Performance assessment of the TDRSS Onboard Navigation System (TONS) experiment on EP/EUVE

The National Aeronautics and Space Administration (NASA) Goddard Space Flight Center (GSFC) is currently developing an operational Tracking and Data Relay Satellite (TDRS) System (TDRSS) Onboard Navigation System (TONS) to provide onboard knowledge of high-accuracy navigation products autonomously to users of TDRSS and its successor, TDRS-2. A TONS experiment has been implemented on the Explorer Platform/Extreme Ultraviolet Explorer (EP/EUVE) spacecraft, launched June 7, 1992, to flight qualify the TONS operational system using TDRSS forward-link communications services. This paper assesses the performance of the TONS flight hardware, an ultrastable oscillator (USO) and Doppler extractor (DE) card in one of the TDRSS user transponders, and the protoype flight software, based on the TONS experiment results. An overview of onboard navigation via TDRSS is also presented for both the EP/EUVE experiment and for future users of TONS. USO and DE short-term and long-term stability performance has been excellent. TONS Flight Software analysis indicates that position accuracies of better than 25 meters root-mean-square are achievable with tracking every one to two orbits, for the EP/EUVE 525-kilometer altitudes, 28.5-degree inclination orbit. The success of the TONS experiment demonstrates the flight readiness of TONS, which is scheduled to provide autonomous navigation for the Earth Observing System (EOS)-AM mission.