Limitations of Pseudolite Systems Using Off-The-Shelf GPS Receivers

Abstract

Pseudolites (PLs) are ground-based transmitters that transmit GPS-like signals. They have been used to test GPS system elements and to enhance GPS in certain applications by providing better accuracy, integrity and availability through the use of PL signals in addition to the GPS signals. PLs are also a promising technology for providing positioning in indoor, high multipath environments where GPS signals are generally unavailable or severely attenuated and of questionable quality. In experiments to date, researchers have almost exclusively used PLs that transmit C/A code on L1/L2 in order to use existing off-the-shelf GPS receivers. This is because no hardware modifications to the GPS receiver are necessary and only minor changes to the receiver firmware are needed to track a PL's signal. However, there are some fundamental issues that limit the effectiveness of a PL system using C/A code on L1/L2. These include the legality of transmitting on L1/L2, cross-correlation between PL and GPS signals, saturation of GPS receiver front-ends, and the limited multipath mitigation offered by C/A codes. When combined with other problems inherent to all PL systems such as near- far, multipath, and synchronization, the issues in using L1/L2 C/A code PL systems further complicates the design and deployment of such systems and places limits on its operational effectiveness. This paper presents the issues which limit PL systems that use GPS hardware and explores the impact of these issues on some common PL applications.