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M.J.B. Scanlan

 

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Early Centimetric Ground Radars - A Personal Reminiscence

This routine was only occasionally interrupted, as for instance when I went to a mini-conference on the microwave stations at Fairlight, near Hastings. The chairman was Squadron Leader Eastwood, then Head of Calibration at 60 Group, later of course Director of Research, both at Marconi Research Centre and for the whole of GEC(12). He invited me to give an account of our measurements with the power meter.

On this visit, I saw the new Type 11 radar, specifically designed to be on the same wavelength (c. 50cm) as the German Wurzburg radars, the theory being that the enemy could not jam the Type 11 without also jamming the Wurzburgs. The Type 11 was highly classified at this time, and never radiated over the sea. Whether inspired by this experience or not, Dr. Eastwood initiated after the war a highly successful series of 50cm radars for air traffic control. He was also sufficiently interested in the waveguide power measurements to offer me a job. He had already been appointed as Chief of Research to the Nelson Research Labs at Stafford: fortunately for us both, by the time I was ready to seek a job, he had moved to the Marconi Research Centre at Gt. Baddow.

In September 1944, working on one of the most southerly of 73 Wing stations, we were able to 'see' (in the radar sense) the air activity connected with the parachute landings at Arnhem. Of course, Arnhem, being about 300 km from the Suffolk coast, was far beyond normal radar range. Even if the radar could see that far, the radar horizon at 300km is about 5km above the ground, and most of the air activity was much lower than this, and would not have been seen. Evidently, anomalous propagation ('anaprop') was occurring, caused by on inverted layer in which the air temperature rises as the height above the ground increases. the radar power is trapped in a duct, perhaps 100m thick, and follows this duct over the earth's surface instead of spreading into space. In this way quite remarkable ranges were sometimes observed, especially in warmer and less windy climates.

The radar picture was most confused, of course, since echoes from a range of 300km, resulting from one transmitter pulse, would arrive back at the same time as the next pulse: echoes at slightly longer range showed up as if they were short range echoes from the second pulse. Since the radar picture was so confused, especially on the PPI, all one could say was that there was considerable air activity at a range of about 300km on a south easterly bearing.

At this time in 73 Wing, there were 11 Type 52 stations, two 27ls and two GCIs which were now equipped with centimetric radars based on the Type 277 transmitter and receiver, as well as the older 200MHz radars. There was therefore more than enough work for a QO party, allowing for lays off, leave and training, to make the rounds very three months. This busy, if rather boring, prospect, was brought abruptly to an end by Operation 'Diver': for the first time, an aerial dimenion was added to our coast-watching role.

Operation 'Diver'

The first Vls (the 'V' was for 'Vergeltungswaffe' or revenge weapon) were launched from northern France against London within a week of the Allied invasion of Europe on June 6th 1944. These flling bombs, each carrying about a ton of explosive were nick-named 'doodlebugs' by their intended victims, based on the recurrent 'phut-phut' of their ram-jet engines, which were cut off when the bomb had flown a prescribed distance. The device then glided for a small distance, crashed to the ground and exploded. Altogether nearly l0,000 bombs were launched from France, causing major activity by fighter and anti-aircraft defences: the anti-aircraft guns were greatly aided by an American 10cm radar, the first radar capable of automatically tracking its target(13) These defences, allied to the unreliability and inaccuracy of the Vl itself, meant that only about a quarter of the bombs launched hit London, whose citizens, having been free of any serious assault for more than three years, were not unnaturally indignant. They took comfort in the fact that the Allied armies would shortly overrun the launch sites.

All of this is fairly well-known, and is within the experience of many. What is not so well-known is that the enemy, also anticipating the loss of thc launching sites in France, began to equip Heinkel III bombers to launch Vls from the air: the main target was still London, the direction of the attack was now from the north east instead of from the south and the weight of the bombardment was of course much reduced. The first air-launched bombs were aimed at London on July 8th, 1944, and by the end of August, about 400 had been launched in this way, of which only 50 or so reached London. This compares with more than 200 launched within twenty-four hours from France in mid-June, of which 73 reached London.

It might well be thought that the threat of air-launched Vls was trivial as long as the rate of arrival in London was less than one a day, on average. However, the threat was taken very seriously, and a considerable redeployment of anti-aircraft guns and fighters took place in an effort to minimize the danger. The enemy tactics were to operate only at night, often in the evening, as if daylight, or at least twilight, was important for take-off: the Heinkels would cross the North Sea at a height of l00m or so, and climb to about 500m at the launch point, generally 60-80 km off shore. The position and direction of launch were important, since the trajectory of the bomb, once launched, could not be controlled either in direction or in distance to be flown. One can have a certain sympathy for the German pilots: to fly hundreds of kilometres very slowly in an almost defenceless aircraft and encumbered with a ton of high explosive must have been a nerve-wracking experience. It is little wonder that the arrival points were somewhat scattered: between 27th and 29th September, eight bombs landed in Essex, two in Cambridgeshire, three in Suffolk and one each in Kent, Sussex and Hertfordshire.


 

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