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

The ionospheric data shall be updated by the CS at least once every six days while the CS is able to upload theSVs. If the CS is unable to upload the SVs, the ionospheric data transmitted by the SVs may not be accurate.3.5.4.2 Subframe 3, Page 2 – GGTO & EOPSubframe 3, page 2, as depicted in Figure 3.5-3, contains the GPS/Global Navigation Satellite System (GNSS) TimeOffset (GGTO) parameters and Earth Orientation Parameters (EOP). The contents of subframe 3, page 2, is definedbelow, and followed by material pertinent to the use of the data.3.5.4.2.1 GGTO Parameter ContentSubframe 3, page 2 shall contain the parameters related to correlating GPS time with other GNSS time. Bits 15through 17 of subframe 3, page 2 shall identify the other GPS-like navigation system to which the offset dataapplies. The three bits are defined as follows:000 = no data available,001 = Galileo,010 = GLONASS,011 through 111 = reserved for other systems.The bit lengths, scale factors, ranges, and units of the GGTO parameters are given in Table 3.5-4. See Figure 3.5-3for complete bit allocation in subframe 3, page 2.The validity period of the GGTO shall be 1 day as a minimum.3.5.4.2.1.1 GPS and GNSS TimeThe GPS/GNSS-time relationship is given by,t GNSS = t E – (A 0GGTO + A 1GGTO (t E – t GGTO + 604800 (WN – WN GGTO )) + A 2GGTO (t E – t GGTO + 604800 (WN – WN GGTO )) 2 )where t GNSS is in seconds, t E and WN are as defined in Section 20.3.3.5.2.4 of IS-GPS-200, and the remainingparameters are as defined in Table 3.5-4.The GGTO parameters provide a global average of the time offset between GPS time and the other GNSS timescales modulo one second. Users must also apply any integer seconds difference between the systems usingdefinitions of each system time scale as defined in respective signal interface documents.54 IS-GPS-800A8 June 2010