See through the clamor of battle

It is possible, by shining a light on a surface and watching the return, to see how noisy it is. (if it's a hard surface you can listen, but this idea doesn't need that level of fidelity)

This laser system would shine an array of lasers onto the ground and detect vibrations. A computer would filter out universal ones and parse the vibrations to draw vectors to the origins and detect what some of those origins are. Short-term vibrations, such as from an explosion, could be filtered out or recognized by recognizing their short-termidness. Universal vibrations (such as from the wind) could be filtered out by recognizing their universality. A great deal of calculus would then process the remaining ones. It's unlikely to be sensitive enough to detect a foot-fall, but an artillery or machine gun emplacement repeatedly firing would easily be detectable, as would a moving tank or truck. Until now this hasn't been possible simply due to the noise of battle involving so many wave-forms. But modern GPUs are up to the task.

edit: Not necessarily aircraft-borne.