Real-Time Software Receiver Tracking of GPS L2 Civilian Signals using a Hardware Simulator

摘要

A dual-frequency civilian L1/L2 GPS receiver has been tested that runs 12 tracking channels in real time using a software correlator. This work is part of an effort to develop flexible receivers that can use the new GPS L2 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 3.4 GHz PC. An analog mixing RF front end composed of separate L1 and L2 signal channels converts the L1 C/A code and L2 CM/CL code signals into two 2-bit digital data streams sampled at 5.714 MHz. The shift registers parallelize the two 2-bit data streams, which the DAQ reads into the PC's memory using direct memory access. The PC performs baseband 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 contributions of this work are a design for an inexpensive dual-frequency GPS civilian L1/L2 RF front end and demonstration of GPS civilian L1/L2 software receiver running on a GPS signal simulator. This RF front end uses two commercially-available GPS RF front ends designed for the L1 C/A code signal. To accommodate the L2 CM/CL code signal, a synthesizer and mixer are located upstream of the RF front end and are used to up-convert the L2 signal to the L1 frequency minus roughly 6 kHz. The successful operation of the receiver while connected to a hardware simulator indicates that the receiver should function when the first civilian L2 signals become available. The GPS civilian L1/L2 software receiver tracks 12 channels in real time and has a navigation accuracy of 1-3 meters. It requires 81% of the processing capabilities of a 3.4 GHz Intel Pentium 4 PC.