AAS 97-641
ANALYSIS OF THE PEGASUS BREAKUP
J.G. Miller - The MITRE Corporation
Abstract
The accuracy of the Simplified General Perturbations 4 (SGP4) element sets for the Pegasus breakup has been examined using the Space Surveillance Performance Analysis Tool (SSPAT). SSPAT is a data warehouse of historical space surveillance data, and a set of analysis tools to measure system performance. It contains historical data on SGP4 element sets, including error growth rate (EGR) values; sensor tasking of the Space Surveillance Network and the sensors' response to tasking; and radar cross section data.
A graph of the average EGR over time for the Pegasus breakup shows pronounced spikes on certain days. Using the technique of database drill down, the average EGR is computed by orbit class. Drilling down further to individual satellites in the SSPAT database, a graph of EGR over time for a Pegasus breakup piece in the dense atmosphere and a graph of EGR for a piece in the tenuous atmosphere illustrate the differences in EGR variations depending on the density of the atmosphere.
Comparing graphs of EGR versus BSTAR (the term in SGP4 that accounts for all unmodeled in-track forces, including drag) for all the Pegasus breakup pieces on a day when the average EGR is low and a day when the average is high shows that the element sets with large BSTAR values are susceptible to large variations in EGR. A graph of the average EGR and daily maximum planetary geomagnetic index Ap over time shows that the spikes in EGR are associated with geomagnetic storms.
To reduce the size of the spikes in EGR, the length of update interval (LUPI) for the batch differential corrections of the element sets was shortened. A graph of EGR over time for a particular breakup piece with batch differential corrections with a 9-day LUPI and 5-day LUPI shows a dramatic reduction in the size of the EGR spikes with the shorter LUPI. To support catalog maintenance of the Pegasus breakup pieces with shorter LUPIs, additional sensors were tasked for observations.