Adaptive Modeling of Receiver Clock for Meter-level DGPS Vertical Positioning

Abstract

A significant improvement in the quality of GPS-derived vertical position estimates has been demonstrated with an approach requiring predictable behavior from the receiver clock. The approach is based on a structure inherent in the GPS measurements: errors in the estimates of vertical position and clock bias obtained from a snapshot of pseudorange measurements are highly correlated. This correlation allows us to compute improved vertical position estimates, and provides a basis for integrity monitoring. The practicality of this approach currently rests on our ability to model the receiver clock behavior accurately using the past GPS measurements. Results are presented on receiver clock modeling from laboratory and field tests for a range of commercial products. Several oven- controlled crystal oscillators and rubidium oscillators have been found to offer the requisite stability, and have provided superior vertical position estimates in field trials, including flight tests. Clock-aided navigation cuts the rms vertical position error by a third to a half, and provides vertical position estimates of a quality comparable with that of the horizontal estimates.