A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Paul Hirose
Date: 2018 Nov 6, 19:12 -0800
The official specification for the GPS signal is an Interface Control Document. I think the one of most interest is the first on the page, Interface Specification IS-GPS-200. https://www.gps.gov/technical/icwg/ See p. 103 in that document for the format of the ephemerides broadcast by the satellites. The orbit parameters have the appearance of the Keplerian orbital elements long used in astronomy, but they actually yield the WGS84 coordinates of the satellites. In other words, the rotation of the Earth and wandering of the pole have been absorbed into the ephemeris. But what about the encrypted military signal ("Y-code")? Does it include additional data for improved accuracy? Apparently not: "The NAV data, D(t), includes SV [Space Vehicle] ephemerides, system time, SV clock behavior data, status messages and C/A to P (or Y) code handover information, etc. The 50 bps data is modulo-2 added to the P(Y)- and C/A- codes; the resultant bit-trains are used to modulate the L1 and L2 carriers. For a given SV, the data train D(t), if present, is common to the P(Y)- and C/A- codes on both the L1 and L2 channels." Upgrades have made new GPS signals available to civil users, such as the "CNAV" data described in Appendix III of the document. They include Earth orientation parameters: x and y of the pole, UT1-UTC, and the time derivatives of those values. See p. 176. I don't know how many satellites broadcast the new data, or to what extent consumer grade devices apply the corrections.