GPS Receiver Block Processing
Proc. of the ION GPS Conf., (1999)Abstract
When a GPS receiver reports a bad measurement, it is often difficult
to analyze the anomaly due to a lack of detailed data surrounding
the event. A system that samples the GPS radio frequency signal at
a rate that preserves all the pertinent signal information allows
for off-line processing to determine the cause of the anomaly. The
GPS Anomalous Event Monitor (GAEM) is designed to capture GPS data
at a rate of 5 mega samples per second for several seconds surrounding
an event. New block processing techniques are used on the 12-bit
samples to assess the quality of the received GPS signal. Block processing
provides estimates of the GPS signal parameters without the use of
tracking loops. It not only demodulates the GPS signals, but also
analyzes the signal quality. This paper describes block processing
algorithms that have been implemented and tested in a software GPS
receiver. The sampled data are processed in 1-millisecond blocks.
Code-phase, frequency, carrier-phase, and signal to-noise ratio are
estimated for each block of data. Advantages of block processing
are discussed and processing results are compared with those for
a sequential-processing GPS receiver. By using block processing techniques,
the satellite signal quality is analyzed in terms of carrier-to-noise
ratio (C / N0 ), shape of the correlation function, false acquisition
peaks, code-carrier divergence, sudden change in code and/or carrier,
and change in the satellite code. Based on the statistical distributions
of the detection parameters, thresholds can be calculated to provide
a specified probability of a false detection, and to calculate a
minimum detectable bias with a specified probability of missed detection.
A detection statistic is introduced for the shape of the correlation
function.