THE PROBLEM OF AVHRR IMAGE NAVIGATION REVISITED

In this study, Earth location errors in AVHRR satellite data and metho ds for their correction are examined with particular application to oc eanic regions far removed from ground control. A general correction pr ocedure, using landmarks or Ground Control Points (GCPs) and taking in to account landmark uncertainties, is presented. Correction functions are derived as expansions for any complete basis. Operationally-availa ble estimates of Earth location are used as a first-guess in developin g the correction procedure. In particular, polynomial expansions are u sed to represent the correction functions which provide the basis for renavigating the satellite data. The coefficients of the polynomial ex pansions are obtained using the method of least-squares. The stability of the correction procedure with respect to local errors in navigatio n, (i.e. within a scene) and bow to select the correct order of the co rrection polynomials are examined. Uncertainty in extrapolating naviga tion corrections over remote regions is examined and quantified. The i mportance of landmark uncertainty in degrading renavigation accuracy i s also addressed. Several parameters are introduced to optimize the ch oice of GCPs and their distributions. The procedures which are develop ed are then applied to simulated and actual AVHRR imagery. Finally, th e impact of local errors in navigation, which most likely arise from r apid variations in spacecraft attitude, on renavigation accuracy is em phasized and one possible solution proposed.