I ask myself if there is a chance to get a response signal whenever a picture is taken.
I read that one is able to sync a number of cameras which sounds appealing for my application. But anyways, I would like to know from the ground whether the 10s interval has been exceeded or not. I imagine, sometimes a picture is taken within a second, sometimes it takes some seconds. Either way, I would like to know when that happens, to not wait in vain or not proceed to early... I think an accoustic signal could be superimposed by environmental noise. Any ideas? Or has this been discussed elsewhere?
I wouldn't expect completely failed shots to happen very frequently. In full auto mode these cameras will sometimes refuse to shoot if they can't get focus, but otherwise they pretty much shoot when you press the button. There are some additional problems you can run into if you are using CHDK script.
If you want to make sure every shot is acceptable you'll probably need to look at them after shooting, although that will depend on the environment.
The only outputs that you have easy control of from CHDK is LEDs and sounds. You won't be able to see camera LEDs from the ground, but you could probably rig up something with a photodiode. The D20 looks like it only has an AF led and one status LED.
It sound like in your situation the AF assist probably wouldn't be needed (3 meters is pretty far, and your subject is presumably quite a bit further), so you might be able to use that.
The status LED will show the normal Canon firmware status activity. That might be enough to tell if the shot was taken, but if it's like my D10, it's actually a couple different color LEDs. In that case, a single photo diode wouldn't give you the same information.
Alternately, if you want to go totally overboard you could use
http://chdk.wikia.com/wiki/PTP_Extension live view to have a remote display on the other end of the pole
