The system was developed at Vanderbilt University's Institute of Software Integrated Systems with funding from Darpa. Obviously this app was designed as a transportable device to be fitted into military helmets. However not they have worked out how to have a phone automatically pulling up a map on a user's smartphone when shots are fired and pointing out the culprit with a red arrow.
The tech uses readings of acoustic measurements to track and pinpoint the origin and trajectory of gunfire. Gunshots can be split into two different acoustic phases. The first, the "muzzle blast", happens when the gun goes off, inducing a flash of light and soundwaves that continue to spread out. The second is the sonic boom produced by the bullets soaring through the air.
The team decided to translate the system to smartphones so it can easily be used in built-up areas. It features a bluetooth headset that acts as a sensor node. The system still needs a series of sensors to pick up the sound from different locations. The sound timings data is sent to the headset, processed and transferred to the smartphone. The appointed location is also then sent to everyone else in the network.
So in other words for it to work, you need to have lots of people who are in the area who are sufficiently worried about being shot by a sniper and who have installed the android app. Not very useful, unless you live in a war zone. Still you have to admire the technology.