Application of accelerometer data to Mars Global Surveyor aerobraking operations

Aerobraking was selected for the Mars Global Surveyor mission as a primary and enabling operation. The application of accelerometer data for determini ng atmospheric density during operations for the first phase of aerobraking is reported. Acceleration was measured along the body z axis, which is the axis nominally into the flow. For a 1-s count time, the data have a resolu tion of 0.332 mm/s, permitting the recovery of density to 3% at nominal aer obraking altitudes near 115 km and on many orbits, permitting the recovery of density to altitudes as high as 180 km. Accelerometer data were analyzed in near real time to provide estimates of density at periapsis, maximum de nsity, density scale height, latitudinal gradient information, and longitud inal wave variations. Summaries are given of the aerobraking phase of the m ission, the accelerometer data analysis methods and operational procedures, some applications to determining thermospheric properties, and some remain ing issues on interpretation of the data. Preflight estimates of 70% 2 sigm a natural variability are shown to be realistic, and predictions that dust storms could produce rapid and large increases in thermospheric density hav e been verified.