measurement of variations in global sea level over cycles 2-51 have an RMS variability of 6.3 mm and a rate of change of -3.5 +/- 8 mm/yr, the uncertainty primarily due to insufficient averaging of the interannual and periodic sea level variations. These results show that the accuracy of the T/P measurements of sea level has dramatically improved over previous missions, with estimated time variable errors of 4 cm or less. Although geographically correlated orbit errors have also been reduced to the few centimeter level, further improvement in determinations of the mean dynamic topography will be difficult to obtain until a more accurate model of the marine geoid is available.

Nerem, R. S.↗