Radio Science Investigations (RS)

Description

The Radio Science Investigations on Mars Global Surveyor have two distinct objectives. The first is to map the planet's gravitational field and the second is to measure the atmospheric pressure and temperature in the polar regions. The gravity field results, combined with topographic data from the MOLA experiment, will allow estimates to be made of the structure of the martian interior, from which studies can be done of the thermal and chemical evolution of the planet and the history of surface features and the atmosphere. The atmospheric experiment will aid in studies of atmospheric dynamics and meteorology, and in detailed studies of transport of water, dust, and carbon dioxide in the atmosphere. The experiments are essentially the same as those which were planned for the Mars Observer, which are discussed in Tyler et al., JGR, v.97, p.7759, 1992.

The gravity experiment is consists of measuring the Doppler shift in radio waves transmitted from the spacecraft to Earth. This gives the velocity of the spacecraft along a line-of-sight (LOS) from Earth. Changes in this velocity give LOS accelerations, which after processing can be interpreted as spatial anomalies in the gravitational field of Mars. Over many orbits a map of the planet's gravitational anomalies can be built up, which can be interpreted in terms of the distribution of mass anomalies on the planetary surface and in its interior.

The radio occultation experiment operates when the Mars Global Surveyor is disappearing behind Mars or reappearing as seen from Earth. At this time, the radio waves transmitted from the spacecraft pass through the martian atmosphere on their way to Earth. The atmosphere effects the signal, and measuring these effects allows properties of the atmosphere to be studied. Because of the polar orbit of the spacecraft, these measurements can only be made near the poles.

The radio science instrument consists of an ultrastable oscillator and the normal Mars Global Surveyor transmitter and reciever. The oscillator provides a frequency reference for the radio science experiments. It operates on frequencies of 7164.624 MHz uplink, 8417.716 MHz downlink (closed loop) and 8416.368 MHz downlink (in-use mode). The oscillator has stabilities (square root of the Allan variance) of 5 x 10^-12 for 0.1 s integration, 1 x 10^-12 for 1.0 s integration, and 4 x 10^-13 for 10 to 1000 s integration. A high accuracy clock generator ensures precise frequencies. In warm-up mode, the oscillator draws 4.5 W.

Information from the radio science team is available on the WWW at:

http://nova.stanford.edu/projects/mgs/dmwr.html