Matlab Generate Gnss Waveform

Generate GNSS Waveform

Generate multi-satellite GNSS baseband waveforms with properly encoded navigation data. Channel impairments (Doppler, delay, noise) can come from a realistic satellite scenario OR from user-specified custom values. Supports GPS, Galileo, and NavIC constellations. The waveform carries real ephemeris so a receiver can decode it for position estimation.

When to Use

• Generating GNSS baseband signals (GPS, Galileo, or NavIC) for receiver testing
• Simulating multi-satellite channels with Doppler and delay from orbital dynamics
• Testing with specific Doppler/delay/SNR values (custom channel mode)
• Building end-to-end GNSS receiver simulations that need decodable nav data

When NOT to Use

• Signal acquisition, tracking loops, nav decoding, or position estimation
• BeiDou/GLONASS (not supported) or SDR hardware transmission

Prerequisites: Helper Files

Copy helper files from MathWorks examples into your project folder. See references/helper-files.md for the full list of source examples and files to copy per constellation (GPS, Galileo, NavIC).

Workflow

The pipeline has 6 steps (0-5). Step 0 determines signal type, channel mode, and data source. The channel mode decides whether Steps 2-3 are needed.

Step 0: Determine signal type + channel mode + data source + nav data intent
Step 1: Configure waveform generator
         ├── Scenario mode → Step 2 (satellite scenario) + Step 3 (compute channel from physics)
         └── Custom mode   → Step 3c (user provides Doppler/delay/SNR directly)
Step 4: Encode navigation data (both modes)
Step 5: Generate waveform with channel (both modes)

Step 0: Determine signal type, channel mode, and data source

If the user's prompt does not clearly specify the signal type, ask:

What GNSS waveform would you like to generate?

GPS: L1 C/A, L1C, L2C, L5 Galileo: E1, E1C (pilot only), E5a, E5b, E5 (wideband) NavIC: L5, S, L1

Also, how should channel impairments (Doppler shift, propagation delay, noise) be applied?

(a) Scenario-driven — I compute realistic values from a satellite scenario. You provide a receiver location (lat/lon) and I handle the orbital dynamics.

(b) Custom values — You provide Doppler (Hz), delay (s), and/or SNR (dB) per satellite. Useful for controlled testing or when you already have your own channel model.

Detection heuristics: Location/city/lat-lon → Scenario. Specific Doppler/delay/SNR numbers → Custom. "Stress test" or "extreme" → Custom. No cues → Ask. If signal type is clear but mode ambiguous, ask only about mode.

Data source selection

Available: gpsAlmanac.txt (always on path), RINEX .rnx files (Nav Toolbox). Require setup: galileoAlmanac.xml (copy from example), fresh almanac (download from USCG NAVCEN).

Decision rules:

1. User provides RINEX → use for both scenario AND nav data.
2. No RINEX → use bundled almanac (gpsAlmanac.txt or galileoAlmanac.xml).
3. Current-epoch needed → offer fresh almanac download (requires internet).
4. RINEX requested but no Nav Toolbox → fall back to almanac or parse manually (see references/rinex-integration.md).

All bundled files are from a fixed past epoch. This is fine for receiver algorithm testing (orbital geometry is still valid) but does not represent today's actual sky. The agent MUST communicate which file and epoch is being used.

Navigation data intent

Navigation data can be:

• Real ephemeris (from almanac/RINEX) — required for end-to-end receiver testing where position estimation must work
• Random/dummy bits — valid for signal-level testing (acquisition, tracking, modulation quality) where nav decode is not the goal

If user's intent is unclear, ask:

Do you need the waveform to carry real navigation data (for position estimation), or are dummy bits fine (for signal-level testing)?

STOP — Present choices before generating

Before writing or executing ANY code, print a standalone configuration summary stating: signal type, center frequency, sample rate, duration, data source + epoch, start time, and nav data type. This MUST appear as visible text BEFORE any code block — not buried in comments or post-run summaries. In interactive mode, wait for user confirmation.

Step 1: Configure waveform generator (constellation-specific)

GPS:

sampleRate = 5e6;
wavegenobj = gpsWaveformGenerator(SampleRate=sampleRate);
wavegenobj.SignalType = "legacy";  % L1 C/A
navDataType = "LNAV";
centerFrequency = 1575.42e6;
stepTime = wavegenobj.BitDuration;

Verify: stepTime should be 0.02 (20 ms) for L1 C/A, 0.01 for L1C/L5.

Galileo:

sampleRate = 25e6;  % Use 60e6 for E5 wideband
wavegenobj = galileoWaveformGenerator(SampleRate=sampleRate);
wavegenobj.SignalType = "E1";  % or "E1C","E5a","E5b","E5"
centerFrequency = 1575.42e6;  % E5 uses 1191.795e6
stepTime = wavegenobj.BitDuration;

Verify: stepTime should be 0.004 (4 ms) for E1/E1C/E5b, 0.02 for E5a/E5.

E5 has critical pitfalls — nav data must be a {fnavbits, inavbits} cell array (not a matrix), and I/NAV needs 5× more rows than F/NAV. See references/galileo-pipeline.md E5 section.

NavIC (R2026a -- no system object yet):

sampleRate = 24e6;
centerFrequency = 1176.45e6;  % L5 (or 2492.028e6 for S, 1575.42e6 for L1)
stepTime = 0.02;  % 20 ms for L5/S, 10 ms for L1

NavIC L1 is completely different from L5/S (SBOC, IZ4 codes, support file download). See references/navic-pipeline.md §NavIC L1 SPS.

See references/gps-pipeline.md, references/galileo-pipeline.md, or references/navic-pipeline.md for full signal type tables.

Step 2: Set up satellite scenario (SCENARIO MODE ONLY)

Skip this step entirely in custom mode.

Create the scenario, ground station, and timing (shared). Then add satellites using the constellation-specific method.

sc = satelliteScenario;
rx = groundStation(sc, lat, lon, Altitude=alt);
rx.MinElevationAngle = 10;

waveDuration = 10;  % seconds
sc.StartTime  = startTime;
sc.StopTime   = sc.StartTime + seconds(waveDuration - stepTime);
sc.SampleTime = stepTime;

Add satellites (constellation-specific). Default to almanac unless the user explicitly has a RINEX file or asks for RINEX. The almanac path avoids a Navigation Toolbox dependency (rinexread). The same source must feed Step 4 (nav data encoding).

GPS (almanac path — preferred):

sat = satellite(sc, "gpsAlmanac.txt", OrbitPropagator="gps");

GPS (RINEX path): Check ~isempty(ver('nav')) first. If Navigation Toolbox is unavailable, use almanac or parse RINEX manually (see references/rinex-integration.md).

rinexdata = rinexread(rinexFileName);
sat = satellite(sc, rinexdata, OrbitPropagator="gps");

Galileo (almanac path — preferred) -- satellite() does NOT accept Galileo XML almanac files. Use the helper:

sat = HelperAddGalileoSatellitesToScenario(sc, "galileoAlmanac.xml");

Galileo (RINEX path): Same toolbox check; fall back to almanac if unavailable.

rinexdata = rinexread(rinexFileName);
sat = satellite(sc, rinexdata);

NavIC -- RINEX only (requires Navigation Toolbox). Use HelperAddSatellite(sc, selectedRows) — see references/navic-pipeline.md.

Critical: Use the same startTime and data source (almanac OR RINEX) for both orbit propagation (this step) and nav data encoding (Step 4).

Verify: After adding satellites, check sat is non-empty. For GPS almanac, expect ~30 satellites total in the scenario.

Step 3: Compute channel parameters from physics (SCENARIO MODE ONLY)

Skip this step entirely in custom mode — go to Step 3c instead.

dopShifts = dopplershift(sat, rx, Frequency=centerFrequency).';
ltncy = latency(sat, rx).';

Compute SNR from free-space path loss:

DtLin = db2pow(12);           % Transmit antenna directivity
DrLin = db2pow(4);            % Receive antenna directivity
k = physconst("boltzmann");
T = 300;                      % Temperature (K)
Pr = Pt*DtLin*DrLin ./ ((4*pi*(centerFrequency + dopShifts).*ltncy).^2);
snrs = 10*log10(Pr/(k*T*sampleRate)) + 3;

Find visible satellites (non-NaN latency at first time step):

satIndices = find(~isnan(ltncy(1,:)));

Verify: numel(satIndices) should be >= 4 (minimum for positioning). If zero, the start time does not match the data source epoch (Gotcha #12).

Step 3c: Set up custom channel parameters (CUSTOM MODE ONLY)

Skip this step entirely in scenario mode.

The user provides Doppler, delay, and/or SNR values directly.

numSats = 4;
dopShifts = [2000, -1500, 800, 3200];      % Hz per satellite
ltncy = [0.072, 0.074, 0.069, 0.078];      % seconds per satellite
snrs = [-10, -12, -11, -9];                 % dB per satellite

% For time-varying custom values, use matrices (numsteps x numSats)
% dopShifts = [...];  % each row is one time step

waveDuration = 10;  % seconds
numsteps = round(waveDuration / stepTime);
satIndices = 1:numSats;

Required from user: number of satellites, Doppler (Hz), delay (s), PRN/SVID list. Optional (defaults): SNR (-10 dB), duration (10 s), time-varying (no, static).

Expand static values for the generation loop:

if isvector(dopShifts) && size(dopShifts,1) == 1
    dopShifts = repmat(dopShifts, numsteps, 1);
    ltncy = repmat(ltncy, numsteps, 1);
    snrs = repmat(snrs, numsteps, 1);
end

No physical constraint validation in custom mode. The user may intentionally provide extreme values. Do NOT assert against physical ranges.

Step 4: Encode navigation data (BOTH MODES, constellation-specific)

Navigation data carries ephemeris the receiver needs for position estimation.

If user needs real ephemeris (position estimation):

• Use the same data source as the scenario (Step 2)
• If RINEX was used for the scenario, use RINEX for nav data too (preferred)
• Almanac supplements non-ephemeris fields only (health bits, almanac pages)
• Start time for nav data encoding MUST match scenario start time — mismatched times produce nav bits that don't correspond to satellite positions, causing silent position estimation errors

If user only needs signal-level testing (acquisition/tracking):

• Random bits are acceptable: navdata = randi([0 1], numsteps, numSats);
• No almanac or RINEX files needed at all
• Waveform will have correct modulation and channel but nav decode will fail

Critical: When RINEX was used for orbit propagation in Step 2, you MUST use HelperGPSRINEX2Config (not HelperGPSAlmanac2Config) for nav data encoding. The same RINEX struct feeds both orbit and ephemeris. Falling back to almanac for nav data while using RINEX for the scenario violates the single-source principle and may produce inconsistent ephemeris.

Scenario mode with real ephemeris: satIndices comes from visible satellite computation in Step 3.

Custom mode with real ephemeris: Use almanac as the data source (no Navigation Toolbox needed). The user must specify which PRN/SVID values to encode. Use the almanac path examples below.

GPS (almanac path):

navcfg = HelperGPSAlmanac2Config(almFileName, navDataType, satIndices, startTime);
visiblePRN = [navcfg(:).PRNID];

% Encode nav data per satellite (HelperGPSNAVDataEncode accepts scalar config only)
tempnavdata = HelperGPSNAVDataEncode(navcfg(1));
navdata = zeros(length(tempnavdata), length(navcfg));
navdata(:,1) = tempnavdata;
for isat = 2:length(navcfg)
    navdata(:,isat) = HelperGPSNAVDataEncode(navcfg(isat));
end

GPS (RINEX path) -- see references/gps-pipeline.md for the full HelperGPSRINEX2Config pattern.

Galileo (almanac path): Use the nav data signal type mapping from references/galileo-pipeline.md (E1→E1B, E5a→E5a, E5b→E5b):

galNavObj = HelperGalileoNavigationData(almFileName, true, SignalType="E1B");
visibleSVIDs = galNavObj.Ephemeris.SatelliteID(satIndices);
[~, inittow] = HelperGPSConvertTime(startTime);
navdata = navdata2bits(galNavObj, visibleSVIDs, inittow, numpages);

Galileo (RINEX path) -- see references/galileo-pipeline.md.

NavIC -- RINEX only. HelperNavICRINEX2Config takes the RINEX data table directly (not a filename):

navicCfg = HelperNavICRINEX2Config(selectedRows);

See the constellation-specific reference files for complete encoding patterns.

Step 5: Generate waveform with channel (BOTH MODES)

This step is identical for both scenario and custom modes. By Step 5, dopShifts, ltncy, and snrs are all numsteps x numSats matrices regardless of how they were obtained.

GPS and Galileo use their system object in a stepped loop:

wavegenobj.PRNID = visiblePRN;  % GPS uses PRNID
% wavegenobj.SVID = visibleSVID;  % Galileo uses SVID

gnsschannelobj = HelperGNSSChannel( ...
    FrequencyOffset=dopShifts(1,satIndices), ...
    SignalDelay=ltncy(1,satIndices), ...
    SignalToNoiseRatio=snrs(1,satIndices), ...
    SampleRate=sampleRate, ...
    RandomStream="mt19937ar with seed", Seed=73);

numsteps = round(waveDuration / stepTime);
samplesPerStep = sampleRate * stepTime;
waveform = zeros(numsteps * samplesPerStep, 1);

for istep = 1:numsteps
    idx = (istep-1)*samplesPerStep + (1:samplesPerStep);
    waveform(idx,:) = gnsschannelobj(wavegenobj(navdata(istep,:)));
    gnsschannelobj.SignalToNoiseRatio = snrs(istep, satIndices);
    gnsschannelobj.FrequencyOffset = dopShifts(istep, satIndices);
    gnsschannelobj.SignalDelay = ltncy(istep, satIndices);
end

NavIC (R2026a) uses HelperNavICBBWaveform or manual modulation (no system object). The helper returns multi-column output (one per satellite) -- pass directly to HelperGNSSChannel, do not sum first. See references/navic-pipeline.md for the full pattern.

Verify: length(waveform) == numsteps * samplesPerStep. Waveform should not be all-zero (would indicate channel SNR too low or nav data mismatch).

Signal Type Quick Reference

See references/signal-type-mapping.md for full tables of SignalType strings, center frequencies, BitDuration, and nav data types per constellation.

Verification (Scenario Mode Only)

After generating a waveform in scenario mode, verify channel parameters are physically realistic. See references/physical-constraints.md for the full verification function and expected ranges per constellation. Key checks: at least 4 visible satellites, Doppler within ±10 kHz (L1), delay 67-86 ms (MEO) or 119-130 ms (GEO/GSO), distinct Doppler per satellite, and time-varying Doppler.

Do not run these checks in custom mode — the user may intentionally provide values outside physical ranges.

Key Functions

Gotchas

1. Random nav bits only if user explicitly agrees. randi([0 1], ...) is acceptable for signal-level testing (acquisition, tracking) but makes position estimation impossible. If user needs end-to-end receiver testing, use the constellation-specific nav data encoder for real ephemeris. Always confirm nav data intent in Step 0.

2. In scenario mode, never hand-roll Doppler or delay. Do not use exp(1j*2*pi*f*t) for Doppler or circshift for delay. Use satelliteScenario -> dopplershift() -> latency() -> HelperGNSSChannel for physics-based channel modeling. In custom mode, user-provided values are passed directly to HelperGNSSChannel.

3. Nav data has more rows than numsteps. Nav data encoders produce the full message (e.g., 37500 rows for GPS LNAV) because they encode the complete nav frame structure. Only the first numsteps rows are consumed in the generation loop. Do not resize or assert equality — excess rows are simply unused.

4. RINEX: pass struct, not filename. satellite(sc, rinexData) expects the struct from rinexread(), not the .rnx filename string. Passing the filename causes a confusing "invalid orbit data" error.

5. Center frequency must match signal type. L1/E1 = 1575.42 MHz, L2 = 1227.60 MHz, L5/E5a = 1176.45 MHz, E5b = 1207.14 MHz, E5 = 1191.795 MHz, S = 2492.028 MHz. Must match in dopplershift().

6. GPS uses PRNID, Galileo uses SVID. Using the wrong property name silently creates a dynamic property on the system object — the waveform generator ignores it and generates with default satellite IDs, producing incorrect spreading codes.

7. Galileo transmit power is 160 W, not GPS's 44.8 W. Using GPS power for Galileo gives incorrect SNR. NavIC uses 50 W.

8. NavIC has no system object in R2026a. Calling navicWaveformGenerator will error. Use manual modulation (interplexmod, bocmod, gnssCACode) or the HelperNavICBBWaveform helper. Ships as a system object in R2026b.

9. satellite() only accepts GPS SEM almanac natively. Passing a Galileo XML or NavIC RINEX file directly to satellite() errors or produces wrong orbits. Use the constellation-specific helpers in Step 2.

10. GPS L5 and L1C: not all satellites broadcast modernized signals. Block IIF+ carry L5; Block III+ carry L1C. Expect ~4-6 visible vs ~8-12 for L1 C/A.

11. Filter PRN/SVID against nav data availability. Intersect visible satellites with the nav data object's available IDs before encoding.

12. Time/epoch consistency is critical. Three times must align: sc.StartTime, the nav data encoder's startTime, and the data source epoch. If sc.StartTime is far from the almanac/RINEX epoch, latency() returns all-NaN (zero visible satellites, no error). If nav data startTime differs from sc.StartTime, ephemeris won't match satellite positions — causing silent position errors. Bundled files are from a fixed past epoch; always communicate which file and epoch is used.

Conventions

• datetime with TimeZone="UTC" for all GNSS time references
• Name-value syntax (Name=Value), not legacy 'Name', Value pairs
• GPS time conversion: HelperGPSConvertTime; Galileo/NavIC: HelperGNSSConvertTime
• Always transpose dopplershift/latency output (.';) — they return column-per-timestep, but the generation loop indexes row-per-timestep
• Pre-allocate the waveform: zeros(numsteps * samplesPerStep, 1)
• Use seconds() for duration arithmetic (not bare numbers)
• Signal type strings are case-sensitive: "legacy" not "Legacy", "l5" not "L5"
• Prefer find(~isnan(ltncy(1,:))) to detect visible satellites — simpler and more robust than elevation angle filtering

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