Georges Balmino

Radio science observations with Mars Global Surveyor: Orbit insertion through one Mars year in mapping orbit

Mars Global Surveyor (MGS) radio science comprises studies of the atmosphere and gravity of the planet. Perturbations of the 3.6-cm λ radio path by the atmosphere during periods of atmospheric occultation provide the vertical temperature-pressure structure T[p(r)] to accuracies at the surface of ΔT ≈ 0.4 K and Δp ≈ 2 Pa, and ∼10 K and ∼0.6 Pa at altitudes of 40–50 km; the error in radius is Δr ≈ 1 m at all levels. Accurate knowledge of the radius permits fixing of the T-p structure to the geopotential and use of the gradient wind equation to calculate components of the wind. Systematic sampling of the atmosphere in combination with the accuracy of the MGS radio system supports studies of the large-scale dynamics of the atmosphere, including seasonal variations of the atmospheric fields and embedded waves such as Kelvin and Rossby waves. Terminator region ionospheric electron density profiles are obtained successfully much of the time but on occasion are undetectable with the MGS system. Two-way radio tracking of MGS with uncertainties in the line-of-sight velocity of several to tens of μm s−1 and less supports solution for spherical harmonic models of the gravity field of order and degree in the range of 75×75, although the degree and order of meaningful terms is limited by the ∼400 km spacecraft altitude to ∼62×62, corresponding to a resolution of a few degrees of arc on the surface. This resolution of gravity is sufficient to support geophysical studies of the planets interior structure and history. Additional radio science investigations include the search for gravitational radiation and observation of very low grazing angle forward scattering by the surface of Mars.

A new global Earth's gravity field model from satellite orbit perturbations: GRIM5‐S1

A new model of the Earths gravity field, called GRIM5-S1, was prepared in a joint German-French effort. The solution is based on satellite orbit perturbation analysis and exploits tracking data from 21 satellites to solve simultaneously for the gravitational and ocean tide potential and tracking station positions. The satellite-only solution results in a homogeneous representation of the geoid with an approximation error of about 45 cm in terms of 5×5 degree block mean values, and performs globally better in satellite orbit restitution than any previous gravity field model. The GRIM5 normals, which were generated taking into account the latest computational standards, shall be the reference for use during the coming geopotential satellite mission CHAMP and should provide new standards in computing orbits of next altimetric missions like Jason and ENVISAT. The GRIM5-S1 normals also give the basis for the tracking/surface data combined solution GRIM5-C1.

Earth Gravity Field and Seasonal Variability from CHAMP

GPS-CHAMP satellite-to-satellite and accelerometry data covering 2.5 years of the CHAMP mission period were exploited to generate the global gravity field model EIGEN-3p revealing considerable improvements in both accuracy and resolution with respect to the previous model EIGEN-2. For the year 2001, CHAMP and satellite laser ranging data of four satellites were combined to recover largest scale monthly gravity field variations that are subsequently analyzed for the annually varying constituents. The temporal gravity field variations observed by CHAMP and the SLR satellites are compared in the spectral and spatial domain with geophysically (atmosphere, ocean, hydrology) predicted gravity variations that do not reflect the large observed scattering in the monthly solutions but are of comparable size and distribution on the annual time scale.

RADIO SCIENCE INVESTIGATIONS WITH MARS OBSERVER

Mars Observer radio science investigations focus on two major areas of study: the gravity field and the atmosphere of Mars. Measurement accuracies expressed as an equivalent spacecraft velocity are expected to be of the order of 100 μm/s (for both types of investigations) from use of an improved radio transponder for two-way spacecraft tracking and a highly stable on-board oscillator for atmospheric occultation measurements. Planned gravity investigations include a combination of classical and modern elements. A spherical harmonic (or equivalent) field model of degree and order in the range 30–50 will be obtained, while interpretation will be in terms of internal stress and density models for the planet, using the topography to be obtained from the Mars Observer laser altimeter. Atmospheric investigations will emphasize precision measurement of the thermal structure and dynamics in the polar regions, which are regularly accessible as a result of the highly inclined orbit. Studies based on the measurements will include polar processes, cycling of the atmosphere between the poles, traveling baroclinic disturbances, small-scale waves and turbulence, the planetary boundary layer, and (possibly) the variability and altitude of the ionosphere. As the radio occultation is insensitive to dust in the atmosphere per se and measures only the resulting change in thermal structure, it is expected that the radio technique can contribute to understanding of dust storm phenomena. Mutual observations of the atmosphere by means of radio occultation and by the pressure modulator infrared radiometer and the thermal emission spectrometer are expected to strengthen the reliability and accuracy of all three investigations.

GRIM5‐C1: Combination solution of the global gravity field to degree and order 120

The new satellite Earth gravity field model GRIM5-S1 was recently prepared in a joint GFZ and GRGS effort. Based on this satellite solution and terrestrial and altimetric gravity anomalies from NMA, a combined model GRIM5-C1, with full variance-covariance matrix up to degree and order 120, was computed. Surface gravity and altimetric gravity data are corrected for several systematic effects, such as ellipsoidal corrections and aliasing. A weighting scheme for gravity anomalies, according to their given standard deviations was developed. From each data set full normal equations were set up and finally combined with the GRM5-S1 normals. To take into account good information from the satellite-only model a procedure was developed to identify such coefficients and appropriately weighed them in the final normal equation system. Internal error propagation and comparisons to external data sets show, that the GRJM5-C1 model represents the best state of long wavelength gravity field models.

Venus gravity and topography: 60th degree and order model

We have combined the most recent Pioneer Venus Orbiter (PVO) and Magellan (MGN) data with the earlier 1978–1982 PVO data set to obtain a new 60th degree and order spherical harmonic gravity model and a 120th degree and order spherical harmonic topography model. Free-air gravity maps are shown over regions where the most marked improvement has been obtained (Ishtar-Terra, Alpha, Bell and Artemis). Gravity versus topography relationships are presented as correlations per degree and axes orientation.

Global Gravity Field Recovery Using Solely GPS Tracking and Accelerometer Data from Champ

A new long-wavelength global gravity field model, called EIGEN-1. has been derived in a joint German-French effort from orbit perturbations of the CHAMP satellite, exploiting CHAMPGPS satellite-to-satellite tracking and on-board accelerometer data over a three months time span. For the first time it becomes possible to recover the gravity field from one satellite only. Thanks to CHAMP’s tailored orbit characteristics and dedicated instrumentation, providing continuous tracking and on-orbit measurements of non-gravitational satellite accelerations. the three months CHAMP-only solution provides the geoid and gravity with an accuracy of 20 cm and 1 mgal, respectively, at a half wavelength resolution of 550 km, which is already an improvement by a factor of two compared to any pre-CHAMP satellite-only gravity field model.

Tectonic fabric and lithospheric strength of northern Eurasia based on gravity data

The isostatic gravity anomalies over the whole former USSR (FSU) from land gravimetry are presented here for the first time at a spatial resolution adequate to study the tectonic fabric and the lithospheric rigidity. The short wavelength gravity anomalies improve the delineation of the regional tectonic units of the FSU, originally identified from the geological and aeromagnetic data. A study of the coherence between the Bouguer gravity anomaly and topography of East European and Siberian Cratons allowed us to estimate the elastic plate thickness Te of 78 to 109 km. Relying on the current knowledge of the thermal state of Archean lithosphere, we inferred from Te the thermal plate thickness Tp of 200–250 km.

Estimation of local planetary gravity fields using line of sight gravity data and an integral operator

Abstract The gravity data coming from planetary probes are Doppler tracking observations which are sometimes reduced to Line-Of-Sight (LOS, the line joining the probe and the observer) gravity data. This direction-dependent representation is quantitatively difficult to use for geodetic and geophysical purposes. We present a new method of mapping LOS gravity data as gravity disturbances (gravity anomalies along a radial direction at a constant altitude). For this, we use an inversion procedure derived from Hotines integral operator, with a Tikhonov-Arsenine regularization method. Different choices of the regularization parameter are presented in relation to data errors and method errors. After a study of a synthetic case, we apply the method to a real case, the region of Gula Mons (Venus), using Pioneer-Venus orbiter data.

Study of Mars dynamics from lander tracking data analysis

After the touchdown of the two Viking landers on Mars, radio tracking measurements have been performed between them and Earth-based stations. With use of the first 9 months of data, we have improved the rotation rate and the mean orientation of the spin axis of Mars, referred to its mean orbit. For the first time, some nutations terms have also been estimated. Nevertheless the precise determination of the spin axis motion will require additional data collected during the extended mission. Our solution includes also the lander locations and the relativistic parameter γ.