ADS-B and Mode-S

The core library provides pulse-processing helpers, an observable message parser, and ADS-B/Mode-S message models. RF acquisition and message interpretation are separate layers: receiving a block of IQ data does not itself produce aircraft tracks.

Parser Input

AdsbMessageParser.ProcessSample(byte) consumes 0/1 pulse decisions at half-bit resolution, including the preamble. It does not accept packed Mode-S frame bytes, ASCII hexadecimal messages, or raw IQ values. Preserve pulse order and use one parser instance per independent input stream.

The parser checks frame CRC, obtains a message identifier, creates a registered model, and publishes deserialized messages. OnError reports parser failures. Registering default messages adds ADS-B identification, position, velocity, status, and related extended-squitter models. It does not register every Mode-S class in the repository. Unknown message types are reported as errors; add a suitable factory with Register only when its model matches the identifier being decoded.

using System;
using System.Reactive.Linq;
using Asv.Sdr;

using var parser = new AdsbMessageParser().RegisterDefaultMessages();
using var messages = parser.OnMessage.Subscribe(Console.WriteLine);
using var errors = parser.OnError.Subscribe(error =>
    Console.Error.WriteLine(error.Message));

// Feed each 0/1 half-bit decision, including the preamble, to parser.ProcessSample.

For typed consumers, use parser.Filter<AdsbAircraftIdentification>() or apply System.Reactive operators to OnMessage. Keep callbacks short and handle parser errors independently from device acquisition errors.

From IQ to Pulse Decisions

The helper chain is:

IQ source -> Magnitude -> AdsbPulseDetector -> AdsbNormalize
          -> AdsbPulseTruncate -> ProcessSample -> OnMessage / OnError

AdsbPulseDetector(sampleRate), AdsbNormalize(sampleRate), and AdsbPulseTruncate(sampleRate) derive pulse width with integer division by 2,000,000. Supply an appropriate integer multiple of that rate. For the current majority decision in AdsbPulseTruncate, use at least two samples per half-bit (for example 4,000,000 IQ pairs/s); a one-sample half-pulse has a zero integer majority threshold. These helpers are not general-purpose resamplers.

Magnitude() retains the interleaved layout (magnitude, 0). The detector reads only the even-index amplitude values and emits a packed scalar pulse buffer. Normalization and truncation then operate on that packed output. Do not pack the magnitudes before AdsbPulseDetector: it would skip every second amplitude. The following method expects a sampled IQ stream at 4 MHz and attaches the pulse stages to an existing parser:

using System;
using System.Reactive.Disposables;
using System.Reactive.Linq;
using Asv.Sdr;

static IDisposable AttachAdsb(
    IReaderIqSubject<double> iq, AdsbMessageParser parser)
{
    const int sampleRate = 4_000_000;
    var magnitude = iq.Magnitude();
    var detected = magnitude.AdsbPulseDetector(sampleRate);
    var normalized = detected.AdsbNormalize(sampleRate);
    var decisions = normalized.AdsbPulseTruncate(sampleRate);
    var subscription = decisions.Subscribe(block =>
    {
        foreach (var decision in block.Span)
            parser.ProcessSample(decision);
    });
    return new CompositeDisposable(
        subscription, decisions, normalized, detected, magnitude);
}

Construct this chain and parser subscriptions before starting acquisition. The returned disposable owns the added stages; the caller owns the sampler, reader, and parser. This is a wiring example, not a validated RF receiver performance claim. Check timing, block boundaries, normalization, CRC statistics, and registered models against known recordings before interpreting live messages.

The streaming parser uses direct equality between calculated and received CRC. Do not assume it accepts every Mode-S parity/address-overlay format simply because a corresponding message model exists. Position reconstruction and aircraft tracking also require application-level state and interpretation.

Source: pulse helpers and parser.

29 October 2026