What's the difference between ARSPD_TYPE=0 and TECS_SYNAIRSPEED=0?

I am a new commer to the ArduPlane and have a question for the airspeed estimation without the airspeed sensor.

The ArduPlane manual says that If there is no sensor, be sure to set the ARSPD_TYPE to 0. ArduPilot calculates an sensor-less airspeed estimate that is used if no sensor is present or fails.

The manual also says in the soaring section that If your plane can’t accommodate an airspeed sensor, it is possible to use the TECS synthetic airspeed estimate TECS_SYNAIRSPEED. To use this feature, set the parameter TECS_SYNAIRSPEED to 1.

What differnce is there between these two parameters?

you’ve basically explained that yourself :wink:

ARSPD_TYPE sets the type of a physical airspeed sensor if present. if no physical sensor is used, set this to 0 = none.
in this case, there will be a synthetic airspeed estimation computed based on grounspeed and sideslip variances when flying turns.
if you want to use this estimated airspeed value in your speed / height controller, set TECS_SYNAIRSPEED = 1.

the code usually provides a very good estimation during turns, though its precision will likely decay while flying longer straight legs. with regard to soaring applications, you will likely get pretty good results while thermalling. maintaining airspeed for maximum performance on longer straight flight periods is something you will most likely need to add a physical airspeed sensor for.

Thank you vierfuffzig,
I had been thinking that if ARSPD_TYPE is set 0, all parameteres including the TECS_SYNAIRSPEED that require airspeed information will take the synthetic airspeed estimation.
Now by your explanation I understand these two parameters are not combined.

By the way, could you explain me the synthetic airspeed estimation a little bit more? I am believing it estimates airspeed even in the head/back wind conditions with GPS and other sensors. But by your explanation, it seems it does not estimate airspeed in these conditions.

in a nutshell, it’s based on wind estimate. afaik most of it is done here: https://github.com/ArduPilot/ardupilot/blob/602388224b45314b20df45503cd783b68c6f42d9/libraries/AP_AHRS/AP_AHRS_DCM.cpp#L962

this is a very informative post on dyidrones: https://diydrones.com/forum/topics/wind-estimation-without-an

and the initial paper by bill premerlani: https://drive.google.com/file/d/0ByvTkVQo3tqXVzBYQUZicUNvbEE/view

“For the method to work, the plane must change attitude.”

always worth a reading :wink:

Thank you vierfuffzig,
These information are very useful for me. I will read them carefully to understand the wind estimation.
Thank you again.