The Case for Narrowband Receivers

Most high performance GPS receivers today are wideband. In most cases, they require the wider precorrelation bandwidth for enhanced multipath mitigation capability. Wideband receivers, however, are still significantly more susceptible to electromagnetic interference. In addition, they are more likely to produce unacceptably large pseudorange errors in the presence of satellite signal anomalies, or “evil waveforms.” Conversely, narrowband receivers are less vulnerable to narrowband interference and more robust against GPS signal faults. However, they tend to have relatively poor multipath performance. Since significant rounding of the correlation peak occurs, narrow correlators and other wideband mitigation techniques at best provide little improvement in multipath performance. In this paper, a case is made for narrowband receivers by describing their interference rejection advantages, and greater robustness against the effects of evil waveforms. Analyses for both classes of receivers under the influence of wideband and narrowband interference are provided. Evil waveforms are modeled and the expected resulting maximum differential pseudorange errors are simulated. The simulation results are given for a range of correlator spacings and receiver bandwidths. The Tracking Error Compensator (TrEC) mitigation technique was investigated as a potential solution to the multipath problem in narrowband receivers. Theoretical bandlimited mitigation performance plots for this approach were generated and compared to those of a conventional delay-lock loop. A TrEC algorithm was developed and implemented on a Mitel Semiconductor receiver having a 2MHz precorrelation bandwidth. A GPS signal generator was used to generate multipath with known characteristics on a prescribed pseudorange. For short-delay multipath, measured pseudorange errors were compared to those estimated by the algorithm. This data suggests that it may be possible to attain both high multipath performance and reject interference in narrowband receivers.