Design and performance of an integrated DGPS/LORAN receiver

Design and implementation of a real-time integrated DGPS/LORAN receiver is described. Previous work by the authors articulated the advantages of integrating these two systems and proposed a tightly coupled Kalman filter architecture; this work examines the essential system interfaces in more detail. Features of these interfaces include a highly stable common frequency reference, hardware measurement of the time offset between GPS and LORAN sampling strobes, data structures incorporating range residuals, and algorithms to align data relative to different assumed positions. Finally, the expected performance of this receiver is briefly described in a variety of operational scenarios. One such operational scenario for integrated DGPS/LORAN is the harbor entrance and approach (HEA) phase of maritime navigation. A critical issue in this scenario is the spatial decorrelation of the LORAN additional secondary factors (ASFs). Most data thus far has focused on the temporal stability of LORAN data and indicates that in evolutions of less than two hours, LORAN may be sufficient to meet the harbor entrance accuracy requirement of 8 to 20 meters. Some preliminary data collected in Connecticut's Thames River are presented and indicates that spatial ASF variations may cause errors much larger than the accuracy requirement. This in turn indicates that precise mapping of these variations using some combination of theoretical calculations and measurements will be necessary in order for integrated DGPS/LORAN to be used in HEA operations.