Spatial and Temporal Measurement of Global Atmospheric Density with the Iridium Constellation
- Paper number
IAC-05-C1.7.10
- Author
Mr. Ryan W. Shepperd, The Boeing Company, United States
- Year
2005
- Abstract
With a constellation of 66 satellites in low-earth orbit, Iridium Satellite LLC provides global telephony. Operated by the Boeing Company, the constellation consists of 6 planes all inclined at 86.4 degrees with a semi-major axis of 7155.8 kilometers. The globally spaced and nearly identical satellites provide an excellent opportunity to measure atmospheric density with both spatial and temporal resolution at the Iridium altitude. When combined with solar flux and geomagnetic indices, these global measurements, in fact, aid Iridium station-keeping operations. As part of routine operations, the orbit of each satellite is measured from several ground tracking stations via a sequential filter of differential time of arrival data. Averaged over a 2-4 day span, a decay rate is determined and corrected with in-track thrusting. The observed decay rate is predominately due to atmospheric drag, and the data correlates well with known solar influences such as the F10.7 solar flux and geomagnetic AP index. During periods of low solar activity, however, solar radiation pressure becomes significant. During such periods, solar sailing has been observed in vehicles with a particular solar array tracking mode. By both modeling solar radiation pressure and by measuring differences in the rate of change of the semi-major axis for vehicles with different array tracking modes, the affects of solar radiation pressure can now be subtracted. Furthermore, the theoretical ballistic number for each vehicle, based on estimated mass and known geometry, can be applied to yield a measurement of atmospheric density. Preliminary results will be presented along with a discussion of the prediction performance of various existing atmospheric models.
- Abstract document