ados-rtcm

RTCM3 framing, CRC-24Q and MAVLink GPS_RTCM_DATA fragmentation for injecting RTK corrections into a flight controller.

An RTK rover needs a steady stream of RTCM3 correction messages from a base station, and a flight controller running ArduPilot or PX4 accepts them over MAVLink as GPS_RTCM_DATA messages of at most 180 bytes each. This crate finds whole, CRC-checked RTCM3 frames in a byte stream that arrives in arbitrary pieces, and splits each frame into the fragments and flag bits the firmware reassembles. Reach for it when you forward corrections from a base receiver, an NTRIP client or a file to a vehicle.

use ados_rtcm::{crc24q, encode_rtcm3, split_rtcm3, Fragmenter, Rtcm3Splitter};

// Frame a 394-byte RTCM3 payload: preamble, 10-bit length, payload, CRC-24Q.
let payload: Vec<u8> = (0..394).map(|i| i as u8).collect();
let frame = encode_rtcm3(&payload);
assert_eq!(frame.len(), 400);

// The trailing three bytes are the CRC-24Q of everything before them.
let (body, crc) = frame.split_at(frame.len() - 3);
assert_eq!(crc24q(body).to_be_bytes()[1..], *crc);

// Junk before the frame is skipped; a stream cut in two still yields one frame.
let mut stream = vec![0x00, 0x11];
stream.extend_from_slice(&frame);
assert_eq!(split_rtcm3(&stream), vec![&frame[..]]);
let mut splitter = Rtcm3Splitter::default();
assert!(splitter.push(&stream[..100]).is_empty());
let frames = splitter.push(&stream[100..]);

// Fragment into GPS_RTCM_DATA payloads: 180 + 180 + 40 bytes.
let mut fragmenter = Fragmenter::default();
let fragments = fragmenter.fragment(&frames[0]).unwrap();
let lens: Vec<u8> = fragments.iter().map(|f| f.len).collect();
assert_eq!(lens, [180, 180, 40]);
assert_eq!(fragments[1].flags, 1 | (1 << 1)); // fragmented, id 1, sequence 0

Highlights

• Zero dependencies, standard library only.
• Stream splitter with resync: junk bytes and frames with a bad CRC are skipped, and the carried partial frame is bounded by one maximum RTCM3 frame (1029 bytes).
• Fragment flags and end-of-message rules match the GPS_RTCM_DATA reassembly in flight-controller firmware, including the zero-length closing fragment.
• Unit tests cover fragment counts at every boundary, sequence id wraparound, CRC check values and splitting at every cut point of a frame.

How it works

RTCM3 frames. A frame is the preamble 0xD3, 6 reserved bits, a 10-bit payload length, the payload (at most 1023 bytes) and a 24-bit CRC-24Q (polynomial 0x1864CFB, initial value 0) over everything before it. encode_rtcm3 builds one and crc24q computes the check.

Splitting. split_rtcm3 returns every whole CRC-valid frame in one buffer and drops an incomplete tail. Rtcm3Splitter does the same for a stream delivered in pieces, keeping the partial frame for the next push. A preamble whose frame fails its CRC is skipped by one byte so the scan resyncs on the next preamble.

Fragmentation. GPS_RTCM_DATA carries at most 180 bytes (FRAGMENT_LEN). A message of up to 180 bytes goes out whole with flags = 0. A longer one is split into 180-byte fragments with flags = 1 | fragment_id << 1 | (sequence_id & 0x1f) << 3, and Fragmenter advances its 5-bit sequence id once per fragmented message, so keep one Fragmenter per output link. The receiver treats a fragmented message as complete at the first fragment shorter than 180 bytes, or once all four fragments arrived. A message that is an exact multiple of 180 bytes below 720 therefore ends with a zero-length fragment, and 720 bytes (MAX_MESSAGE_LEN) is four full fragments. Longer messages return TooLong. message_fragments and fragment_count count the messages ahead of sending.

Part of ADOS

Used in production by the ADOS Drone Agent, the onboard software of the ADOS software-defined drone platform. All ADOS crates · Docs · Discord

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License

Licensed under either of Apache License, Version 2.0 or MIT license at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.