Group delay and phase delay in GNSS systems

Abstract

GNSS signals have previously been modulated using binary phase shift keying but this modulation scheme is being replaced by binary offset carrier (BOC) modulation. Research has considered how the BOC signals might be affected differently when passed through a surface acoustic wave (SAW) filter. The concern has been that because of the split spectrum nature of the BOC signals, the upper and lower side-lobes will be delayed significantly differently. This was suggested because SAW filters have nonlinear phase characteristics and therefore different frequencies are delayed differently. It was suggested that this difference in delay will result in greater distortion of the correlation triangle. A delay magnification effect was also mentioned when analyzing the delay of a BOC signal. It was not understood why the theoretical delay calculations did not match up with the actual results in both hardware and simulation. This paper clarifies some of the confusion and explains why the “delay magnification” applies to phase delay but not group delay. This paper also takes a look at how the code phase delay can vary with frequency and correlator spacing as a result of the SAW filter properties.

Keywords:

• group delay
• phase delay
• SAW filter
• non-linear phase
• GNSS delay compensation

Acknowledgments

This research is being carried out within iNsight (www.insight-gnss.org). It is a collaborative research project funded by the UK’s Engineering and Physical Sciences Research Council (EPSRC) to extend the applications and improve the efficiency of positioning through the exploitation of new global navigation satellite systems signals. It is being undertaken by a consortium of 12 UK University and industrial groups: Imperial College London, University College London, the University of Nottingham, the University of Westminster, EADS Astrium, Nottingham Scientific Ltd, Leica Geosystems, Ordnance Survey of Great Britain, QinetiQ, STMicroelectronics, Thales Research and Technology UK Limited, and the UK Civil Aviation Authority.

Thanks are also due to Alper Ucar from the University of Westminster for providing MATLAB code for GPS signal generation.