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accuracies to the ranging accuracies obtainable with a specific satellite based on historical data.
23/11/06 APP B-4
Appendix B Annex 10 — Aeronautical Communications
Table B-2. Subframe 1 parameters
Parameter
Number
of bits**
Scale factor
(LSB)
Effective
range*** Units
Week number 10 1 weeks
Satellite accuracy 4
Satellite health 6 1 discretes
TGD 8* 2–31 seconds
IODC 10
toc 16 24 604 784 seconds
af2 8* 2–55 seconds/second2
af1 16* 2–43 seconds/second
af0 22* 2–31 seconds
* Parameters so indicated are two’s complement, with the sign bit (+ or –) occupying the MSB.
** See Figure B-6 for complete bit allocation.
*** Unless otherwise indicated in this column, effective range is the maximum range.
Table B-3. User range accuracy
URA Accuracy
0 2 m
1 2.8 m
2 4 m
3 5.7 m
4 8 m
5 11.3 m
6 16 m
7 32 m
8 64 m
9 128 m
10 256 m
11 512 m
12 1 024 m
13 2 048 m
14 4 096 m
15 Do not use
3.1.1.3.1.3 Health. The transmitting satellite 6-bit health indication shall be provided by bits 17 through 22 of word 3.
The MSB shall indicate a summary of the health of the navigation data, where:
a) 0 = all navigation data are valid; and
b) 1 = some of the navigation data are not valid.
APP B-5 23/11/06
Annex 10 — Aeronautical Communications Volume I
The 5 LSBs shall indicate the health of the signal components in accordance with 3.1.1.3.3.4. The health indication shall be
provided relative to the capabilities of each satellite as designated by the configuration code in 3.1.1.3.3.5. Any satellite that does
not have a certain capability shall be indicated as “healthy” if the lack of this capability is inherent in its design or it has been
configured into a mode which is normal from a receiver standpoint and does not require that capability. Additional health data
shall be given in subframes 4 and 5.
Note.— 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.
3.1.1.3.1.4 Issue of data, clock (IODC). Bits 23 and 24 of word 3 in subframe 1 shall be the 2 MSBs of the 10-bit
IODC term. Bits 1 through 8 of word 8 in subframe 1 shall contain the 8 LSBs of the IODC. The IODC shall indicate the
issue number of data set. The transmitted IODC shall be different from any value transmitted by the satellite during the
preceding 7 days.
Note.— The relationship between the IODC and the Issue of Data, Ephemeris (IODE) terms is defined in 3.1.1.3.2.2.
3.1.1.3.1.5 Estimated group delay differential. Bits 17 through 24 of word 7 shall contain the correction term, TGD, to
account for the effect of satellite group delay differential.
Note.— TGD does not include any C/A to P(Y) code relative group delay error.
3.1.1.3.1.6 Satellite clock correction parameters. Bits 9 through 24 of word 8, bits 1 through 24 of word 9, and bits 1
through 22 of word 10 shall contain the parameters needed by the users for apparent satellite clock correction (toc, af2, af1 and af0).
3.1.1.3.1.7 Reserved data fields. Reserved data fields shall be as indicated in Table B-4. All reserved data fields shall
support valid parity within their respective words.
3.1.1.3.2 Subframes 2 and 3 — satellite ephemeris data. Subframes 2 and 3 shall contain the ephemeris representation
of the transmitting satellite.
3.1.1.3.2.1 Ephemeris parameters. The ephemeris parameters shall be as indicated in Table B-5. For each parameter in
subframe 2 and 3, the number of bits, the scale factor of the LSB, the range, and the units shall be as specified in Table B-6.
3.1.1.3.2.2 Issue of data, ephemeris (IODE). The IODE shall be an 8-bit number equal to the 8 LSBs of the 10-bit
IODC of the same data set. The IODE shall be provided in both subframes 2 and 3 for the purpose of comparison with the
8 LSBs of the IODC term in subframe 1. Whenever these three terms do not match, as a result of a data set cutover, new data
shall be collected. The transmitted IODE shall be different from any value transmitted by the satellite during the preceding
six hours (Note 1). Any change in the subframe 2 and 3 data shall be accomplished in concert with a change in both IODE
words. Change to new data sets shall occur only on hour boundaries except for the first data set of a new upload. Additionally,
the toe value, for at least the first data set transmitted by a satellite after an upload, shall be different from that transmitted
prior to the change (Note 2).
Table B-4. Subframe 1 reserved data fields
Word Bit
3 11 – 12
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