Using GPS Multicorrelator Receivers for Multipath Parameters Estimation

The performance of GPS may be degraded by many perturbations such as jamming, the effects of the ionosphere, and multipath. Many studies have been done to reduce the effect of multipath on the GPS measurements. Some of these are based on the radiation pattern of the receiving antenna, while most of them concentrate on the characteristics of the receiver tracking loops. A well-known implementation enabling the characterization of multipath effects on the tracking loops through the analysis of the shape of the correlation peak using a multicorrelator receiver has been proposed in [ 1 ]. The widespread use of multicorrelator receivers gave us the idea to try our own implementation of an identification and mitigation technique. The aim of the proposed paper is to describe a least squares method to identify different multipath parameters using multicorrelator outputs, and to present results of the application of this technique. The paper starts with a brief review of the impact of several reflected rays on the code and phase tracking loops. Then, the principle of multicorrelator receivers is described, and the particular structure of the multicorrelator firmware of the NovAtel Millenium receiver is given as an illustration. After this, the least squares technique used to estimate the relative amplitude, relative code and phase delays, code and phase tracking errors is presented. Results of the application of this technique on real data collected on a real receiver connected to a Spirent GPS generator are shown. These results illustrate the overall good performance of the method and its limitations. Finally, a conclusion is drawn on this technique and its possible capacity to improve the performance of tracking loops by removing multipath components in correlator outputs. I. MODEL AND IMPACT OF MULTIPATH Multipath model The multipath phenomena are encountered in most radio propagation. This phenomenon happens when the received signal is a contribution of a direct ray and one or more other reflected rays, which follow indirect paths. The effect depends on the application we deal with. In the case of the Global Positioning System (GPS), mutipath deteriorate the tracking quality of both PLL and DLL inside the GPS receiver. Consequently, the system performance, especially its range measurement accuracy may be diminished dramatically, This measurement error caused by multipath ranges form centimeters to several meters, In the case of GPS applications it is difficult to give a statistical model to describe the received signal in presence of multipath. However, many hypotheses can be made. For instance, the reflected signals are delayed with respect to the direct one, as they travel a longer path. Furthermore, only signals with a delay less than one chip are considered. This latter hypothesis may be justified by the fact that signals with a code delay larger than roughly one chip are uncorrelated with the direct ray. In addition, the reflected ray is supposed to have less power than the direct one. In the presence of N-1 reflected rays, the received signal at the input of the tracking loops may be written as follows: N-1 s(t)=A.~' a~.d(t-r~ )c f (t-r~).cos(2rfct-O~) (1.1) i=0 where • c~,z'i and t~ represent the amplitude, the time delay and the phase delay of the i 'h path with respect to the direct one (the index zero is used for the direct path). • fc is the carrier frequency • c f ( t ) i s the Pseudo Noise code waveform filtered by the RF front-end filter • d(t) represent the payload data. 1.2 In the absence of multipath, the matched filter output is close to the symmetric correlation function of the PN code. This symmetry is needed to obtain reliable time delay estimation. Nevertheless, in the presence of multipath, this symmetry is lost (as shown in figure 1.1), consequently, the propagation delay becomes harder to estimate, thus, the range measurement accuracy is diminished. MULTIPATH E F F E C T 1.4