GPS (Global Positioning System) Orbit Determination: Bootstrapping to Resolve Carrier Phase Ambiguity

摘要

For Global Positioning System (GPS) satellite orbit determination, the most accurate observable available is carrier phase, differenced between observing stations and between satellites to cancel both transmitter and receiver related errors. For maximum accuracy, the integer cycle ambiguities of such observations must be resolved. To perform this ambiguity resolution, a bootstrapping strategy is effective. This strategy requires the tracking stations to have a wide ranging progression of spacings. Then, by conventional integrated Doppler processing of the observations from the most widely spaced stations, the orbits can be determined well enough to permit resolution which can reduce the uncertainty of the orbit determination enough to enable ambiguity resolution for more widely spaced stations, which will reduce the orbital uncertainty further, and enable ambiguity resolution for still more widely spaced stations, and so on. This strategy is tested with two different tracking networks. This limited ambiguity resolution reduced both the formal and the actual errors of GPS orbit determinations by a factor of two. In the second twelve stations were arranged in a spiral with geometrically increasing spacings from 10 to 330 km. By bootstrapping, all ambiguities for baselines up to about 100 km long were resolved. The distance was limited by strong ionospheric variability. Still, orbit-determination uncertainty (3 sigma) was reduced to about 1:1,000,000. Improved handling of ionospheric effects in ambiguity resolution and the use of observations spanning more than one day should further reduce the uncertainty.