OBTAINING EARTH SURFACE GRAVITY DISTURBANCES FROM A GPS-BASED HIGH-LOW SATELLITE-TO-SATELLITE TRACKING EXPERIMENT

A 'high-low' satellite-to-satellite tracking measurement process, involving a single low orbiter (300 km altitude) and the GPS constellation (20 189 km altitude), has been proposed by Jekeli & Upadhyay (1990) to estimate Earth surface gravity disturbances. This paper examines the potentiality of such a mission by initially conducting an error analysis based on signal-to-noise ratios of the spectra of intersatellite line-of-sight (los) accelerations possessing an assumed white noise process. Using a 60 day mission duration, los acceleration data rates ranging from 1 to 60 s and data noise levels from 0.1 to 0.3 mGal, corresponding errors of commission and omission propagating into the spectrum of a desired set of 1-degrees and/or 2-degrees mean Earth surface vertical gravity disturbances (residual to harmonic degree and order n = m = 6) were obtained. The resulting total estimated errors (in a global average sense) of the 1-degrees mean disturbances ranged from 6 to 12 mGal, and for 2-degrees values the range was 1.5 to 6 mGal, the lower error bounds corresponding to a 1 s data rate and a 0.1 mGal data noise level. An optimal least-squares collocation simulation was next carried out. The simulated los accelerations reflected all gravimetric perturbations due to an adopted true gravity field extending to n = m = 180. A downward continuation approach was used in conjunction with singular value decomposition techniques to predict 1-degrees mean disturbances over two separate regions on the Earth's surface. For a 10-degrees by 10-degrees region possessing a fairly tranquil local gravity field, the overall RMS of 100 'true-predicted' mean differences, using the aforementioned los data rates and noise levels, ranged from 5 to 9 mGal and was only 1 mGal when the data were noise-free. For a more turbulent 15-degrees by 20-degrees region, the RMS of 300 differences was 1.5 mGal using perfect data and was in the 6 to 11 mGal range using noisy data. Again, the lower error bounds corresponded to 1 s and 0.1 mGal data rate and noise levels. If the low satellite takes on a polar orbit one could therefore obtain respectable 1-degrees mean disturbances and highly reliable 2-degrees mean values over all previously inaccessible Earth surface areas.