There are lots of small, hand-launched roboplanes around these days, usually intended to let foot soldiers get an idea what lies over the hill or around the corner. There are also lots of acoustic gunshot spotter rigs nowadays, some of them very fancy indeed. The FNS900 Seeker, as used with the A-HFI shotspotter system Shooting …
Stick them on the soldiers' helmets...
Sure... you still end up dead, but your buddies won't have to look at your bulletriddled body to tell where the enemy is hidden...
Why not just
Let the soldiers on the ground carry the sensor package with them?
Are just as much of a threat on modern battlefields as they've been since they were invented.
Surely the key difference here is that they are of no significant threat if the person using them isn't close enough for you to be able to see where they're firing from without resorting to gadget assistance (e.g. standing on the edge of your foxhole with the nasty end pointed in your direction).
Kill you just as dead as supersonic ones.
Less of a threat in terms of range, but in close quarters - such as an urban environment - they're pertty lethal. And I'm pretty sure that a CQB is where most soldiers would want this kind of tech, as there's more hidey-holes for baddies to hide in than on an open battlefield. Great for Helmand, maybe, but not so much help in Baghdad or Islamabad*...
*if Bush was still in charge, we'd be "assisting" Pakistan "root out the terrorists" by now....
How long before...
... it returns fire? No need to have soldiers on the ground. Just wait for someone dumb enough to fire at the UAV and then it takes them out.
Also, the yanks would have another excuse for when wedding celebrations get rudely interrupted...
Swings and roundabouts
With personal body armour, subsonic rounds are a lot less dangerous than they used to be, but they're far from harmless.
What I wonder is how close the bullet has to be to be detected. It sounds silly, but sensors on every soldier's helmet might be more useful than some mini RPV. It might at least give them a direction to look, while they're praying for thinner buttons.
Subsonic rounds not much threat *to aircraft*
I doubt you'll be using your trusty sidearm to go picking off little UAVs a few hundred yards away, which is why they aren't much threat.
This is a system to primarily designed to protect the UAV, not the meatbags on the ground.
Aforementioned meatbags also have rather more to worry about from snipers and the like than from someone packing a pistol... and as any fule kno, freedom fighters use AKs, which this system would detect. No-one liberates their people from the oppressors using Uzis.
just incorporate sonic location equipment into the guns held by ground troops. Campers always get shot first :P