The desired_accel is set again 11 lines lower so this line did nothing.

Prevents possible loss of attitude reference for flights without optical flow and GPS.

The optical flow measurement timeout can reset the velocity states which decouples the position states from IMU errors and therefore significantly reduces the amount of attitude error correction.

don't use the 1kHz timer as it conflicts with other I2C devices

this allows for support of multiple sensors on a board

This enables the filter to report the last known position after disarm.
The LLH location of the filters NED origin is published and should be logged every time the vehicle is armed to assist with post-flight trajectory reconstruction.
The LLH location of the filters NED origin can be set externally whilst the vehicle is disarmed.

The last known position was being output on the velocities when in constant position mode.