docs/schema: server-side GNSS lever-arm correction - #680
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Document the data-model contract for server-corrected navigation.position when GPS antenna offsets are configured. Adds sensors.<id>.sourceRef so sensor metadata can be linked to its data stream; describes navigation.position as the Common Coordinate Reference Point (CCRP) at center-of-vessel on the centerline (body coordinates (length/2, 0) measured from the bow) when correction is active; documents the meta.gpsOffsetCorrection entry that preserves the raw value and the correction provenance. Motivated by SignalK/signalk-server#2399, which adds the multi-GPS configuration UI and the in-server lever-arm corrector.
Rename the linkage field on the sensor object to $source per the $-prefix reference convention, the correction meta to gnssOffsetCorrection with $sensor referencing the sensor instance, and the surrounding prose from GPS to GNSS. Matches the server implementation in SignalK/signalk-server#2399.
dirkwa
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July 9, 2026 05:58
Update the spec to match the server implementation: correction is an opt-in server option (replace or both modes) rather than always-on, and is not signalled in-band. Drop the meta.gnssOffsetCorrection description - position deltas are high volume so no per-delta marker is added; the 'both' mode keeps the raw antenna value as a separate <id>.ccrp source instead. Document the notifications.navigation.gnss.headingUnavailable notification raised when correction is active but heading is missing.
fromCenter was the only signed athwartships quantity in the specification declaring +ve to port. Every other signed lateral or rotational key -- rudderAngle, attitude.roll, attitude.yaw, rateOfTurn, thrustAngle, keel.angle, environment.wind.angle* -- is +ve to starboard. The wording dates to fa162f7 (2014) and has never been revisited; there is no issue or discussion in the repository recording a rationale for it. The clash became load-bearing with server-side lever-arm correction, which combines fromCenter with a heading taken from the same body frame as attitude.yaw. A port-positive offset against a starboard-positive rotation forces a non-standard rotation matrix; under this change it collapses to the standard NED form. Implementations already disagree with the schema and with each other: n2k-signalk emits +ve to starboard while signalk-to-nmea2000 encodes +ve to port, so an AIS reference point round-trips mirrored across the hull. NMEA 2000 itself carries no sign here -- positionReferenceFromStarboard is unsigned -- so the signed form is a Signal K invention and there is no external convention to preserve. Also documents the body frame explicitly (x forward, y to starboard, right-handed) in data_model.md, and syncs the stale inline descriptions in the AIS test fixture with the schema. This inverts the meaning of existing data and is a MODEL-level change under SchemaVer.
steering.autopilot.target.windAngleApparent and windAngleTrue declared "+port -starboard", while environment.wind.angleApparent, angleTrueGround and angleTrueWater all declare "negative to port". These describe the same physical quantity -- measured apparent wind angle and the target apparent wind angle an autopilot steers to -- so an autopilot comparing the two had to negate one or steer to the mirror image of the requested angle. The measured side is already starboard-positive in practice: n2k-signalk maps the NMEA 2000 wind angle (0..2pi clockwise from the bow) into (-pi, pi], so 0..pi is starboard. This is a description-only clarification. No consumer of the autopilot target wind angle keys was found in signalk-server, signalk-autopilot, signalk-to-nmea2000 or Freeboard-SK, and the only test fixture holds positive values, so no existing data encodes a side.
This was referenced Aug 11, 2026
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Summary
Document the data-model contract for server-corrected
navigation.positionwhen GNSS antenna offsets are configured.sensors.<id>.$source(new, optional) links a sensor instance to the source of its data stream — needed when multiple sensors of the same class are present (e.g. multi-GNSS). Named per the$-prefix reference convention established by$sourcein deltas.navigation.positiondescription expanded to define the Common Coordinate Reference Point (CCRP) as center-of-vessel on the centerline (body coordinates(design.length.value.overall / 2, 0)measured from the bow), matching the NMEA 2000 / radar / AIS industry convention.meta.gnssOffsetCorrectionentry that travels with correctednavigation.positiondeltas so history-recording consumers can preserve both the corrected value and the raw antenna measurement (with full provenance:$sensorreferencing the sensor instance, fromBow, fromCenter, lengthOverall, headingTrue, rawValue).mdbook/src/data_model.mdexplaining the lever-arm correction model end-to-end.Motivation
This is the specification side of SignalK/signalk-server#2399, which adds the multi-GNSS configuration UI and the in-server lever-arm corrector. Source priorities mean the active position source can flip between antennas with different offsets, which is fragile to correct client-side; the server has the active-source resolution and the heading, so it applies the correction in-process and emits the raw value plus correction provenance in
metafor downstream history.Update: athwartships sign convention (RFC #685)
Two further commits, added after review feedback from @keesverruijt on SignalK/signalk-server#2399, who pointed out that
fromCenterreads backwards versus every other signed lateral quantity in the spec.fix: make sensors fromCenter positive to starboard— flipssensors.*.fromCenterto+ve to starboard, -ve to port, documents the body frame explicitly indata_model.md(x forward, y starboard, right-handed), and syncs the stale inline descriptions in the AIS test fixture. This inverts the meaning of existing data and is a MODEL-level change under SchemaVer.fix: align autopilot target wind angles with environment.wind—steering.autopilot.target.windAngle*said+port -starboardwhileenvironment.wind.angle*says the opposite, for the same physical quantity. Description-only, ADDITION-level, kept as a separate commit so it can be split out and landed independently.Full argument, provenance and blast radius in #685 — including that the AIS round trip is already broken today because
n2k-signalkandsignalk-to-nmea2000disagree with each other.This PR should not merge until #685 is settled. If the RFC resolves the other way, these two commits come off and the rest of the PR stands on its own.
Downstream: SignalK/signalk-server#2943, SignalK/signalk-to-nmea2000#162.
npm testpasses (207).