2011 International Technical Meeting
Session B3: Land and Marine Based Applications

Title: Analysis of L1C Acquisition by Combining Pilot and Data Components over Multiple Code Periods
Author(s): K.C. Seals, U.S. Coast Guard Academy; W.R. Michalson, Worcester Polytechnic Institute; P.F. Swaszek, University of Rhode Island; R.J. Hartnett, U.S. Coast Guard Academy
Date/Time: Tuesday, January 29, 2013, 4:30 p.m.

One of the new features of modern GNSS signals is that they generally have a pilot component and data component. The pilot component is a carrier with no data whereas the data component carries the navigation data. A unique aspect of the GPS L1C signal is that it has an unequal power split between the pilot and data components. In addition, the L1C signal transmits the the pilot and data components in phase, with orthogonality achieved by code division multiplexing (two distinct spreading codes). This paper analyzes the acquisition performance of acquisition methods for GPS L1C signal that combine the pilot and data components over multiple spreading code periods.The design of the new civil signal in the L1 band, called L1C, was initiated in August 2003 and completed in April 2006. The first launch of a GPS Block III satellite with this signal payload is expected to occur within a few years. It has the same carrier frequency of 1575.42 MHz as the legacy L1 C/A code signal but many innovative design features separate this signal from its counterpart on the same frequency (that was designed thirty years prior). As the most recent of the modernized GPS signals, L1C has acquired many advancements seen in other modern signals including WAAS, L5, and L2C. 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. In addition to the spreading code, the pilot component uses 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.Acquisition of GNSS signals typically requires a two-dimensional search for code delay and Doppler frequency of the incoming signal. For modern dual-component signals, the conventional non-coherent GNSS acquisition scheme could be used on either component, correlating the received signal with either the pilot or data spreading code. However, one obvious disadvantage of this approach is the wasting of signal power. Previous papers have proposed channel combining techniques to acquire modern GNSS signals using both components.Acquisition techniques for combined pilot and data components of modern GNSS signals include noncoherent combining, coherent combining, and differentially combining. In noncoherent combining, separate correlations of the incoming signal with the pilot and data spreading codes are squared and added. Alternatively, 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 composite codes, the data spreading code plus the pilot spreading code and the pilot spreading code minus the data spreading code. Finally, the original differentially coherent acquisition technique combined correlations on two consecutive blocks of the received signal and used the inner product of the two correlations as the decision variable. As proposed for signals with two components, differentially coherent combining uses the correlations of the pilot and data signals instead of two consecutive blocks of incoming signal in the decision variable.Performance of acquisition schemes is often analyzed using single trial false alarm and detection probabilities. Recent work has studied the theoretical performance of these channel combining acquisition techniques for modern signals that have an equal power split between the data and pilot components which are in phase quadrature. This paper conducts an analysis and performance comparison of GPS L1C acquisition techniques that combine pilot and data components over multiple code periods taking into account the unique aspects of the L1C signal.In this paper, the optimal detector for GPS L1C acquisition over multiple code periods in additive white Gaussian noise is derived. Single channel acquisition and noncoherent channel combining acquisition along with their respective detection and false alarm probabilities are provided for comparison purposes. False alarm and detection probabilities are derived for coherent channel combining. Coherent channel combining with unequal power compensation is proposed to improve the performance of the previously proposed coherent combining technique for dual-component signals with equal power. Monte Carlo computer simulations are used to compare the performance of the optimal detector to other channel combining detection schemes as well as to validate the analytically derived false alarm and detection probabilities. It is shown that the coherent channel combining with unequal power compensation detector nearly approaches the performance of the optimal detector.

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