Using Both GPS L1 C/A and L1C: Strategies to Improve Acquisition Sensitivity

K.C. Seals, W.R. Michalson, P.F. Swaszek, R.J. Hartnett

Abstract:	GPS users will have access to two different civil signals at the same frequency within the next several years. The legacy GPS L1 C/A code signal will be joined by the GPS L1C signal with the launch of GPS Block III satellites. L1C is the most recent of the modernized GPS signals and has both a pilot and data component like most other modern GNSS signals. This paper proposes and analyzes various strategies to combine the L1 C/A code and L1C signals for joint acquisition with the goal to improve acquisition performance relative to using either signal alone. Performance is evaluated using the detection and false alarm probabilities. The L1C signal is split into two components with 75% power in the pilot component and 25% power in the data component.  Spreading codes with a length of 10,230 chips and a period of 10 ms at a chipping rate of 1.023 Mcps are based on Weil codes. Not only does each satellite have unique spreading codes, but different codes are also used for the pilot and data components as the two components are transmitted with the same phase. In addition to the spreading code, the pilot component has an 18 second 1800-bit overlay code.  One bit of this overlay code and one bit of the navigation data on the data component both have a duration of 10 ms which corresponds to one period of the spreading code.  Both components of the L1C signal use binary offset carrier (BOC) modulation. Acquisition of GNSS signals requires a two-dimensional search for code delay and Doppler frequency of the incoming signal. With the onset of new GNSS signals that have pilot and data components, various joint acquisition schemes have been previously investigated. A simple acquisition scheme can use either component, correlating the received signal with either the pilot or the data spreading code. One obvious disadvantage of this approach is the wasting of signal power; hence, more sophisticated techniques for signal combining or joint acquisition of the pilot and data components have been proposed. This paper describes methods for improving acquisition sensitivity by using both civil GPS signals on the L1 frequency. Specifically, the strategies developed for joint pilot/data acquisition are extended for joint acquisition of GPS L1 C/A and L1C. In a 2010 paper, Macchi-Gernot, Petovello, and Lachapelle proposed a combined acquisition scheme that uses the FFT to perform parallel code phase search using four different local composite codes which are linear combinations of the C/A, L1C pilot, and L1C data spreading codes to cover all possible relative signs between them. The performance of each of the joint acquisition strategies presented here are compared to this scheme as well as noncoherently combining the signals and using just one of the signals. Acquisition schemes previously considered for joint pilot/data acquisition include noncoherent combining, semi-coherent combining and various forms of differential combining. Noncoherent combining uses correlators for each component separately and combines the power from the correlators. Coherent combining takes advantage of the fact that the relative sign between the data and pilot components can be estimated by correlating the received signal with two different local composite codes, the data spreading code plus the pilot spreading code and the pilot spreading code minus the data spreading code. Semi-coherent integration refers to noncoherently combining these coherent combinations (every 10 ms code period for the case of GPS L1C). A detector that maintains the differential phase information between successive correlator outputs uses differentially coherent integration which consists of the product of the current correlator output and the complex conjugate of the previous correlator output. Directly transferring these schemes proposed for joint pilot/data acquisition to joint L1 C/A and L1C acquisition has some limitations based on the different signal structures of these two signals which were designed thirty years apart from each other. L1 C/A has spreading code that repeats every 1ms with navigation bit duration of 20 ms. L1C has a bit duration (navigation data or pilot overlay code) that is the same as the spreading code period of 10 ms. The joint acquisition schemes presented in this paper, take into account these unique aspects of the GPS L1 civil signals as well as their different power levels. The main contribution of this paper is an analysis of the performance of various joint acquisition schemes for GPS L1 C/A and GPS L1C signals. Performance is measured and compared using the probability of detection and probability of false alarm. Theoretical expressions for the probabilities of detection and false alarm are developed where possible. Analytical results are verified using computer simulation.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	92 - 106
Cite this article:	Seals, K.C., Michalson, W.R., Swaszek, P.F., Hartnett, R.J., "Using Both GPS L1 C/A and L1C: Strategies to Improve Acquisition Sensitivity," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 92-106.
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