Radio Science (RS)

Description

The Radio Science investigation on Mars Observer was intended to study: (1) the gravity field of Mars; and, (2) the atmospheric structure of Mars with a special emphasis on temporal changes near the polar regions. In addition to the nominal goals of the radio science investigation, one cruise phase period was used to participate in multi-spacecraft investigations looking for gravity waves.

The experiment utilized the components of the on-board telecommunications system plus the stations of the Deep Space Network (DSN). The radio system was built around a pair of redundant X-band transponders. Reception of signals from a DSN station occurred at a nominal frequency of 7.1646 GHz with a peak antenna gain of 35.6 dBi. The frequency at which the signal was transmitted to the ground station was driven either by the exact frequency of the signal received from a ground station (coherent mode) or by one of several on-board oscillators (noncoherent mode). In coherent mode, the turnaround ratio (from which the transmitted signal was derived) was 880/749, nominally resulting in a transmitted frequency of 8.4177 GHz with a peak gain (via the high-gain antenna) of 40.2 dBi.

Each transponder contained a separate auxiliary oscillator which could be used to generate a noncoherent downlink carrier. However, the frequency stability of these auxiliary oscillators was too poor to allow them to be useful to the radio science experiment. Therefore, for noncoherent radio science signal transmission, an ultrastable oscillator (USO) was specially provided so that a precision frequency reference was available. The USO consisted of a quartz crystal resonator contained in a temperature-controlled titanium dewar. The output frequency of this oscillator was multiplied by a factor of 440 to produce the downlink carrier signal at 8.4232 GHz. The USO on Mars Observer was an improved model over those on Voyager and Galileo and was expected to produce a more stable frequency over the lifetime of the mission as a result. In all cases (received/transmitted, coherent/noncoherent), the signal was right circularly polarized.

Contact with Mars Observer was lost for unknown reasons on August 21, 1993, three days before scheduled orbit insertion, so only cruise mode data were returned for this investigation.