Radio science investigations with Mars Observer

Abstract

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.

An improved solution of the gravity field of Mars (GMM‐2B) from Mars Global Surveyor

“…In this case the signal originated at the spacecraft from the MGS ultrastable oscillator (USO) [Tyler et al, 1992] and was transmitted via the low-gain antenna (LGA). In this mode, lock-up on the signal from MGS could be achieved earlier than if the data were collected in two-way mode.…”

Mars without Tharsis

“…Kaula et al [1989] asserted that this argument should also hold for Mars. Goldreich and Toomre's condition for stability of the rotation axis due to global-scale mass re-distribution as given by equation (9) Anew Mars gravitational field of globally uniform spatial resolution will be derived from X band Doppler tracking of the Mars Global Surveyor spacecraft [Tyler et al, 1992], which will be in a polar orbit with an altitude of approximately 400 km. This gravity field, in combination with an even higher resolution topography field that will also be obtained in the mission [Zuber et al, 1992], will enable detailed analysis of the geophysics of Mars using many techniques, including the one presented here.…”

Characteristics of Near-Polar Atmosphere/Thermosphere of Mars at Altitudes of 125–145 km: The Data of a Radio Occultation Experiment onboard the Mars Global Surveyor Spacecraft

“…The radio occultation experiment data are obtained, in the main, on circular orbits at practically constant local time and are described in [11][12][13]. Practically constant local time of the radio occultation experiment means that latitude and solar zenith angle χ are related to each other.…”