How Old Is Your GPS Navigation Message?

Steven T. Hutsell, Brian K. Brottlund and Clifford A. Harris

Abstract:	Most designs of GPS user equipment make use of the several different types of information available in the broadcast navigation message. In addition to the ephemeris and clock corrections in subframes 1-3, the GPS navigation message also provides health summary, almanac data, time transfer corrections, and ionosphere parameters for single frequency users, in subframes 4 and 5. Many users benefit when subframe 1-5 information is the most current. The relative ages of subframe 1-5 data for given satellites will differ widely, depending on how recently the GPS control segment has refreshed given satellites with navigation uploads. Though some of the available subframe 4-5 parameters have associated reference time tags (toa and tot), contrary to popular belief, these reference tags do not indicate relative satellite upload age. Often, many satellites uploaded at different times may, in given instances, broadcast the exact same reference tags. Until recently, normal GPS users were not able to select the most current subframe 4-5 information from all satellites in view, simply because typical users were not able to properly assess the relative ages of navigation messages between satellites. On 20 October 1997, the GPS program office revised the GPS Space Segment – User Interface Control Document, ICD-GPS-200, to reflect recent changes in the navigation message, in support of Navigation Message Correction Tables (NMCT). When available, users can now extract information from NMCT-related parameters, in order to assess the relative ages of the navigation messages between broadcasting satellites in view. This paper will present a recipe for utilizing this new information, as well as commentary on potential windfalls from making use of this recipe.
Published in:	Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000) September 19 - 22, 2000 Salt Palace Convention Center Salt Lake City, UT
Pages:	2556 - 2561
Cite this article:	Hutsell, Steven T., Brottlund, Brian K., Harris, Clifford A., "How Old Is Your GPS Navigation Message?," Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000), Salt Lake City, UT, September 2000, pp. 2556-2561.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In