Radar, Gun Laying, Mk. I and Mk. II

I for short, was a pre-World War II radar system developed by the British Army to provide range information to associated anti-aircraft artillery.

Based on the early Chain Home radar's electronics, GL used separate transmitters and receivers located in wooden cabins mounted on gun carriages, each with its own antennas that had to be rotated to point at the target.

The first mention of radar in the UK was a 1930 suggestion made by W. A. S. Butement and P. E. Pollard of the Army War Office's Signals Experimental Establishment (SEE).

[6] The Cell was initially given the task of improving anti-aircraft fire, and was told that the main problem to address was the accurate measurement of range.

When these sets demonstrated the ability to easily pick up ships in the English Channel, the Army Cell started a second group to adopt these systems to the Coast Defence role (CD), providing both range and angle measurements with enough accuracy to blind-fire their shore batteries.

By the time GL was ready to begin testing, the system was able to operate at wavelengths between 3.4 and 5.5 m (11 and 18 ft)[9] reducing the antenna size to a more manageable several-metre length.

[6] CH-type radar displays use a time base generator to produce a sawtooth wave voltage that is fed to one of the inputs of a cathode ray tube (CRT).

The return signal is amplified and then sent into the CRT's other channel, typically the Y-axis, causing the spot to deflect away from the straight line being created by the time base.

The single half-wave dipole antenna was mounted on a short vertical extension at one end of the cabin, with the "line-of-shoot" along the long axis.

[13] In the field, the transmitter would be aimed in the expected direction of attacks, and the receiver placed some distance away to help protect it from the signal being reflected off local sources.

[18] There were enough parts left behind for Wolfgang Martini's radar team to piece together the design and determine the basic operational capabilities of the systems.

[18] Luftwaffe radars for both early warning (Freya) and gun-laying (Würzburg) were significantly more advanced than their British counterparts at that time,[19] operating on much shorter wavelengths around 50 cm.

Despite being aware of Chain Home, German reports on the state of the Royal Air Force written just before the Battle of Britain did not even mention radar at all.

[18] The GL team had already started plans for a greatly improved version of the system that could also provide accurate bearing and elevation information.

After considerable study, using reflectors hung from balloons and testing against the occasional aircraft, it became clear that the main problem was the levelling of the ground around the station.

For this role he chose Patrick Blackett, who had World War I experience in the Royal Navy and had since demonstrated considerable mathematical ability.

Blackett planned to study the AA problem from a purely mathematical standpoint, a concept that proved extremely valuable in other areas of air defence, and would ultimately develop into the general field of operational research.

Blackett deliberately chose members from different backgrounds, including physiologists David Keynes Hill, Andrew Huxley and Leonard Ernest Bayliss, mathematical physicists Arthur Porter and Frank Nabarro, astrophysicist Hugh Ernest Butler, surveyor G. Raybould, physicist I. Evans and mathematicians A. J. Skinner and M. Keast, the only woman on the team.

[33] Their goals were neatly summed up by Blackett: ...the first task was to work out the best method of plotting the [radar] data and predicting the future enemy position for the use of the guns on the basis only of pencil and paper, range and fuse tables.

[35] The Circus soon added a fourth trailer to some AA sites in the London area, dedicated solely to recording the inputs to the predictors, the numbers of rounds fired, and the results.

The official history, published just after the war, noted that between September and October 1940, 260,000 AA rounds had been fired with the result of 14 aircraft destroyed, a rate of 18,500 rounds-per-kill.

[32][36] Pile commented on the improvements by noting: The initial difficulties had largely been smoothed away, and on May 11–12 [1941], when the raids were so widespread that we were given greater scope, we obtained 9 victims, with one probable and no fewer than 17 others damaged.

[21] Displays were located in a wooden cabin below the receiver array, including separate CRTs for range, bearing and elevation, allowing continual tracking throughout the engagement.

[37] The introduction of the cavity magnetron in 1940 allowed radars to operate effectively at much shorter microwave wavelengths than was possible with earlier vacuum tube designs.

Using a technique known as conical scanning, a rotating version of lobe switching, this could be further reduced to well under ½ a degree, more than enough to directly lay the guns.

Normally a time base is triggered to start its sweep as soon as the signal from the transmitter is seen, but as noted above, this would not provide the accuracy required for this role.

The output of the magslip was used to directly turn the controls on the predictor, allowing the radar to continually update the range measurement.

[41] The bearing operator would then turn the entire receiver cabin using a gear set connected to bicycle pedals, looking for the point when the signal disappeared, indicating that the target was now perfectly aligned between the two antennas.

The upper antenna of the vertical pair was able to be moved up and down the ladder-like extension, causing the lobe pattern to shift and thus allow the altitude angle to be measured.

The wide-pattern antenna would be used during the initial search, and once a target was selected a switch was thrown to move the transmission to the narrow beam.