TDRSS system design

The TDRSS (Tracking and Data Relay Satellite System), composed of four identical satellites in circular orbit, will become operational in 1980. The four satellites 'look down' on low and medium altitude user satellites to provide data collection command, and control in real time over more than 85 percent of the earth's surface for the lowest altitude user satellites and 100 percent coverage for satellite altitudes above 1200 km. Two of the four satellites are designated as Tracking and Data Relay satellites, one is designated the Advanced Westar commercial satellite (providing C- and K-band commercial domestic satellite communications); the fourth satellite acts as a spare for both missions. All satellites can be configured for either mission by command. The details of the forward and return links are discussed, along with the TDRS communication subsystem simplified block diagram.