IS-GPS-800A - US Coast Guard Navigation Center

3.5.3 Subframe 2Subframe 2 provides users with the requisite data to correct SV time and to calculate SV position. Nominally, thesubframe 2 data is invariant for the nominal transmission interval of two hours. The contents of the SV ephemerisrepresentation, clock correction, and accuracy parameters are defined below, followed by material pertinent to theuse of the data.The general format of ephemeris data of subframe 2 consists of data fields for reference time tags, a set ofgravitational harmonic correction terms, rates and rate corrections to quasi-Keplerian elements, and an accuracyindicator for ephemeris-related data.The ephemeris parameters describe the orbit of the transmitting SV during the curve fit interval of three hours. Thenominal transmission interval is two hours, and shall coincide with the first two hours of the curve fit interval. Theperiod of applicability for ephemeris data coincides with the entire three-hour curve fit interval. Table 3.5-1 givesthe definition of the orbital parameters using terminology typical of Keplerian orbital parameters; it is noted,however, that the transmitted parameter values are expressed such that they provide the best trajectory fit in Earth-Centered, Earth-Fixed (ECEF) coordinates for each specific fit interval. The user shall not interpret intermediatecoordinate values as pertaining to any conventional coordinate system.Any change in the subframe 2 ephemeris and clock data shall be accomplished with a simultaneous change in the t oevalue. The SV shall assure that the t oe value, for at least the first data set transmitted by an SV after an upload, isdifferent from that transmitted prior to the cutover. The eight LSBs of t oe for each data set shall be different fromthe eight LSBs of t oe transmitted during the previous six hours by the SV.The general format of clock data in subframe 2 consists of data fields for SV clock correction coefficients. Theclock parameters of subframe 2 describe the SV time scale during the period of validity. The parameters areapplicable during the time in which they are transmitted. Beyond that time, they are still applicable, however, themost recent data set should be used since the accuracy degrades over time.38 IS-GPS-800A8 June 2010