Validation of theoretical GPS multipath bias characteristics

Although GPS multipath error envelopes have been well-explored both mathematically and in hardware environments, the range biases caused by Doppler-shifted multipath pulling on the GPS receiver tracking loops have long remained the target of purely analytical discussion. This paper examines the effect of so-called fast-fading multipath on GPS code phase tracking performance and proposes a test method for isolating system range biases in an off-the-shelf receiver. The biases, which may be on the order of several meters, are based on the use of a noncoherent delay-lock loop (NCDLL) in the receiver's code tracking structure. In addition, existing models show a complete elimination of these bias errors in the presence of fast-fading multipath if a coherent delay-lock loop (CDLL) architecture is employed in the receiver. Bench data shows the existing theoretical model to be incomplete and a revised model is presented in order to explain the observed results.