The GPS-Linked Transponder - A Command, Telemetry, and Positioning System for Small Spacecraft

A proposed GPS-Linked Transponder (GLT) Command, Telemetry, and Positioning System is described that offers significant advantages over present systems. The new system would replace the standard coherent transponder and modify existing ground-based systems to provide the U.S. space industry with significantly smaller and lighter-weight flight systems in addition to simplified ground stations with reduced operating costs. The GLT comprises a NASA STDN/ DSN-compatible or AFSCN/SGLS-compatible command receiver/detector, a 20-Mbps-capable PCM/PSK telemetry transmitter with a selectable-rate FEC encoder and optional encryptor, and a dual-mode spacecraft positioning subsystem including a full GPS receiver/navigator and/or GPS trans digitizer. The GLT design is based on similar hardware developed by APL for SDlO. The system will recover high accuracy spacecraft position and time data-either in real time autonomously or in near-real time on the ground-using advanced GPS positioning techniques. A simplified command receiver option is also available when compatibility with existing standards is not required and improved capability is desired. In the transdigitizer-only mode, mass and size are reduced to nearly one-tenth of existing transponder systems. Other advantages include reduced complexity and significantly higher uplink and downlink data rate communications than presently supported. Five ground-station configurations are defined, each providing varying levels of spacecraft positioning accuracies to the user.