Artillery sound ranging

Method of determining the location of an enemy battery from the sound of its weapons firing

In land warfare, artillery sound ranging is a method of determining the coordinates of a hostile battery using data derived from the sound of its guns (or mortar or rockets) firing, so called target acquisition. The same methods can also be used to direct artillery fire at a position with known coordinates, so called fire control. Sound ranging is an application of sound (or acoustic) location, which is the identification of the source of sounds that may originate in the air, on the ground or on or below the water's surface. Sound ranging was one of three methods of locating hostile artillery that rapidly developed in World War I. The others were aerial reconnaissance (visual and photographic) and flash spotting. A sound ranger used aural and stop watch methods which first emerged before World War I. Stop watch methods involved spotting a gun firing, measuring the bearing to it and the length of time it took the sound to arrive. Aural methods typically involved a person listening to a pair of microphones a few kilometres apart and measuring the time between the sound arriving at the microphones. This method appears to have been used by the Germans throughout that war, but was quickly discarded as ineffective by the western allies, who developed scientific methods of sound ranging whose descendants are still used. The basis of scientific sound ranging is to use a sensor post, consisting of at least a pair of microphones, to produce a bearing to the source of the sound. When using a number of sensor posts, the intersection of these bearings gives the location of the battery. The bearings are derived from the differences in the time of arrival at the microphones located in each of these sensor posts. Typically, the sensor posts have three microphones placed in a triangular shape, typical size around 10 meters, a distance needed to obtain optimal signal to noise ratio in the lower frequency range. Since 2018, a new approach is being developed, using so called Acoustic Multi Mission Sensors, that contain both microphones and particle velocity sensors. As the particle velocity sensors have broad banded directionality, also for lower frequencies, the large (ground based) sensor post can be shrunk to the size of a "molehill".

British research continued between the wars as it did in other nations. It appears that in Britain this led to better microphones and recording apparatus using heat sensitive paper instead of photographic film. Radio link was also developed, although this could only connect the microphones to the recording apparatus, it did not enable the APs to switch on the recorder. Another innovation in the late 1930s was development of the comparator, a mechanical computer that calculated first order differential equations. It provided a fast means of comparing the coordinates of the fall of shot located by sound ranging with the coordinates of the target and hence deduction of a correction to the fall of shot.

During World War II, sound ranging was a mature technology and widely used, particularly by the British (in corps level artillery survey regiments) and Germans (in Beobachtungsabteilungen). Development continued and better equipment was introduced, particularly for locating mortars. At the end of the war the British also introduced multiplexing, which enabled microphones to share a common field cable to the recording apparatus. In 1944 it was found that radar could be used to locate mortars, but not guns or rockets. Although the radar should 'see' the shells their elliptic trajectories could not be solved. The US Marines included sound ranging units as standard parts of their defense battalions. These sound ranging units were active in the Marines both before and during World War II. The US Army also used sound locators. US Army sound ranging units took part in nearly all battles in which the army participated after November 1942. By the end of the war there were 25 observation battalions with 13,000 men. During the Okinawa campaign, the US Army used its sound ranging sets to provide effective counter battery fire. The Japanese tried to counter this effective counter battery fire with the tactic of "shoot and scoot," which means shooting a small number of rounds and leaving the firing position before the counter battery fire could arrive. While an effective tactic against counter battery fire, this approach tends to reduce the effectiveness of artillery fire. During World War II, the British made extensive use of sound ranging. There are a number of excellent memoirs that address their use of sound ranging for artillery spotting available on the web, including "The 4th Durham Survey Regiment: Sounds like the Enemy" and "Communications for Artillery Location". A "Communications for Artillery Location" article describes the electronic equipment involved with these operations. A very comprehensive account of British sound ranging units including their positions is Massimo Mangilli Climpson's 2007 account.

Sound ranging of artillery was done in Korea, but mainly was supplanted by counter mortar radar and aircraft based artillery observers. Since anti radar countermeasures were limited at this time and the UN had air superiority throughout the war, these approaches were simpler and more accurate.

Most counter battery work in Vietnam was with artillery spotting done using radar or aircraft. Australia deployed a sound ranging detachment from 1967 to 1970 in Vietnam, which operated a cross base to provide all round observation. Also, during this period the British deployed ad hoc "Cracker" batteries, with sound ranging and mortar locating radars, to Borneo and Oman. In the early 1970s an effective VHF radio link was introduced that enabled the APs to switch on the recording apparatus. Soon after, advances in electronics meant that the manual plotting of bearings and some other calculations were replaced by electronic calculators.