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GL Radar

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GL Radar, or gun laying radar, was a World War II development used to target anti-aircraft guns. Early radar merely provided targeting information, which the operators manually input to the fire control system, while the later and more advanced radars were able to send their information directly to the control system, making them considerably quicker in operation.

The Royal Artillery Museum gives a brief introduction to this targeting system's basic layout, and also suggests an alternative meaning for the GL acronym:

GL means Ground Locating, the first name given to the anti-aircraft locating equipment which began to be developed in early 1939. The first set, GL Mark 1 later became Equipment Radar AA No 1 Mark 1. Each of the stations had a Transmitter and a Receiver unit and both of these were housed in a wheeled cabin. Because the signals received from the target were affected by the lie of the land around the receiver cabin a wire mesh net was erected on wooden stakes around the cabin so that the return signals from the target arrived on level ground. The mat was about 5 feet above the ground and because of this the Cabin had to be raised above ground level, sitting on a concrete block. To get the cabin up on the block a ramp was required. The ramp is not therefore a GL Mat Ramp but a ramp up to the block on which the GL Receiver stood.
- The Royal Artillery Museum, Royal Arsenal, Woolwich, London SE18 6ST.

The earliest GL radars were not notable for their effectiveness, and while they could provide ranging information, were not capable of targeting the guns:

The GL effort proved less impressive. About 400 GL Mark I sets were made, followed by about 1,600 GL Mark IIs. They were crude radars, operating at in the range of 5.5 to 3.5 meters (54.6 to 85.7 MHz). They were capable of ranging but not targeting, which still had to be done by eye. The limitations of GL reflected the entire army radar effort. For the first years of the war, the British Army lived up to the stereotype of stodginess that the Air Ministry had transcended.

The GL Mark II did have its fans. When the Soviet Union joined the war against Hitler after the Nazi invasion of the USSR in June 1941, the British would send the Soviets a large quantity of GL Mark IIs. While the Soviets had developed relatively crude "RUS-1" and "RUS-2" fixed-station radar sets and fielded them in small numbers, the GL Mark II was simple, effective to a degree, and far better than anything else the Soviets had. They designated the set the "SON-2", produced a limited number themselves, and were given hundreds of GL.IIs by the British. They would be given improved Western radars later.
- The British Invention Of Radar[1]

Although there are many references to GL Radar, very few go on to describe the system, its many variations, method of operation, or technical basis. For these reasons, the account given below is of particular interest. For those of a more advanced technical background, it should be noted that the article appears exactly as published, with any apparent typos and technical slips or errors also carried over, hence these need not be reported.

GL radar memories

The following is reproduced under the 'fair dealing' terms offered by the BBC in association with their WW2 People's War project.

WW2 People's War is an online archive of wartime memories contributed by members of the public and gathered by the BBC. The archive can be found at this link

Radar No 2, Gopshall Hall, Twycross

Gopshall Hall, Twycross, was an incredibly lonely place, and resident students impressed this on the new intakes by greeting them in the drive by swinging from the trees and making simian noises. Here I had my first introduction to actual equipment - a GL (gun-laying) MkII AA (anti-aircraft) system, with transmitter (tx) and receiver (rx) display in separate trailer cabins.

We were highly impressed with security, for the place was circled with a double layer of barbed wire and armed guards. Mk1* was met occasionally but in my time it had already given way and memory does not come to the front to describe differences. Being used to seeing radios with a maximum of 9 valves, it was an enormous shock to my system and gave a feeling of near despair to be shown equipment with about 66 of them (? that figure seems to stick in my mind). The course was to me, immensely interesting. Given that, the final exam was no problem.

The GL transmitter was a pair of air blown triodes operating at 5 K volts, whose modulator was a blocking oscillator operated at about 1500 c/s, later wobbulated to avoid jamming , and whose pulse of about 1S duration was determined by the blocking oscillator grid capacitor. The short duty cycle put the real power limitation (about 0.25 Mwatt) on the space charge of the filaments.

The Lecher line tuning was push pull, and was bent into a circle so that a shorting bar could be rotated from the front of the cabinet. Both the transmitter and Receiver/Display Unit were mounted in cabins, which were rotatable on the travelling trailer chassis. A pillar came up from the floor, with bicycle pedals hand operated. The chain went down the pillar to the cabin turning gear. The Mark of the equipment seemed to be determined more by the presentation of the received signal.

The rx allowed the use of straight Ally Pally TV RF receivers, tuned to around 60MHz followed by an IF at 15MHz. All the RF and IF valves seemed to have been EF50 pentodes - the type which subsequently flooded the surplus market after the war. In a surrounding metal can they carried their own screening, and had screw on retaining rings.

More technical resources

I believe I remember 5 aerials, with the highest having a reflector that could be shorted at the centre. With an otherwise 180 deg uncertainty, the change of signal strength (with and without reflector) allowed the right polarity of direction to be determined. The four operational aerials were horizontal dipoles in a flat vertical array mounted on outriggers turning with the cabin, and had a narrower beam (about 20 degrees) than a simple dipole. The array had a vertical lobe pattern complicated by the ground reflection. Signal balance (upper and lower) was effected by means of a cam operated goniometer, between upper and lower aerials, so that maximum signal from the target was obtained at the correct azimuth setting.

'Bedford Bastard'

I don't think the system was much good until Bedford of Cossor came up with the 'Bedford Bastard'. It was so obviously a late addition, being bolted onto the front of the display unit, and a scandalous rumour circulating at the time said it was conceived overnight in an alcoholic haze. Still, it was very much better than anything else we had, and we actually started shooting down planes. Until then 'ack-ack' had been more of a frightener than a threat.

There were two 5" display tubes, bearing and elevation. The time base was a pure exponential charge on a capacitor, of a voltage nearly equating the tube supply, and the equally large X shift potential required to bring the spot back centrally was obtained from a massive hand-wound, oil-filled exponential-law potentiometer, about the size of a small car engine.

The TB was left exponential, since the law was accurately predictable and repeatable. Range was measured from the hand operated rev. counter, which turned the exponential X shift pot. I cannot remember a time when all data was not sent automatically to the predictor via mag slips (Selsyns), and certainly, later in the war, the guns were laid via mag slips and oil motors by the predictor.

Selsyns is a trade name for a rotary device with a three phase field. The rotary pick-up coil of the transmitter version passes on a signal, whose phase is an indication of the position of the rotor. The receiver is an indicator that sets itself to a position where, by cancellation with the transmitted signal, there is no residual flux giving torque to the rotor. Or it may have been a control to the oil motors which turned a transmitter until the signals cancelled (can't remember).

Remembering the detail

Memory is often dodgy. We must remember that all this was 50 years ago, and there is uncertainty about what, precisely, Bedford (Cossor's Ch Eng) came up with, but I seem to remember a common shutter rotating over the two CRT's, presumably to blank out fly back, stationary spots and the tube not under consideration. It would help if I could remember which power supplies were floating. I can't believe that the TB and the range pot output just went to the X-plates.

To have put the X-plates at a potential massively different from the rest of the CRT would have produced focusing and brightness problems. I have given thought to the problem, and now believe the supply was isolated, heavily insulated and fed only the TB capacitor and range potentiometer. The same motor that operated the blanking (?) disc also switched aerial outputs, and rotated the aerial lobe by phase switching, and putting a small shift voltage in turn on each of the four X-plates. This gave a split image on each of the tubes.

Bearing was established by rotating the cabin, using the hand-operated pedals, until the split bearing signals matched for amplitude, and also was the split elevation signals using the cam-operated goniometer. The control of the latter was a linear handle rotation, and had, additionally, a shaft protruding from the side of the cabin, which could mount a telescope used to sight a balloon carrying a reflector when the station was being calibrated.

To ensure best standardisation of ground effect on the aerials, sites of a semi-permanent nature had a mattress of wire netting, mounted on short stakes, over the area of the site. It was about 50 yards in diameter, unbroken except for a walkway to the cabins. Oddly enough, security has remained pretty firm over the years, and in spite of reading every book on radar that I could lay my hands on, I have never found reference to Bedford`s achievement.

Move for security

After Gopshall Hall, I was posted to an Ordnance Depot at Old Dalby, in the Midlands, near Nottingham, which was receiving equipment of all kinds from the manufacturers, then modifying it up to date, and despatching it where required round the world. It had REME base workshops, of which the radar part was about 40 strong. It will be appreciated that security was very strong on radar, and although factories were making and testing assemblies, no civilian company seemed to be putting them together and testing them as complete stations.

This then, for security, was our function, and we became the last part of a production line. In spite of no instruction (I don't know who could have given it) we were given handbooks which had to be locked in the company safe until actually required, so we were able to muddle through.

Army ways

The army is renowned for the stupidity of some of the things it does, and apart from a week's detachment on loan to an AA Regiment modifying their GLs for shipment to Madagascar (May 42), this was the last I ever saw of a GL Mk II, or any AA station. So you could say our GL course was practically wasted time.

Some centrimetric radar, with 2 paraboloids of about 3 foot diameter, of Canadian manufacture (Mk III probably 3cm), was coming through just before I left Old Dalby. I have since come to understand, without verification, that the receiver dipole was off-centre, and rotated such that its lobe was rotated in a circle - obviously ideal for AA work.

However, I was told that I had been selected to go on the Tizard Commission to America, as a mechanic, to service and repair where necessary the equipment that was going with them. But this posting was baulked by my colonel. I felt at the time this was to keep know-how within the unit, but it could have been that he had received forewarning that we were going to assemble and test centrimetric stuff for Coast Artillery, which is what happened three months later.

Centrimetric

The equipment we then started assembling was Naval 10cm microwave, about which we had been taught nothing. It was 271P, in small numbers, initially, and then 271Q. Memory says the difference was that the latter totally eliminated coax cable, which was terribly lossy, and the route from the transmitter, with its branch through a TR (transmit/receive) cell to the receiver, was wave-guide all the way up to the single 6 foot paraboloid dish.

Two bits of bent wire about 1 inch long protruded into the dish from the back, as monitor pickup aerials. These went to a thermocouple, whose dc output was read on a meter on the front panel, but was not in itself a measure of transmitter power. I remember a problem of keeping the reading down (because of now increased high powers involved), tying these aerials into knots to diminish their performance. The overall equipment performance was judged from the height of local permanent echoes.

The wave-guide was about 2.5 x 10cm, and became a circular wave-guide for the rotating joint. Since the Germans had no wave-guide technology, and it would have been well-nigh impossible anyway for the 50cm, which is the lowest they could achieve. They had to use coaxial cable, which limited aerial rotation to 360deg, as on 'Freya'. The rx pickup crossed the wave-guide, and a 2.5 x 10 cm ashtray sliding in the wave-guide (Adjustable) became the reflector. The 'Bed-of-Violets', from memory, was a string of 4.7Mohm across the charge capacitor in the transmitter as a bleeder for safety.

Centrimetric training, at last

We had been building 271 sets at Old Dalby for some time before some organiser at HQ or Records realised that none of our little team had ever had a centrimetric course. Common-sense said this should be at No.1 Radio Mechanics School, so off we duly went. Trouble was, none of the staff there had any knowledge or experience of it. They had a system that had been delivered for a week, and it was assembled but had not yet been switched on.

The teaching staff were apologetic, and explained that both they and us students were going to have to learn about it together as best we could. There was a great deal of relief on their part, when they learned that we students from Old Dalby knew a fair bit about it already, having been assembling and testing them for some months already. Our roles were therefore switched, and we set our own final exam paper, in which I did so well - knowing all the answers beforehand - that I was trade-test upgraded to Class I Coast Artillery, which at least increased my Army pay by about 50%.

West Africa

We had not been back at Old Dalby for long, before I was asked to construct a prototype with the paraboloid in a lantern (impregnated wood and Perspex), test it, disassemble it, and take a detachment to Beachy Head, on the south coast, to reassemble it there and get it operational. I was allowed to go home on leave first, on the understanding that if the equipment arrived at Beachy Head in my absence I would return immediately.

In the middle of the week I returned back with Syb from the pictures, to find a telegram recalling me. At Old Dalby I found the recall was not for Beachy Head at all, but to go to some foreign part, and I was immediately returned home on leave for the rest of the week - I was to call it embarkation leave. The issue of tropical kit suggested Burma and the Far East, but we hadn't been on the boat for long before we knew it was West Africa - Freetown to be precise - where we were headed.

At No 3 West African Base Workshops, in the grounds of Wilberforce Girls School, we enjoyed really good food (after UK rations) since it was also the home of a cookery school. There we learned our objective was to install a 10cm gun-laying equipment (new to us since our experience was only with the surface watching 271 types). The Coast Artillery Regiment was very civilised (they had two tennis courts, which I was free to use when I could find a partner), and occupied the buildings built for Frank Buck when he filmed 'Bring-'em Back Alive', a documentary about capturing animals for zoos.

Sierra Leone is the home of the leopard (Leopard-men is the name of a secret society) and also the gorilla. The Coast Artillery Unit had 2 x 9.2-inch guns overlooking Lumley Beach, and guarding the entrance to the harbour. They had a (trailer-mounted) 271P equipment, which was used for surface watching, and our posting was to install in a newly built pillbox a Coast Artillery gun-laying equipment based on 271Q (but with a very sophisticated display unit of which we had no experience). This was on its way, and since the 271P already had its own mechanics we would not be required until the CA GL arrived.

In the meanwhile, the base workshops would find us odd jobs. One of these was changing all the .1Fd capacitors in the type 18 Infantry radios used by the Sierra Leone Infantry (ruined by the damp). Another was preparing a disc recording lorry, touring round the tribes to cut message discs for the SL Infantry lads in Burma. My own responsibility was the maintenance of electronic gear at the two hospitals, including the X-ray equipment. This continued for six months, for we were told that our station had been sunk on the way out. I just felt thankful we did not travel with it.

Soldiering on

When it did arrive we moved to the CA Regiment, and took over the maintenance also of the 271P, as well as the installation of the CA. The mechanics' tour of duty was over, and they did not bother to replace them since the unused staff were to hand. Not for long - during the wait, one was moved to the Gambia and another boarded home with mental breakdown brought on by a 'Dear John' letter.

I was able to soldier on alone, since there were two REME instrument mechanics to handle the mag slips, who were a great help in spite of knowing nothing about the radar. The Regiment's Master Gunner was also a real king, the kind of bloke around which a unit operates like a Swiss watch. With the advice of a real expert at the heavy stuff (like dismounting 9.2-inch guns), and with unlimited local labour all on one shilling and six pence a day, moving seven tons of aerial turning gear on to the roof of the pillbox was a doddle.

- © Beverley, WW2 People's War[2]

American GL radar systems

As there is little online technical information providing any details of British GL radar, the following provides information for similar system used by the Americans, and which were used in Britain.

The most well-known use of American GL radar was on the south coast of England, where their radar controlled guns, used in conjunction with shells fitted with their latest proximity fuzes, were eventually able to down some 90% of the V1 flying bombs before they could reach London. The first tests were carried out while the configuration of the American batteries and their proximity fuzes were still being kept from the British gunners, who took part in comparison tests between the two system to determine the effectiveness of the American system. While the British gunners used conventional altitude/time fuzes, and shot down few V1s, the were bemused to see the Americans hit almost every target they fired at.

Combating The V-I Flying Bomb. Twenty SCR-584s were assigned to England's Channel coast to reduce the number of V1 Flying Bombs hitting London. The eighty 90mm guns, using the new proximity-fused shells, destroyed hundreds of those weapons, including one August, 1944 Sunday when 97 of 101 were shot out of the sky. Overall, less than 10 percent of V-Is reached their targets.

- Chrysler and radar: the SCR584 radar-mounted anti-aircraft guns[3]
SCR-584 with gun battey,  http://www.vectorsite.net/ttwiz_05.html#m4

The Canadians and the British Army did complete their own centimetric gun-laying radars, which were designated the "GL Mark 3C", where "C" meant "Canadian", and "GL Mark 3B", where "B" meant "British". They were simply not in the same league with the "whizzbang" SCR-584, having separate transmit and receive antennas and no automatic tracking, and the British ended up obtaining the SCR-584 as the "GL Mark 3A", where "A" meant "American". The Soviets were provided with the SCR-584 under Lend-Lease and were also appropriately impressed by it, putting it into production themselves as the "SON-4".

- [5.4] Centimetric Gun-Laying Radar: SCR-584[4]

References

1 [1.0] The British Invention Of Radar Retrieved May 09, 2011.

2 Radar No 2, Gopshall Hall, Twycross

3 Chrysler and radar: the SCR584 radar-mounted anti-aircraft guns. Dr David George Briant.

4 The Wizard War: WW2 & The Origins Of Radar

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