Global Positioning System Reference
In-Depth Information
midnight on the night of January 5, 1980/morning of January 6, 1980. Users
must count the rollover if it is over 1,023 weeks.
2. User range accuracy (73-76) : These four bits give the predicted user range
accuracy and its value N ranges from 0 - 15. The accuracy value X is:
ž
= 2 ( 1 + N/ 2 )
If N is 6 or less, X
(rounded-of-values N
= 1, X
= 2 . 8; N
=
3, X
= 5 . 7; N
= 5, X
= 11 . 3).
If N is 6 or more, but less than 15, X = 2 (N 2 ) .
ž
= 15 will indicate the absence of an accuracy prediction and will
advise the user to use that satellite at the user's risk.
3. Satellite health (77-82) : These six bits represent the health indication of
the transmitting satellite. The MSB (bit 77) indicates a summary of the
health of the navigation data, where bit 77 equals:
0
N
ž
=
All navigation data are OK.
Some or all navigation data are bad.
The five LSBs indicate the health of the signal components in Table 5.7.
Additional satellite health data are given in subframes 4 and 5. The data
given in subframe 1 may differ from that shown in subframes 4 and/or 5
of other satellites, since the latter may be updated at a different time.
4. Issue of data, clock (IODC) (83-84 MSB, 211-218 LSB) : These 10-bit
IODC data indicate the issue number of the data set and thereby provide
the user with a convenient means of detecting any change in the correction
parameters. The transmitted IODC will be different from any value trans-
mitted by the satellite during the preceding seven days. The relationship
between IODC and IODE (in both subframes 2 and 3) will be discussed in
the next section.
5. Estimated group delay differential T GD (197-204) : This eight-bit informa-
tion is a clock correction term to account for the effect of satellite group
delay differential. It is used in Equation (4.38).
1
=
TABLE 5.7 Codes for Health of Satellite Signal Components
MSB
LSB
00000
All signals OK.
11100
Satellite is temporarily out - do not use this satellite
during current pass.
11101
Satellite will be temporarily out - use with caution.
11110
Spare
11111
More than one combination would be required to
describe anomalies.
All other combinations
Satellite experiencing code modulation and/or signal
level transmission problem - modulation navigation
data valid; however, user may experience intermittent
tracking problems if satellite is acquired.
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