Acoustic gunshot detectors have become common in the past few years, and some have been reduced in size to where a single soldier can wear one on his uniform and be cued-in to an enemy's location as soon as he fires.

Engineers in Tennessee, however, are touting the idea of tieing a unit of footsoldiers' acoustic shot-spot sensors together in a wireless net. They think this would offer several benefits: the system wouldn't be confused by echoes or multiple enemies firing at once, and it would be able to locate gunmen who weren't in line-of-sight from an individual soldier. Perhaps even more impressively, the networked sonic system is able to distinguish the calibre of weapons fired, and even in some cases identify different weapons firing the same kind of ammunition.

Vanderbilt University's inhouse Exploration mag reported last week on the gunshot-locator net developed by the uni's Institute for Software Integrated Systems (ISIS). It involves mounting a small electronics package on each soldier's helmet, running on four AA batteries.

In each package is a wireless network node, of a type dubbed a "smart-dust mote" for its small size and cheapness. There are also four separated microphones, for picking up the acoustic signatures of flying bullets, and a GPS satnav location system. The GPS isn't accurate enough to act as a basis for properly pinning down opposing gunmen, so the Vanderbilt boffins added a crafty radio interferometry enhancement system of their own - apparently of such cunning that it has attracted as much interest as the rest of the system on its own.

The system works by picking up the distinctive conical shockwave trailing behind a passing supersonic bullet - the same phenomenon which produces a sonic boom behind a plane at Mach 1+. This is then related to the muzzle blast from the weapon which fired it, trailing slightly behind (the two noises are heard by people under fire as "crack-thud", or "crack-bang"). A software algorithm in the unit can work out a range and bearing to the enemy weapon's muzzle.

So far, so ordinary: a system much like the Qinetiq-America "Ears" or others already under trial with US forces.

But then various special sauces developed by ISIS prof Akos Ledeczi and his team kick in. All the smart-dust node hats in the squad or platoon net pass their information back and forth, and a special patented filter strips out false muzzle-blast reports - the great bugbear of such technology. The supersonic bullet-booms are very distinctive, it seems, but the muzzle blasts are much harder to distinguish from random banging sounds or echoes - especially with lots of guns firing at once in a built-up area.

As every node has a good idea exactly where it is, owing to its embedded radio-interferometry-enhanced GPS, the combined reports can thus be boiled down to locate all the guns firing nearby within a metre or two - enough to pinpoint which window, corner or whatever each enemy is shooting from. Apparently it still works even in the case of crafty snipers lurking well back from windows - the usual method favoured by the pros. Nor is the system bothered by guns firing out of line of sight - hidden behind walls or buildings or whatever.