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NASA NTRS1986

Analysis and application of frozen orbits for the TOPEX mission

This paper identifies and analyzes frozen orbits for use by the Topography Experiment (TOPEX) mission. Frozen orbits are characterized by almost no long-term change in eccentricity or argument of periapsis. The standard method of frozen orbit prediction is shown to be inadequate for TOPEX inclinations due to the effect of higher degree zonal harmonic gravity terms. A method is described from which long-term motion in mean eccentricity, argument of periapsis, and inclination can be predicted without numerical integration and is used to locate frozen orbits. A zonal gravity field of degree 13 is shown to be necessary and sufficient for TOPEX frozen orbit prediction. Results are verified by numerical integration methods. Frozen orbits are available for most TOPEX orbits under consideration and short-term altitude variations and maximum altitude rates are specified. Alternatives to the frozen orbit which may result in lower altitude variations and rates are briefly examined.

Smith, J. C., Jr.↗

NASA NTRS1989

On-board attitude determination for the Topex satellite

This paper presents an overall technical description of the on-board attitude determination system for The Ocean Topography Experiment (Topex) satellite. The stellar-inertial attitude determination system being designed for the Topex satellite utilizes data from a three-axis NASA Standard DRIRU-II as well as data from an Advanced Star Tracer (ASTRA) and a Digital Fine Sun Sensor (DFSS). This system is a modified version of the baseline Multimission Modular Spacecraft (MMS) concept used on the Landsat missions. Extensive simulation and analysis of the MMS attitude determination approach was performed to verify suitability for the Topex application. The modifications to this baseline attitude determination scheme were identified to satisfy the unique Topex mission requirements.

Dennehy, C. J.↗

NASA NTRS1990

Minimizing selective availability error on Topex GPS measurements

GPS measurements made at Topex/Poseidon and the accompanying ground tracking sites will be affected by the selective availability. Although in principle the effects may be removed by differencing between receivers observing the same GPS satellites, this requires accurate synchronization of all receiver clocks. In the case of Topex/Poseidon application, there are two sources of imperfect clock synchronization. The first and larger is due to the constantly drifting clock onboard Topex, which may cause a residual effect as large as 10 cm on Topex carrier phase and 1 m on Topex pseudorange. The second is due to light-time differences between receivers observing the same GPS satellites, which may amount to a few mm error. In this paper a data reduction scheme which incorporates a low-order polynomial interpolation and carrier phase smoothing on pseudorange acquired at Topex and ground receivers is described; a simulation analysis is given demonstrating the effectiveness of the scheme for reducing the GPS S/A effects; and comparison with other schemes is discussed.

Wu, S. C.↗

NASA NTRS1982

TOPEX satellite option study

The basic design of the fleet satellite communication spacecraft (FLTSATCOM) can easily accommodate any of the three payload options for the ocean dynamic topography experiment (TOPEX). The principal mission requirements as well as the payload accommodations and communications systems needed for launching this payload are reviewed. The existing FLTSATCOM satellite design is identified and the approaches for the proposed propulsion system are described in addition to subsystems for mechanical; power; attitude and velocity control; and telemetry, tracking and control are described. The compatability of FLTSATCOM with the launch vehicle is examined and its capabilities vs TOPEX requirements are summarized. Undetermined changes needed to meet data storage, thermal control, and area to mass ratio requirements are discussed. Cost estimates are included for budgetary and planning purposes. The availability of the described design is assessed based on the continuing production of FLTSATCOM spacecraft during the schedule span planned for TOPEX.

Source record↗

NASA NTRS1983

TOPEX watershed coming in oceanography

The NASA Ocean Topography Experiment (TOPEX) will use precision radar altimetry to determine topographic features of the global oceans from which currents may be deduced. TOPEX will coincide with the World Ocean Circulation Experiment (WOCE), which will be conducted at the end of this decade and shall involve ships, fixed and drifting buoys, aircraft observations, and satellite remote sensing, to resolve fundamental questions about the flow of water in the global ocean. TOPEX will contribute to WOCE the measurement of satellite height above the sea surface, and the precise radial position above a reference ellipsoid for the earth. The combination of these two measurements with the marine geoid yields the topographic data sought. Three years of topographic data, together with conventional oceanographic data and theoretical ocean models, will be needed to derive the mean and variable components of ocean circulation.

Cleven, G. C.↗

NASA NTRS1983

TOPEX ground data system

The TOPEX Project is a proposed oceanographic mission to measure the topography of the sea surface for a period of three years. This mission is sponsored by the National Aeronautics and Space Administration and managed by the Jet Propulsion Laboratory. Measurements of topography are used to study ocean currents, tides, bathymetry and the oceanic geoid. Several of the primary goals of this mission are to process and verify the altimetric data, and distribute them within days to the science investigators. This paper describes the TOPEX end-to-end ground data system. In addition to controlling the TOPEX satellite, the ground data system has been designed to minimize the time from data acquisition to science processing and data distribution. A centralized design supports the favorable response time of the system and also allows for operational efficiencies. Networking of real time and non-real time elements of the data system provides for more effective data processing.

Rosell, S. N.↗

NASA NTRS1986

Precise positioning capabilities for TOPEX using differential GPS

NASA's Ocean Topographic Experiment (TOPEX), to be launched in 1991, is the first mission designed to reach the decimeter accuracy needed for the solution of the general mean circulation problem. An experimental tracking capability for TOPEX is studied using differential measurements with satellites of the U.S. DOD's Global Positioning System (GPS). Two data types are studied: (1) integrated Doppler from GPS carrier phase, and (2) GPS P-code pseudo-range. Results of covariance analysis predict that with differential GPS techniques, 5-10 cm average TOPEX altitude accuracies can be achieved over data arcs of two hours.

Lichten, S. M.↗

NASA NTRS1988

Design and predicted performance of the GPS demonstration receiver for the NASA Topex satellite

NASA's Ocean Topography Experiment (Topex) satellite will perform topographic ocean mapping. Topex will carry an experimental tracking system based on the use of GPS (Global Positioning System). An onboard GPS Demonstration Receiver (GPSDR) will be used with a network of up to ten receivers at precisely known ground sites. Following a brief overview of the Topex mission and its derived accuracy requirements, the authors describe the design of the GPSDR. The major hardware and software components are reviewed and discussed. Projected receiver tracking and navigation performance measures are presented and discussed.

Carson, Lance↗

NASA NTRS1989

Design of the TOPEX Earth Pointing Safe Hold Mode

A technical description of the design and analysis of the Earth Pointing Safe Hold mode (EPSHM) for the Ocean Topography Experiment (TOPEX) satellite is presented. The EPSHM serves as the primary hardwired analog backup controller for safe-haven satellite operations in the event of on-orbit anomalies. The EPSHM is a modified version of the baseline Multimission Modular Satellite (MMS) EPSHM used on the Landsat-4 mission. Modifications to the MMS baseline EPSHM used on Landsat-4 were required to satisfy the unique TOPEX operational requirements. One such TOPEX operational requirement is that the EPSHM provide a spacecraft yaw-axis slewing capability to maintain adequate illumination of the solar array. The EPSHM architecture, constituent hardware components, performance requirements and predicted on-orbit performance are described.

Dennehy, C. J.↗

NASA NTRS1990

Toward decimeter Topex orbit determination using GPS

Several practical aspects of precision GPS-based Topex orbit determination are investigated. Multipath signals contaminating Topex pseudorange data are greatly reduced by placing the GPS antenna on a conducting backplate consisting of concentric choke rings to attenuate signals coming in from the Topex horizon and below, and by elevating it on a boom to keep it well above all reflecting surfaces. A proper GPS antenna cutoff view angle is chosen so that a sufficient number of GPS satellites with good geometry are in view while reception of reflected signals is minimized. The geometrical strength of the tracking data is optimized by properly selecting GPS satellites to be observed so as to provide data with moderate continuity, low PDOP, and common visibility with ground tracking sites. The tracking performance is greatly enhanced when three complementary sites are added to the minimum ground tracking network consisting of the three NASA DSN sites.

Wu, Sien-Chong↗

NASA NTRS1990

How well can gravity be recovered using Topex and GPS data?

When Topex is launched in mid-1992 it will carry a high quality GPS receiver which will operate in concert with a worldwide network of precision GPS ground receivers. The data from these receivers can be used to recover new information about the earth's gravity field at longer wavelengths. Software and algorithms have been developed which will allow this gravity field information to be recovered with much greater efficiency than with traditional techniques. The basis for these algorithms is the gravity bin formulation and related filtering techniques that exploit the repeat orbit of Topex and the sparse matrix structure of the problem. This new software has been used to evaluate the expected improvement in the gravity field using multiple ten-day arcs of GPS data from Topex.

Bertiger, Willy↗

NASA NTRS1994

Preliminary estimates of Gulf Stream characteristics from TOPEX data and a precise gravimetric geoid

TOPEX sea surface height data has been used, with a gravimetric geoid, to calculate sea surface topography across the Gulf Stream. This topography was initially computed for nine tracks on cycles 21 to 29. Due to inaccurate geoid undulations on one track, results for eight tracks are reported. The sea surface topography estimates were used to calculate parameters that describe Gulf Stream characteristics from two models of the Gulf Stream. One model was based on a Gaussian representation of the velocity while the other was a hyperbolic representation of velocity or the sea surface topography. The parameters of the Gaussian velocity model fit were a width parameter, a maximum velocity value, and the location of the maximum velocity. The parameters of the hyperbolic sea surface topography model were the width, the height jump, position, and sea surface topography at the center of the stream. Both models were used for the eight tracks and nine cycles studied. Comparisons were made between the width parameters, the maximum velocities, and the height jumps. Some of the parameter estimates were found to be highly (0.9) correlated when the hyperbolic sea surface topography fit was carried out, but such correlations were reduced for either the Gaussian velocity fits or the hyperbolic velocity model fit. A comparison of the parameters derived from 1-year TOPEX data showed good agreement with values derived by Kelly (1991) using 2.5 years of Geosat data near 38 deg N, 66 deg W longitude. Accuracy of the geoid undulations used in the calculations was of order of +/- 16 cm with the accuracy of a geoid undulation difference equal to +/- 15 cm over a 100-km line in areas with good terrestrial data coverage. This paper demonstrates that our knowledge or geoid undulations and undulation differences, in a portion of the Gulf Stream region, is sufficiently accurate to determine characteristics of the jet when used with TOPEX altimeter data. The method used here has not been shown to be more accurate than methods that average altimeter data to form a reference surface used in analysis to obtain the Gulf Stream characteristics. However, the results show the geoid approach may be used in areas where lack of current meandering reduces the accuracy of the average surface procedure.

Rapp, Richard H.↗

NASA NTRS2004

TOPEX Radar Altimeter Engineering Assessment Report Update: Side B Turn-On to January 1, 2004

This is the eleventh in a series of TOPEX Radar Engineering Assessment Reports, The initial TOPEX Radar Altimeter Engineering Assessment Report, in February 1994, presented performance results for the NASA Radar Altimeter on the TOPEX/POSEIDON spacecraft, from the time of its launch in August 1992 to February 1994. Since the time of that initial report and prior to this report, there have been nine interim supplemental Engineering Assessment Reports, issued in March 1995, May 1996, March 1997, June 1998, August 1999, September 2000, June 2001, March 2002 and again in May 2003.The sixth supplement in September 2000 was the first assessment report that addressed Side B performance, and presented the altimeter performance from the turn-on of Side B until the end of calendar year 1999. This report extends the performance assessment of Side B to the end of calendar year 2003 and includes the performance assessment of Jason-1, the TOPEX follow-on mission, launched on December 7, 2001.

Hancock, David W., III↗

NASA NTRS2006

TOPEX Radar Altimeter Engineering Assessment Report Final Update-Side B Turn-On to End-of-Mission on October 9, 2005

This is the thirteenth and final report in a series of TOPEX Radar Altimeter Engineering Assessment Reports. The initial TOPEX Radar Altimeter Engineering Assessment Report, in February 1994, presented performance results for the NASA Radar Altimeter on the TOPEX/POSEIDON spacecraft, from its launch in August 1992 to February 1994. Since the time of that initial report and prior to this report, there have been eleven interim supplemental Engineering Assessment Reports, issued in March 1995, May 1996, March 1997, June 1998, August 1999, September 2000, June 2001, March 2002, May 2003, April 2004 and September 2005. The sixth supplement in September 2000 was the first assessment report that addressed Side B performance, and presented the altimeter performance from Side B turn-on until the end of calendar year 1999. This report extends the performance assessment of Side B to the final collection of data on October 9, 2005, and includes the performance assessment of Jason-1, the TOPEX follow-on mission, launched on December 7, 2001. This report provides some comparisons of Side A and Side B performance.

Lockwood, Dennis W.↗

NASA NTRS1981

Orbit determination requirements for TOPEX

The error sensitivity of orbit calculations in support of the NASA Ocean Surface Topography Mapping Experiment (TOPEX), which require an accuracy on the order of 5 cm, is investigated. The contributions of errors in the gravitational, atmospheric drag and solar radiation pressure models to the computed orbit are analyzed for the cases of an ideal data distribution and realistic laser ranging data coverage. It is found that the major contributor to radial orbital error is the error in the geopotential model, accounting for orbital errors of 30 to 70 cm, with the effects of solar radiation pressure, drag modeling, tracking station coordinate errors making lesser contributions. It is concluded that TOPEX accuracy goals cannot be met using ground-based laser ranging data without improving the geopotential model.

Tapley, B. D.↗

NASA NTRS1983

Flight path design issues for the TOPEX mission

The proposed Ocean Topography Experiment (TOPEX) is an earth satellite mission currently under consideration by NASA. The primary purpose of the experiment is to determine the general circulation of the oceans and its variability. High precision, space based altimeter measurements will be combined with surface measurements and ocean models to accomplish the mission objectives. The paper will discuss mission requirements on orbit design, orbit selection space, derived requirements on navigation and satellite design issues which impact orbit selection. Unique aspects of the TOPEX orbit design are highlighted, such as high precision repeating orbits, 'frozen orbit' values of eccentricity and periapses, precise maneuver and orbit determination requirements and insuring crossing arcs over a calibration-site.

Frautnick, J. C.↗

NASA NTRS1982

Topex: Observing the Oceans from Space

Measurement of global ocean topography by a radar altimeter aboard the TOPEX satellite is discussed. Technical aspects of satellite altimetry as they pertain to the measurement of ocean circulation are described. The TOPEX mission is explained and a general history of oceanography is included.

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NASA NTRS1984

TOPEX - Observing the oceans from space

Born, G. H.↗