Estimation of of the Electromagnetic Bias from Retracked TOPEX Data
We examine the EM bias by using retracked TOPEX altimeter data.
TOPEX↗
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TOPEX orbit determination using GPS signals plus a sidetone ranging system
The GPS orbit determination was studied to see how well the radial coordinate for altimeter satellites such as TOPEX could be found by on board measurements of GPS signals, including the reconstructed carrier phase. The inclusion on altimeter satellites of an additional high accuracy tracking system is recommended. It is suggested that a sidetone ranging system is used in conjunction with TRANET 2 beacons.
Bender, P. L.↗
Technical draft study report for TOPEX satellite options study, volume 1
The use of two spacecraft for adaptation to the TOPEX mission, namely the P80-1 and the GPS phase 2 are considered. The mission involved three mission options, each option varying in payload definition, payload weight, orbital altitude and payload power requirements. The P80-1 spacecraft is an Air Force Space Test Program satellite which carries a number of payloads to an orbital altitude of 400 n.mi. at a minimum inclination of 72.5 deg, and which has an orbital life capability of three years. The GPS phase 2 spacecraft is the operational satellite for the Global Positioning NAVSTAR navigation constellation provided for all service (and commercial) use.
Source record↗
TOPEX orbital radiation study
The space radiation environment of the TOPEX spacecraft is investigated. A single trajectory was considered. The external (surface incident) charged particle radiation, predicted for the satellite, is determined by orbital flux integration for the specified trajectory. The latest standard models of the environment are used in the calculations. The evaluation is performed for solar maximum conditions. The spacecraft exposure to cosmic rays of galactic origin is evaluated over its flight path through the magnetosphere in terms of geomagnetic shielding effects, both for surface incident heavy ions and for particles emerging behind different material thickness. Limited shielding and dose evaluations are performed for simple infinite slab and spherical geometries. Results, given in graphical and tabular form, are analyzed, explained, and discussed. Conclusions are presented and commented on.
Stassinopoulos, E. G.↗
TOPEX orbit determination using GPS signals plus a sidetone ranging system
The GPS orbit determination was studied to see how well the radial coordinate for altimeter satellites such as TOPEX could be found by on board measurements of GPS signals, including the reconstructed carrier phase. The inclusion on altimeter satellites of an additional high accuracy tracking system is recommended. It is suggested that a sidetone ranging system is used in conjunction with TRANET 2 beacons. Previously announced in STAR as N83-14606
Bender, P. L.↗
Topex - A spaceborne ocean observing system
The key to the ocean's influence on life is its general circulation. For the study of this circulation, geostrophic currents at the sea surface can be mapped from space with the aid of satellite altimeters, which measure the height of the sea surface and its variations in time and space. In view of the usefulness of satellite altimetry, the European Space Agency, Japan, France, and the United State have plans to launch Seasat-class altimeters. The similarity between the U.S. and the French programs and goals for satellite altimetry has led the respective agencies to study the possibility of combining NASA's Ocean Topography Experiment Topex with CNES's Poseidon Project. The objectives of the combined Topex/Poseidon mission involve an enhancement of the understanding of ocean dynamics on the basis of precise and accurate observations of the oceanic topography for a period of three years. The mission is to provide the foundation for a continuing program concerned with long-term observations of oceanic circulation.
Born, G. H.↗
GPS-based tracking system for TOPEX orbit determination
A tracking system concept is discussed that is based on the utilization of the constellation of Navstar satellites in the Global Positioning System (GPS). The concept involves simultaneous and continuous metric tracking of the signals from all visible Navstar satellites by approximately six globally distributed ground terminals and by the TOPEX spacecraft at 1300-km altitude. Error studies indicate that this system could be capable of obtaining decimeter position accuracies and, most importantly, around 5 cm in the radial component which is key to exploiting the full accuracy potential of the altimetric measurements for ocean topography. Topics covered include: background of the GPS, the precision mode for utilization of the system, past JPL research for using the GPS in precision applications, the present tracking system concept for high accuracy satellite positioning, and results from a proof-of-concept demonstration.
Melbourne, W. G.↗
GPS-based precision orbit determination - A Topex flight experiment
Plans for a Topex/Poseiden flight experiment to test the accuracy of using GPS data for precision orbit determination of earth satellites are presented. It is expected that the GPS-based precision orbit determination will provide subdecimeter accuracies in the radial component of the Topex orbit when the extant gravity model is tuned for wavelengths longer than about 1000 kms. The concept, design, flight receiver, antenna system, ground processing, and data processing of GPS are examined. Also, an accurate quasi-geometric orbit determination approach called nondynamic or reduced dynamic tracking which relies on the use of the pseudorange and the carrier phase measurements to reduce orbit errors arising from mismodeled dynamics is discussed.
Melbourne, William G.↗
Precision orbit determination for Topex
The ability of radar altimeters to measure the distance from a satellite to the ocean surface with a precision of the order of 2 cm imposes unique requirements for the orbit determination accuracy. The orbit accuracy requirements will be especially demanding for the joint NASA/CNES Ocean Topography Experiment (Topex/Poseidon). For this mission, a radial orbit accuracy of 13 centimeters will be required for a mission period of three to five years. This is an order of magnitude improvement in the accuracy achieved during any previous satellite mission. This investigation considers the factors which limit the orbit accuracy for the Topex mission. Particular error sources which are considered include the geopotential, the radiation pressure and the atmospheric drag model.
Tapley, B. D.↗
Sunrise/sunset thermal shock disturbance analysis and simulation for the TOPEX satellite
It is shown here that during normal on-orbit operations the TOPEX low-earth orbiting satellite is subjected to an impulsive disturbance torque caused by rapid heating of its solar array when entering and exiting the earth's shadow. Error budgets and simulation results are used to demonstrate that this sunrise/sunset torque disturbance is the dominant Normal Mission Mode (NMM) attitude error source. The detailed thermomechanical modeling, analysis, and simulation of this torque is described, and the predicted on-orbit performance of the NMM attitude control system in the face of the sunrise/sunset disturbance is presented. The disturbance results in temporary attitude perturbations that exceed NMM pointing requirements. However, they are below the maximum allowable pointing error which would cause the radar altimeter to break lock.
Dennehy, C. J.↗
Design of the TOPEX-NFEP utilizing the functional component approach
JPL is involved in the TOPEX/Poseidon mission, a project to study ocean circulation from an orbiting satellite. One subsystem, called the NASCOM Front End Processor (NFEP), is tasked with accepting spacecraft telemetry data relayed via the NASCOM Network and performing certain functions before passing it to the Telemetry and Command. This paper describes the NFEP system architecture and the TOPEX-NFEP configuration.
Chesney, James R.↗
General method for assessing the geographically correlated error for an orbiter with applications to Topex
An estimation based computational algorithm for evaluating geographically dependent orbit determination errors is presented. A derivation is included of the generalized geographically correlated error analysis algorithm and the algorithm is applied to a Topex simulation. Then a general numerical technique which provides the capability to analyze any type of mismodeled parameter with the geopotential as a special case is described. The numerical algorithm is compared with the analytic method of Rosborough (1986) and it is shown that both methods generate consistent geographically dependent orbit error profiles when the ephemeris error is dominated by the mismodeled geopotential.
Wolff, P. J.↗
Geographically correlated orbit error for the Topex satellite using GPS tracking
This paper reports on ongoing investigations of geographically correlated orbit error performed in support of precise orbit determination for the Topex/Poseidon mission. Tracking data is assumed to be generated with the Global Positioning System. Geographically correlated orbit error due to errors in modeling the gravity field is described and demonstrated. A method for rational selection of the gravity model coefficients which produce significant orbit error is also presented. The effects of these coefficient errors and other error sources is evaluated using a consider covariance approach. Finally, the covariance analysis is used to evaluate the orbit accuracy for Topex using dynamic, reduced dynamic, and kinematic GPS tracking. The reduced dynamic and kinematic approaches to orbit determination are found to have significant advantages with respect to reducing the level of geographically correlated orbit errors.
Rosborough, George↗
Topex high-gain antenna system deployment actuator mechanism
A deployment actuator mechanism was developed to drive a two-axis gimbal assembly and a high-gain antenna to a deployed and locked position on the Jet Propulsion Laboratory Ocean Topography Experiment (TOPEX) satellite. The Deployment Actuator Mechanism requirements, design, test, and associated problems and their solutions are discussed.
Jones, Stephen R.↗
Reducing S/A Errors In TOPEX GPS Measurements
Proposed scheme for postprocessing of data acquired by receivers monitoring broadcasts from Global Positioning System (GPS) satellites reduces those errors in computed positions of receivers caused by pseudorandom offsets introduced into transmitted signals. Offsets canceled exactly with help of codes available to privileged users and intended to degrade accuracies available to nonpriviledged users, pursuant to policy called "selective availability" (S/A). Proposed scheme devised primarily for GPS receivers aboard TOPEX satellite and at associated ground stations.
Wu, Sien-Chong↗
TOPEX Microwave Radiometer - Thermal design verification test and analytical model validation
The testing of the TOPEX Microwave Radiometer (TMR) is described in terms of hardware development based on the modeling and thermal vacuum testing conducted. The TMR and the vacuum-test facility are described, and the thermal verification test includes a hot steady-state segment, a cold steady-state segment, and a cold survival mode segment totalling 65 hours. A graphic description is given of the test history which is related temperature tracking, and two multinode TMR test-chamber models are compared to the test results. Large discrepancies between the test data and the model predictions are attributed to contact conductance, effective emittance from the multilayer insulation, and heat leaks related to deviations from the flight configuration. The TMR thermal testing/modeling effort is shown to provide technical corrections for the procedure outlined, and the need for validating predictive models is underscored.
Lin, Edward I.↗
Topex Microwave Radiometer thermal control - Post-system-test modifications and on-orbit performance
The Topex Microwave Radiometer has had an excellent thermal performance since launch. The instrument, however, went through a hardware modification right before launch to correct for a thermal design inadequacy that was uncovered during the spacecraft thermal vacuum test. This paper reports on how the initially obscure problem was tracked down, and how the thermal models were revised, validated, and utilized to investigate the solution options and guide the hardware modification decisions. Details related to test data interpretation, analytical uncertainties, and model-prediction vs. test-data correlation, are documented. Instrument/spacecraft interface issues, where the problem originated and where in general pitfalls abound, are dealt with specifically. Finally, on-orbit thermal performance data are presented, which exhibit good agreement with flight predictions, and lessons learned are discussed.
Lin, Edward I.↗
Analysis of TOPEX laser retroreflector array characteristics
The joint U.S./French TOPEX/POSEIDON mission was successfully launched on August 10, 1992 for the study of ocean height variation using microwave altimetry. Accurate determination of the satellite orbit is paramount to the determination of the above phenomenon. To accomplish this, using laser ranging, the satellite is equipped with a laser retroreflector array (LRA) around the altimeter antennae. The goal of laser ranging is to obtain precision orbits with a radial accuracy of 13 cm to the center of mass of the satellite. This requires the laser range correction to the LRA reference be known at the sub-cm level and is quite a challenge considering the geometry of the LRA. Detailed studies were initiated by the TOPEX project office (Christensen) under the auspices of the CDP/DOSE project (Degnan) at Goddard Space Flight Center.
Varghese, Thomas K.↗