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3. GNSS ELEMENTS
3.1 Global Positioning System (GPS)
Standard Positioning Service (SPS) (L1)
3.1.1 NON-AIRCRAFT ELEMENTS
3.1.1.1 RADIO FREQUENCY (RF) CHARACTERISTICS
3.1.1.1.1 Carrier phase noise. The carrier phase noise spectral density of the unmodulated carrier shall be such that a
phase locked loop of 10 Hz one-sided noise bandwidth is able to track the carrier to an accuracy of 0.1 radian (1 sigma).
3.1.1.1.2 Spurious emissions. In-band spurious emissions shall be at least 40 dB below the unmodulated L1 carrier over
the allocated channel bandwidth.
3.1.1.1.3 Correlation loss. The loss in the recovered signal power due to imperfections in the signal modulation and
waveform distortion shall not exceed 1 dB.
Note.— The loss in signal power is the difference between the broadcast power in a 2.046 MHz bandwidth and the signal
power recovered by a noise-free, loss-free receiver with 1-chip correlator spacing and a 2.046 MHz bandwidth.
ANNEX 10 — VOLUME I APP B-1 23/11/06
Annex 10 — Aeronautical Communications Volume I
3.1.1.1.4 Coarse/acquisition (C/A) code generation and timing. Each C/A code pattern Gi(t) shall be formed by the
Modulo-2 sum of two 1 023-bit linear patterns, G1 and G2i. The G2i sequence shall be formed by effectively delaying the G2
sequence by an integer number of chips to produce one of 36 unique Gi(t) patterns defined in Table B-1. The G1 and G2
sequences shall be generated by 10-stage shift registers having the following polynomials as referred to in the shift register
input:
a) G1: X10 + X3 + 1; and
b) G2: X10 + X9 + X8 + X6 + X3 + X2 + 1.
The initialization vector for the G1 and G2 sequences shall be “1111111111”. The code phase assignments shall be as shown
in Table B-1. The G1 and G2 registers shall be clocked at a 1.023 MHz rate. Timing relationships related to the C/A code
shall be as shown in Figure B-1.*
3.1.1.2 Data structure. The navigation message shall be formatted as shown in Figure B-2. Each page, as shown in
Figure B-6, shall utilize a basic format of a 1 500-bit-long frame with up to 5 subframes, each of 300 bits in length. All words
shall be transmitted most significant bit (MSB) first.
3.1.1.2.1 Subframe structure. Each subframe and/or page of a subframe shall start with a telemetry (TLM) word
followed by a handover word (HOW). The HOW shall be followed by 8 data words. Each word in each frame shall contain
6 parity bits. The TLM word and HOW formats shall be as shown in Figures B-3 and B-4, respectively.
3.1.1.2.2 End/start of week. At the end/start of week:
a) the cyclic paging of subframes 1 through 5 shall restart with subframe 1 regardless of which subframe was last
transmitted prior to the end/start of week; and
b) the cycling of 25 pages of subframes 4 and 5 shall restart with page 1 of each of the subframes, regardless of which
page was transmitted prior to the end/start of week. All upload and page cutovers shall occur on frame boundaries
(i.e. Modulo 30 seconds relative to the end/start of week).
Note.— New data in subframes 4 and 5 may start to be transmitted with any of the 25 pages of these subframes.
3.1.1.2.3 Data parity. Words 1 through 10 of subframes 1 through 5 shall each contain 6 parity bits as their least
significant bits (LSBs). In addition, two non-information bearing bits shall be provided as bits 23 and 24 of words 2 and 10
for parity computation purposes.
3.1.1.2.4 Telemetry (TLM) word. Each TLM word shall be 30 bits long, occur every 6 seconds in the data frame and be
the first word in each subframe. The TLM format shall be as shown in Figure B-3. Each TLM word shall begin with a
preamble, followed by 16 reserved bits and 6 parity bits.
3.1.1.2.5 Handover word (HOW). The HOW shall be 30 bits long and shall be the second word in each subframe/page,
immediately following the TLM word. A HOW shall occur every 6 seconds in the data frame. The HOW format and content
shall be as shown in Figure B-4. The full time-of-week (TOW) count shall consist of the 19 LSBs of the 29-bit Z-count
(3.1.1.2.6). The HOW shall begin with the 17 MSBs of the TOW count. These 17 bits shall correspond to the TOW count at
the 1.5-second epoch that occurs at the start (leading edge) of the next following subframe.
3.1.1.2.5.1 Bit 18. On satellites designed by configuration code 001, bit 18 shall be an “alert” flag. When this flag is
raised (bit 18 is a “1”), it shall indicate to the user that the satellite user range accuracy (URA) may be worse than indicated in
subframe 1 and that use of the satellite is at the user’s risk.
*All figures are located at the end of the appendix.
23/11/06 APP B-2
Appendix B Annex 10 — Aeronautical Communications
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