M.J.B.Scanlan; Early Centimetric Ground Radars

The following article has been reproduced from the "GEC Review", vol 10, no.1, 1995 with the kind permission of the Editor:

by M. J. B. SCANLAN, B.Sc., ARCS.
(Formerly at GEC-Marconi Research Centre)

After graduating in Physics from Imperial College, London. M. J. B. Scanlan served four years in the RAF radar branch, working on decametric and centimetric radars in the UK and the Far East. He joined the Marconi Company in 1952. and ran the Applied Physics Group in what is now the GEC-Marconi Research Centre from 1963 to 1982 working on a great variety of systems, sub-systems and components. He began editing the Marconi Review in 1979, and became the first editor of the GEC Journal of Research in 1983 and of the GEC Review in 1985. He retired from full time employment in 1986.

A previous paper⁽¹⁾ showed that the Chain Home (CH) radar system, although based largely on Marconi technology of the 1920s, sufficed to be invaluable to the air defence of Britain in 1940. The paper also described, largely from personal recollection, how an East Coast CH station, the earliest and most elaborate of all CH stations, was organized, how it operated on a daily basis, how it was calibrated and how it was maintained. Although those recollections date from 1942-43, and are based on experience of the station at Ottercops Moss in Northumberland, they would not have been very different, except only in the intensity of air activity, from an account of a station in S.E. England in 1940, when the CH system was put to its most severe test. It passed that test, despite its deficiency in low cover and inaccuracy in measuring position and height, mainly because the enemy onslaught was generally in daylight and at a great height. Some other contributory factors are given in the previous paper.

The value of the CH system was greatly diminished, however, when in mid-September, 1940, the enemy turned to bombing by night, having evidently given up all hopes of achieving air supremacy as a prelude to an invasion. Given the rudimentary state of AI (airborne interception) radar at this time, CH was not accurate enough to direct night-fighters to within interception range: much less was it capable of directing anti-aircraft gunfire.

As early as 1935, when early experiments at Bawdsey had raised hopes that the battle against the day bomber could be won, attention had been turned to the night bomber problem, because it seemed likely that if the enemy could be frustrated in his daylight attacks, he would turn to night bombing. It was Tizard⁽²⁾ who, in 1936, suggested an attempt to make a radar set small enough to go in an aircraft. The first crude AI radars were installed, first in a Heyford bomber and later in an Anson: one of these sets, in September, 1937, tracked the Home Fleet as it passed up the Channel. Thus was born ASV (air-to-surface-vessel) radar which in due course (and in later and more refined versions) played such an important role in the battle against the U-boats. AI radar was important, no doubt, but ASV was to be crucial.

The progress of AI radar is vividly described by Rowe⁽³⁾. By the summer of 1940, AI Mk. III was operational in Blenheim aircraft. Its maximum range was about 3km, and its minimum range about 250 m, any closer range being obscured by the recovery time of the receiver after the transmitter pulse. The set was also liable to 'squint', that is, to give a false indication of the position of the target relative to the centre-line of the fighter. Moreover, the Blenheim was too slow to overtake its target easily: one pilot flying a Blenheim chased a Dornier bomber for 80km, followed it over the sea to make his attack and landed at his base with empty fuel tanks⁽⁴⁾. By the end of October, 1940, only one bomber had been shot down using AI.

However, Mk. IV AI sets, with a maximum range of 6km, and a minimum range of less than 200m, installed in much faster Beaufighter aircraft, began to be available towards the end of 1940. In parallel, a crash programme to produce a dozen GCI (ground control interception) radars was initiated. The GCI set, on 200 MHz, was a derivative of the CHL (Chain Home Low) sets used around the coast to improve the low-cover capability of the chain: however, GCI sets were positioned inland, preferably in a shallow saucer of ground two or three kilometres in diameter. Hence there were no permanent echoes (PEs) from beyond the rim of the saucer. The set was equipped with a PPI (plan position indicator) display on which any aircraft within range appeared as a bright 'sausage', and an A-scope on which signals from upper and lower aerials were displayed side-by-side, thus giving a height on the target. Since the PPI radial timebase was generated magnetically by a coil rotating around the CRT neck, it was always difficult to generate a linear timebase, and thus to get an accurate display. Fortunately, any errors applied equally to the fighter and its target, and it was therefore possible for a controller to direct a fighter to within AI range.