GPS Non-Standard Codes

Karl Kovach

Abstract: The Navstar Global Positioning System (GPS) Non-Standard Codes (NSCs) have a long history behind them, but surprisingly little is actually known about them. They are an essential element in maintaining GPS integrity in the event of an unexpected failure, but their effectiveness as an alerting mechanism varies from NSC to NSC. There is an NSC for the L1 C/A-code signal, two NSCs for the L2C signal, two for the L5 signal, and two for the L1C signal. This paper describes each of these NSCs in detail, derives their varying levels of effectiveness, and explains why the NSC for C/A-code has nearly ideal properties as a ‘blind’ alerting mechanism for all GPS receivers. The other NSCs can be nearly as effective in delivering an alert, but only when the GPS has been designed to be able to ‘see’ the particular NSC. The paper concludes with the fundamental recipes – published here for the first time anywhere in the open literature -- that allow GPS receivers to better respond to each NSC in both ‘blind’ and ‘seeing’ implementations.
Published in: Proceedings of the 34th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2021)
September 20 - 24, 2021
Union Station Hotel
St. Louis, Missouri
Pages: 1654 - 1671
Cite this article: Kovach, Karl, "GPS Non-Standard Codes," Proceedings of the 34th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2021), St. Louis, Missouri, September 2021, pp. 1654-1671. https://doi.org/10.33012/2021.17953
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