TARGET ACQUISITION AND COUNTER BOMBARDMENT

 Last updated 9 August  2002

Introduction

Counter bombardment (CB) was the general term used to attack and defeat the enemy's artillery.  But before the enemy's artillery can be attacked it has to be found.  Artillery normally deploys where it can't be directly seen by the other side's observation posts or other elements along the forward defence lines (FDLs).  This means that other methods have to be used to find the enemy's guns.

The CB battle in WW2 was nowhere near as intense as it had been in WW2.  The German artillery was relatively far smaller than in the previous war because the Luftwaffe had been given a major role in delivering firepower.  Nevertheless some 75% of British casualties in N Africa were caused by artillery and mortar fire and in the final year of the war they caused some 61% of Red Army casualties.  While airpower worked in the early years of the war the Luftwaffe gradually lost the ability to achieve even local air superiority, but Germany lacked the resources to re-build their field artillery arm.  Gun production concentrated on 88-mm FLAK, which was used in the field role.   

Nebelwefers (multiple rocket launchers) were introduced in late 1942 and mortars, being cheap and simple, were also expanded and British operational research had found that a medium mortar was worth two MGs.  However, while these presented a negligible CB threat they were a major threat and cause of casualties to the infantry and had to be dealt with.  Despite being generally smaller than the Allies German artillery was a significant force in Italy, often attacking Allied targets and was also heavily used in the Normandy campaign.

The Japanese artillery appears to have lacked any significant CB capability, both in guns and target acquisition.  In Burma their usual method was to attack gun positions with fighting patrols.  

Target Acquisition

As in WW1 air observation was a key means of observed fire against hostile batteries (HB) and other targets beyond the view of ground observers, although both flash-spotting and sound ranging were used.  AOPs, particularly in Burma, played the vital role once they became available, and were later able to take useful photographs as well.  Air photo reconnaissance was also a major target acquisition asset and other RAF aircraft could report and engage ground targets, including HBs, using the Arty/R procedure.

However, the corps survey regiment was the primary artillery target acquisition element, particularly for CB purposes. Initially survey regiments comprised three specialist batteries, one each for sound ranging, flash spotting and survey.  The regiment deployed as a regiment across the corps front in defence but provided a composite sub-unit under the flash spotting and sound ranging battery HQs to each of the two forward divisions when operations were less static. However, in 1941 some survey regiments were re-organised into two identical survey batteries, each with troops for flash spotting and sound ranging and survey that could be assigned to the forward divisions.  This organisation became standard in late 1943, and with it part of the flash-spotting battery became an observation troop in RHQ. This undertook cross-observation tasks when air-burst ranging was used against targets out of sight to ground observers.  They also had a capability for air photo analysis and deducing target co-ordinates from air photos.  The detailed organisation of this regiment is linked to 'Artillery Organisations'.  

The typical layout of a survey battery assigned to a forward division is shown in Figure 1.  It shows the sound ranging (SR), flash spotting (FS), and survey (Svy) troops from a survey battery and a ranging (Rng) section from the regimental HQ troop.  This RHQ troop ranged friendly artillery fire onto targets by using cross observation, usually of airburst shells.  The SR troop had 2 APs, its microphone base and computing centre, the FS troop had 4 FSPs and a computing centre, and the ranging section 3 OPs and a computing centre.  The survey troop had 2 survey sections, each 3 pairs of surveyors and a computing centre.  The FS and SR troops would have direct communications to the ACBO, where battery HQ was sometimes collocated.

Figure 1 - Schematic of a Survey Battery and RHQ Ranging Section in a Divisional Area

 After D-Day the German mortars rook an increasingly significant toll of British infantry and the counter mortar (CM) battle became increasingly important.  This led, in late 1944, to divisional counter mortar batteries being formed, equipped with CM radars and new sound ranging equipment.

Flash Spotting

Flash spotting originated and was widely used in WW1 where, in the British Army, it was operated by the Royal Engineers.   It became an artillery responsibility after WW1 when the Royal Artillery created its own survey organisation.  

The essence of flash spotting to locate HBs was to get accurate cross-observation of muzzle flashes from at least three different positions and the real trick was to ensure that the observers were all looking at the same flashes.  Flash spotting troops were organised into an observation section of several flash spotting posts (FSP) that reported the observed bearings to muzzle flashes to their troop HQ and reporting centre that plotted the bearings to find HB's location.  Flash spotting became harder with increasing use of flashless propellant, longer range guns deploying further back also made it difficult, as did the practice of deploying guns behind a ridge.

The deployment of the FSPs depended on the terrain in the divisional area.  The posts needed height so they could see well into enemy territory.  In some cases they used pre-fabricated towers to gain height and therefore visibility.  The flash spotting 'base' (line of posts) was usually 3,000 - 10,000 yards long, but could be up to 15,000.  Accurate HB locations required observations from three FSPs, and the troop had four FS parties.  The FSPs also reported other enemy activity that they observed and undertook crater analysis of hostile fire, from which bearings to the HB and the calibre of the guns could be found.

The basic equipment of a FSP was 'Instrument, Flash-spotting, No 4, Mk 1'.  This was a tripod-mounted binocular device with a bearing scale.  Posts were also equipped with more conventional instruments including a tripod-mounted high power spotting telescope that provide several magnifications up to ×30.  Communications could be by line or radio, the latter obviously far quicker to deploy.  However, line enabled use of the 'flash & buzzer switchboard'.  FSPs had telephones (D Mk3) with a 'flash key' (basically a buzzer button), the switchboard had light and buzzer displays that responded to signals from the 'flash key', when they functioned together then it meant that the FSPs were observing the same HB firing flashes.

Once it was clear that the FSPs were all observing the same apparent source then their observation bearings were reported to the troop HQ/plotting centre where they were plotted to locate the HB.  The  HQ/plotting centre usually used three different plotting boards for various purposes, including one large scale one for maximum accuracy and one that also showed all known HBs so that only their activity had to be reported.

There were several different deployment modes, which traded deployment time for accuracy of results, although deployment could be progressive to get to a fully accurate base.

• The fastest and least accurate was a single 'double post'.  An FSP established a sub-post 200 or 300 yards away and did its own plotting of the two bearings.  This could produce HB locations with about a 'B' accuracy out to about 10,000 yards and took little more than an hour to be ready to observe.
• A three post base, with the FSPs positions determine from the map and using wireless could be ready to observe in about 4 hours.  In could produce 'Z' accurate locations of actual muzzle flashes to about 20,000 yards and sky reflection flashes of about 'B' accuracy.
• A four post base, fully surveyed and using line communications could be ready to observe in about 12 hours.  In could produce 'W' accurate locations of actual muzzle flashes to about 20,000 yards and sky reflection flashes of about 'A' accuracy.

Although FS was the main method of visual observation of HB, it was not the only one.   All-arms were responsible for reporting hostile shelling and mortaring.  This used   ‘Shelreps’ (hostile shelling reports) and 'Moreps' for mortars.   This method was not particularly accurate for locations (the reported bearings were usually based on sound and rarely sufficiently accurate), but was useful in providing information about HB activity, particularly when the 'area shelled' was reported as well. ‘Moreps’ were of less value because hostile mortars tended to move more often.

Sound Ranging

The Russians claim to have successfully used sound ranging (SR) on a training range in 1910.  It evolved rapidly in WW1.  Initially most armies tried 'aural' methods, basically large ear trumpets.  The British and French quickly abandoned this method for microphone methods although the Germans retained it throughout the war.  Microphone sound ranging needed two technologies, one to detect the low frequency sound (about 20 hertz) of guns firing, the other to record the signals.  The first needed the invention of the low frequency microphone using a heated platinum filament; the second was provided by a pen recorder, the 'pens' plotted a trace-line on a 'film' (electrical on sensitive paper) .

Sound ranging used a base of several microphones connected to a pen recorder in a plotting centre. This recorded the relative times of arrival at each microphone of the sound of guns firing.  The were several methods of converting the time difference into the HB location.  A firing gun produces three different sets of sound waves:

• The gun-wave, the actually noise of the gun firing.
• The shell-wave, the noise of the shell in flight particularly when in is supersonic, this is a higher frequency sound and the need to avoid being swamped by these sound waves is what made the low frequency microphone so important.
• The burst-wave from the shell bursting.

The last could be used to range friendly fire onto a HB location.  The pen recorder film recorded the duration of the sound (to 1/100 second) and the strength of its pressure wave.  It wasn't practical to have the pen-recorder running continuously, therefore there had to be a way of switching it on when HBs were firing.  This required Advance Posts (AP) forward of the microphones.  Figure 2 illustrates the basic set-up. 

Figure 2 - Sound Ranging

In addition to using the relative time to deduce location, the recorded sound shape of a gun-wave on the film was often distinctive for each calibre of gun, which provided additional important intelligence information. It also made it difficult to spoof with HE explosions.  However, recording the gun-wave and the burst-wave enabled the time of flight  to be determined and graphs were used to find the calibre by this means as well. The problem with sound ranging was that lots of guns firing together was liable to 'swamp' the recording, particularly with their shell-waves.

The sound waves are affected by wind and temperature (the speed of sound varies with temperature) and so the data read from the film had to be corrected in the plotting centre.  This need for meteorological data was the main reason for each SR troop having a detachment of RAF meteorologists to provide the necessary meteorological data.

The microphone base, up to 6 microphones, was about 3000 - 5000 metres behind the FDLs and ideally behind most of friendly field artillery positions, with the APs at least 1000 metres in front of the microphones.  A 'regular' base, where the microphones were equidistant was best from a plotting perspective because it was simplest.  A regular base could be either straight line or curved.   Each microphone was paired with its neighbours and each pair was a 'sub-base'.

A straight line base would be up to about 8,000 metres, while a curved one could be as short as an arc of about 1500 metres at a 16000 metre radius.  The problem was fitting the base to the terrain, not too difficult in the desert but a real challenge in Burma and a fair one in Europe. Siting microphones in good listening positions was very important and the terrain could have many undesirable effects that interfered with it. When a regular base was impossible then an irregular one had to be used.  It was also important to accurately 'fix' by survey the position of each microphone.  This was to a far higher level of accuracy than was necessary for gun positions.

Initially all communications between APs, plotting centre and microphones was by line, although it wasn't necessary to lay a line from the plotting centre to each microphone and each AP.  The APs had as 'AP unit' that enabled them to switch on the recorder in the plotting centre.  One line could serve a cluster of three microphones and an AP.  However, mid-war radio link equipment was introduced that could support up to five radio linked microphones and could be used in combination with line linked ones.  The radios used were No 11 (SR) with a 'sender SR' at each microphone, these each communicated on their own frequencies to their own R 105 receiver in the plotting centre.   However, the APs could not switch on the recorders remotely by using their radio.  At the end of the war a Mk 2 radio link was introduced that used a single multi-channel receiver in the plotting centre.  The SR variant of the No 11 wireless used a different frequency range to the standard set, but the problem was that these radios needed frequent re-tuning not to mention battery changing, see Communications.  They were not well suited to unattended operation.

In the plotting centre the line or radio links were connected through a Control Unit SR No 1 to the Recorder SR No 1 Mks 3 or 4, which could be switched on by the AP unit.  Once a film trace had been produced it had to be analysed, corrected for met and the results plotted.  There were several plotting methods ('circle', 'graph', 'secant' and 'asymptote'), but the asymptote method gradually emerged as the preferred one.  This method deduced the bearing from each sub-base and plotted them.  The plotting process didn't use paper and pencil methods but fine strings that could be quickly laid out on the plotting board to give a ‘cats-cradle’ and the HB location.  Plotting wasn't used to range CB fire onto a HB, instead a mechanical device called a ‘comparator’ was used, it solved first-order differential equations.

The accuracy of this sound ranging system, expressed as a Probable Error in range was approximately the square of the distance to the HB in thousands of metres.  The maximum range for locations was about 15,000 metres in an arc about 10,000 metres wide in front of the base.  A base of 6 microphones fully surveyed could produce 'Z' accuracy HB locations and took 6 - 12 hours to deploy.  Five microphones with 1000 metre sub-bases fixed from the map gave 'B' accuracy locations but could be deployed in about 2 hours.  Of course once deployed it could be properly surveyed.

The problem with the 6-pen equipment was that it was not sensitive enough to detect and locate medium (8.2-cm) mortars.  However, in the second half of 1944 the 4- pen recorder (Recorder SR No 2 Mk 1) was introduced.  The following photo shows, from the left, the recorder, power supply and amplifier.  Associated with this were new microphones and a new control unit (Control Unit SR No 2 Mk 1).  

The 4 microphones were placed in shallow pits (typically 400 to 1000 yards apart) and covered with windshields, radio link was not used.  

These additional elements were provided as an extra 4 sections per survey regiment, although it did not happen all at once.  Table 1 gives a comparison of German and British equipment, it was published in March 1945.

Table 1 - German and British 4-pen Recorders

In 1945 a further enhancement was introduced, 'Carrier Link SR'.  This enabled all microphones and APs to be connected to the plotting centre by a single telephone line, the transmitter units at each microphone each used a different carrier frequency.

Sound ranging was far more effective that flash spotting in terms of the number of HBs located.  However, they were complimentary because flash spotting could be used when there was heavy firing by friendly artillery, which tended to swamp sound ranging.

Radar

 In 1943 the British started mortar locating experiments with radars and in 1944 their use became quite extensive in both NW Europe and Italy. The radars used were AA No 3, Mk 2 (GL III ) and AA No 3, Mk 5 (SCR 584).  The latter were better and could produce locations accurate to about 25 yards.  The former was also used to track meteorology balloons and hence the production of meteor telegrams.  Radars designed for observing artillery fire, ground surveillance and mortar locating were advanced in development when the war ended. However, experimental observation of fire radars were successfully deployed early in 1945 to Italy and NW Europe.  In mid 1944 British and Canadian army radar batteries were formed in NW Europe, primarily in a counter-mortar role.  The next table summarises the field artillery radars.

 Table 2 - Radar Summary

 Counter Bombardment

From early in the war each corps had a CB staff some 30 strong including seven officers, the senior (a major) being the CBO.  The doctrine was to handle CB at corps level, devolving to division level in mobile operations.  The corps CB staff could provide sections comprising an Assistant CB Officer (ACBO) and staff to two forward divisions where they worked with the HQ of a survey battery.  However, at the beginning of 1944 a third section was added and this worked with the air-photo interpretation section (APIS) in corps HQ.  This CB section had direct wireless communications with an airfield so that it could task CB sorties.  The CBO also had VHF communications to Arty/R aircraft.

The CB staff were provided with dedicated radio communications and were not just concerned with locating HB and attacking them.  They were also an intelligence agency responsible for CB Intelligence and providing intelligence derived from analysis of HB to the General Staff (Intelligence) branch. They used various specialist techniques to deduce information about the enemy order of battle and impending activities from the deployment and firing patterns of their guns and mortars.  These included the Shelling Plot, Hostile Battery History Sheet, Gun Density Trace, and Shelling Connectivity and Activity Trace (which showed which HBs fired where and when).  These techniques could be useful against the Germans with their short range infantry guns firing in support of their infantry regiment and small divisional artillery, because if an ARKO (artillery HQ with non-divisional artillery) appeared it was probably an indicator of an impending operation.

CB policy could be either active or passive.  Active meant attacking HB when they were located. Passive meant not attacking them until an appropriate time.  In practice it was seldom this simple.  One factor in deciding whether or not to attack an HB was the accuracy of its location.  The standard accuracies were the following distances from 'truth':

• J - 10 yards
• W - 25 yards
• Z - 50 yards
• A - 100 yards
• B - 200 yards
• C - 300 yards
• D - 400 yards

As the AGRAs evolved they had an increasingly important role and in the final stages of the war took responsibility for the CB targets in major formation fire plans with the corps CBO working from the AGRA HQ.

HB could be destroyed or neutralised.  Destruction was best achieved by using destruction procedures with a single gun engaging a single gun, towed guns being notoriously difficult to destroy. Neutralisation, in the sense of suppression, was in fact a misnomer; the method of attack was short bursts of fire at irregular intervals, typically using a ratio of initially two but later 5 - 10 guns against one.   The objective was to cause casualties and damage to achieve a degree of neutralisation.  

Predicted fire was seldom really effective for CB, even when 'sweep and search' procedures were used to increase the size of the area shelled.  As in WW1, air observation was the best solution and AOPs were extensively used for observed CB fire although they often relied on seeing guns firing, and it needed luck to get the aircraft into position when HB fired only a few rounds at a time or not fire if an Auster was in view.  Sound ranging could also range guns onto located batteries using the comparator and the ranging sections from the HQ of the survey regiment could also range fire onto HBs.

CB fire used a special type of fire plan - the 'Bombard'.  The CB staff prepared HB Lists and issued them to batteries, which produced data to engage them, excluding corrections for non-standard conditions, which were produced when the HB was engaged.  The ‘bombard’ procedure, could be on-call or scheduled during a fire plan or invoked any time against a troublesome HB, used the HB List.  However, in July 1944 neutralisation was generally abandoned for CB fire in NW Europe, not least because the British had defeated the German artillery. Air observed one-on-one-destruction shoots became normal.

Using the bombard procedure, the CB staff ordered a mix of batteries or troops to engage a listed HB using predicted fire. Aim points were distributed depending on the information available about the HB, including aiming ‘gun-on-gun’ if their co-ordinates were known. Short bursts of fire at irregular intervals were applied.  In Italy, German guns were often concealed in caves and bunkers so neutralisation was ineffective.  This led to the use of AOP observed destruction shoots. The problem was that these took a long time, perhaps only one per sortie, this in turn led to the Festa system where an AOP engaged several protected guns simultaneously during each sortie.

Counter Mortar

Mortars became a major menace (as were nebelwerfers when they appeared) and being short range their deployment was very different to guns. This led to the creation of separate counter-mortar (CM) organisations as part of divisions in Italy (in about May 1944) and NW Europe.  Late in 1944 divisional counter-mortar batteries were formally authorised.  These were equipped with both radars and 4-pen recorders and subsumed the existing counter-mortar organisation.  A CMO was created at divisional HQ and worked with the ACBO as part of HQRA and there was an ACMO at each brigade HQ, often an infantry officer.  

The brigade CM section included listening posts and a sergeant responsible for crater analysis.  Airborne divisions also received a CM capability by adding listening posts, 2 × 4-pen recorder sections and a plotting centre to the Forward Observation Unit (Airborne).

Nebelwerfers didn’t fit neatly into either CB or CM and one or other would take responsibility by local agreement.    

 Copyright © 2002 Nigel F Evans. All Rights Reserved.