Is there any detailed explanation about local position estimator?

I am trying to utilize local_position_estimator(LPE) with optical flow sensor however it does not work as expected.
I checked lidar range data is streaming as well as flow sensor but LPE keeps saying lidar timeout and baro timeout.

I also found flow data is not corrected and always return because agl()<flow_min_agl does not met.
I think agl stands for above ground level is that correct?
But agl is not correctly represent my range measurement data and oscillates near zero value.

Is there any documentations I can refer to about using LPE?