Counterbattery is a technique in artillery operations, classically defined as applying lethal force to enemy artillery that threatens, or has fired upon, one's own forces. This was relatively simple when artillery was only direct fire, so if it could hit its target, the target could see it. With the advent of indirect fire artillery toward the end of the 19th century, and as artillery range increased, the side being attacked, at first, could take no effective counteraction.
Current counterbattery is only one of the techniques used as "active defense" against enemy artillery. If facing a relatively advanced enemy who is using radar-based proximity fuzed ammunition, electronic countermeasures may be able to predetonate the rounds. The latest technique is counter-rocket, artillery and mortar (C-RAM), which actually intercepts the enemy projectiles in midair and destroys them harmlessly. C-RAM may be the only alternative when guerrillas are firing from urban areas; counterbattery and C-RAM are complementary in conventional warfare.
Detecting enemy batteries
Modern counterbattery techniques were introduced in the First World War by Canadian forces at the Battle of Vimy Ridge in April 1917. Although operations research is usually described as having started in WWII, this has been described as a predecessor. Techniques developed under Andrew McNaughton were able, with only primitive electronics and no computers, to compute the location of German artillery in approximately three minutes. The techniques identified the sound or flash of a cannon firing, and the geographic location was calculated.
While now primarily of historical interest, one of the first applications of acoustic and optical MASINT was locating enemy artillery by the sound and flash of their firing, a technique pioneered by Canadian Forces under Gen. Arthur Currie, with Andrew McNaughton in a key staff role.  The combination of sound ranging (i.e., acoustic MASINT) and flash ranging (i.e., before modern optoelectronics) gave information unprecedented for the time, in both accuracy and timeliness. Enemy gun positions were located within 25 to 100 yards, with the information coming in three minutes or less.
For some modern applications, however, acoustic and electro-optical methods have had a revival, complementing radar systems. The U.S., indeed, continued sound ranging in WWII and Korea, with limited use in Vietnam. 
Initial WWI Counterbattery Acoustic Systems
In the "Sound Ranging" graphic, the Listening Post, which is well forward of the microphone stations, sends an electrical signal to the microphone stations (MS) when the LP operator hears the gun's sound at time T0. Either manually or electrically, each MSx sends a starting pulse to an oscillograph. When the MS operator hears the sound, he stops the signal sent to the oscillograph. The oscillograph operator can then compute a time of arrival Ax, which is the difference between the T0 and TMx. Without computer assistance, the range had to be computed manually.
The positions of the microphone stations and listening posts are precisely known. Each Ax can be graphed as a hyperbola. Where the asympotes of the hyperbola meet is the position at which the gun is assumed to be located.
Where sound ranging is a time-of-arrival technique not dissimilar to that of modern multistatic sensors, flash ranging used theodolites to take bearings on the flash from the presurveyed flash observation post. The location of the gun was determined goniometrically, where the bearings intersected. Flash ranging, today, is part of electro-optical artillery detection MASINT. A related electro-optical technique are space-based Staring Infrared Sensor arrays; which detect the heat (i.e., infrared light) of missile launches and gave the first warning of SS-1 SCUD launches in the Gulf War.
Artillery sound and flash ranging remained in use through WWII and into the postwar years, until mobile counterbattery radar, itself a MASINT radar sensor, became available. These techniques anticipated, and then paralleled, radio direction finding in SIGINT, which first was goniometric and now, with the precision time synchronization from GPS, is often time-of-arrival.
If the observation was at night, the Canadian master gunner could compare the sound and flash, while only sound was available in daylight. The Canadian units still had to estimate wind, temperature, and barometric pressure on the trajectory to the German artillery. and manually -- and quickly -- compute firing orders. Optimal times were on the order of three minutes.
U.S. forces had poorer cooperation between the target acquisition personnel, initially assigned to the Corps of Engineers, and the Field Artillery.
LT Charles B. Bazzoni, an American physicist who commanded Sound Ranging Section No. 1, complained that battery officers received insufficient instruction about the potential of his service. In his opinion sound ranging deserved more than a 20-minute dissertation to 90 or 100 officers who had forgotten whether a hyperbola was animal, vegetable, or mineral. In 1922, the Field Artillery took over the sound ranging mission.
WWII, Korea and Vietnam
"In France and Germany several German railroad guns were located by sound ranging out to distances up to 55,000 meters. In Italy the "Anzio Express," a German railroad gun that shelled the Anzio Beachhead from 40 to 50 kilometers away, was located by sound ranging...75.6 percent of all enemy gun locations in the corps area were made by sound ranging and that 51.5 percent of all corps artillery counterbattery fire was based on sound ranging locations. " Sound ranging was considered the most important intelligence source for counterbattery operations. 
Due to shortages of personnel and equipment, as well as the mountainous terrain that caused echoes, sound ranging was less effective in Korea, but still was estimated to have provided 60 percent of the counterbattery targeting.
Use in Vietnam was limited to two sites on the Demilitarized Zone. Sound ranging never became operational at the Battle of Khe Sanh because the microphone wires were frequently cut by fire.
Modern Artillery Locators
Artillery positions now are located primarily with counterartillery radar, such as the US (and allied) short-range omnidirectional AN/TPQ-46, and longer-range but directional AN/TPQ-36 and AN/TPQ-37. These are part of Target Acquisition Batteries.
Other classical intelligence disciplines, such as imagery intelligence and signals intelligence, with the latter divided into subdisciplines COMINT for firing orders, and ELINT for such things as weather radar.
Much current development, however, comes from measurement and signature intelligence (MASINT). in both acoustic and electro-optical systems to complement counterartillery radar; the counterartillery mission has expanded to what the U.S. Army calls counter-rocket, artillery and mortar (C-RAM). Partially to solve Middle Eastern problems with guerrilla-launched rockets and mortars, the traditional idea of counterbattery fire, or return fire on the launching location, is changing. Especially when the launch is in an urban area, counterfire may be too dangerous to civilians, and if the rocket was on a timer, the firing crew is no longer there. In some cases, it may be possible to send troops to the firing site, but the new C-RAM approaches actually destroy the projectile in midair.
Acoustic sensors have come a long way since WWI. Typically, the acoustic sensor is part of a combined system, in which it cues radar or electro-optical sensors of greater precision, but narrower field of view.
The UK's hostile artillery locating system (HALO) has been in service with the British Army since 2002. HALO is not as precise as radar, but especially complements the directional radars. It passively detects artillery cannon, mortars and tank guns, with 360 degree coverage and can monitor over 2,000 square kilometers. HALO has worked in urban areas, the mountains of the Balkans, and the deserts of Iraq.
The system consists of a distributed array of up to 12 acoustic pressure sensors, which can compute location data on up to 8 rounds per second, and forwarding the data to the system operator. Assuming typical sensor dispersion, three or more sensors will measure the pressure wave, and the triangulation of the system computer can match a signature and help the AN/TPQ-36 and TPQ-37 Firefinder radars, which are not omnidirectional, to focus on the correct vector.
Another acoustic system is the US Army [[Po(UTAMS), which detects detect mortar and rocket launches and impacts. UTAMS has three to five acoustic arrays, each with four microphones, a processor, radio link, power source, and a laptop control computer. UTAMS, which was first operational in Iraq , first tested in November 2004 at a Special Forces Operating Base (SFOB) in Iraq. UTAMS was used in conjunction with AN/TPQ-36 and AN/TPQ-37 counter-artillery radar. While UTAMS was intended principally for detecting indirect artillery fire, Special Forces and their fire support officer learned it could pinpoint improvised explosive device (IED) explosions and small arms/rocket-propelled grenade (RPG) fires. It detected Points of Origin (POO) up to 10 kilometers from the sensor.
Analyzing the UTAMS and radar logs revealed several patterns. The opposing force was firing 60mm mortars during observed dining hours, presumably since that gave the largest groupings of personnel and the best chance of producing heavy casualties. That would have been obvious from the impact history alone, but these MASINT sensors established a pattern of the enemy firing locations.
This allowed the US forces to move mortars into range of the firing positions, give coordinates to cannon when the mortars were otherwise committed, and to use attack helicopters as a backup to both. The opponents changed to night fires, which, again, were countered with mortar, artillery, and helicopter fires. They then moved into an urban area where US artillery was not allowed to fire, but a combination of PSYOPS leaflet drops and deliberate near misses convinced the locals not to give sanctuary to the mortar crews.
Originally for a Marine requirement in Afghanistan, UTAMS was combined with electro-optical MASINT to produce the Rocket Launch Spotter (RLS) system useful against both rockets and mortars.
In the Rocket Launch Spotter (RLS) application, each array consists of four microphones and processing equipment. Analyzing the time delays between an acoustic wavefront’s interaction with each microphone in the array UTAMS provides an azimuth of origin. The azimuth from each tower is reported to the UTAMS processor at the control station, and a POO is triangulated and displayed. The UTAMS subsystem can also detect and locate the point of impact (POI), but, due to the difference between the speeds of sound and light, it may take UTAMS as long as 30 seconds to determine the POO for a rocket launch 13 km away. In this application, the electro-optical component of RLS will detect the rocket POO earlier, while UTAMS may do better with the mortar prediction.
Electro-optical artillery detection MASINT
Both electro-optical and radar sensors have been coupled with acoustic sensors in modern counter-artillery systems. Electro-optical sensors are directional and precise, so need to be cued by acoustic or other omnidirectional sensors. The original Canadian sensors, in the First World War, used visual observation and plotting of flash, as well as acoustic sensors.
Complementing counter-mortar radar is the Israeli Purple Hawk mast-mounted electro-optical sensor, which detects mortars and provides perimeter security. The device, remotely operated via fiber optics or microwave, is intended to have a laser designator.
Rocket Launch Spotter
A newer U.S. system couples an electro-optical and an acoustic system to produce the Rocket Artillery Launch Spotter (RLS). RLS combines components from two existing systems, the Tactical Aircraft Directed Infra-Red Countermeasures (TADIRCM) and the UTAMS . The two-color infrared sensors were originally designed to detect surface-to-air missiles for TADIRCM. Other TADIRCM components also have been adapted to RLS, including the computer processors, inertial navigation units (INU), and detection and tracking algorithms.
It is an excellent example of automatic cueing of one sensor by another. Depending on the application, the sensitive but less selective sensor is either acoustic or nonimaging electro-optical. The selective sensor is forward-looking infrared (FLIR).
RLS uses two TADIRCM sensors, an INU, and a smaller field-of-view single-color (FLIR) camera on each tower. The INU, which contains a GPS receiver, allows the electro-optical sensors to align to the azimuth and elevation of any detected threat signature.
The basic system mode is for rocket detection, since a rocket launch gives a bright flare. In basic operation, RLS has electro-optical systems on three towers, separated by 2 to 3 kilometers, to give omnidirectional coverage. The tower equipment connects to the control stations using a wireless network.
When a sensor measures a potential threat, the control station determines if it correlates with another measurement to give a threat signature. When a threat is recognized, RLS triangulates the optical signal and presents the Point of Origin (POO) on a map display. The nearest tower FLIR camera then is cued to the threat signature, giving the operator real-time video within 2 seconds of detection. When not in RLS mode, the FLIR cameras are available to the operator as surveillance cameras.
Integrated detection and neutralization
More modern techniques still may use a sound or flash to start the tracking system and cue precision projectile-tracking radar or sensors that locate the projectile in flight. Once the projectile's trajectory is measured, computers backtrack to the point of origin. In the 1970s, the target acquisition team had to pass the position to the gunners, who would manually compute a fire pattern.
Current techniques automate aiming the howitzers or multiple rocket launchers at the enemy weapon, fast enough that counterbattery fire is often in the air before the enemy projectile lands. If counterbattery is fast enough and lethal enough, a hostile artillery unit will not get a second chance to fire. As a consequence, "shoot and scoot" techniques are used against forces that have modern counterbattery capabilities: artillery pieces such as the U.S. M109A6 self-propelled howitzer or the M270 Multiple Launch Rocket System multiple rocket launchers are mobile, and, seconds after firing, are moving to their next position.
Since artillery batteries are often spread-out targets, counterbattery will, at a minimum, involve multiple blast-fragmentation shells fuzed for airburst over the target area. A typical U.S. response, especially when the enemy artillery is in an area where there may be civilians, is six M107 high-explosive shells from 155mm howitzers.
When the enemy weapon is in open areas, as with desert warfare during Operation DESERT SHIELD, the counterbattery munition of choice has been cluster submunitions, that spread out more efficiently than airbursts. A significant number of antipersonnel or "dual-purpose improved conventional munition" bomblets do not detonate on impact, but remain hazardous if disturbed. Effectively, such munitions create an antipersonnel minefield, and there are strong international initiatives to ban antipersonnel mines.
- JS Finan and WJ Hurley (1997), "McNaughton and Canadian operational research at Vimy", Journal of the Operational Research Society 45: 10-14
- Fred Cameron. A Century of Operational Analysis for Commanders in the Canadian Army. Military Operations Research Society. Retrieved on 2007-10-03.
- William R. Bursell (November-December 1981), "American Sound Ranging In Four Wars", Field Artillery Journal: 51-53
- Daniel W. Caldwell, Radar planning, preparation and employment of 3-tiered coverage: LCMR, Q-36 and Q-37. Retrieved on 2000-10-19
- Justino Lopez, Jr. (July-August, 2005), "Terrain denial missions in OIF III", Field Artillery. Retrieved on 2007-12-01
- Mabe, R.M. et al., Rocket Artillery Launch Spotter (RLS), U.S. Naval Reearch Laboratory. Retrieved on 2007-12-01
- Daniel W. Caldwell, Radar planning, preparation and employment of 3-tiered coverage: LCMR, Q-36 and Q-37
- Mabe, R.M. et al.. Rocket Artillery Launch Spotter (RLS).