GPS C/A Code Signal Structure - Fundamentals of Global Positioning System Receivers: A Software Approach - page 95

The information in subframes 4 and 5 and its applications are listed below:

1. Subframe 4 :
ž Pages 2, 3, 4, 5, 7, 8, 9, and 10 contain the almanac data for satellite
25 through 32. These pages may be designated for other functions. The
satellite ID of that page defines the format and content.
ž Page 17 contains special messages.
ž Page 18 contains ionospheric and universal coordinated time (UTC).
ž Page 25 contains antispoof flag, satellite configuration for 32 satellites,
and satellite health for satellites 25 - 32.
ž Pages 1, 6, 11, 12, 16, 19, 20, 21, 22, 23, and 24 are reserved.
ž Pages 13, 14, and 15 are spares.
2. Subframe 5 :
ž Pages 1 - 24 contain almanac data for satellites 1 through 24.
ž Page 25 contains satellite health for satellites 1 through 24, the almanac
reference time, and the almanac reference week number.
3. Almanac data : The almanac parameters provided in subframes 4 and 5 are:

e s , t oa , ˙ , √ a s , e , ω , M 0 , a f 0 ,and a f 1 . The almanac data are much less
accurate than the detailed ephemeris data of subframes 2 and 3. However,
the almanac data are valid for longer periods of time and do not require
frequency updates. The almanac data and their application will be further
discussed in Section 12.12.
4. Translation of GPS time to UTC time : In page 18 of subframe 4 the param-
eters in Table 5.11 are included.
The GPS/UTC time relationship is given by: ( 3 , 7 )

t UTC = (t E − t UTC ) { modulo 86,400 seconds } ( 5 . 4 )

TABLE 5.11 UTC Parameters

Parameter ∗∗ Effective Range ∗∗∗ Number of Bits Scale Factor Units

32 ∗ 2 − 30

A 0 seconds

24 ∗ 2 − 50 A 1 sec/sec

t LS 8 1 seconds

2 12 t ot 8 602,112 seconds

WN r 8 1 weeks

WN LSF 8 1 weeks

8 ∗∗∗∗ DN 1 7 days

8 ∗

t LSF 1 seconds

∗ Parameters so indicated are two's complement, with the sign bit (

+ or − ) occupying the MSB.

∗∗ See Figure 5.9a for bit allocation in subframe 4.
∗∗∗ Unless otherwise indicated in this column, effective range is the maximum range attainable with
indicated bit allocation and scale factor.
∗∗∗∗ Right justified.