feat(gpsl2c): add GPS L2C (CNAV) decoder

CONTEXT.md

@@ -15,6 +15,18 @@ code; this file is for naming and meaning only.
   non-systematic convolutional code (G1 = 171₈, G2 = 133₈), convolved
   *continuously* across message boundaries (no tail bits). Message = 300 bits
   = 600 channel symbols at 100 sps = 6 seconds (IS-GPS-705).
+- **GPS L2C** (`GPSL2CM`) — 25 bps CNAV broadcast on the L2 CM (civil-moderate)
+  code; the time-multiplexed L2 CL code is a dataless pilot. The CNAV message
+  is *bit-for-bit identical* to GPS L5I's (IS-GPS-200N §30 ≡ IS-GPS-705J
+  §20.3.3): same preamble, FEC, CRC-24Q, message-type layouts, π and `TOW × 6`.
+  Only the signal layer differs — 25 bps → 50 sps, 300-bit message = 600
+  symbols = 12 seconds — which is purely time-domain, so the symbol-domain
+  decoder is the same 600-symbol message / 616-symbol sync window. Shares the
+  GPS CNAV core (`src/gps/cnav.jl`) and the `GPSCNAVData` container with L5I;
+  the only decode difference is that `is_sat_healthy` reports the L2 health bit
+  (MT10 bit 53) instead of the L5 bit (54). `GNSSDecoderState(::GPSL2CM, prn)`
+  maps here (`GPSL2CL` is the pilot); `GPSL2CMDecoderState(prn)` is the
+  equivalent direct constructor.
 - **Galileo E1B** (`GalileoE1B`) — 250 sps I/NAV nominal pages over a K=7
   rate-1/2 convolutional code plus 30×8 block interleaver. Page = 250 channel
   symbols = 1 second. Two consecutive pages (even+odd) carry one word.
@@ -112,3 +124,6 @@ lives in the `cache`.
   domain (the FEC runs continuously across messages), so each sync attempt
   Viterbi-decodes the buffered 616-symbol window and requires the preamble at
   both ends of the decoded 308-bit window plus a clean CRC-24Q.
+- **L2C**: identical to L5I — the same shared GPS CNAV `try_sync`
+  Viterbi-decodes the 616-symbol window and gates on preamble + CRC-24Q. (The
+  symbol rate, 50 vs 100 sps, does not enter the symbol-domain sync.)

Project.toml

@@ -20,7 +20,7 @@ DataStructures = "0.18, 0.19"
 Dictionaries = "0.4"
 DocStringExtensions = "0.6, 0.7, 0.8, 0.9"
 Downloads = "1"
-GNSSSignals = "2.3"
+GNSSSignals = "2.6"
 Random = "1"
 Test = "1"
 Tracking = "2"

README.md

@@ -16,6 +16,7 @@ magnitudes need not be normalized.
 Currently implemented:
  * GPS L1 C/A (LNAV)
  * GPS L1C-D (CNAV-2)
+ * GPS L2C (CNAV)
  * GPS L5I (CNAV)
  * Galileo E1B (I/NAV)
  * Galileo E5a (F/NAV)

docs/src/api.md

@@ -14,6 +14,7 @@ GalileoE1BDecoderState
 GalileoE5aDecoderState
 GPSL1C_DDecoderState
 GPSL5IDecoderState
+GPSL2CMDecoderState
 ```
 
 ## Decoding
@@ -84,14 +85,22 @@ GPSL1C_DMidiAlmanac
 GPSL1C_DDifferentialCorrection
 ```
 
-### GPS L5I
+### GPS CNAV (shared by L5I and L2C)
+
+GPS L5I and GPS L2C carry the identical CNAV message, so they share the decoded
+[`GPSCNAVData`](@ref) container (and its almanac/correction records) and one
+constants struct, [`GPSCNAVConstants`](@ref GNSSDecoder.GPSCNAVConstants). The
+per-signal constants are type aliases that fix its signal tag; they differ only
+in which signal-health bit [`is_sat_healthy`](@ref) reports.
 
 ```@docs
+GNSSDecoder.GPSCNAVConstants
 GNSSDecoder.GPSL5IConstants
-GPSL5IData
-GPSL5IReducedAlmanac
-GPSL5IMidiAlmanac
-GPSL5IClockDifferentialCorrection
-GPSL5IEphemerisDifferentialCorrection
-GPSL5IIntegritySupportMessage
+GNSSDecoder.GPSL2CMConstants
+GPSCNAVData
+GPSCNAVReducedAlmanac
+GPSCNAVMidiAlmanac
+GPSCNAVClockDifferentialCorrection
+GPSCNAVEphemerisDifferentialCorrection
+GPSCNAVIntegritySupportMessage
 ```

docs/src/index.md

@@ -6,6 +6,7 @@ A Julia package for decoding GNSS (Global Navigation Satellite System) navigatio
 
 - **GPS L1 C/A**: Decodes the 50 bps LNAV data stream from GPS L1 civil signals
 - **GPS L1C-D**: Decodes the 100 sps CNAV-2 data stream from the modernized GPS L1C signal's data component
+- **GPS L2C**: Decodes the 50 sps CNAV data stream from the GPS L2 CM (civil-moderate) signal component
 - **GPS L5I**: Decodes the 100 sps CNAV data stream from the GPS L5 in-phase signal component
 - **Galileo E1B**: Decodes the 250 bps I/NAV data stream from Galileo E1B Open Service signals
 
@@ -128,7 +129,31 @@ julia> state.prn
 1
 
 julia> typeof(state)
-GNSSDecoderState{GPSL5IData, GNSSDecoder.GPSL5IConstants, GNSSDecoder.GPSL5ICache}
+GNSSDecoderState{GPSCNAVData, GNSSDecoder.GPSCNAVConstants{:GPSL5I}, GNSSDecoder.GPSCNAVCache}
+
+julia> state = decode(state, Float32[+1, -1, +1, +1, -1, -1, -1, -1, -1, +1], 10);  # Decode 10 soft symbols
+
+julia> GNSSDecoder.num_bits_buffered(state)
+10
+```
+
+### GPS L2C Decoding
+
+GPS L2C broadcasts the *same* CNAV message as GPS L5I (IS-GPS-200N §30), on the
+L2 CM component at 50 sps. Decoding therefore reuses the shared GPS CNAV core,
+and decoded fields land in the same [`GPSCNAVData`](@ref) container; only the
+health check differs (it reports the L2 signal-health bit):
+
+```jldoctest l2c_example
+julia> using GNSSDecoder
+
+julia> state = GPSL2CMDecoderState(1);  # Initialize decoder for PRN 1
+
+julia> state.prn
+1
+
+julia> typeof(state)
+GNSSDecoderState{GPSCNAVData, GNSSDecoder.GPSCNAVConstants{:GPSL2CM}, GNSSDecoder.GPSCNAVCache}
 
 julia> state = decode(state, Float32[+1, -1, +1, +1, -1, -1, -1, -1, -1, +1], 10);  # Decode 10 soft symbols
 

src/GNSSDecoder.jl

@@ -11,12 +11,13 @@ export decode,
     GPSL1C_DMidiAlmanac,
     GPSL1C_DDifferentialCorrection,
     GPSL5IDecoderState,
-    GPSL5IData,
-    GPSL5IReducedAlmanac,
-    GPSL5IMidiAlmanac,
-    GPSL5IClockDifferentialCorrection,
-    GPSL5IEphemerisDifferentialCorrection,
-    GPSL5IIntegritySupportMessage,
+    GPSCNAVData,
+    GPSCNAVReducedAlmanac,
+    GPSCNAVMidiAlmanac,
+    GPSCNAVClockDifferentialCorrection,
+    GPSCNAVEphemerisDifferentialCorrection,
+    GPSCNAVIntegritySupportMessage,
+    GPSL2CMDecoderState,
     GalileoE1BDecoderState,
     GalileoE5aDecoderState,
     is_sat_healthy,
@@ -66,9 +67,17 @@ include("galileo/e5a.jl")
 # `decode` framework hooks (`try_sync`, `decode_syncro_sequence`, …).
 include("gps/l1c_d.jl")
 
-# GPS L5I (CNAV) decoder. Included after `gps/l1ca.jl` (reuses its `UInt320`
-# packed-word type) and after `gps/l1c_d.jl` (reuses the `_deque_slice` and
-# `_merge_keyed` helpers), and consumes `crc24q` from the shared utilities
-# above.
+# Shared GPS CNAV core (the 300-bit message broadcast identically on GPS L5I,
+# IS-GPS-705J §20.3, and GPS L2C, IS-GPS-200N §30): FEC, window-Viterbi sync,
+# CRC-24Q, and all per-message-type parsing, plus the shared `GPSCNAVData`
+# container. Consumes only shared utilities included above — `crc24q` and the
+# `UInt320` / `_merge_keyed` primitives in `gnss.jl` — so it is independent of
+# the sibling signal files.
+include("gps/cnav.jl")
+
+# GPS L5I and GPS L2C signal layers — each a thin wrapper over `gps/cnav.jl`
+# (their own `*Constants` type, decoder-state constructor, and `is_sat_healthy`
+# health-bit selection). Included after the shared core they consume.
 include("gps/l5.jl")
+include("gps/l2c.jl")
 end

src/gnss.jl

@@ -357,3 +357,25 @@ function decode(
     end
     return state
 end
+
+# ---- Shared GPS decoder primitives ------------------------------------------
+#
+# Signal-agnostic primitives used by more than one GPS signal decoder. They
+# live here (a shared file included before every signal) rather than in a
+# per-signal file so that no signal decoder has to be included after another
+# just to borrow them.
+
+# Packed-word integer type shared across the GPS decoders: a GPS L1 C/A
+# subframe, a GPS L1C-D subframe, and a GPS CNAV message (GPS L5I / L2C) each
+# pack into a single `UInt320` — 300 data bits plus up to 8 trailing sync bits.
+# `BitIntegers.@define_integers` also defines the signed companion `Int320`.
+BitIntegers.@define_integers 320
+
+"""
+Insert/overwrite `value` keyed by `key` in a (possibly `nothing`) `Dictionary`, returning the updated copy.
+"""
+function _merge_keyed(dict::Union{Nothing,Dictionary{Int,V}}, key::Int, value::V) where {V}
+    out = isnothing(dict) ? Dictionary{Int,V}() : copy(dict)
+    set!(out, key, value)
+    return out
+end