Collaborative Tracking of Weak GPS Signals Using an Open-loop Structure

Weak GPS signal tracking is studied in this paper. To utilize the spatial correlation of the GPS signal among local receivers, a collaborative signal tracking structure is proposed. More specifically, the GPS signal parameter, carrier Doppler from a nearby receiver is provided to the target receiver as reference. An open-loop batch processing technique is used in this study, mainly to take advantage of its high-sensitivity and immediate signal reacquisition. As the major limitation of the open-loop batch processing technique is its heavy computation due to the large uncertainty space in frequency and time domain, the reference signal parameters, that is, carrier Doppler and code phase are provided to limit the search range, and hence to reduce the complexity. In this paper, another benefit of the collaborative open-loop tracking, namely the capability to block a strong interference in a nearby frequency band, is presented, as well. INTRODUCTION Weak GPS signal acquisition and tracking remains an active research area even though the GPS reception performance has been improved tremendously in the past. Various techniques and architectures are designed and implemented to utilize the time and spatial correlation of the GPS signal either to improve a GPS receiver’s sensitivity, the final positioning accuracy, or to reduce the time of outage of a receiver. The general way to exploit the time correlation of the GPS signal is to increase the integration time at the baseband signal processing stage as a longer integration time results in a higher signal to noise ratio for a given carrier to noise power density. As the integration time exceeds the navigation bit duration (20 ms for GPS L1 C/A), bit synchronization is required as is knowledge of the (relative) sign of the data bits. Another limiting factor is the dynamics of the receiver, which is equivalent to the stability of the carrier frequency viewed by the receiver. If the carrier frequency has a large dynamics over the integration time such that it cannot be tracked accurately, the advantage of the long integration time will be reduced dramatically. To overcome this drawback, modern GPS receivers use a separate set of sensors to track the dynamics, for example, inertial sensors to track the velocity and acceleration of a receiver such that the Doppler frequency and Doppler frequency rate can be estimated accurately. Examples could be found in Babu et al (2008), Cox (1980), and Petovello et al (2007, 2005). The general way to utilize the spatial characteristics of the GPS signals is to receive GPS signals from multiple antennas and to process the multiple GPS signals in either the signal processing domain or the navigation processing domain. The former takes advantage of signal combining methods to achieve spatial diversity or to perform beamforming (e.g. Brown and Gerein 2001), and thus to increase the overall signal to noise ratio. The latter shares measurements amongst receivers for correcting common errors such as satellite orbit error and atmosphere effects (RTO-AG-160-V21).