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Cezar T's avatar

Regarding knowing the altitude in order to be able to estimate speed using optical flow

While absolute ground speed estimation is tricky because there-s an unknown wind speed component to it, it is easy to estimate speed variation, e.g. when the aircraft accelerates or decelerates, variance of speed over a short period (a few seconds) can be computed from IMU, sensed from airspeed sensor or other means e.g. inferred from propeller rpm, craft attitude and air density (temperature, pressure) measured during some initial calibration flights.

Then if the robot knows its speed increased with e.g, 2m/s between Ta and Tb it has (Va, Oa) and (Vb, Ob), where Oa, Ob is optical flow measured "magnitude" at points a and b, Va is unknown velocity at Ta and Vb = Va+2m/s. While the ratio Ob/Oa can also be measured, then Va can be inferred, simply because in horizontal flight optical flow measured vertically under the craft is proportional to ground speed.

All this assumes the craft also maintains fixed altitude or uses some means to measure variance in altitude between Ta and Tb (IMU, altimeter) and account for it too.

Cezar T's avatar

I recall that in sailing when one's own boat is in motion and another boat is seen as "staying" at the same horizon angle relative to your boat's moving direction, then you are on a collision course. Even when the other boat is stationary, this simple visual clue means your boat is for some reason spiraling towards it and will eventually collide. The exception is when both ships have straight parallel paths with exact same speed but that's quite improbable. I think this could be a simple landing strategy to use when the "insect" is able to visually track the desired landing spot - somehow keep the optical flow of the target at zero or close.

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