| Popis: |
GPS (Global Position Systems) have been widely used on microsatellites to provide the ephemerid but availability of the GPS measurement cannot be guaranteed. Orbit estimators based on celestial objects from available attitude sensors with a fraction of additional computing power are attractive alternatives but lacks of accuracy. On the other hand, an orbit propagator with frequent measurement updates can provide the needed accuracy. Solving the ephemerid at the entrance and exit point of eclipse, which is used to readjust the orbit propagator is suggested in this paper. The Keplerian elements are split into slow varying elements [a, e, i] and fast varying elements [w, W, u] where the slow varying elements are constantly being estimated using a Kalman filter from magnetometer data. The true anomaly, u, is then solved from the epoch time measured at the entrance or exit point of the eclipse later used to solve the other elements. This is possible because the uniqueness of the equatorial orbit configuration at eclipse for a given sun vector, will result in a unique solution set where the most probable answer can be selected. Having the ephemerid at eclipse point, the subsequent orbit elements in between the eclipse points can be obtained using orbit propagator while frequent updates to this orbit propagator will maintain its accuracy within its limit. |
