GPS C/A Code Signal Structure - Fundamentals of Global Positioning System Receivers: A Software Approach - page 90

TABLE 5.8 Ephemeris Parameters in Subframe 1

Number
of
Bits

Scale
Factor
(LSB)

Effective
Range ∗∗ Parameter Location Units

WN: Week number 61 - 70 10 1 week

Satellite accuracy 73 - 76 4

Satellite health 77 - 82 6 1

IODC: Issue of data, clock 83 - 84 10

211 - 218

8 ∗ 2 − 31

T GD : Satellite group delay differential 197 - 204 seconds

2 4

t oe : Satellite clock correction 219 - 234 16 604,784 seconds

8 ∗ 2 − 55 sec/sec 2 a f 2 : Satellite clock correction 241 - 248

16 ∗ 2 − 43

a f 1 : Satellite clock correction 249 - 264 sec/sec

a f 0 : Satellite clock correction seconds

∗ Parameters so indicated are two's complement, with the sign bit ( + or − ) occupying the MSB.
∗∗ Unless otherwise indicated in this column, effective range is the maximum range attainable with
indicated bit allocation and scale factor.

271 - 292 22 ∗ 2 − 31

6. Satellite clock correction parameters : This subframe also contains the
four additional satellite clock correction parameters: t oc (219 - 234), a f 0
(271 - 292), a f 1

(249 - 264), and a f 2 (241 - 248). They are used in

Equation (4.37).
7. In subframe 1 there are some reserved data fields and their locations are
71 - 72; 91 - 114; 121 - 144; 151 - 174; 181 - 196. All reserved data fields
support valid parity within their respective words.

The ephemeris parameters in subframe 1 are listed in Table 5.8.

5.13 NAVIGATION DATA FROM SUBFRAMES 2 AND 3 ( 3,7 )

Figures 5.9b and c show the following ephemeris data contained in subframes 2
and 3:

1. The issue of data, ephemeris (IODE) : This parameter has 8 bits and is
in both subframes 2 (61 - 68) and 3 (271 - 278). The IODE equals the
8 LSB of the IODC, which has 10 bits. The IODE provides the user
with a convenient means for detecting any change in the ephemeris rep-
resentation parameters. The transmitted IODE will be different from any
value transmitted by the satellite during the preceding six hours. Whenever
these three terms, two IODEs from subframes 2, 3 and the 8 LSBs of the
IODC, do not match, a data set cutover has occurred and new data must
be collected.
Any change in the subframe 2 and 3 data will be accomplished in concert
with a change in both IODE words. Cutovers to new data will occur only