Robust Tracking of Attenuated GPS Signals
Over the past decade GPS has become the single most important and ubiquitous utility in modern navigation. Given that GPS-based systems can provide centimeter-level accuracy today, the principle research issues for modern aerospace navigation are not related to accuracy per se, but robustness, which is quantified by the metrics of integrity risk and continuity risk. Integrity risk is the likelihood of an undetected navigation error or failure that results in hazardously misleading information. Continuity risk is the probability of a detected but unscheduled navigation function interruption after an operation has been initiated. One of the major threats to GPS integrity and continuity is signal power attenuation caused by unintentional interference from other radio frequency signals, malicious jamming or signal obstruction in urban environments.
We have been exploring techniques whereby information from sources external to a GPS receiver can be used to affect the performance of the signal tracking process inside a receiver. In particular, we have been exploring a technique known as Dopppler aiding which can be implemented using the system architecture shown in the figure on ther right. The fundamental idea behind this approach is that signal attenuation caused by interference which is wide-band in nature can be dealt with by narrowing the bandwidth of the signal tracking loops inside a receiver. While bandwidth narrowing mitigates the effect of wide-band interference, it amplifies the effect of correlated errors which contain power in the low-end of the frequency spectrum (e.g., oscillator phase errors). More importantly, however, it removes information about the relative motion between the user and the GPS satellites relevant to the navigation solution. This information is manifested as a Doppler shift in the GPS radio frequency carrier. Since the GPS satellite orbits are well known and modeled an accurate estimate of the user’s velocity from another source can be used to estimate this Doppler shift. This estimate is then used in a feed-forward fashion in the carrier tracking loops.
Our work has quantified the margins against signal attentuation that can be gained by Doppler-aiding (see figure on the right). This gain is a function of various tracking loop design parameters and the interactions between them have been identified. For example, one of the important findings was that when carrier tracking loops use an arctan phase discriminator, the effect of a longer pre-detection coherent integration time on the weak signal tracking performance is not significant as it would be otherwise.
Relevant Publication
Gebre-Egziabher, D., A. Razavi, 2005, Performance and Sensitivity Analysis of Doppler Aided GPS Carrier Tracking Loops, ION Journal of Navigation, Vol. 52, No. 2, pp. 49-60. More Details
Alireza Razavi, Demoz Gebre-Egziabher, Dennis M. Akos, 2007, Carrier Loop Architectures For Tracking Weak GPS Signals, to appear in IEEE Transactions on Aerospace Electronic Systems, Vol. 43, No. 4. More Details, Request PDF
Last Modified: 2007-08-07 at 09:28:36 -- this is in International Standard Date and Time Notation
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