GNSSDecoder.jl

A Julia package for decoding GNSS (Global Navigation Satellite System) navigation messages.

Supported Systems

• GPS L1 C/A: Decodes the 50 bps LNAV data stream from GPS L1 civil signals
• Galileo E1B: Decodes the 250 bps I/NAV data stream from Galileo E1B Open Service signals

Installation

using Pkg
Pkg.add("GNSSDecoder")

Quick Start

GPS L1 Decoding

Initialize a decoder and process bits from your tracking loop:

julia> using GNSSDecoder

julia> state = GPSL1DecoderState(1);  # Initialize decoder for PRN 1

julia> state.prn  # Access PRN
1

julia> typeof(state)
GNSSDecoderState{GNSSDecoder.GPSL1Data, GNSSDecoder.GPSL1Constants, GNSSDecoder.GPSL1Cache, GNSSDecoder.UInt320}

Process incoming bits and check the decoder state:

julia> state = decode(state, UInt8(0b10001011), 8);  # Decode 8 bits

julia> state.num_bits_buffered  # Bits are now buffered
8

In a real application, you would decode bits from a tracking loop:

for i in 1:iterations
    # Track signal (e.g., with Tracking.jl)
    track_res = track(signal, track_state, state.prn, sampling_freq)
    track_state = get_state(track_res)

    # Decode navigation message
    state = decode(state, get_bits(track_res), get_num_bits(track_res))
end

# After decoding completes, access the data
if !isnothing(state.data.TOW)
    println("Time of Week: $(state.data.TOW)")
end

Galileo E1B Decoding

julia> using GNSSDecoder

julia> state = GalileoE1BDecoderState(1);  # Initialize decoder for PRN 1

julia> state.prn
1

julia> typeof(state)
GNSSDecoderState{GNSSDecoder.GalileoE1BData, GNSSDecoder.GalileoE1BConstants, GNSSDecoder.GalileoE1BCache, GNSSDecoder.UInt288}

julia> state = decode(state, UInt16(0b0101100000), 10);  # Decode 10 bits (preamble)

julia> state.num_bits_buffered
10

State Management

Resetting After Signal Loss

If signal tracking is lost and reacquired, use reset_decoder_state to clear buffers while preserving previously decoded ephemeris:

julia> using GNSSDecoder

julia> state = GPSL1DecoderState(1);

julia> state = decode(state, UInt8(0xff), 8);  # Some decoding

julia> state.num_bits_buffered
8

julia> state = reset_decoder_state(state);  # Reset after signal loss

julia> state.num_bits_buffered  # Buffers are cleared
0

julia> state.prn  # PRN is preserved
1

Checking Satellite Health

julia> using GNSSDecoder

julia> state = GPSL1DecoderState(1);

julia> is_sat_healthy(state)  # Health not yet decoded
false

julia> state = GalileoE1BDecoderState(1);

julia> is_sat_healthy(state)  # Health not yet decoded
false

Data Fields

GPS L1 Data

After successful decoding, state.data contains:

Galileo E1B Data

Similar ephemeris and clock parameters are available for Galileo, plus: