THE PROBLEM OF AVHRR IMAGE NAVIGATION REVISITED

In this study, Earth location errors in AVHRR satellite data and methods for their correction are examined with particular application to oceanic regions far removed from ground control. A general correction procedure, using landmarks or Ground Control Points (GCPs) and taking into account landmark uncertainties, is presented. Correction functions are derived as expansions for any complete basis. Operationally-available estimates of Earth location are used as a first-guess in developing the correction procedure. In particular, polynomial expansions are used to represent the correction functions which provide the basis for renavigating the satellite data. The coefficients of the polynomial expansions are obtained using the method of least-squares. The stability of the correction procedure with respect to local errors in navigation, (i.e. within a scene) and bow to select the correct order of the correction polynomials are examined. Uncertainty in extrapolating navigation corrections over remote regions is examined and quantified. The importance of landmark uncertainty in degrading renavigation accuracy is also addressed. Several parameters are introduced to optimize the choice of GCPs and their distributions. The procedures which are developed are then applied to simulated and actual AVHRR imagery. Finally, the impact of local errors in navigation, which most likely arise from rapid variations in spacecraft attitude, on renavigation accuracy is emphasized and one possible solution proposed.