Showing papers by "Frank G. Lemoine published in 2001"

Mars Orbiter Laser Altimeter: Experiment summary after the first year of global mapping of Mars

Abstract: The Mars Orbiter Laser Altimeter (MOLA), an instrument on the Mars Global Surveyor spacecraft, has measured the topography, surface roughness, and 1.064-μm reflectivity of Mars and the heights of volatile and dust clouds. This paper discusses the function of the MOLA instrument and the acquisition, processing, and correction of observations to produce global data sets. The altimeter measurements have been converted to both gridded and spherical harmonic models for the topography and shape of Mars that have vertical and radial accuracies of ~1 m with respect to the planet's center of mass. The current global topographic grid has a resolution of 1/64° in latitude × 1/32° in longitude (1 × 2 km^2 at the equator). Reconstruction of the locations of incident laser pulses on the Martian surface appears to be at the 100-m spatial accuracy level and results in 2 orders of magnitude improvement in the global geodetic grid of Mars. Global maps of optical pulse width indicative of 100-m-scale surface roughness and 1.064-μm reflectivity with an accuracy of 5% have also been obtained.

Crossover analysis of Mars Orbiter Laser Altimeter data

Abstract: In its first 15 months of continuous operation, the Mars Orbiter Laser Altimeter (MOLA) instrument aboard Mars Global Surveyor ranged to Mars over 330 million times, generating more than 5000 orbital profiles, with a ranging precision of 0.4 m over smooth terrain. The accuracy of the profiles depends on knowledge of the spacecraft position, orientation, and observation time, which are subject to errors. We model these errors via the analysis of over 24 million altimetric crossovers. A quasiperiodic, once per revolution adjustment of the ground tracks as a function of time in three locally orthogonal directions minimizes the altimetric residuals via least-squares. Using a sparse matrix technique, computational effort scales linearly with the number of crossovers and only marginally with the number of parameters. Orbital errors mainly result from poor modeling of spacecraft thrusting events in the absence of tracking. Seasonal effects, likely due to changing thermal environment, as well as residual miscalibrations, are evident in the pointing solutions. Incorporating multiple parameters per revolution significantly improves crossover residuals, and resolves pointing aberrations during orbital transitions from night to day. Altimetry from the adjusted tracks generates a topographic model whose accuracy is typically better than 1 m vertically with respect to the center of mass of Mars. The centroid position of each MOLA shot is typically accurate to ∼100 m horizontally. Terrain models from accurately located lidar data can be gradient-shaded to illuminate geological structures with 1 in 1000 slopes that are invisible to cameras. Temporal changes in elevation (e.g., frost deposition/ablation) at decimeter levels may also be assessed using crossovers, but results must be interpreted with caution due to uncertainties in range walk correction.

Density structure of the upper thermosphere of Mars from measurements of air drag on the Mars Global Surveyor spacecraft

Abstract: We present measurements of the density of the Martian atmosphere between 170- and 180-km altitude above the high northern latitudes over a 6-month period in 1998, when the solar cycle was beginning to rise out of its activity minimum. These measurements were made from the observed orbital decay of the Mars Global Surveyor (MGS) spacecraft during its Science Phasing Orbits (SPO) (April to September 1998) using X band Doppler tracking observations. The densities that we retrieve are comparable to model values given by Culp and Stewart [1984], Stewart [1987], Mars-GRAM 3.7 [Justus et al., 1996], and recent Mars Thermospheric Global Circulation Model (MTGCM) simulations [Bougher et al., 2000]. However, the SPO period can be divided into two time periods (separated at Ls ≈ 355°∼0°) that are characterized by significantly different orbit-to-orbit variability that is not predicted by these earlier models. The first time period corresponds to the time during which the MGS orbit perifocus moved toward the north pole while the local solar time was 1000–1100; during this period, orbit-to-orbit variability is 50–70%, and our average measured density at 175 km is 0.018±0.007 kg km−3 (between 67° and 72°N and Ls = 315° to 320°). The second time period corresponds to the time during which the orbit perifocus moved south from the north pole and the local time was 1700–1730; during this period, orbit-to-orbit variability is 40–20%, and our average measured density at 175 km is 0.024±0.004 kg km−3 (between 62° and 69°N and Ls = 17° to 28°). For both time periods the observed latitudinal gradient of density on a constant altitude surface exhibited a factor of 3–4 decrease between 60° and 90°N. This gradient is comparable to that expected by the polar vortex (high-latitude wind) effect modeled by the MTGCM for solar medium conditions at southern summer solstice [Bougher et al., 2000]. A southern hemisphere dust storm that the MGS Thermal Emission Spectrometer (TES) observed at Ls = 309° is distinguishable in our data set as a 100% rise in density at 180 km above the 70° northern latitudes 7 days later (Ls = 313°).

Precise Orbit Determination for GEOSAT Follow-On Using Satellite Laser Ranging Data and Intermission Altimeter Crossovers

Abstract: The US Navy's GEOSAT Follow-On Spacecraft was launched on February 10, 1998 with the primary objective of the mission to map the oceans using a radar altimeter. Following an extensive set of calibration campaigns in 1999 and 2000, the US Navy formally accepted delivery of the satellite on November 29, 2000. Satellite laser ranging (SLR) and Doppler (Tranet-style) beacons track the spacecraft. Although limited amounts of GPS data were obtained, the primary mode of tracking remains satellite laser ranging. The GFO altimeter measurements are highly precise, with orbit error the largest component in the error budget. We have tuned the non-conservative force model for GFO and the gravity model using SLR, Doppler and altimeter crossover data sampled over one year. Gravity covariance projections to 70x70 show the radial orbit error on GEOSAT was reduced from 2.6 cm in EGM96 to 1.3 cm with the addition of SLR, GFO/GFO and TOPEX/GFO crossover data. Evaluation of the gravity fields using SLR and crossover data support the covariance projections and also show a dramatic reduction in geographically-correlated error for the tuned fields. In this paper, we report on progress in orbit determination for GFO using GFO/GFO and TOPEX/GFO altimeter crossovers. We will discuss improvements in satellite force modeling and orbit determination strategy, which allows reduction in GFO radial orbit error from 10-15 cm to better than 5 cm.