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Session A3b: Next-Generation Satellite Navigation Technology

Modernizing C/A Code
Philip A. Dafesh, Gourav K. Khadge, and Jason W. Zheng, The Aerospace Corporation
Location: Beacon A
Date/Time: Wednesday, Jan. 29, 4:46 p.m.

Since the initial design of the Course Acquisition (C/A) code signal, it was recognized that the 1023 chip length of the C/A code was a compromise between ease of acquisition and short code cross-correlation performance as described in Cahn et al., (1972) and Parkinson et al., pg. 95. (1996.).
The higher cross-correlation levels and 1 kHz spectral lines of C/A code have motivated many investigations on the so-called C/A on C/A effect, which has been characterized as significant or negligible, dependent on specific receiver assumptions and use cases including: coherent integration length during track, tracking loop bandwidth, coherent integration length during acquisition, and relative Doppler conditions as detailed in Van Dierendonk et al., (2002), Van Dierendonk et al., (2013), Golshan et al., (2014), Cerruti et al., (2014) and Hegarty (2020).
The C/A-on-C/A effect and lack of a pilot channel for enhanced carrier tracking and acquisition has further motivated some authors to seek replacements of the C/A code that are not backward compatible with most fundamental C/A code receiver operations, including code tracking and bit sync as first proposed in Betz (2010) and later in Stansell e. al. (2015). The proposed C/A code evolutions have ranged from combining C/A code with 20 chip overlay codes having periods of 20 ms or greater, to replacing C/A code with a longer data-less spreading code signal that serves only to enhance L1C’s pilot component Stansell (2015). Such approaches are equivalent to “sunsetting” the C/A code signal, so that its only purpose is as an enhancement to L1C. Longer overlay codes, that are multiples of the C/A code bit period, have also been proposed. Unfortunately, these codes have limited benefit in terms of resolving C/A code spectral lines and do not provide immediate bit sync like the 20 chip overlay codes used in enhanced C/A (eC/A) that are described in Betz (2010).
Despite C/A code’s shortcomings, it has been successfully used in a wide range of applications and is stated to be “near optimal for consumer applications” as evidenced by the fact that every MGNSS receiver today includes C/A code. In some cases, the sensitivity and TTFF performance of C/A code in MGNSS receivers meets or exceeds that of modernized signals like B1C or Galileo’s E1 signal by leveraging decades old techniques to overcome C/A code’s shortcomings as outlined in Van Diggelen (2009) and Van Diggelen (2014) and demonstrated in the C/A performance specification.
Rather than replacing C/A code outright, this paper proposes a backward compatible approach to modernizing the C/A code signal that can be implemented using algorithms and codes already resident in today’s MGNSS receivers. This allows for the addition of advanced features to C/A code, such as a pilot channel that provides a coherent tracking channel with high reliability bit sync, and frame sync, while retaining backward compatibility with legacy receivers in all modes of operation, and simultaneously improving cross-correlation correlation properties as compared to legacy C/A code.
This work will provide the design, performance simulation, and measurements of a new backward compatible approach to modernizing C/A code that simultaneously reduces C/A on C/A effects, improving spectral line behavior and providing C/A code with many enhancements of modernized signals. The paper will compare the modernized C/A approach with alternative approaches such as enhanced C/A (eC/A) in terms of performance, backward compatibility, mutual interference, and spectral line behavior. An incremental approach to implementing the modernized C/A code will also be described.

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