Enable loiter type offboard position setpoint for Fixedwing vehicles

[Jaeyoung-Lim]

Describe problem solved by this pull request
This PR Fixes #13180 , where sending the loiter type offboard position setpoint on a fixed wing vehicle will result in a fly away. Loiter setpoints can be useful for fixed wing offboard mode as normal position setpoints are passed as a reference to the l1 controller so there are no explicit constraints on the radius of the vehicle while keeping close to the position setpoint.

The reason the it wasn't working was

• Commander thinks it is a valid setpoint, as it is receiving enough SET_POSITION_TARGET_LOCAL_NED messages, therefore the vehicle is able to switch to offboard mode.
• The fixed wing position controller treats the setpoint as a loiter setpoint as it is marked here as a lotier setpoint.
• However, as the loiter_radius and loiter_direction is not set from SET_POSITION_TARGET_LOCAL_NED, it is left as zero.

Describe your solution
Set the loiter_radius and lotier_direction if loiter_radius<=0it is invalid (which probabiliy means it is in offboard mode following position setpoints). It uses the default NAV_LOTIER_RAD parameter as default loiter radius

Test data / coverage
This log shows a flight which is being sent a offboard local position setpoint at [0.0m, 0.0m, 20.0m] and being switched between mission mode and offboard mode

[Jaeyoung-Lim]

@hamishwillee The types of position setpoints are in general UNDOCUMENTED. I realized this is a thing by going through the code. Now that it works would it make sense to add this to the documentation?

[hamishwillee]

The types of position setpoints are in general UNDOCUMENTED. I realized this is a thing by going through the code. Now that it works would it make sense to add this to the documentation?

@Jaeyoung-Lim
How does the "type" of the setpoint get determined? I.e. I can't see it in https://mavlink.io/en/messages/common.html#SET_POSITION_TARGET_LOCAL_NED so it looks entirely internal to PX4.
Can someone determine what type of setpoint they are dealing with?

Upshot, I can't answer this yet, because I don't understand where these come from and how they are used.

[Jaeyoung-Lim]

@hamishwillee Exactly. It seems to be coming from #3405 and the type definitions didn't make it through the mavlink spec. The definitions just use raw bitfields so it is hard to track where it is coming from.

I do like the fact that we can use the type_mask to configure how the setpoint can be followed.

In my opinion, this should either be pushed to the mavlink standard or at least be documented somewhere.

[hamishwillee]

@Jaeyoung-Lim Thanks for the clarification. I don't know if this should be public or not, but if it is then IMO it has been done incorrectly.

I have created mavlink/mavlink#1250 to discuss whether it should be public or not. Depending on how that falls out will define how this is documented.

Can you tell me the difference between the idle and loiter setpoints, and why knowing the setpoint type is useful (as an answer to the linked PR?)

[Jaeyoung-Lim]

@hamishwillee Specifying the position of the setpoint does not always fully define the expected behavior of the vehicle.

• An idle setpoint will make the vehicle be in idle state - zero throttle, zero roll / pitch.
• A Loiter setpoint will make the vehicle circle around the setpoint.
• A normally passed postion setpoint will make the vehicle use the position setpoint as a reference to the L1 controller, so it will fly in a flower like pattern.

All three types use the same position setpoint, but the behavior is handled differently in the fixed wing position controller.

[sfuhrer]

Looks reasonable to me. Haven't tested it in offboard, but tested it a bit in VTOL, doesn't seem to break anything in the other modes.

[hamishwillee]

@Jaeyoung-Lim What do the setpoints do on other vehicle types?

[Jaeyoung-Lim]

@hamishwillee I have replied in the user guide!

[Jaeyoung-Lim]

@sfuhrer Can we get this merged?

[sfuhrer]

@sfuhrer Can we get this merged?

Yes!

[dagar]

This really makes me think that loiter radius should be a FW parameter rather than navigator.