Rapid Signal Acquisition Techniques for Civilian Military User Equipments Using DSP Based FFT Processing

摘要

Use of a general purpose DSP for GPS receiver implementation allows processing approaches not commonly seen in more traditional architectures. In Medium and High PRF radar systems, efficient FFT based methods are used in creating Range/Doppler maps for detection purposes. This paper explores performance and design issues associated with an FFT based GPS signal search modeled along the same lines. The results are rather stunning; a full C/A code sky search can typically be done in less than one second with excellent sensitivity whereas a traditionally architected GPS receiver might take 1 to 5 minutes to accomplish the same task. Furthermore, these results are obtained with an approach optimized for simplicity rather than top speed. Extensions to the technique find applicability in high performance military systems where the use of C/A code is undesirable but is often required due to a poor knowledge of time. Direct P(Y) code acquisition performance is analyzed for a variety of sustaining oscillator types and interference levels with the conclusion that even modest timing accuracy (1 second) is sufficient for direct P(Y) acquisition under jammed conditions. It should thus be possible to acquire signals directly, without the use a C/A code, for periods of several weeks since last tracking satellites providing a low power, sustaining oscillator is included. Snapshot techniques for improved jamming resistance during acquisition are also discussed in the context of a dynamic interference environment. Taking advantage of short periods of low jamming due to obscuration, antenna nulls, etc., FFT-based acquisition schemes can outperform more traditional massive correlator architectures.