ADS-B and native Mode S

AdsbProtocol.RegisterAdsbProtocol registers AdsbParser for receiver-framed 56-bit and 112-bit Mode S downlinks. The protocol ID remains ADS-B. This is a byte-stream codec for AVR and Beast receiver output, not an RF 1090 MHz demodulator or a UAT decoder. ASTERIX CAT021 uses a separate ASTERIX transport.

Registration and input

using System.Text;
using Asv.Gnss;
using Asv.IO;
using R3;

var protocol = Protocol.Create(builder => builder.Protocols.RegisterAdsbProtocol());
using var parser = protocol.CreateParser(AdsbProtocol.Info.Id);
using var subscription = parser.OnMessage.Subscribe(message =>
{
    var downlink = (AdsbDfMessage)message;
    Console.WriteLine($"DF{downlink.DownlinkFormat}: {downlink.ExtendedSquitter?.Identification?.Callsign}");
});
foreach (var value in Encoding.ASCII.GetBytes("*8D4840D6202CC371C32CE0576098;"))
{
    parser.Push(value);
}

The example contains the published KLM1023 identification vector. A transport frame can arrive across any number of input blocks; Reset discards an incomplete candidate. The parser keeps at most 64 receiver-frame bytes. Invalid candidates are reported through OnError, and subsequent complete frames can still be read.

Input	Accepted structure
AVR `*` or `:`	Native frame as 14 or 28 hexadecimal digits, followed by `;`
AVR `@` or `%`	12 hexadecimal receiver-clock digits, then the native frame and `;`
AVR `<`	12 clock digits, two signal-level digits, native frame and `;`
Beast type `2`	`0x1A 0x32`, six clock bytes, one signal byte, seven native bytes
Beast type `3`	`0x1A 0x33`, six clock bytes, one signal byte, fourteen native bytes

Beast data bytes equal to 0x1A are escaped by doubling them, including inside clock and signal fields. Complete Mode A/C frames (AVR four-digit records and Beast type 1) and Beast type 4 receiver-status records are skipped; they are not ADS-B messages. Other Beast extension types are not decoded. AVR line endings and noise outside candidates are ignored. Hexadecimal input is case-insensitive. The % receiver marker never bypasses host-side CRC validation.

Message model and encoding

Every five-bit DF key is registered by default to AdsbDfMessage. This preserves unrecognized payload formats as native bytes rather than claiming a semantic decoder for every DF. Id is the DF as ushort; DownlinkFormat exposes it as a byte. Factory Remove and Clear are respected, with no fallback recreation.

RawData is message-owned ReadOnlyMemory<byte>. SetRawData copies native bytes, validates length and applicable parity, and refreshes ExtendedSquitter. Deserialize instead consumes one AVR or Beast frame, leaving the rest of the input span untouched. Failed validation leaves the previous message data intact. Tags uses the normal ProtocolTags contract.

FrameFormat, AvrMarker, ReceiverTimestamp and SignalLevel describe the receiver envelope. The clock is an unmodified 48-bit receiver value, not a UTC instant. Signal is the raw 0–255 receiver level, not calibrated dBm.

GetByteSize and Serialize produce the selected receiver envelope. AVR output uses uppercase hex and a trailing semicolon without CR/LF. Beast output includes all required escape bytes. Missing clock/signal fields for a selected envelope, out-of-range timestamps and undersized output buffers are rejected. Serialization retains native parity bits; it never silently repairs the received downlink.

Parity and identity

DF	Validation and identity
17, 18	Direct 24-bit CRC must match; corrupt frames are rejected
11	CRC XOR parity must fit the seven-bit interrogator overlay; reported as `InterrogatorParity`
Other DF	Address/data parity remains `AddressParityUnverified`; a nonzero remainder is not a CRC failure by itself

ParitySyndrome contains computed payload CRC XOR transmitted parity, and Parity preserves the transmitted 24-bit field. No aircraft-address cache or bit error correction is hidden in the parser. IcaoAddress is exposed only for DF11, DF17 and DF18 control value 0. Relay addresses whose IMF semantics have not been decoded, anonymous/non-ICAO DF18 addresses and an unverified parity-derived value are never presented as an ICAO identity.

Typed ADS-B payloads

DF17 and DF18 control values 0 or 1 provide typed nested ADS-B models. DF18 control values 2, 5 and 6 provide AdsbExtendedSquitter with its type code and raw message element only: relays repurpose flag positions for IMF and other semantics, so ordinary ADS-B flag meanings are not applied to those records. Other DF18 control layouts remain raw. All message models and decoded payload models implement IVisitable with cached typed metadata, English descriptions, units and read-only markers.

Type code	Model	Decoded data
1–4	`AdsbIdentification`	Callsign and category; type code retains the category set
5–8	`AdsbSurfacePosition`	Movement, speed/track availability, speed in knots, track in degrees and raw CPR
9–18	`AdsbAirbornePosition`	Surveillance/quality bits, raw altitude, Q=1 pressure altitude in feet and raw CPR
20–22	`AdsbAirbornePosition`	GNSS height in meters and raw CPR
19, subtypes 1–2	`AdsbVelocity`	East/north velocity, ground speed, track and vertical-rate information
19, subtypes 3–4	`AdsbVelocity`	Heading, indicated/true airspeed, vertical-rate information
Other type codes	`AdsbExtendedSquitter`	Type code and unmodified seven-byte `MessageElement` hex

Unavailable values use nullable numeric fields rather than zero. Surface movement code 124 indicates a lower bound on speed through GroundSpeedIsLowerBound. The raw version-dependent quality bit is retained; it is not automatically converted into an ADS-B-version-specific integrity estimate.

The implementation does not reconstruct geographic coordinates from CPR pairs, track aircraft over time, or decode Q=0 Gillham pressure altitude. For Q=0 the original AltitudeCode remains available and BarometricAltitudeFeet is null. Emergency, target-state and operational-status message elements are preserved raw rather than advertised as typed decoders.

File tools and inspector fields

The shell ProtocolFileReader registers ADS-B by default, so asv-gnss file accepts AVR and Beast recordings. Other applications can enable the same parser through RegisterAdsbProtocol; consumers pinned to an older package need an updated library build. Visitors expose receiver metadata, DF, explicit address, parity status, original bytes and decoded message-element fields without mutating the received data.

References and validation

AdsbProtocolTest, AdsbMessageTest and AdsbPayloadTest cover published external vectors, independently constructed CRC/parity cases, streaming fragmentation, escaping, recovery, transport round trips, unavailable values, typed visitors, metadata reuse and unchanged raw storage. The shell integration has a mixed AVR/Beast fixture in ProtocolFileReaderTest.

22 October 2026