Bit and Secondary-Code Sync

Until a signal's bit boundary (or, for a pilot, its secondary-code phase) is found it integrates one primary code period at a time. Sync unlocks longer coherent integrations (e.g. 20 ms for GPS L1 C/A), decoded bits, and the code phase modulo a whole symbol, which a pseudorange needs.

Users only need has_bit_or_secondary_code_been_found per signal (see the per-signal accessor table) to gate get_soft_bits. The detectors and the bit buffer are TrackingLoops'; see BitBuffer.

Code-phase wrap period

The shared TrackedSat.code_phase wraps at the least common multiple of the signals' wraps (#129), each of which depends on the signal's sync state:

• Before sync: its primary code length, since the bit or secondary chip of the current period is unknown.
• After sync: one symbol period, primary × secondary_code_length for a pilot and primary × blocks_per_data_bit for a data signal.

mod(code_phase, primary) is a signal's replica phase, div(code_phase, primary) its position within the symbol. max_code_length is the post-sync bound, current_code_wrap the runtime wrap.

max_code_length(signals)

current_code_wrap(signals)

Sync detection

Each signal's detector (detect_bit_or_secondary_code_sync) comes in one of three families:

• Soft, maximum-energy CFAR: GPS L1 C/A's bit edge and every short secondary code (1 < N ≤ 100). It locks when the peak hypothesis beats its runner-up at the confidence get_bit_edge_detection_confidence (default 0.999), needs two periods first, and fires on a period boundary, so it reports phase = 0.
• Hard rotation sweep: the 1800-chip overlay pilots GPS L1C-P and BeiDou B1C-P. It matches the prompt signs against the overlay at every rotation within get_bit_edge_or_secondary_code_tolerance, locks after one period at any alignment and reports the recovered chip phase.
• Trivial: one symbol per primary period (Galileo E1B, E6-B, GPS L1C-D, L2CM, BeiDou B2b-I, B1C-D), and Galileo E5a-QP (no data, no overlay), sync on the first block. GPS L2CL, a pilot without secondary code, never syncs and stays pre-sync.

Min-to-fire is the number of blocks before a detector can lock.

BeiDou B1I/B3I GEO satellites (PRN 1-5, 59-63) carry no NH20 overlay and broadcast D2, so the detector never locks on them: they track and range but stay pre-sync (see Known limitations).

The confidence (soft detectors) and the tolerance (hard sweep only) can be overridden per signal; the hard sweep reads its tolerance on every call, so no TrackState rebuild is needed:

julia> using Tracking, TrackingLoops, GNSSSignals

julia> TrackingLoops.get_bit_edge_or_secondary_code_tolerance(GPSL1C_P())  # default
0.025

julia> # Loosen the L1C-P ceiling to 5 % (= 90 errors over 1800 blocks) for low-C/N₀ work.
       TrackingLoops.get_bit_edge_or_secondary_code_tolerance(::GPSL1C_P) = 0.05;

julia> TrackingLoops.get_bit_edge_or_secondary_code_tolerance(GPSL1C_P())  # after override
0.05

Code-phase seeding

When a signal with a secondary code syncs, its secondary_phase snaps TrackedSat.code_phase into the right secondary-code window, by a multiple of the primary length; the within-period phase is kept (#117). Of several synced signals, the one with the longest primary × secondary length wins.

A signal without secondary code (e.g. GPS L1 C/A) needs no snap: its detector locks at a true bit boundary, so the widened wrap starts at the bit's first period, and the bit buffer's block counter, which reset preserves, keeps the bit cadence across track! calls.