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fix(mavlink): handle multi-GPS HIL_GPS id extension and prevent HIL message forwarding

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Co-authored-by: mrpollo <317648+mrpollo@users.noreply.github.com>
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copilot-swe-agent[bot]
2026-03-13 01:19:24 +00:00
co-authored by mrpollo
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docs/en/sim_airsim/index.md
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@@ -14,6 +14,17 @@ It provides physically and visually realistic simulations of Pixhawk/PX4 using e
<!-- datestamp:video:youtube:20170216:AirSim Demo -->
## MAVLink Compatibility Note
When using AirSim with PX4 for HITL simulation, you may encounter a `Message length 37 doesn't match expected length 36` error.
This is caused by a MAVLink version mismatch: the `HIL_GPS` message was extended with an `id` field (to support multiple GPS inputs) in a newer version of the MAVLink spec.
When this extension field is non-zero (e.g. when a simulator sends a second GPS instance with `id=1`), the message length becomes 37 bytes instead of 36 bytes.
Older MAVLink implementations that are unaware of this extension may reject the message.
PX4 now properly handles both the old 36-byte and new 37-byte `HIL_GPS` messages (where 37 bytes occurs when the `id` extension field is non-zero and the `yaw` extension field is zero), and no longer forwards HIL sensor messages between MAVLink instances to avoid triggering this error in connected clients.
If you are still experiencing this error, ensure your AirSim version uses a MAVLink library that supports the `HIL_GPS` `id` extension field.
## PX4 Setup
[PX4 Setup for AirSim](https://microsoft.github.io/AirSim/px4_setup/) describes how to use PX4 with AirSim using both [SITL](https://microsoft.github.io/AirSim/px4_sitl/) and [HITL](https://microsoft.github.io/AirSim/px4_setup/#setting-up-px4-hardware-in-loop).