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.
Analysis of early late phase in single-and dual-frequency GPS receivers for multipath detection
Much of the research into multipath detection and mitigation has not considered the carrier phase delay between the line of sight (LOS) and reflected signals. A new variable referred to as early late phase (ELP) has recently been proposed to exploit this phase difference. It has been found that in a receiver tracking the L1 GPS signal, the probability of detecting multipath is lower when the carrier phase difference between the LOS and a reflected signal is an integer multiple of π. Since the pseudorange error caused by the multipath’s presence is the highest in this case, we propose to exploit the coexistence of another GPS civilian signal, the L2C. We present an analysis of ELP for the L1 and L2C signals, and a combination of both, for detecting multipath. The multipath detection performance has been compared using probabilities of false alarm and detection. An ideal algorithm should have lower probability of false alarm and higher probability of detection. However, it has been found that using dual-frequency ELP increases both probabilities. Thus, receiver operator characteristics (ROC) curves, and the area under the ROC curves, have been used for effective comparison. It has been found that the L2C signal individually gives worse performance than L1 because of its weaker signal strength. However, the combination of L1 and L2C gives the best overall performance, and thus it can be claimed that ELP using dual-frequency receivers is a more effective approach for detecting multipath.
Open GNSS Receiver Platform
This paper presents results from the open GNSS receiver platform that has been under development at the School of Surveying and SIS at the University of New South Wales since mid 2004. The aim of the project is to create the first open source GNSS receiver design based on Field Programmable Gate Array (FPGA) technology for research and development. The project is on target with many parts complete and initial results looking good. It is hoped that eventually the hardware components along with the FPGA design files and GPS firmware will become available as a development kit and possibly as an integrated IP block available from one of the FPGA providers. The platform consists of a custom circuit board, a baseband processor design and application firmware. Version one of the circuit board has been built and tested. It has a GPS L1 RF front end, Altera FPGA chip and support components. The baseband design is complete and shown to operate correctly. The application firmware has been ported to the new platform and testing and tuning are on-going. Design aspects and results from each component is discussed, followed by ideas for the future.