Camera Trigger Interval

This is the small equation that turns a geometric plan into two settings an aircraft can actually fly: an interval and a speed. $t = B/V_g$, the air base divided by the ground speed. It is trivial arithmetic and it is where a surprising number of flights go wrong.

Ground speed is not airspeed. The base in this equation is a distance over the GROUND, so the speed must be a ground speed. A fixed-wing aircraft commanded to hold 15 m/s of airspeed with a 5 m/s tailwind is covering ground at 20, and every base is a third longer than planned — which turns a 75% endlap into 67%, quietly, on exactly one leg of every back-and-forth pattern while the other leg gets more overlap than it needs. Flight controllers that target a ground speed handle this correctly; those that target airspeed, and manual flights, do not. This is the same wind triangle that navigation has always had to solve, and it does not go away because the aircraft is small.

The better answer, where the payload supports it, is not to trigger on time at all. Distance-based triggering fires the shutter from the GNSS position — every twelve and a half metres, say — and the wind problem disappears entirely, along with the terrain problem, because the planner can adjust the spacing as the ground rises. Most current mission planners do this by default. The time equation is still the right way to CHECK such a plan, because it tells you the rate the camera will be asked to sustain.

And the camera is often the binding constraint. A payload with a mechanical shutter, autofocus and a large file to write may need a second and a half to two seconds per frame, and that gets worse as the card fills, as the buffer backs up, or as the sensor warms. When a camera cannot keep up it does not announce it: it simply skips the exposure. The overlap collapses at those frames only, the block develops a weak patch that looks random, and the flight log looks perfect. If this equation returns an interval shorter than your payload's honest cycle time, the fix is to slow the aircraft down — the base is what the geometry needs, and the speed is the free variable.

The other constraint on speed is motion blur, and the two are independent. The camera's cycle time sets a maximum speed from above through the interval; the smear budget sets one from below through the exposure time. In bright sun the blur limit is usually generous and the camera is the constraint. In overcast light, in early morning or late afternoon, or under any lens working at a small aperture, the blur limit tightens sharply and becomes the binding one. Work out both, take the smaller, and plan the flight around that.