Nmea0183 Knowledge Patch

NMEA 0183 Knowledge Patch

Baseline: NMEA 0183 through 4.11, including common sentences and checksums; covered range: additional implementation details and changes through 4.30.

Use this patch when implementing, reviewing, configuring, or diagnosing NMEA 0183 links, emitters, parsers, and navigation integrations. Start with the quick references below, then read the topic file that matches the task.

Reference index

Breaking and compatibility-critical changes

Update fixed lookup tables for 4.30

NMEA 0183 4.30 replaces 4.11 and expands the GNSS suite for GPS, GLONASS, Galileo, BDS, QZSS, and NavIC. Update fixed talker, formatter, GNSS identification, and observation-code tables. In particular, recognize the new or expanded GIR, GRP, GGC, GCF, GSN, RLM, and SMV coverage. Read gnss-and-receiver-output.md before changing a decoder table.

Keep protocol syntax separate from electrical compatibility

• Treat v2.x and later links as EIA-422-A differential A/B, normally 4800 8N1.
• Never ground a conventional differential talker's driven B/- output.
• Allow one continuously driving talker per pair; use a multiplexer for several talkers and a buffer for isolated fan-out.
• Do not infer signal levels from sentence version, product labels, connector names, or wire colors.
• Use optical isolation for computer connections where ground loops or induced spikes are possible.

Read protocol-and-wiring.md before connecting legacy single-ended equipment.

Do not confuse high-speed 0183 with NMEA 2000

NMEA 0183-HS is 38400 8N1 serial NMEA 0183. NMEA 2000 is a 250 kbps CAN network with binary PGNs. Require a protocol gateway for sentence/PGN conversion; neither a multiplexer nor a level adapter performs it.

Preserve nulls and validate versioned suffixes

Keep commas for unavailable fields. For v2.0 and later, require the two-digit checksum and calculate the XOR over every byte between, but excluding, the opening $ or ! and *. For NMEA 2.3 modes, do not equate a syntactically valid sentence with an active navigation solution: only A and D are active/reliable in the covered navigation behavior; E, N, S, and null are not.

Framing quick reference

$ttsss,d1,,d3*HH\r\n
!ttsss,fields*HH\r\n
$Pmmm,data*HH\r\n
$CCGPQ,GGA*2B\r\n

• Parse a standard address as two talker characters plus a three-character formatter.
• Parse $P plus a three-character manufacturer identifier as a proprietary address.
• Parse a query as requester talker, target talker, Q, then requested formatter.
• Accept printable ASCII 0x20 through 0x7e with the high data bit clear.
• Retain the exact opening delimiter for checksum and sentence-class handling.
• Expect CRLF; device command envelopes may reject otherwise valid input without it.

GNSS talkers and multi-constellation output

Do not assume GP means the receiver tracks only GPS. A configuration may force GP while the solution uses several constellations. Conversely, AUTO commonly chooses GN for a combined solution and constellation-specific identifiers such as GP, GL, GA, GB/BD, GI, and GQ for single-system data.

Treat GSV as a common exception: Emlid, NovAtel, and SBG output it under separate constellation talkers rather than GNGSV. For GSA, prefer an explicit NMEA 4.10 System ID over satellite-number ranges because identifiers overlap.

GGA is not semantically uniform

Before consuming GGA, identify the configured source and profile:

Do not enable multiple Fixposition GGA solution variants on one port: they are indistinguishable on the wire. Read gnss-and-receiver-output.md and inertial-and-motion.md for field-level rules.

Navigation sentence traps

APB

Parse both status fields, cross-track error and steering direction, arrival and perpendicular flags, origin-to-destination bearing, waypoint, present-position-to-destination bearing, and heading-to-steer references. Some autopilots interpret the origin-to-destination field with APA semantics, so verify interoperability when far from the route leg.

RTE and RMB

• Reassemble RTE with total/current message numbers. Mode c is complete; mode w is working and begins with the waypoint just left, then the destination.
• Interpret RMB closing velocity as waypoint VMG, SOG * cos(COG - bearing), not raw SOG.

BOD, XDR, and legacy route data

• Keep BOD fixed to the selected waypoint-to-waypoint leg; it is not present-position bearing.
• Parse every repeating type,value,unit,name group in XDR.
• Support R00 plus alternating WPL where legacy active-route exchange is required.
• Apply an STN source ID to the sentence immediately following it.

Read sentences-and-navigation.md for exact layouts and sign conventions.

Direction, sign, and validity

Do not normalize signs without the sentence/profile convention:

• Negative ROT means port; negative RPM pitch means astern.
• Negative RSA means port; negative VBW longitudinal/transverse values mean astern/port.
• Negative VPW means downwind.
• SBG native heave is commonly positive down, while its WASSP PASHR, PHLIN, and some compatibility forms use positive up.
• SBG body velocity is forward/starboard/down, but PHLIN sway is positive left.

Honor accompanying validity fields; a parsed number is not automatically usable.

Device command safety

• Preserve null command fields when they mean “no change.”
• Send the exact checksum and CRLF required by SiRF $PSRF<MID> commands.
• Remember that SiRF input commands work only after the receiver is in NMEA mode.
• Treat reset numbering as platform-specific; SiRFLoc and non-SiRFLoc assignments differ.
• Treat Garmin baud and binary-phase changes as deferred until reset or power cycle where documented.
• Do not identify two same-address PASHR layouts by the address alone; use configured-log context.

Read garmin-and-sirf.md before emitting configuration commands and inertial-and-motion.md before decoding proprietary motion output.

Parser selection and error handling

Rust nmea0183 0.6.0

Use parse_from_byte for streaming and parse_from_bytes for chunks. Distinguish Ok(ParseResult::<type>(None)) from Err: the former is a valid sentence with no navigation solution. Check actual decoder support rather than inferring it from public data structures. Account for the API spelling Source::Gallileo, string errors, and the strict 79-character versus non-strict 120-character limit.

Hazer and gpstool

On a negative Hazer parse result, inspect errno: zero means valid input with no valid solution; positive values classify rejection. Keep stream state machines alive across recoverable errors. Use gpstool masks deliberately when mixed NMEA/UBX/RTCM/CPO data is present, and do not treat 0. trace placeholders as measured zero.

Read parsers-and-tools.md for APIs, error mappings, routing switches, trace priority, and build steps.

OpenCPN integration checks

• Do not assume OpenCPN core and Dashboard accept the same sentence set.
• Do not send routes expecting core RTE import; incoming RTE is not processed.
• Enable the AIS option explicitly before treating WPL as an APRS position report.
• Expect TTM and TLL radar targets to flow through the AIS decoder.
• Validate Dashboard MDA and XDR against its narrow field, range, unit, and name rules.
• For heading, apply the documented HDG variation fallback and accept only autonomous THS mode A.

Read opencpn-integration.md before mapping NMEA inputs to OpenCPN features.