Rotational and Orbital Dynamics of Mars
The Deep Space Network
(DSN), by using two-way X-Band and doppler tracking of the Mars Pathfinder lander will be
able to address a variety of orbital and rotational dynamics questions. Ranging involves
sending a ranging code to the lander on mars and measuring the time required for the
lander to echo the code back to the Earth-based station. dividing this time by the speed
of light results in an accurate measurement (within 1-5 meters) of the distance from the
station to the spacecraft. As the lander moves relative to the tracking station, the
velocity between the spacecraft and Earth causes a shift in frequency (doppler shift).
Measuring this frequency shift provides an accurate measurement of the distance from the
station to the lander. Within a few months of observing these features, the Mars
Pathfinder lander location can be determined within a few meters. Once the exact location
of Pathfinder has been identified, the orientation and precession rate (regular motion of
the pole with respect to the ecliptic) of the pole
can be calculated and compared to measurements made with the Viking landers 20 years ago.
Measurement of the precession rate allows direct calculation for the moment of inertia,
which is in turn controlled by the density of the martian rock with depth. Measurements
similar to these are used on earth to determine the makeup of the earth's interior.