Software Receiver Strategies for the Acquisition and Re-Acquisition of Weak GPS Signals

Abstract

The acquisition of weak GPS signals is a challenging problem. The lack of a priori information regarding the signal parameters typically necessitates a search through an uncertainty space in order to find the signal of interest. To acquire weak signals requires longer observation intervals to accumulate sufficient energy from the signal of interest. Perversely, a direct increase in the observation interval also leads to a narrowing of the Doppler bins, thereby effectively increasing the size of the uncertainty space. This paper investigates various weak signal acquisition strategies, with particular emphasis on software receivers. The acquisition problem is treated as a whole, rather than focusing solely on signal detection aspects. The effect of residual frequency errors and data bit transitions are considered, and various techniques for alleviating the problems associated with these signal effects are discussed. Some design rules for constructing weak signal acquisition strategies are introduced. Finally, the application of the ideas discussed is demonstrated with live satellite signals collected in a benign environment. The signals are passed through a variable attenuator which provides attenuation of up to 60 dB. The receiver antenna is in motion during the entire attenuated portion of the test. The reliable acquisition of signals with C/N0 as low as 20 dB-Hz is demonstrated for two acquisition strategies. The first strategy treats the data modulation as a nuisance parameter, modeling it as an effective loss in C/N0. The second strategy uses a method of alternating coherent integrations to eliminate the effect of data bit transitions from the decision variable. The first strategy is shown to provide a similar level of reliability, with a significant improvement in mean acquisition time.