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.
Receiving the L2C Signal with "Namuru" GPS L1 Receiver
L2C is the first modernised GPS signal for civilians, broadcast by the new generation of satellites, designated Block IIR-M. Currently three of these satellites (PRN-17, PRN-31 and PRN-12) have been declared operational by the Air Force Space Command at Schriever Air Force Base, Colorado. The structure of the L2C signal is designed such that it achieves a number of advantages over the GPS L1 signal alone. This includes weak signal acquisition, ionospheric error elimination when both L1 and L2C are used, quicker operations and improved tracking performance. It was suggested that it would be interesting as well as challenging to acquire and track the L2C signal with a standard GPS L1 receiver. “Namuru” is an FPGA-based GPS L1 receiver, developed at School of Surveying & Spatial Information Systems, UNSW. In this work, the RF front end of “Namuru” has been modified by hardware augmentation, to process real L2C signals from Block IIR-M satellites. The modified RF front end provides real time samples of the L2C signal. These samples are recorded in a memory module on “Namuru” board and further processed in Matlab environment to perform the acquisition of L2C signal. The new structure of the L2C signal requires novel approaches for its acquisition. The L2C signal consists of two codes, L2C CM and L2C CL. The L2C CM-code is modulo-2 added to data and the resultant sequence is time-multiplexed with the L2C CL-code on a chip by chip basis. In this work, the replica code used for acquisition primarily contains CM chips while CL chips being replaced by zeros. This replica code is referred to as return to zero (RZ) CM code and can be used for signal search across any 20 milliseconds segment of an L2C signal record. The L2C signal is successfully acquired from all the three operational satellites and “Namuru” is now equipped with L2C acquisition capability.
An Analysis of L1-C/A Cross Correlation & Acquisition Effort in Weak Signal Environments
Modern mobile devices are now equipped with GPS-receivers to provide E911 and other location based services. Working indoors, the GPS receiver is likely to experience a situation where some of the received signals are stronger than others. These strong signals can prevent the acquisition of a desired weak signal or lead to false acquisition, due to high cross correlation between the strong and weak signals. The C/A Gold code used in GPS L1 signal has an inherent correlation protection of about 24 dB. This becomes inadequate for weak signal acquisition in the presence of multiple strong signals. Various cross correlation mitigation techniques are used to deal with this problem. The impact of cross correlation on weak signal acquisition largely depends on the relative power level and carrier offset of the desired weak signal with reference to the strong interfering signals. This paper first presents an analysis of the L1-C/A cross correlation in the presence of multiple strong signals under different conditions of relative Doppler offset and dwell time. Acquisition of weak signals with different SNR levels is then performed under these conditions, using the “Tong” search algorithm and the results of acquisition are presented. The results indicate that, for certain relative Doppler offsets, extended dwell times assist in combating the cross correlation noise hence improving the detection probability.