A novel SET estimation algorithm for precise geosynchronous orbital solutions

Abstract

Global Positioning System (GPS) accuracy, availability, reliability and integrity depends on the synchronisation of onboard atomic clock with GPS system time. The lack of syncronisation (satellite clock offset) impacts Signal Emission Time (SET) which not only degrade the orbital solutions but also manifest as large pseudorange error when it is scaled by speed of light, which results in inaccurate assessment of navigation solution. Another important parameter that impacts Signal Emission Time (SET) is the change in one of the Keplerian orbit elements i.e. eccentricity. Geosynchronous satellite orbits are elliptical in shape which causes the change in eccentricity. This results in change in satellite altitude which adds an error to SET. In this paper, the signal emission time from satellite antenna phase centre is modeled by considering the clock correction parameters, signal reception time at the receiver and eccentricity. The SET is estimated for a typical day (24 Hrs ephemerides data of year 2011), which is solar peak activity year. Though many researches were done on signal propagation path delays, pseudorange correction, instrument bias errors and integer ambiguity resolution, not much attention is paid on the satellite clock error and eccentric corrections impact on geosynchronous orbital and navigation solutions. In critical navigation applications like CAT I/II aircrafts landing and missile navigation the proposed SET algorithm can be implemented to achieve the required accuracy. In all these applications, higher positional accuracy is required compared to the existing 16.5 meters horizontal and 4.5 meters vertical accuracy. Hence for precise navigation applications accurate timekeeping is inevitable.