GPS protocol

Scope

For an RF challenge focused on legacy GPS LNAV as seen by a basic receiver (L1 C/A, L2 P(Y)). No L2C/L5.

Structure (navigational message)

Subframe (SF1..SF5): 10 words × 30 bits = 300 bits = 6 s.
Frame: 5 subframes = 1500 bits = 30 s (SF1..SF5).
Superframe: 25 frames = 12.5 min.

Each 30-bit word = 24 data bits + 6 parity bits (shortened Hamming). Tables below list the 24 data bits only; every word carries +6 parity at the end.

Legend (units, scales, sign)

Angles are in semi-circles unless stated otherwise. Convert: radians = value × π.
Times:
- TOW (HOW): start time of the next subframe, step 6 s (17 bits, truncated).
- t_oc, t_oe: seconds-of-week, step 16 s (16 bits).
- WN: GPS week mod 1024 (since 1980-01-06).
Scales (ICD style):
- Δn: semi-circles/s, scale 2^-43 (16 b).
- M₀, ω, Ω₀, i₀: semi-circles, scale 2^-31 (32 b).
- Ω̇, IDOT: semi-circles/s, scale 2^-43 (24/14 b).
- e: unitless eccentricity, scale 2^-33 (32 b).
- √A: sqrt(m), scale 2^-19 (32 b).
- C_rs, C_rc: meters, scale 2^-5 (16 b signed).
- C_us, C_uc, C_is, C_ic: radians, scale 2^-29 (16 b signed).
- TGD: seconds, int8 × 2^-31 s (8 b signed).
Signed fields are two’s-complement.
Padding (t): tail bits with no nav meaning (used by the parity mechanism).

Header - Common to each frames

Word 1 (TLM): Preamble(8) | Message(14) | Integrity(1) | Reserved(1)

Word 2 (HOW): TOW(17) | Alert(1) | Anti-Spoof(1) | ID(3) | Padding(t,2)

Header notes

Preamble: 10001011
Integrity: 0 = legacy, 1 = enhanced
TOW: start of next subframe (6 s step)
Alert: 1 ⇒ URA not guaranteed
Anti-Spoof: 1 ⇒ P-code encrypted (Y-code)
ID: subframe index 1..5

Subframe 1 — Clock & status

Word 3: WN(10) | L2 code(2) | URA(4) | Health(6) | IODC

Word 4: L2 P-data flag(1) | Reserved(23)

Word 5: Reserved(24)

Word 6: Reserved(24)

Word 7: Reserved(16) | TGD(8)

Word 8: IODC | t_oc(16)

Word 9: af2(8) | af1(16)

Word 10: af0(22) | Padding(t,2)

Clock model & statuses

L2 code: 01 P-code on L2; 10 C/A on L2 (others invalid).
URA: user range accuracy index (error bound class).
Health: SV health flags (L1/L2).
IODC: Issue Of Data — Clock (version of clock dataset).
Clock correction (for transmit time t): Δt_sv(t) = af0 + af1·(t − t_oc) + af2·(t − t_oc)^2

Subframe 2 — Ephemeris (1/2)

Word 3: IODE(8) | C_rs(16)

Word 4: Δn(16) | M₀

Word 5: M₀

Word 6: C_uc(16) | e

Word 7: e

Word 8: C_us(16) | √A

Word 9: √A

Word 10: t_oe(16) | Fit(1) | AODO(5) | Padding(t,2)

Notes (SF2)

IODE: ephemeris dataset version (should match IODC LSBs when clock/orbit uploaded together).
Fit: 0 = 4 h fit interval, 1 = > 4 h.
AODO: Age Of Data Offset (900 s step).

Subframe 3 — Ephemeris (2/2)

Word 3: C_ic(16) | Ω₀

Word 4: Ω₀

Word 5: C_is(16) | i₀

Word 6: i₀

Word 7: C_rc(16) | ω

Word 8: ω

Word 9: Ω̇

Word 10: IODE(8) | IDOT(14) | Padding(t,2)

Notes (SF3)

Ω₀ / Ω̇: RAAN at epoch / RAAN rate.
i₀ / IDOT: inclination at epoch / rate.
ω: argument of perigee.
Harmonics: C_ic, C_is (inclination); C_rs, C_rc (radius); C_uc, C_us (arg. latitude).

Subframes 4–5

Constellation/almanac pages — not required for my use case.

/RF/ /satellite/