HAPPI - a High Accuracy Pseudolite/GPS Positioning Integration

NovAtel, in conjunction with Stanford Telecom, have developed a prototype positioning system that combines multiple GPS and Pseudolite signals in a pseudo range/carrier based solution. The system is designed to give supplementary coverage during times of reduced GPS availability, to give better position accuracy during periods of poor geometry, to aid in ambiguity resolution enabling rapid centimeter level positioning, and to provide receiving remote stations with differential data collected at base stations. This paper describes the prototype system, including the components, the integration and theoretical and realized performance. Unique problems occur when using signals from pseudolites. These manifest themselves as both signal processing and as filtering problems. Transmitted multipath is a characteristic of pseudolite signals which does not occur, at least to the same extent, in GPS signals. The effects of this condition on both signal processing (tracking) and filtering algorithms are described and possible remedies are suggested. Furthermore, the geometry associated with pseudolite transmitters is significantly different than that expected from a set of GPS orbital transmitters. The effects of this geometrical difference, and the approach used to incorporate pseudolite measurements in a floating ambiguity position filter are described. Finally, simulation and real world test results from various manifestations of the system are presented along with descriptive analysis of the results generated in the different scenarios.