docs(Position): correct ground_speed unit to km/h (matches firmware)#963
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docs(Position): correct ground_speed unit to km/h (matches firmware)#963Yeraze wants to merge 1 commit into
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The comment documents ground_speed as m/s, but the firmware transmits it in
km/h. In meshtastic/firmware, GPS::lookForLocation() is the only place the GPS
path sets the field:
// src/gps/GPS.cpp
p.ground_speed = reader.speed.kmph(); // TinyGPS++ .kmph() -> km/h
PositionModule.cpp copies the value through unchanged. So the on-the-wire unit
is km/h, and clients decoding it as m/s over-report speed by 3.6x. This updates
the doc comment to match the implementation.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Randall Hand <randall.hand@gmail.com>
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This was referenced Jun 26, 2026
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Summary
Position.ground_speedis documented as m/s, but the firmware transmits it in km/h. This is a one-word doc-comment fix to make the proto match the implementation. No wire-format or value change.Evidence (firmware)
The GPS path sets
ground_speedin exactly one place, and it assigns the TinyGPS++ km/h accessor:// src/gps/GPS.cpp — GPS::lookForLocation() p.ground_speed = reader.speed.kmph();https://github.com/meshtastic/firmware/blob/a4b2021782d3e323b8fdc6da166f4305c53c6181/src/gps/GPS.cpp#L1824
TinyGPSSpeed::kmph()returns kilometers per hour (TinyGPS++ also offers.mps(),.knots(),.mph(); the firmware chose.kmph()).PositionModulethen copies the value through without any conversion:// src/modules/PositionModule.cpp p.ground_speed = localPosition.ground_speed;https://github.com/meshtastic/firmware/blob/a4b2021782d3e323b8fdc6da166f4305c53c6181/src/modules/PositionModule.cpp#L260
So the value on the wire is km/h.
Impact
A client that follows the current doc and decodes
ground_speedas m/s over-reports speed by a factor of 3.6 (e.g. a wire value of90is 90 km/h, but rendered as90 m/s= 324 km/h). Aligning the comment with the firmware lets implementers convert correctly.Note on the existing inconsistency
There's a pre-existing units disagreement inside the firmware that this PR does not touch (and which likely seeded the confusion):
src/GPSStatus.hlogs the value asground_speed * 1e-2, i.e. as if it were cm/s, and the ATAK PLI path inPositionModule.cppfeeds the km/h value into a CoT.speedfield that expects m/s. Those are firmware-side issues; this PR only corrects the protobuf documentation to reflect what is actually transmitted today (km/h). If maintainers would prefer the firmware be changed to emit m/s instead, happy to redirect — but that would be a breaking change for every existing node/client, whereas this doc fix is non-breaking.Discovered while debugging inflated speeds in MeshMonitor.