Modeling the effect of pressure altimetry on geostationary satellite surveillance accuracy

In the ranging and processing mobile-satellite (RAPSAT) system, aircraft position is estimated from the aircraft altitude in conjunction with the aircraft range (signal transit time) to two surveillance satellites. Due to large variations in atmospheric conditions, altimeter readings deviate significantly from the true geometric altitudes. As a result, the aircraft position estimation accuracy of satellite surveillance systems, such as RAPSAT, may be degraded. The authors model the spatial and temporal deviations between pressure altitudes and true geometric altitudes, and the results are applied to characterize the position estimation error of the RAPSAT system. The results show that the geometric altitude deviated widely from the pressure altitude at different latitudes and altitudes, but the altitude difference remained constant at the same latitudes and altitudes. As a result, these altitude deviations can be modeled as a constant correction over the same latitudes and altitudes. These altitude deviations were observed to be reasonably stable over several days. With the adjustment of altimeter readings, the errors in the estimation of geometric altitudes, including instrumentation errors, were computed to be less than 310 ft (one standard deviation) across conterminous US. The corresponding root mean square error in aircraft position (2-D) estimated by the RAPSAT system varied from 135 ft to 297 ft for six selected locations representing the cross section of the country