RTCM v2

Connect the parser

The stream parser is RtcmV2Parser and its message model is RtcmV2MessageBase. With the standard Asv.IO builder, registration also installs RtcmV2Encoder for outgoing streams. See Quick Start for router, port, and subscription setup. The receiver must already be configured to emit the required data.

Default message coverage

The default registration covers message types 1, 9, 14, 15, 17, 21, and 31.

Encode and transmit

The project references the NuGet packages Asv.IO and Asv.Common 3.7.0. RtcmV2Encoder implements the Asv.IO IProtocolEncoder contract: it adds GPS parity, data inversion, and six-of-eight transport framing to normalized message data. Standard ports use the registered encoder automatically when sending RTCM v2 messages.

For direct encoding, create one encoder for each outgoing stream and reuse it in transmission order:

The previous word's D29/D30 bits affect the next word, including across message boundaries. Encoder instances are not thread-safe. Serialize encoding and sending in the same order, and create a new instance when starting a new independent stream. GetMaxByteSize does not advance state; rejected encoding leaves state unchanged. A failed transport write can leave the remote stream incomplete: restart or resynchronize that stream before continuing.

RtcmV2MessageBase.Serialize and GetByteSize operate on normalized 24-bit data words, matching Deserialize; they exclude parity and transport framing. Use the encoder for receiver-ready bytes. A normalized packet contains 6 to 99 bytes; its transport representation contains 10 to 165 bytes. Parsed messages retain unmodeled and fill bits when the normalized packet size remains unchanged.

Model details and limitations

Types 1, 9, and 31 encode 40-bit satellite corrections and select the fine or coarse correction scale as needed. PRN 32 is encoded as zero. NaN correction values encode an unavailable correction; out-of-range finite values are rejected.

Type 14 uses GpsTime as GPS time, without an additional UTC conversion. Its week is encoded modulo 1024, and LeapSeconds supplies the six-bit GPS-minus-UTC offset. Set ZCount separately to the seconds within the hour, in 0.6-second increments. The decoded header includes reference-station ID, sequence number, modified Z-count, GPS time, and station health exposed through the existing Udre property. GPS hour and week reconstruction use the current time, so historical replay requires care.

Type 15 preserves the existing API conversion formulas: IonosphericDelay = rawUnsigned14 * 0.001 and IonoRateOfChange = rawSigned14 * 0.05. These legacy model values are not the specification's SI scaling; convert explicitly when interpreting them physically. Records include the two reserved leading bits and occupy 36 bits each.

Type 17 uses the specification's 480-bit payload layout. WeekNumberRaw supplies the ten-bit week, Toc supplies the GPS clock-reference time, and A is the semi-major axis (the encoder writes its square root). Existing raw-value properties such as Cus and Toes retain their public representation. The corrected payload layout replaces the former extra skipped bits in the decoder.

Type 21 exposes its unmodeled payload as RawData, consisting of complete three-byte data words. It supports opaque payload replay and transport encoding; individual high-accuracy correction fields are not modeled.

Implementation reference

Coverage above follows Protocols/Rtcm/V2/RtcmV2Protocol.cs under src/Asv.Gnss. Inspect concrete message classes for field names, units, and supported payload revisions.

Specification message catalog

Source: Protocols/Rtcm/V2/Docs/c10402.3 + Amend 1 - RTCM VER 2-3 (3).pdf, RTCM 10402.3 with Amendment 1, Chapter 4.3 and its contents on PDF p. 8. All named types and undefined/reserved ranges in that contents list are represented. Type 34 covers partial differential corrections for N > 1 and a null frame for N <= 1, as confirmed by the message summary on PDF p. 50. The amended type 27 is included.

Status: Model present means a concrete payload model exists in the current checkout; it does not certify complete field, revision, encoding, or firmware coverage. Partial identifies a known incomplete decoder. Not implemented means no matching concrete model was found. Default reports whether the standard registration adds that model. Models outside the default set require explicit registration. See the limitations above before transmitting messages.

Page references below are PDF page numbers (1-based), including covers and front matter.

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