Carrier Loop Architectures For Tracking Weak GPS Signals
by
Alireza Razavi, Demoz Gebre-Egziabher, Dennis M. Akos
in
IEEE Transactions on Aerospace Electronic Systems, Vol. 43, No. 4., 2007.
Category: Journal Article
Keywords: Global Positioning System (GPS), Phase Lock Loop, Frequency Lock Loop, Phase Jitter Estimator, Error Models
Click here to request an electronic copy of this paper.
Abstract:
In this paper the performance of various carrier recovery loop architectures (phase lock loop, Doppler-aided phase lock loop, frequency lock loop, and Doppler-aided frequency lock loop) in tracking weak GPS signals are analyzed and experimentally validated. The effects of phase or frequency detector design, oscillator quality, coherent averaging time and external Doppler aiding information on delaying loss-of-lock are quantified. It is shown that for phase lock loops the metric of total phase jitter is a reliable metric for assessing low C/N performance of the tracking loop provided the loop bandwidth is not too small (> 5 Hz). For loop bandwidths that are not too small, total phase jitter accurately predicts Carrier to Noise ratio (C/N) at which loss of lock occurs. This predicted C/N is very close to the C/N predicted by Bit Error Rate (BER). However, unlike BER, total phase jitter can be computed in real-time and an estimator for it is developed and experimentally validated. Total phase jitter is not a replacement for BER, since at low bandwidths it is less accurate than BER in that the receiver loses lock at a higher C/N than predicted by the estimator. Similarly, for frequency lock loops operating at small loop bandwidths, it is found that normalized total frequency jitter is not a reliable metric for assessing loss of lock in weak signal or low C/N conditions. At small loop bandwidths, while total frequency jitter may indicate that a loop is still tracking, the Doppler estimates provided by the frequency lock loop will be biased.
This entry has been accessed 1805 times.