A tracking and data relay satellite (TDRS) is a type of communications satellite that forms part of the Tracking and Data Relay Satellite System used by NASA and other United States government agencies for communications to and from independent "User Platforms" such as satellites, balloons, aircraft, the International Space Station, and remote bases like the Amundsen-Scott South Pole Station. This system was designed to replace an existing worldwide network of ground stations that had supported all of NASA's manned flight missions and unmanned satellites in low-Earth orbits. The primary system design goal was to increase the amount of time that these spacecraft were in communication with the ground and improve the amount of data that could be transferred. The TDRS is designed and built to be launched to and function in geosynchronous orbit, 35786 km above the surface of the Earth.
The TDRSOD.m program performs a least-squares orbit determination using TDRS four-way range measurements. Based on an appropriate set of measurements the orbital parameters of a single user spacecraft and up to two relay satellites can be adjusted simultaneously. Tracking data are provided in the TDRSOD.dat file, which contains one record per time. A single header line serves to label each column and is skipped upon reading. Following the epoch of the measurement the ground station and spacecraft identification numbers as well as the four-way relay range are given in each line.
A supplementary setup file TDRSOD.txt provides a priori state vectors and spacecraft-related parameters for the user and TDRS satellites as well as relevant auxiliary information.
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