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Nightfighting in World War Two
By Lee Brimmicombe-Wood

Part 2. Ground Controlled Intercept

Key to perfecting nightfighting tactics was the development of radar. With it, ground-based fighter controllers could direct nightfighters to intercept a raider and shoot it down.

In the late 1930s the British, in desperation, adopted a third-rate radar system and built a world-class fighter control system around it. Chain Home looked out to sea and was accurate enough to place day fighters within a few miles of a raid, where they could pick it up visually. However, Chain Home was a metre-wave radar and its enormous wavelengths, around 10m to 15m, gave it poor performance over land. It also had problems detecting low-level raiders and lacked the precision for night intercept.

The Germans had a superior ground-based radar in Freya. With a shorter wavelength, around 2.4m, it traded off a modest amount of range compared to Chain Home, but had much better discrimination. In 1940 Freya early warning radars were lined from the Danish to the Swiss frontiers, but unlike the British system they did not report to a central command post. Though they had a better radar the Germans lacked an integrated system of warning and control to hook the radars into. It took them a while to comprehend the potential of radar for control of fighters.

The earliest German attempts at fighter control were crude. If a target was picked up by Freya, a nightfighter was ordered to intercept on a bearing. But without constant updates of precise target information, successes were few.

PHOTO: A FuMG 80 Freya radar of the kind that eventually lined the coast from Denmark to the Atlantic. Initially used just for early warning, they were soon adapted with AN circuits to provide close control of fighters for interception.

In November 1939 a Luftwaffe signals detachment began to experiment with trying to control fighters using the Freya radar. Unfortunately Freya did not give the target position to the level of precision needed for a night intercept. These early radars were difficult to use, using primitive cathode ray scopes that gave distance information, but required careful interpretation to determine the bearing in azimuth.

The result of experimentation was the development of Freya-AN. This used a technique known as 'lobe switching' to give much greater precision. The radar's receiving aerial was divided into two parts, left and right, which were switched between at the rate of 75 times a second. The cathode ray scope would show whether the signal was stronger on the left or right side of the radar beam. If the signal was symmetrical it would mean the contact was dead centre in the beam.

ILLUSTRATION: The Freya-AN lobe switching system. The left-most example shows the contact in the right-hand lobe of the radar. As the aircraft moves toward the centre the signal on the left and right of the scope becomes more symmetrical.

Despite problems with height-finding (later solved by pairing the Freya with W├╝rzburg radar) Freya-AN had sufficient precision to permit night interception. The fighter controller operated the radar. He could track both the bomber and the target on his scope and give directions to the nightfighter pilot. It was highly skilled and demanding work, because the controller might only have ten minutes to complete the intercept before the bomber passed out of the radar's view. The first kills were achieved in September 1940. Now nightfighters could intercept without the aid of searchlights and the first 'dark' night fighting zones (or Dunkelnachtjagdgebeite, abbreviated to Dunaja) were established in front of the Freya chain along the North Sea coast.

ILLUSTRATION: This sequence shows the Freya-AN scope registering the fighter and bomber at different angles from the radar (to the left). In the topmost, the fighter is behind and to the port of the bomber. In the second row it is crossing left to right. In the bottom-most example the fighter is crossing but approaching firing range. The blob at the bottom of each scope is the permanent ground echo. A skilled operative could use this information to give course corrections to the fighter.

The British were also experimenting with fighter control using the more accurate 1.5m Chain Home Low radar. In the National Archive I came across plot sheets from 1940 for interception exercises off Foulness on the coast of England. There were also attempts to employ anti-aircraft gun-laying radars to guide fighters, along the lines of Freya-AN. The product of these developments was the deployment of the first Ground Control Intercept (GCI) radar, in January 1941.

GCI had many radar features that we would now recognize as 'modern', with a rotating aerial array and the Plan Position Indicator (PPI), a scope that gave a map-like plan view around the radar, which was positioned at the centre of the screen. Unlike the Battle of Britain fighter control system, where positions had to be calculated from bearings and gradually filtered through to a plotting table, a PPI scope could provide accurately filtered information in real time, permitting the controller to pass timely instructions directly to the fighter.

PHOTO: A graphic showing the features of a PPI scope.

Like Freya-AN, GCI had the disadvantage of only being able to control one fighter at a time (though the addition of a scope for a second controller soon increased capacity). But it was part of an integrated system of night defence that included high-frequency communications, well-trained crews and a vital new technology, Airborne Intercept (AI) radar, which could be carried on aircraft.

Next: The birth of AI radar.