Real-Time Software Receiver Tracking of GPS L2 Civilian Signals
using a Hardware Simulator
Proc. of the 18th ION GNSS Conf., (2005)Abstract
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.