Combined L1 / L2C Tracking Scheme for Weak Signal Environments

Cyrille Gernot, Kyle O’Keefe, Gerard Lachapelle

Abstract:	GPS receiver performances are strongly dependent on the user environment. For instance, attenuation of 20 to 30 dB or more can easily be reached indoor or in urban canyon environments. In order to overcome these limitations, one usually has to significantly increase the coherent integration time used. However, even though one was to use Assisted-GPS or the pilot channel proposed by new signals to eliminate the problem of data bit transitions, the use of long dwelling time remains limited by dynamics factors such as oscillator stability or user motion. On the other hand, GPS modernization duplicates the number of signals available. Therefore, the coherent integration time required to increase the receiver sensitivity can significantly be reduced through the combination of signals transmitted from the same satellite. In this paper, two methods combining L1 C/A and L2C signals energy at the tracking loop level are presented. One of the main difficulties when combining the signals transmitted at different frequencies in the tracking loop arises due to the ionosphere. Indeed, typical L1 / L2 tracking loop basically assumes that the Doppler frequency ratio between L1 and L2 is directly related to the theoretical scale factor f2 / f1 = 0.7792 and uses one signal to perform aiding on the other. However, the Doppler shift induced by the ionosphere is a function of the inverse transmitted frequency, thereby relating L1 and L2 Dopplers through the factor f1 / f2. In order to solve this problem, the output of the DLL and PLL discriminators are combined through a least-squares method estimating the TEC change and L2 code and phase error. In order to apply this method, specific considerations are given to the observation covariance matrix of the least squares method and an analysis of the PLL atan and DLL normalized early minus late envelope discriminators statistics is proposed
Published in:	Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008) September 16 - 19, 2008 Savannah International Convention Center Savannah, GA
Pages:	1758 - 1772
Cite this article:	Gernot, Cyrille, O’Keefe, Kyle, Lachapelle, Gerard, "Combined L1 / L2C Tracking Scheme for Weak Signal Environments," Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 1758-1772.
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