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A Real-Time GPS Civilian L1/L2 Software Receiver

A dual-frequency civilian GPS receiver has been developed that runs 10 tracking channels in real time using a software correlator. This work is part of an effort to develop a flexible receiver that can use the new GPS L2C CM/CL signals as they become available on L2 without the need for new correlator hardware. The receiver consists of an RF front end, a system of shift registers, a digital data acquisition (DAQ) system, and software that runs on a 3.2 GHz PC. The direct RF sampling front end uses intentional aliasing and converts the GPS L1 C/A codes and L2 CM and CL codes into a 1-bit digital data stream sampled at a frequency between 8 and 12.5 MHz. The shift registers parallelize the 1-bit data stream, which the DAQ reads into the PC’s memory using direct memory access. The PC performs base-band mixing and PRN code correlations in a manner that directly simulates a hardware digital correlator. It also performs the usual signal tracking and navigation functions, under the control of a real-time Linux operating system. The main contribution of this work is a method for realtime generation of over-sampled bitwise-parallel representations of PRN codes via a table look-up function. The over-sampled PRN codes need to be in a bitwiseparallel format in order to be used in the software receiver’s bitwise-parallel correlation algorithms. On-line generation of the over-sampled codes is required because it is impractical to pre-compute and store over-sampled representations of very long codes, such as the L2 CL code. The GPS civilian L1/L2 software receiver tracks 10 channels in real time and has a navigation accuracy of 2–5 meters. It requires 80% of the processing capabilities of a 3.2 GHz Intel Pentium 4 PC.