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the 24 MSBs of words 4 through 7, and the 16 MSBs of word 8, all in page 25 of subframe 4. The first MSB of each field
shall be reserved. The 3 LSBs shall indicate the configuration of each satellite using the following code:
Code Satellite configuration
001 Block II satellite
3.1.1.3.3.6 UTC parameters. Page 18 of subframe 4 shall include:
a) the parameters needed to relate GPS time to UTC time; and
b) notice to the user regarding the scheduled future or past (relative to navigation message upload) value of the delta
time due to leap seconds (tLSF), together with the week number (WNLSF) and the day number (DN) at the end of
which the leap second becomes effective. “Day one” shall be the first day relative to the end/start of week and the
WNLSF value consists of the 8 LSBs of the full week number. The absolute value of the difference between the
untruncated WN and WNLSF values shall not exceed 127.
Note.— The user is expected to account for the truncated nature of this parameter as well as truncation of WN, WNt and
WNLSF due to rollover of the full week number (3.1.1.2.6.2).
3.1.1.3.3.6.1 The 24 MSBs of words 6 through 9, and the 8 MSBs of word 10 in page 18 of subframe 4 shall contain
the parameters related to correlating UTC time with GPS time. The bit length, scale factors, ranges, and units of these
parameters shall be as specified in Table B-11.
3.1.1.3.3.7 Ionospheric parameters. The ionospheric parameters that allow the GPS SPS user to utilize the ionospheric
model for computation of the ionospheric delay shall be contained in page 18 of subframe 4 as specified in Table B-12.
3.1.1.3.3.8 Special message. Page 17 of subframe 4 shall be reserved for special messages.
23/11/06 APP B-12
Appendix B Annex 10 — Aeronautical Communications
Table B-11. UTC parameters
Parameter
Number
of bits**
Scale factor
(LSB)
Effective
range*** Units
Ao 32* 2–30 seconds
A1 24* 2–50 seconds/second
ΔtLS 8* 1 seconds
tot 8 212 602 112 seconds
WNt 8 1 weeks
WNLSF 8 1 weeks
DN 8**** 1 7 days
ΔtLSF 8* 1 seconds
* Parameters so indicated are two’s complement, with the sign bit (+ or –) occupying the MSB.
** See Figure B-6 for complete bit allocation in subframe.
*** Unless otherwise indicated in this column, effective range is the maximum range attainable with the indicated bit
allocation and scale factor.
**** Right justified.
Table B-12. Ionospheric parameters
Parameter
Number
of bits**
Scale factor
(LSB)
Effective
range*** Units
α0 8* 2–30 seconds
α1 8* 2–27 seconds/semi-circle
α2 8* 2–24 seconds/semi-circle2
α3 8* 2–24 seconds/semi-circle3
β0 8* 211 seconds
β1 8* 214 seconds/semi-circle
β2 8* 216 seconds/semi-circle2
β3 8* 216 seconds/semi-circle3
* Parameters so indicated are two’s complement, with the sign bit (+ or –) occupying the MSB.
** See Figure B-6 for complete bit allocation in subframe.
*** Unless otherwise indicated in this column, effective range is the maximum range attainable with the indicated bit allocation
and scale factor.
3.1.1.3.3.9 Reserved data fields. All bits of words 3 through 10, except the 58 bits used for data ID, satellite (page) ID,
parity (six LSBs of each word) and parity computation (bits 23 and 24 of word 10) of pages 1, 6, 11, 12, 13, 14, 15, 16, 19,
20, 21, 22, 23 and 24 of subframe 4, and those almanac pages assigned satellite ID of zero shall be designated as reserved.
Other reserved bits in subframes 4 and 5 shall be as shown in Table B-13. Reserved bit positions of each word shall contain a
pattern of alternating ones and zeros with a valid word parity.
3.1.2 DEFINITIONS OF PROTOCOLS FOR DATA APPLICATION
Note.— This section defines the inter-relationships of the data broadcast message parameters. It provides definitions of
parameters that are not transmitted, but are used by either or both non-aircraft and aircraft elements, and that define terms
applied to determine the navigation solution and its integrity.
APP B-13 23/11/06
Annex 10 — Aeronautical Communications Volume I
Table B-13. Reserved bits in subframes 4 and 5
Subframe Pages Words Reserved bit position in word
4 17 10 17 – 22
4 18 10 19 – 22
4 25 8 17 – 18
4 25 10 19 – 22
5 25 10 14 – 22
Table B-14. Parity encoding algorithms
D1 = d1 ⊕ D*
30
D2 = d2 ⊕ D*
30
D3 = d3 ⊕ D*
30
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