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

3.5.3.1 Transmission Week NumberBits 1 through 13 of subframe 2 shall contain 13 bits that are a modulo-8192 binary representation of the currentGPS week number at the start of the data set transmission interval (see paragraph 6.2.2).3.5.3.2 ITOWBits 14 through 21 of subframe 2 shall contain 8 bits representing ITOW count defined as being equal to the numberof two-hour epochs that have occurred since the transition from the previous week. The count is short-cycled suchthat the range of the ITOW-count is from 0 to 83 2-hour epochs (equaling one week) and is reset to zero at the endof each week. The ITOW-count's zero state is defined as that 2-hour epoch which is coincident with the start of thepresent week. This epoch occurs at (approximately) midnight Saturday night-Sunday morning, where midnight isdefined as 0000 hours on the UTC scale that is nominally referenced to the Greenwich Meridian. The occurrence ofthe “zero state epoch” may differ by a few seconds from 0000 hours on the UTC scale since UTC is periodicallycorrected with leap seconds while GPS time is continuous without such correction.3.5.3.3 Data Predict Time of WeekBits 22 through 32 of subframe 2 shall contain the data predict time of week (t op ). The t op term provides the epochtime of week of the state estimate utilized for the prediction of satellite quasi-Keplerian ephemeris parameters.3.5.3.4 L1C Signal HealthThe one-bit health indication in bit 33 of subframe 2 refers to the L1C signal of the transmitting SV. The health ofthe signal is indicated by:0 = Signal OK,1 = Signal bad or unavailable.The predicted health data will be updated at the time of upload when a new data set has been built by the CS. Thetransmitted health data may not correspond to the actual health of the transmitting SV. In real time, if the L1Csignal becomes unhealthy, the status change will normally be indicated by the broadcast of non-standard code or beindicated by the health bits as described in subframe 2.Additional SV health data are given in the almanac in subframe 3, pages 3 and 4. The data given in subframe 2 maydiffer from that shown in other messages of the transmitting SV and/or other SVs since the latter may be updated ata different time. Subframe 2 data is the most reliable; subframe 3 data is intended only to aid acquisition.3.5.3.5 SV AccuracyBits 34 through 38 of subframe 2 shall contain the ephemeris User Range Accuracy (URA oe ) index of the SV.URA oe index shall provide the ephemeris-related user range accuracy index of the SV as a function of the current42 IS-GPS-800A8 June 2010