HALO, in service with the British Army since 2002, is an acoustic weapon locating system developed by BAE Systems.
It uses a distributed array of up to twelve sensor posts to detect the pressure wave that is generated when artillery weapons fire or shells explode. Sensor Posts are usually deployed some two to four km apart.
Data on the detected passing pressure wave together with the prevailing meteorological conditions at the Sensor Post are relayed back to a Command Post. Here, data associated with the same event that has been detected at the other Sensor Posts is used to compute the location of the source of the sound. HALO uses the meteorological data to generate a large-area model of the effects of the weather on the propagation of the sound which, together with a digital terrain database, is used to compute the future flight path of the sound. By this means HALO is able to accurately calculate locations.
HALO incorporates a specially designed surface mount sensor. As HALO is used in desert, mountainous and arctic environments they needed a wind speed and direction sensor that would work in all of these environments.