Satellite Ephemeris Correction via Remote Site Observation for Star Tracker Navigation Performance Improvement

摘要

In order for celestial navigation techniques to provide accurate positioning estimates, precise ephemerides of the observed satellites are necessary. This work analyzed a method to correct for satellite ephemeris to be used in celestial navigation applications. This correction is the measured angle difference between the expected location of the satellite, which is given by propagating publicly available Two-Line Element sets (TLE), and their observed angles from a precisely known reference site. Therefore, the angle difference can be attributed completely to satellite ephemeris error assuming instrument error was accounted for. The intent is to calculate this correction from the reference site and relate it to remote sites that have visibility of the same satellite, but where its own location is known with some uncertainty. The effects of increased baseline distances from the reference site are studied, as well as time delays. Satellite observations were simulated and propagated using TLE. This simulated data was used to calculate the angle difference and project that angle to the viewpoint of the remote site. This corrected observed angle was integrated using an extended Kalman filter (EKF) with an inertial measurement unit (IMU) and a barometric altimeter. The performance of the position solution in the navigation filter was calculated as the error from simulated truth. The satellite ephemeris error measured at a reference location becomes less observable by a remote user according to the line-of-sight transformation due to the reference-satellite-remote geometry. A mathematical formula for calculating the applicability of projecting the remote site observation to other locations is developed and compared to simulated ephemeris errors. This formula allows a user to define geographic regions of validity through ephemeris error tolerance. Estimating the ephemeris error with regular updates from a reference site resulted in a reduction of IMU drift and a distance root mean squared (DRMS) error of 100 m.