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

3.5.4.3 Subframe 3, Page 3 & Page 4 – AlmanacThe almanac parameters are provided in any one of subframe 3, pages 3 and 4. Page 3 provides the reducedalmanac parameters and Midi almanac parameters are provided in page 4. The reduced almanac parameters (i.e.subframe 3, page 3) for the complete set of SVs in the constellation will be broadcast by a SV using shorter durationof time compared to the broadcast of the complete set of Midi almanac parameters (i.e. subframe 3, page 4). Theparameters are defined below, followed by material pertinent to the use of the data.3.5.4.3.1 Almanac Reference WeekBits 15 through 27 of subframe 3, pages 3 and 4 shall indicate the number of the week (WN a-n ) to which the almanacreference time (t oa ) is referenced (see paragraph 3.5.4.3.2). The WN a-n term consists of 13 bits which shall be amodulo-8192 binary representation of the GPS week number (see paragraph 6.2.2) to which the t oa is referenced.Bits 28 through 35 of subframe 3, pages 3 and 4 shall contain the value of t oa , which is referenced to this WN a-n .3.5.4.3.2 Almanac Reference TimeSee paragraph 20.3.3.5.2.2 of IS-GPS-200.3.5.4.3.3 SV PRN NumberBits 36 through 43 of subframe 3, page 4 and bits 1 through 8 in each packet of reduced almanac shall specify PRNnumber of the SV whose almanac or reduced almanac, respectively, is provided in the message or in the packet.3.5.4.3.4 Signal Health (L1/L2/L5)The three, one-bit, health indication in bits 44, 45 and 46 of subframe 3, page 4 and bits 31, 32 and 33 of each packetof reduced almanac refers to the L1, L2, and L5 signals of the SV whose PRN number is specified in the message orin the packet. For each health indicator, a “0” signifies that all navigation data are valid and “1” signifies that someor all navigation data are invalid. The predicted health data will be updated at the time of upload when a newreduced almanac has been built by the CS. The transmitted health data may not correspond to the actual health ofthe transmitting SV or other SVs in the constellation.3.5.4.3.5 Reduced Almanac DataSubframe 3, page 3, Figure 3.5-4, shall contain reduced almanac data packets for 6 SVs. The reduced almanac dataof a SV is broadcast in a packet of 33 bits long, as described in Figure 3.5-9. The reduced almanac data are a subsetof the almanac data which provide an ephemeris with less precision than that derived from parameters in subframe2. The reduced almanac data values are provided relative to pre-specified reference values. The bit lengths, scalefactors, ranges, and units of the reduced almanac parameters are given in Table 3.5-6.The reduced almanac parameters shall be updated by the CS at least once every 3 days while the CS is able toupload the SVs. If the CS is unable to upload the SVs then the accuracy of the reduced almanac parameters57 IS-GPS-800A8 June 2010