Terra Onboard Navigation - The TDRSS Onboard Navigation System (TONS)

Terra is flying an autonomous onboard navigation system to provide accurate orbital parameters to the spacecraft in real time. The system, known as the TDRSS Onboard Navigation System or TONS, is another first for NASA by flying onboard navigation as the operational system for orbit solutions.

TONS measures the Doppler data off the forward communications signal from Tracking Data Relay Satellite System (TDRSS) and processes it in onboard software with a sequential estimation algorithm to produce the real time outputs. The accuracy requirement for TONS is 150 meters in position and 0.16 meters per second in crosstrack velocity, 3 sigma. TONS performance has far exceeded the requirement. TONS compares to traditional orbit determination methods to 7 meters, one sigma in position and better than 0.015 meters per second in crosstrack velocity. TONS provides Terra with onboard position knowledge to better than 5 meters, one sigma and 20 meters, 3 sigma. TONS performance was monitored during the Terra ascent maneuvers and indicated excellent recovery within the first few measurement updates after the maneuver. TONS has since been used as the operational solution during the station-keeping maneuvers, staying within the 150 meter position requirement. TONS also estimates the local oscillator frequency, a drag correction factor, and a TDRS measurement bias for Terra. All on-orbit requirements have been exceeded and navigation operations have been performed nominally. TONS data was also used to calibrate the spacecraft clock to aid the operational clock correlation system. Terra scientists obtain the real-time TONS navigation solution in their ancillary data.