Abstract
This paper investigates cold start acquisition schemes for a GPS L5
software receiver. Since the chipping rate of L5 PRN code is 10.23
MHz, which is 10 times that of the L1 C/A-code, one major concern
for the software signal processing is the computation load. The proposed
algorithm should have a reasonable computation burden in order to
be implemented into a software receiver. In particular, two new methods
are proposed. The main idea of the first approach is to eliminate
the frequency information of the incoming signal, thus only one dimension
search of the initial point of the PRN code is required in the first
step. The acquisition of the Doppler frequency and NH code can be
easily fulfilled after the detection of the chip offset. When the
signal strength is normal or above, the computation burden of this
method is very low, which makes it well suited for a software signal
acquisition. The second approach can detect the L5 signal in a very
low signal-to-noise ratio, such as indoors. Longer coherent integration
is expected using Q5 correlator outputs for a higher correlation
gain. A combined L5 codes and Neumann Hoffman (NH) codes acquisition
algorithm is proposed by Hegarty et al. 2003 using very long coherent
integration times (i.e. 20 ms or longer) on dataless channel. However,
this method is not practical for software signal processing due to
its heavy computation load. A shortened fast Fourier transform (FFT)
version of the above method, which is suitable for the software receiver,
can greatly reduce the computation burden without degradation on
performance. In case of a weak signal, the coherent integration time
can be extended for a very long period to get enough correlation
gain. The only limitation is the quality of the receiver's oscillator.
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