This paper discusses the narrowbandlwideband mix of potential interferences to GPlS and describes the results of a recent successful technology demonstration n hich established the efficacy of a precorrelalion temporal filter solution to narrowband interference. In (addition to describing the theory of operation of the temporal filter solution and its interference suppression performance in laboratory tests, this paper discusses potential dual use applications of the Air ForcelMayflower prccorrelalion temporal filter.It is anticipated that the unintentional interference problem in commercial GPS applications is characteristic of narronband interference, and therefore if can be fully mitigated by the temparal filter solution discus*,ed in this paper.
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 mGA1 = 10-Sm/s 2 -• 1 tag) 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 2ø-mean gravity anomalies on the Earth's surface to an accuracy of about 5 mGal. Accuracy in lø-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). Paper number 90JB00573. 0148-0227/90/90JB-00573505.00 mode [Kaula, 1970] where one satellite in high orbit tracks a low-altitude satellite. Compared to the low-low mode, it has the disadvantage that global coverage of high resolution requires up to twice as much time on account of the data gaps created when the Earth prevents intervisibility of the two satellites. The advantages of this concept are that the high satellite is virtually (atmospheric) drag free and tracking can be done with fewer ground stations. Tracking the low satellite is still a problem unless it can be tracked by a satellite navigation system (see below). The European Space Agency (ESA) recently studied the possibility of tracking a GRM-type satellite with the positioning satellite, called POPSAT [Reigber et al., 1987]. The apparent demise of GRM has created a temporary vacuum in efforts to map the gravity field globally. Satellite gravity gradiometer missions are being initiated (ESA's Aristoteles [Benz et al., 1988] and NASA's Superconducting Gravity Gradiometer Mission (SGGM) [Morgan and Paik, 1988]), but the technology required to achieve the necessary high precision (10 -• to 10 -13 S -2) in gravity gradient measurements at satellite altitude is still unproven. To fill the void, the U.S. Air Force has begun an earnest investigation of using the Global Position System (GPS) for high-low satellite-to-satellite tracking [Upadhyay et al., 1989] where the motivation was the possibility of obtaining reasonable accuracies for gravity model improvement with minimal cost. High-low tracking using GPS was also sugge...
For the quadratic cost, non-linear, adaptive stochastic control problem with linear plant and measurement models excited by white gaussian noise, and unknown t.imeinvariant model parameters, the optimal stochastic control is obtained and shown to separate ('non-linear separation theorem') into a bank of model.conditionel deterministic controller gains and a corresponding bank of known non-linear functionals of the model-conditional, causal, mean-square state-vector estimates. This separation may also be viewed as a decomposition of the optimal, non-linear adaptive control into a bank of model-conditional, optimal, non-adaptive linear controls, one for each admissible value fof the unknown parameter 8 and a non-linear part, namely the bank of a posteriori model probabilities, which incorporate the adaptive nature, of the optimal adaptive control. Results are given for a special case of the above problemnamely, uncertainty in the measurement matrix-that exhibit drastically reduced computational requirements. In this special case, we have explicit separation between control and estimation, and it is 8ho\\-'11 that only one detenninistic controller is required to be used with the non-linear, adaptive, mean-square state-vector estimate.
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