Gravity estimation from STAGE, a Satellite‐to‐Satellite Tracking Mission

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Abstract
An analysis was performed to determine the accuracy of estimating gravity anomalies using satellite‐to‐satellite tracking between the Space Transportation System (NASA's space shuttle) and the constellation of GPS satellites (STAGE: Shuttle‐GPS Tracking for Anomalous Gravitation Estimation). Second time derivatives of the observed ranges, obtained from carrier phase measurements, between the low and high satellites provide in situ line‐of‐sight components of gravitation, as long as on‐board accelerometers are used to correct for the nongravitational signals. Absolute (nondifferential), single‐difference, and double‐difference modes of tracking the GPS satellites were considered in the analysis, which showed that the last yields the highest gravity estimation accuracy. With instrumentation already (or soon to be) available, it is anticipated that the gravitational acceleration at altitude can be measured to an accuracy of about 0.3 mGal (1 mGAl = 10−5m/s2≃ 1 μg) for an integration time of 60 s. Double‐difference acceleration data collected by the shuttle at 300 km altitude could, depending on mission duration and data processing techniques, yield estimates of 2°‐mean gravity anomalies on the Earth's surface to an accuracy of about 5 mGal. Accuracy in 1°‐mean anomalies of about 4–5 mGal would be achievable using a free‐flyer at 160 km altitude and assuming a potentially feasible measurement accuracy of 0.1 mGal (30 s integration time).