HSGPS Reliability Enhancements Using a Twin-Antenna System

High sensitivity GPS (HSGPS) receiver technology enables GPS signals to be acquired in harsh signal environments such as urban canyons and indoors where previously impossible. However, the measurements are biased and noisy due to multipath and noise associated with the low-power signals. The occurrence of simultaneous blunders might cause traditional RAIM (Receiver Autonomous Integrity Monitoring) schemes, relying on the assumption of only one erroneous satellite signal, to fail. In addition, the poor geometry due to blocked satellites under the harsh signal environments may fail to provide the redundancy required for RAIM. This paper presents the result of an investigation based on the use of a twin-antenna system for improving HSGPS performance in terms of accuracy and reliability. The key idea is to use two identical antennas with known inter-antenna distance and exploit signal spatial diversity. Although the system requires the use of an extra antenna, it provides more redundancy and potentially a better chance of detecting and isolating reflected measurements because these are highly localized. An epoch-by-epoch least-squares estimation algorithm is used in order to conduct a better sensitivity analysis. The known inter-antenna distance serves as a constraint in the least-squares estimation process. It is also used as a final solution check for integrity monitoring. In addition, a user-level integrity monitoring overview and analysis for the twin-antenna system are provided and a FDE (Fault Detection and Exclusion) scheme suitable to some extent for multiple blunder detection is presented. The proposed system is tested and assessed using high-sensitivity GPS data collected in a relatively harsh indoor environment.