Performance Analysis of Doppler Aided Tracking Loops in Modernized GPS Receivers
The GPS modernization program deploys new generations of satellites designated Block IIR-M, Block IIF and Block III-A, equipped to transmit multiple civil signals. The coexistence of multiple civilian signals in a GPS receiver has been a focus in recent research activities. In the literature to date, correlator outputs of individual channels are combined in ways either to increase the power available for processing or to reduce the error in tracking measurements. These combinations are typically made at the discriminator level, after the discriminator or after the loop filter. The combinations have been considered both for channels operating on the same carrier frequency (e.g. data and pilot channels of L5) as well as channels across different carrier frequencies. In a modernized GPS receiver, tracking multiple civil signals, the relationship between Doppler frequencies (proportional to the corresponding carrier frequencies) of all signals is known and therefore Doppler estimates of one PLL can be utilized by another PLL tracking a different frequency. While such benefits are conventionally achieved through ultra-tight GPS/INS coupling, this paper presents performance analysis of a Doppler-aided tracking loop architecture, where the Doppler assistance is obtained from another loop in the same receiver. L1 C/A and L2C are the two civil signals selected for the analysis, because this pair of signals will be available long before any other combination. The analysis, however, is relevant to any pair of signals from a single satellite. The L2C carrier, having the pilot channel and relatively slower dynamics, is used as aiding source in the considered arrangement. The L2C PLL, operated with a wide loop bandwidth, absorbs the dynamic stress and provides Doppler estimates to the L1 PLL (affected by wide-band interference) operating at narrow-band. The tracking error associated with such collaboration between two tracking loops is analyzed. It is shown that a 7 dB-Hz RFI margin (equivalent to that achieved with a moderate quality inertial sensor), can be achieved through the local Doppler-assistance.
Combined L1 / L2C Tracking Scheme for Weak Signal Environments
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. INTRODUCTION GPS satellites are roughly orbiting 20,000 km above the earth surface. As such, signals have to undergo a tremendous amount of free space loss before reaching the users. At the same time, signals have to go through ionosphere and troposphere. Whereas all these difficulties were taken into account during the design of the system and as such the -158.5 dBW of the legacy L1 C/A signal is easily acquired and tracked by receivers under open sky conditions, urban canyon and indoor environments add
Real-Time Software Receiver Tracking of GPS L2 Civilian Signals using a Hardware Simulator
A dual-frequency civilian L1/L2 GPS receiver has been tested that runs 12 tracking channels in real time using a software correlator. This work is part of an eort