Article,
A 12-Channel Real-Time GPS L1 Software Receiver
Proc. of the 18th ION GNSS Conf., (2005)
Abstract
A GPS receiver has been developed that runs 12 tracking channels in
real-time using a software correlator. This work is part of an effort
to develop a flexible receiver that can use new GPS signals as they
become available 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) card, and software that runs on
a 1.73 GHz PC. The commercial RF front-end down converts the signal
into a 2-bit digital data stream at 5.714 MHz. The shift registers
parallelize the magnitude and sign data bit streams into separate
words, 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 software
correlator receives frequency commands for simulated carrier and
code NCOs and, in effect, uses these to reconstruct carrier and code
replicas which it mixes with the input data stream. The resulting
signals are summed to produce the standard in-phase and quadrature,
prompt and early-minus-late accumulations. These, along with the
phases of the 2 NCOs, are sent back to the part of the code that
executes the tracking loops and the navigation functions. The contributions
of this work are a set of special high-speed algorithms for doing
the correlations in software. They make use of bit-wise parallelism
so that a single C-code command (partially) processes 32 samples
at a time. This system has been tested using a roof-mounted antenna.
When operating with 12 channels, the entire re ceiver uses less than
50% of the capacity of the 1.73 GHz processor and navigates to an
accuracy of 10 meters.
