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

3.5.3.10.1 Integrity Status Flag (ISF)Bit 566 of subframe 2 shall be the Integrity Status Flag (ISF). A “0” in bit position 566 indicates that the conveyingsignal is provided with the legacy level of integrity assurance. That is, the probability that the instantaneous URE ofthe conveying signal exceeds 4.42 times the upper bound value of the current broadcast URA index, for more than5.2 seconds, without an accompanying alert, is less than 1 x 10^-5 per hour. A “1” in bit position 566 indicates thatthe conveying signal is provided with an enhanced level of integrity assurance. That is, the probability that theinstantaneous URE of the conveying signal exceeds 5.73 times the upper bound value of the current broadcast URAindex, for more than 5.2 seconds, without an accompanying alert, is less than 1 x 10^-8 per hour. The probabilitiesassociated with the nominal and lower bound values of the current broadcast URA are not defined.In this context, an “alert” is defined as any indication or characteristic in the conveying signal, as specifiedelsewhere in this document, which signifies that the conveying signal may be invalid and should not be used, suchas, not Operational-Healthy, Non-Standard Code, parity error, etc.3.5.4 Subframe 3Subframe 3 provides users with other navigation data which is commutated over multiple pages. The contents ofsubframe 3 data are defined below.Every subframe 3 page begins with an 8-bit PRN number of the transmitting SV with a range of 0 (00000000) to255 (11111111). Each subframe 3 page is identified by a 6-bit page number provided in bits 9 through 14.3.5.4.1 Subframe 3, Page 1 – UTC & IONOAs depicted in Figure 3.5-2, subframe 3, page 1 contains the UTC and ionospheric correction parameters. Thecontents of subframe 3, page 1, are defined below, and followed by material pertinent to the use of the data.3.5.4.1.1 UTC Parameter ContentSubframe 3, page 1 shall contain the parameters related to correlating UTC(USNO) time with GPS Time. The bitlengths, scale factors, ranges, and units of these parameters are given in Table 3.5-3. See Figure 3.5-2 for completebit allocation in subframe 3, page 1.The parameters relating GPS time to UTC(USNO) shall be updated by the CS at least once every three days whilethe CS is able to upload the SVs. If the CS is unable to upload the SVs, the accuracy of the UTC parameterstransmitted by the SVs will degrade over time.3.5.4.1.1.1 UTC and GPS TimeSubframe 3, page 1 includes: (1) the parameters needed to relate GPS Time to UTC(USNO), and (2) notice to theuser regarding the scheduled future or recent past (relative to navigation message upload) value of the delta time52 IS-GPS-800A8 June 2010