A Complex-Ambiguity Function Approach to a GPS Receiver

摘要

Standard GNSS signal correlators form three fixed- o?set lags:an early,prompt,and late.This is typ- ically adequate for general signal tracking,though in most GNSS receivers,the user has little insight,or control,over the signal processing that eventually pro- duces observables.As a result of our investigations into multipath,we are interested in the development of real-time correlators with many,possibly hundreds, of lags.While many-lag correlators have been used with GNSS signals to accelerate signal acquisition or for o?ine analysis,these systems have historically op- erated at very low duty cycles and in parallel with a traditional early,prompt,late correlator because the output data is not directly suitable for real-time signal tracking or precise observable generation. rnThe algorithmic approach we have developed is re- ferred to as the GNSS Complex Ambiguity Function (GCAF)Engine.A Fast Number Theoretic transform is used to e?ciently compute hundreds or thousands of correlator lags for a GNSS signal.These can be fur- ther processed using conventional Fast Fourier Trans- forms to produce a complex ambiguity function.The GCAF Engine is a hardware implementation that runs in real-time and provides bit-for-bit identical results to a traditional correlator architecture. rnWe have developed a prototype GPS C/A code re- ceiver in which the GCAF algorithms are implemented in a Field Programmable Gate Array(FPGA)pro- grammed with custom firmware which computes 512 points along the correlation curve for each tracked sig- nal.As an example of the value of this approach,we describe a multipath investigation that we have con- ducted in which we were able to localize and then iden- tify the source of the multipath.