Artillery is dangerous because it is hard to find, hard to destroy, and has the ability to very quickly attack targets via indirect fire. Importance and difficulty of finding the enemy artillery was noticed early in World War I; French were the first to try to locate the enemy artillery, by locating the gun flash produced when firing. This was hard to do if the artillery was far away (reflections had to be used) and unreliable. This it was quickly replaced by sound detection techniques, which were also adapted by British and German militaries as well.

The basis of this technique is a row of 6-7 microphones in a 12 kilometre line. Since the sound spreads in the circles, microphones all detect the sound of a gun at different times, which allows calculating location of the origin of the sound. Original system was based on ordinary stopwatch and telephone cable or radio link, but it worked very well. Similar, but automated, system is in use today. Such system has major advantages in that it is low-cost and completely passive, making it very hard to find and destroy. However, it also has numerous disadvantages. Atmospheric state determination is crucial for its correct operation. The refractive index of air has to be found along the entire sound path, and wind affects system’s performance. Up until the appearance of GPS, setting up the system was also very difficult. Due to these and other issues, it is an inherently defensive technique, difficult to use in rapid movement (albeit it is possible to have two teams of “sounders” alternately set up the line ahead of each other).

Fixing these problems began in World War II through utilization of a then-new sensor – radar – for the role. First proof of concept was during Anzio landings, where radar operators completely accidentally detected shells fired by ships providing fire support. Germans noticed it as well, so both sides began a work on putting it into practice, albeit unsuccessfully. In mid 1944 British and Canadian army radar batteries were formed in NW Europe, primarily in a counter-mortar role. But it was only after the end of the war that United States and Great Britain started serious development. First practical radar appeared in 1950.; it was the US radar MPQ-10, designed for detecting mortars. Between 1951. and 1954., the US Army received 485 radars of that type. It worked in frequencies of 2.740 – 2.980 MHz, allowing good resolution along with small dimensions of the antenna. It was quickly followed by General Electric’s MPQ-4A using the 16 GHz (16.000 MHz) frequency, which was widely used in the Vietnam War. Higher frequency limited its range, but that compromise was acceptable for the purpose.

British Royal Radar Establishment also undertook research of counter-battery radars. At first, efforts were focused on the development of Blue Diamond anti-mortar radars, to be continued with Green Archer radar from mid-1950s, which was more or less identical in performance to MPQ-4A. After that, development stagnated until British Army requested a lighter and simpler-to-use radar with the ability to detect 81 mm mortars up to 10 km with error no larger than 40 m, and 120 mm mortars up to 12 km distance with same error parameters. Result of the request was CYMBELINE by Thorn EMI, which actually improved upon the required characteristics. It has vertical scan page width of 720 milliradians, and can determine the location of a mortar upon two intercepts of the mine. Effectiveness against cannon and howitzer fire is far lesser due to significantly higher velocities and flatter projectile path.

United States had similar requirement, but they decided to use a phase radar. After eight years of development, the result was the TPQ-36/37 system, where TPQ-36 was optimized against short-range weapons and TPQ-37 against long-range artillery. Typical unit has three TPQ-36 in the front and two TPQ-37 in the rear. It includes electronically scanned array, which results in good ability of rejecting false signals as well as simultaneous tracking of multiple targets along with path extrapolation, and covers 90* azimuth. Step-scan when combined with stable coherent transmitter allows good clutter rejection. Due to quick scanning ability, the possibility of missed detection is significantly reduced. However, large antenna presents a very vulnerable target, easily damaged even by small fragments, despite the kevlar armoring present. Also, as the requirements of detecting small, fast projectiles – especially in a cluttered / jammed environment – require high emission power, counterbattery radars are relatively easy to discover.

In 1986., France, Germany and UK agreed on a list of requirements for new counterbattery radar. The result of that was COBRA AESA system, while at the same time Norway and Sweden developmed a smaller, more mobile ARTHUR system. COBRA is a mobile long-range system with a detection range of 40 km and coverage of 1.600 km2. It is capable of locating and classifying up to 40 batteries in two minutes.

Radar is typically attached to an artillery battery or their support groups, and can also be used for correcting battery’s own fire. Basic technique is tracking a projectile for a sufficient time to record a portion of the trajectory; this can be either a continuous track or a composition of several individual interceptions. In the latter case, by measuring azimuth and distance of a projectile, flight path and its origin are calculated. Once a trajectory segment is captured it can then be processed to determine its point of origin on the ground by overlaying extrapolated trajectory with digital 3-D maps of the terrain.

One very basic problem is locating projectile in the first place. Modern AESA radars can cover a wide area, but with conventional radars (especially early manually-operated ones), acoustic detection was used to point radar in the right direction. Once located, the radar tracks it; due to small sizes involved, radars are typically of higher frequencies than usual, operating in C, S and Ku bands (though X band is also common).

More modern systems can detect howitzer shells at 30 km and rockets/morar shells at 50 km. They also use INS and GPS for precise determination of radar’s own coordinates, necessary to accurately determine target’s location, and can use datalinks to pass data for counterbattery fire. This allows quick counterbattery fire as soon as the point of origin of projectiles has been determined.

This has forced the artillery to switch away from its stationary tactcs, increasing the importance of mobile artillery systems. These systems practice what is known as “shoot-and scoot” tactics, where artillery must be able to rapidly engage the enemy, switch to new firing position and reengage the enemy. This allows them to avoid counterbattery fire, which takes about two minutes to hit back, and even that only if everything works perfectly (15 s to detect and calculate origin of the rounds, 2 s to send call of fire, 13-15 s to calculate return fire, 15 s to open fire and 15 s for projectiles to reach the targets). If the artillery pieces are towed and not ready, it can take 10-12 minutes for them to return fire (15 s to detect and calculate origin of the rounds, 2 s to send call of fire, 13-15 s to calculate return fire, 7 min to hook off the guns and get the battery ready, 3 min for the first gun to fire) – times recorded by the_shadow. That being said, towed artillery is difficult to physically destroy, so it can still be effective, assuming that adequate cover for crews and ammunition is available.

Counterbattery radars are also important in naval application, as landing forces require from a shipboard system to fill the gap between landings and the operational capability of the radars brought ashore. This can be done through either provision of dedicated shipboard systems, or sparing some resources from multipurpose AESA arrays already installed on ships.
Another application where counterbattery radars are increasing in importance is defense against unmanned aerial systems. AN/TPQ-53 has proved itself capable of tracking and identifying multiple drones while at the same time tracking incoming ordnance.

Further reading

Hrvatski Vojnik, Broj 81, 13. Siječnja 1995., ISSN 1330-500X (Croatian Soldier, No.81, 13. January 1995., ISSN 1330-500X)

http://nigelef.tripod.com/tgtacqcb.htm

http://www.usni.org/magazines/proceedings/1998-04/counterbattery-sea

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