Showing papers by "Frank G. Lemoine published in 1999"

The Global Topography of Mars and Implications for Surface Evolution

Abstract: Elevations measured by the Mars Orbiter Laser Altimeter have yielded a high-accuracy global map of the topography of Mars. Dominant features include the low northern hemisphere, the Tharsis province, and the Hellas impact basin. The northern hemisphere depression is primarily a long-wavelength effect that has been shaped by an internal mechanism. The topography of Tharsis consists of two broad rises. Material excavated from Hellas contributes to the high elevation of the southern hemisphere and to the scarp along the hemispheric boundary. The present topography has three major drainage centers, with the northern lowlands being the largest. The two polar cap volumes yield an upper limit of the present surface water inventory of 3.2 to 4.7 million cubic kilometers.

The Gravity Field of Mars: Results from Mars Global Surveyor

Abstract: Observations of the gravity field of Mars reveal a planet that has responded differently in its northern and southern hemispheres to major impacts and volcanic processes. The rough, elevated southern hemisphere has a relatively featureless gravitational signature indicating a state of near-isostatic compensation, whereas the smooth, low northern plains display a wider range of gravitational anomalies that indicates a thinner but stronger surface layer than in the south. The northern hemisphere shows evidence for buried impact basins, although none large enough to explain the hemispheric elevation difference. The gravitational potential signature of Tharsis is approximately axisymmetric and contains the Tharsis Montes but not the Olympus Mons or Alba Patera volcanoes. The gravity signature of Valles Marineris extends into Chryse and provides an estimate of material removed by early fluvial activity.

The Use of Laser Altimetry in the Orbit and Attitude Determination of Mars Global Surveyor

Abstract: Altimetry from the Mars Observer Laser Altimeter (MOLA) which is carried on board Mars Global Surveyor (MGS) has been analyzed for the period of the MOS mission known as Science Phasing Orbit 1 (SPO-1). We have used these altimeter ranges to improve orbit and attitude knowledge for MGS. This has been accomplished by writing crossover constraint equations that have been derived from short passes of MOLA data. These constraint equations differ from traditional Crossover constraints and exploit the small foot print associated with laser altimetry.

Orbit Determination for Mars Global Surveyor During Mapping

Abstract: The Mars Global Surveyor (MGS) spacecraft reached a low-altitude circular orbit on February 4, 1999, after the termination of the second phase of aerobraking. The MGS spacecraft carries the Mars Orbiter Laser Altimeter (MOLA) whose primary goal is to derive a global, geodetically referenced 0.2 deg x 0.2 deg topographic grid of Mars with a vertical accuracy of better than 30 meters. During the interim science orbits in the' Hiatus mission phase (October - November 1997), and the Science Phasing Orbits (March - April, 1998, and June - July 1998) 208 passes of altimeter data were collected by the MOLA instrument. On March 1, 1999 the first ten orbits of MOLA altimeter data from the near-circular orbit were successfully returned from MGS by the Deep Space Network (DSN). Data will be collected from MOLA throughout the Mapping phase of the MCS mission, or for at least one Mars year (687 days). Whereas the interim orbits of Hiatus and SPO were highly eccentric, and altimeter data were only collected near periapsis when the spacecraft was below 785 km, the Mapping orbit of MGS is near circular, and altimeter data will be collected continuously at a rate of 10 Hz. The proper analysis of the altimeter data requires that the orbit of the MGS spacecraft be known to an accuracy comparable to that of the quality of the altimeter data. The altimeter has an ultimate precision of 30 cm on mostly flat surfaces, so ideally the orbits of the MGS spacecraft should be known to this level. This is a stringent requirement, and more realistic goals of orbit error for MGS are ten to thirty meters. In this paper we will discuss the force and measurement modelling required to achieve this objective. Issues in force modelling include the proper modelling of the gravity field of Mars, and the modelling of non-conservatives forces, including the development of a 'macro-model', in a similar fashion to TOPEX/POSEIDON and TDRSS. During Cruise and Aerobraking, the high gain antenna (HGA) was stowed on the +X face of the spacecraft. On March 29, 1999 the HGA will be deployed on a meter long boom which will remain Earth-pointing while the instrument panel (including the MOLA instrument) remains pointed at nadir. The tracking data must be corrected for the regular motion of the high gain antenna with respect to the center of mass, and the success of the MGS determination during Mapping will depend on correctly accounting for this offset in the measurement model.

Further analyses towards the introduction of ocean circulation model information into geopotential solutions

Abstract: Two representation methods for the Dynamic Ocean Topography (DOT) are compared. One uses surface spherical harmonics, the other Proudman functions, which form an ocean domain-specific orthonormal basis. The DOT implied by the temporally averaged output of the POCM―4B ocean circulation model, provided the data for the implementation and testing of the two methods. Using these data a spherical harmonic representation was developed, to degree 30, and a Proudman function decomposition employing 961 basis vectors, so that both representations involve an equal number of parameters. The input DOT field had an rms value of ±66.6 cm. The recovered rms DOT was ±66.1 cm for the spherical harmonic case, ±66.3 cm for the Proudman function case, while the rms difference between the two cases was ±4.2 cm. Although in an overall sense the two representations (with equal number of parameters) yield similar results, in the proximity of the ocean domain boundary the Proudman functions approximate the input DOT field better than the surface spherical harmonics.

Intercomparison and evaluation of some contemporary global geopotential models

Abstract: The performance of five global Earth gravitational models, published after 1995, was examined through tests with data (mostly) withheld from the development of these models. We considered the models: JGM-3 (Tapley et al., 1996), GRIM4-C4 (Schwintzer et al., 1997), TEG-3 (Tapley et al., 1997), EGM96 (Lemoine et al., 1998) and GPM98A (Wenzel, 1998). The test data that we used for model evaluations include satellite tracking measurements acquired over several spacecraft at various inclinations and altitudes, geoid undulations (or height anomalies) determined from GPS positioning and leveling observations, Dynamic Ocean Topography (DOT) information implied by an ocean circulation model, as well as hydrographic estimates of (relative) DOT. Over 9307 GPS/leveling geoid undulation values distributed over North America, Europe and Australia, EGM96 (to degree 360) outperforms all other models tested, yielding a standard deviation of the undulation differences of ±37.2 cm. Considering that the available GPS/leveling data are located over some of the best surveyed areas (gravimetrically), this value is consistent with the predicted (commission plus omission) geoid error of EGM96, whose global rms value equals ±45.3 cm. Over the ocean, the performance of EGM96 is superior to that of all other models tested, as judged by the results of comparisons with both the POCM―4B circulation model DOT output and with the hydrographic DOT estimates. GPM98A was found to be inaccurate over medium wavelengths, and is not considered suitable for orbit determination applications.

Precise Orbit Determination for the GEOSAT Follow-On Spacecraft

Abstract: The US Navy's GEOSAT Follow-On spacecraft was launched on February 10, 1998 with its primary mission objective to map the oceans using a radar altimeter. The spacecraft tracking complement consists of GPS receivers, a laser retroreflector and Doppler beacons. Since the GPS receivers have not yet returned reliable data, the only means of providing high-quality precise orbits has been though satellite laser ranging (SLR). SLR has tracked the spacecraft since April 22, 1998, and an average of 7 passes per day have been obtained from US and foreign stations. Since the predicted radial orbit error due to the gravity field is only two to three cm, the largest contributor to the high SLR residuals (10 cm) is the mismodelling of the non-conservative forces. The SLR residuals show a clear correlation with beta prime (solar elevation) angle, peaking in mid-August 1998 when the beta prime angle reached -80 to -90 degrees. We report in this paper on the analysis of the GFO tracking data (SLR, Doppler, and if available GPS) using GEODYN, and on the tuning of the non-conservative force model and the gravity model using these data.

The Removal of Periodic Gravitational Perturbations from Mars Global Surveyor's Science Phasing Orbits Applied to the Study of the Martian Exosphere

Abstract: The martian exosphere and upper atmosphere exert measurable drag on the Mars Global Surveyor (MGS) spacecraft. Using the Goddard Space Flight Center's GEODYN orbit determination software, the science phasing orbits (SPO) were analyzed to determine the atmospheric drag and hence, measure the average density near the orbits' perifocus (170-180 km altitude above high northern latitudes). Future work will include the gravity calibration and mapping periods as well. Additional information is contained in the original extended abstract.