Application of Accelerometer Data to Mars Odyssey Aerobraking and Atmospheric Modeling

Aerobraking was an enabling technology for the Mars Odyssey mission, even though it involved risk due primarily to the variability of the Mars upper atmosphere. To reduce the risk, numerous analyses, based on various data types, were performed during operations. The use of one such data type, measurements from spacecrafts accelerometers, for determining atmospheric density during Odyssey aerobraking operations is reported. Accelerometer data were analyzed in near real time to provide estimates of density at periapsis, maximum density, density scale height, latitudinal gradient, longitudinal wave variations, and location of the polar vortex. Summaries of the aerobraking phase of the mission, the accelerometer data analysis methods and operational procedures, applications to determining thermospheric properties, and several remaining issues on interpretation of the data are discussed. Although acceleration was measured along three orthogonal axes, only data from the component along the axis nominally into the flow were used during operations. For a 1-s count time, the rms noise level, derived from the acceleration, varied from 0.07 to 0.5 mm/sq s, permitting density recovery to between 0.15 and 1.1 kg/cu km, or about 2 percent of the mean density at periapsis during aerobraking. Preflight estimates of natural variability based on Mars Global Surveyor accelerometer measurements proved reliable in the midlatitudes but overestimated the variability inside the polar vortex.