Article,

Acquisition Schemes for a GPS L5 Software Receiver

, and .
Proc. of the 17th ION GNSS Conf., (2004)

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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