Skip to content

protocols.md

Latest commit

 

History

History
156 lines (117 loc) · 8.41 KB

protocols.md

File metadata and controls

156 lines (117 loc) · 8.41 KB
Raw

Marine Instrument Protocols

Reference for the communication protocols used to connect with onboard marine instruments.

NMEA 0183

The dominant legacy protocol. ASCII text sentences over serial (RS-422) or WiFi (TCP/UDP).

Sentence format

$TTYYY,field1,field2,...,fieldN*CC<CR><LF>
  • TT = talker ID (GP GPS, SD depth sounder, WI weather, HC compass, AI AIS)
  • YYY = sentence type
  • *CC = XOR checksum (hex)
  • Max 82 bytes per sentence

Key sentences

Sentence Data Example fields
GGA GPS fix time, lat, lon, fix quality, satellites, HDOP, altitude
RMC Recommended minimum time, status, lat, lon, SOG (knots), COG, date, mag variation
DBT Depth below transducer depth in feet, meters, fathoms
DPT Depth depth (m), transducer offset, max range
MWV Wind speed & angle angle, R/T reference, speed, units, status
HDG Heading magnetic heading, deviation, variation
VDM/VDO AIS data fragment count, fragment num, channel, 6-bit payload, fill bits

WiFi transport

WiFi multiplexers (Yacht Devices, Digital Yacht, Vesper) bridge serial instruments to the network. They either create their own AP or join the boat's WiFi, then serve NMEA sentences as plain-text lines over:

  • TCP on port 10110 (IANA-registered) — guaranteed delivery, typically 4-8 concurrent clients
  • UDP broadcast on port 10110 — fire-and-forget, unlimited clients

Data rate: 4800 baud default (~5-6 sentences/sec). High-speed variant at 38400 baud.

NMEA 2000

Modern binary protocol on CAN bus at 250 kbit/s. Messages identified by PGN (Parameter Group Number). Mobile devices cannot connect directly — requires a gateway that bridges N2K to WiFi.

Common gateways

Product Manufacturer Notes
YDWG-02 Yacht Devices N2K to WiFi, TCP + UDP, bidirectional, ~$200
W2K-1 Actisense N2K to 0183 over WiFi, SD card logging
iKommunicate Digital Yacht N2K + 0183 to Signal K (JSON), acts as Signal K server

All gateways convert N2K to NMEA 0183 sentences over TCP/UDP, so the app treats them identically to a direct 0183 WiFi connection.

Signal K

Open-source, JSON-based marine data standard. A Signal K server (typically Node.js on a Raspberry Pi) ingests NMEA 0183/2000 and exposes data via standard web protocols.

Why Signal K is ideal for a mobile app

  • JSON is native to JavaScript — no binary parsing
  • WebSocket streaming works out of the box in React Native
  • REST API via standard fetch() for one-off queries
  • Server handles all NMEA protocol translation
  • mDNS/Bonjour discovery (_signalk-http._tcp, _signalk-ws._tcp)
  • All values in SI units (m/s, meters, radians, Kelvin)
  • AIS targets included as additional vessel entries

Connection flow

  1. Discover via mDNS or manual IP entry
  2. GET /signalk — returns endpoint URLs
  3. Connect WebSocket to /signalk/v1/stream?subscribe=self
  4. Receive delta messages with changed values
  5. Subscribe to specific paths with configurable update rates

Key data paths

Path Unit Description
navigation.position decimal degrees {latitude, longitude}
navigation.speedOverGround m/s SOG
navigation.courseOverGroundTrue rad COG
navigation.headingTrue rad True heading
environment.depth.belowSurface m Depth
environment.depth.belowTransducer m Depth
environment.depth.belowKeel m Depth under keel
environment.wind.speedApparent m/s AWS
environment.wind.angleApparent rad AWA (relative to bow)
environment.wind.speedTrue m/s TWS
environment.wind.directionTrue rad TWD
environment.water.temperature K Water temp

AIS targets appear under vessels.<mmsi> with the same path structure.

Delta message format

{
  "context": "vessels.self",
  "updates": [
    {
      "timestamp": "2026-03-19T10:30:00.000Z",
      "values": [
        {
          "path": "navigation.position",
          "value": { "latitude": 47.6, "longitude": -122.3 }
        },
        { "path": "environment.depth.belowTransducer", "value": 8.7 }
      ]
    }
  ]
}

Signal K v2 server APIs

The Signal K v2 specification includes higher-level REST APIs beyond raw instrument data:

API Endpoint Purpose
Resources /signalk/v2/api/resources/ CRUD for waypoints, routes, notes, regions, charts
Course .../navigation/course Active navigation — set destination, follow route
History /signalk/v2/api/history/ Time-series queries for tracks, depth, conditions
Autopilot .../autopilots/ Unified autopilot control (engage, heading, tack)
Notifications /signalk/v2/api/notifications/ Instrument alerts — depth alarm, anchor drag, AIS
Radar .../radars/ Radar control and spoke data streaming

AIS (Automatic Identification System)

AIS data arrives via NMEA !AIVDM sentences containing 6-bit armored ASCII payloads that must be decoded.

Key message types

Type Class Data
1-3 Class A position MMSI, nav status, ROT, SOG, position, COG, heading
5 Class A static IMO, call sign, name, type, dimensions, draught, destination, ETA
18 Class B position SOG, position, COG, heading
19 Class B extended Adds name, type, dimensions
21 AtoN Aid-to-navigation position, type, status
24 Class B static Name (Part A), type/dimensions/call sign (Part B)

AIS hardware for small boats

Vesper XB-8000, Digital Yacht AIT5000, Quark-elec QK-A026. All broadcast NMEA over WiFi (UDP/TCP). Some devices like Digital Yacht NavLink Blue use BLE.

Online services

Service Data Cost Notes
AISStream.io Real-time AIS via WebSocket Free Subscribe by bounding box, all message types as JSON
NOAA CO-OPS Tides, currents, water level, weather Free No API key needed. U.S. coastal stations.
Open-Meteo Marine Wave height/period/direction, swell, currents Free (non-commercial) No API key. 7-day forecast, hourly.
StormGlass.io Comprehensive marine weather Free 10 req/day; paid commercial Aggregates multiple weather models
GEBCO Global bathymetry (15 arc-sec grid) Free WMS tiles for map overlay. NetCDF/GeoTIFF download.