Big Ben - V2 Watch

By 1942 British Intelligence began to take seriously the threat of rocket weapons being developed by Germany. At first it was considered that a long range rocket, i.e. the V2, was being developed by Germany; it was not until 1943 that it became clear that Germany was working on two weapons (the V1 and V2).

Consequently, a programme codenamed ‘BODYLINE’ was set up to watch for the expected long range rocket. The equipment used was the existing Chain Home (CH) Radar stations equipped with Cathode Ray Direction-Finding equipment (CRDF). During July 1943 the station at Rye was installed with CRDF. It would take a while to equip other stations earmarked for CRDF so by the end of July all CH stations between Dover to Branscombe were on BODYLINE watch. The object of the watch was to provide information on the firing points of the rocket, to allow counter measures to be planned.

As nothing happened, the BODYLINE watch was suspended but immediately reactivated when the first V1 flying bomb landed. With Allied advances on the Continent, the westerly stations on the BODYLINE watch became redundant and the focus moved to the East Coast. In Suffolk, Bawdsey Chain Home Low became involved in the Big Ben watch (but for only nine days) as well as High street CH.

By November 1944, the CH stations could give a warning of five minutes by plotting the firing points. However many of these would be false alarms as there was no way in telling which rockets were intended for targets on the Continent, which rockets would fall into the sea or which would land harmlessly in a farmers field. A system was needed to plot the rockets trajectory and predict where they would land.

Any defence against the long range V2 rocket after it had been launched would require accurate information on its trajectory. This required either a location on the rockets path to be continuously identified or at certain points on its path. It was realised it would take several months to develop equipment designed specifically for this purpose. As an interim measure it was decided to employ G.L. (Gun Laying) II’s as an early warning system as a chain of these radars could be established within weeks.

The minimum period of warning for the public was considered to be 60 seconds. Allowing for a delay of 15 seconds from the first observation to sounding the alarm, any radar used in an early warning system had to be able to detect a rocket with not less than 75 seconds remaining flight time. Of course 60 seconds was useless as a warning for the public and in the end no early warning system was developed, but valuable lessons were learned in developing a defence against the long range rocket.

G.L.II was chosen because of its wide beam and long maximum range. G.L. III had a maximum range of 30,000 yards, S.L.C. about 20,000 yards, Z.P.I. about 50,000 yards and L.W. about 90,000 yards. G.L. II could detect a target at 100,000 yards but such a result was only possible at a height of 200,000 ft owing to ground reflection. Due to the height that V2’s obtained, G.L II was the natural choice.

Long wave equipments had a long range when fixed beam coverage was required as they had bigger aerials and consequently received a bigger voltage from the returned signal. The alternative was to use short wave equipment, with a very narrow beam to increase range and to obtain the desired space coverage with rapid scanning. A further consideration was the shape of the rocket itself. An aircraft had numerous reflecting surfaces for radar detection. The V2 rocket was basically cylindrical and hence aspect was crucial – the best chance for detection was when the aspect was broadside-on and long wave lengths worked better than short wave lengths in spreading the broadside-on to make aspect less critical.

Right - The Big Ben chain of GL II's and the
echoing area of the V2 in relation to aspect.
Comparing polar diagrams of various wave
lengths shows that in order to spread the
broadside-on maximum, the longer the wave
length the better.

With these considerations in mind, the G.L. II’s in the first warning chain were fitted with vertical stacks on receiver and transmitter at 70 Mc/s to give a high looking beam at 56° A/S. They were deployed in a chain around the South-east coast looking in a sideways direction to the possible approach of rockets towards London, which at the time included all directions from the S to NNE. The sideways look ensured that observations would be made at a favourable aspect.

When the first V2 rocket arrived on Sept 8th, the G.Ls failed to detect it. Of the next 14 incidents, only one was seen which landed not far from the Felixstowe G.L. which detected it. Two reasons were identified for this failure; firstly the receivers were off tune and secondly the aerials were not connected in the correct phase. When these faults were rectified results improved markedly and from October 1944 onwards, no incidents passed undetected.

With the Allied advance through Belgium and France, many of the southerly G.L. sites became redundant as the only launching sites now concerned The Hague and Walcheren Island. Five of these sites were redeployed to the north of the Thames to strengthen coverage of the easterly sites, while three were transferred to Ostend so as to give increased warning times. Further modifications were made to transmitter pulse and receiver bandwidth and at some sites Yagi’s were tried instead of stacks, all of which yielded better results.

Right: The Big Ben chain of GL II's following the Allied advance
through Belgium and France.

An AA trial of G.L II’s in London more or less confirmed the theoretical expectations with detection of rockets possible up to a range of 150,000 yards. It was found that results of head-on aspects were better than expected, with detection possible up to a range of 50,000 yards. Although it was not possible for the G.L.II’s to follow a rocket continuously, plots of range at five second intervals allowed the end of the trajectory to be determined.

During December 1944 two notable events occurred in the Big Ben Watch programme. Firstly, nine out of the 12 Big Ben G.L II’s sets were sent over to the Continent at the request of SHAEF in order to warn on rockets fired at Antwerp. Secondly, AA Command at this time decided to attempt trials to identify a point on the rocket trajectory to allow practical AA measures to be taken. This was to be accomplished by fitting bearing split to the remaining G.L sets at Aldeburgh to determine range and with a combined reading from a set at Foreland would enable a point on the trajectory to be fixed.

The first attempt to fit bearing split to the G.L.II’s consisted of siting three Yagis side by side to replace the range and bearing aerials of the G.L. receiver. This did not yield the expected results and further trials were carried out at Richmond where the best solution was discovered to be three Yagis arranged in the same plane, the range Yagi being displaced forward with respect to the bearing Yagis. These results showed it was now possible for continuous following, which yielded the crucial information that the actual trajectory of the V2 rocket often varied from the theoretical trajectory. As well as the determining the firing point, two points would now have to be fixed on the rockets trajectory for any prediction scheme to work.

A further trial carried out in December 1944 was an attempt to achieve automatic following using a SCR 584 modified to give an extended range measurement of 96,000 yards. For S band waves, the aspect needed to be within 20° of broadside-on, so equipment was sited at Southminster looking towards London, with a search arc of 30° at an elevation of 60° to ensure a chance of following a rocket at a favourable aspect. The operators were warned of the approach of a rocket by Big Ben G.L.’s ; when the SCR picked up a target, an elevation depression rate of 2° per second could be applied allowing time for the operators to get onto the target and switch to automatic following. Successful results were achieved in some cases, at a range of about 60,000 yards but self-searching for targets proved problematic. It was decided to move the set to Aldeburgh where it could operate with “putting-on” equipment i.e. Big Ben G.L II’s with bearing split.

Similar trials with SCR 584 were also carried out by SHAEF where better results were obtained due to the more favourable aspect resulting from the vertical launch of the rocket. This allowed not only the launching point to be determined when the track was plotted but also enabled an immediate firing point to be determined, of great value to AA Command.

The final tracking system was in place in March 1944 and AA Command was seeking permission to open fire on the V2 rockets. The final plan utilised an SCR at Steenbergen which gave the approximate firing area. The Big Ben G.L. II at Aldeburgh indicated when it was on a bearing of 180° to sites at Wrentham and Walmer, which read the range. This fixed a point on the trajectory. An additional range when the Aldeburgh set was on a bearing of 200° fixed a second point to enable the necessary prediction of a 2 ½ km map square. All the information was coordinated by the G.O.R where the prediction was made with specially prepared tables.

Right: AA Command's final tracking
system and the echoing area of S
Band as used by the SCR 584.

In the end AA Command did not receive permission to open fire. The V2 attacks also came to an end at the end of March so the refusal to open fire was academic. Further trials planned at Aldeburgh (the SCR 584 trials which had not yet commenced, the use of SCR 545 as an alternative to SCR 584 and a 55 Mc/s split system using two Yagis) were ready but the attacks ended before they could take place. However, much useful information on Radar had been gathered for what was in effect the world’s first ground based anti-missile defence scheme.

References:
Building Radar, C Dobinson, Methuen, 2010
Army Radar for Big Ben Warning and AA Prediction, TNA

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