Historical note on H2SA.C.B. Lovell, Ph.D.

Indexing terms: History, Radar and radionavigation

Abstract: In March 1942, the first radar aid to navigation GEE was introduced. However, GEE could bejammed, its range was limited and positional accuracy was not good enough for the needs of the RAF. Againstthis background H2S was developed, and the paper describes the early research and operational work carriedout on the system.

1 Origins

Historically, the development of H2S will be closely associ-ated with the rapid development of centimetric techniqueswhich took place in the second half of 1940 and 1941. Theambition to enable an aircraft to be navigated by a self-contained radar device relying on information fromground and town reflections (as distinct from informationfrom ground stations, e.g. GEE) dates from before thepresent war, however. In the summer of 1939 the Officer inCharge of the airborne group at Bawdsey Research Stationflew in a Blenheim equipped with a modification of Mark I1.5 m AI (airborne interception) from Martlesham to theWest Coast of Wales and was able to present the pilotwith an accurate account of the route flown. Also, in thewinter of 1939-40, experiments were carried out from St.At nan on 5/10 m in an Anson in an effort to detect thepresence of towns. No serious attention was given to thesubject, however, until the critical situation of our nightbombing efforts became known late in 1941. Fortunately,by that time, the remarkable development of the centi-metre magnetron had enabled considerable power to beobtained on wavelengths which made a realistic technicalsolution of the problem more probable.

An experimental 9 cm AI system had flown early in1941 and, by the end of the year, the first centimetre prog-ramme for the RAF had been well launched (AI MarkVII). A competitive experimental 9 cm AI system using adifferent form of scanning and presentation (helical scan asdistinct from the spiral scan of AI Mark VII) had not faredso well as an AI, and, in November 1941, when theurgency of town detection really became apparent, thisequipment was diverted to the new problem. The aircraft(Blenheim V6000) flew with the centimetre beam tilteddownwards, rotating at 300 rev/min, and isolatedresponses were immediately observed on the range/azimuth presentation. These critical experiments carriedout from Christchurch aerodrome were of the utmostimportance in that they demonstrated that a centimetreairborne apparatus could give discrete returns from certainareas of ground as distinct from the general groundreturns which had been the bugbear of all AIs. There wasone uncertainty, however; the responses on the tube werecomparatively numerous, and it was evident that manyobjects other than towns were giving responses (e.g. alanding screen near Salisbury gave a particularly strongresponse, as did the many military camps on SalisburyPlain). Were the signals obtained in separate flights defi-nitely associated with specific ground objects? Before thesecrucial experiments could be completed, the Secretary ofState for Air called a meeting on 23rd December 1941;gave instructions for six specific flights to be made imme-diately to 'determine whether the signals obtained inseparate flights could be definitely associated with specific

ground objects', and for the initiation of the H2S prog-ramme.

The results of these historic flights were published in aTRE Report 12/106, dated 23rd April 1942. Its first con-clusion was that 'The H2S scanning system offers the likel-ihood of successful target selection and accurate locationwith some possibility of selective bombing within thetarget area'. A sample of some tube photographs publishedwith this report and obtained in these early flights isshown in Fig. 1.

2 The development of H2S

The first active steps to put the scanning H2S system intoa heavy bomber were taken after the Secretary of State'smeeting at the end of 1941. The Blenheim equipment wasdesigned essentially for AI purposes and was, in manyrespects, unsuitable for H2S purposes as they were thenenvisaged. In particular, the scanner installation gaveforward looking only and the presentation was range-azimuth, whereas all-round looking and plan positionpresentation were clearly more desirable for H2S. The firstcontacts with Handley Page were made on 4th January1942 and the experimental installation of a Perspex cupolain the under-turret position was initiated on two Halifaxes.The first of these, Halifax V9977, landed at Hum on 27thMarch 1942. (See Figs. 2 and 3.)

This Halifax was installed with an experimentalequipment built in TRE, using a magnetron transmitter/receiver (TR) box of Mark VII AI design. It took off for itsfirst flight in the evening of 16th April 1942, but theequipment refused to work due to a hidden switch in the80 V supply. The next morning's flight was successful,however, and ranges of 4 or 5 mile (6.4 or 8 km) wereobtained on towns from 8000 ft (2438 m) altitude.

In the meantime, work on the design of an H2Sequipment for manufacturing had proceeded with vigourand the firms chosen were well ahead with development.Many design alternatives (such as high-speed against low-speed scanning) were settled by local discussion, but oneoutstanding question had deep political repercussions.This was the use of the magnetron over enemy territory.This valve is comparatively undestructible, and it was con-sidered that it would be a grave mistake to present theenemy with information on our advanced technique in thecentimetre field. An early decision had, in fact, been giventhat the magnetron must not be used, but a klystron wasto be used instead, the principles of which were believed tobe known by the enemy and which was in any casedestructable. The klystron had only been developed,however, as a local oscillator, and the forms then existingcould only produce a very small power. Urgent develop-ment was undertaken to make a klystron suitable for use

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as a transmitter, and, in a few months, klystrons capable ofgiving 5 to 10 kW peak were produced. Many technical

Fig. 1 Some of the first H2S responses obtained with Blenheim V6000from 7000 ft (2134 m)Note that the presentation is range-azimuth Crown Copyright reserved

Fig. 2 A Halifax heavy bomber at Defford with one of the later install-ationsCrown Copyright reserved

difficulties associated with the use of this valve were over-come, and, by July 1942, a prototype equipment existedwhich had interchangeable magnetron and klystron TRheads.

The Air Staff expressed faith in the development of H2S,however, and issued the following directive:

'2 (a) That the system should be accurate enough toguarantee that bombs would fall within an industrial orother area selected as a target

(b) That the Air Staff would be satisfied in the firstinstance if the range of the device enabled the aircraft tohome on a built-up area from 15 mile (24 km) at 15000 ft(4572 m)

3 Subject to there being no delay or interference withthe development of the equipment and its introductioninto the Service in a form which will fulfil this aim, it wasagreed that details in design to enable it to be used as anavigational aid to determine a specific area or targetcould be incorporated during the later stages of develop-ment and operational trial.'

This 'vote of confidence' did nothing to lessen our troubles.In quick succession followed the disturbing move toMalvern, and, on 7th June, a major disaster when HalifaxV9977 crashed in South Wales killing five of the compara-tively small team then working on H2S. The loss of thesevital personnel was a most overwhelming tragedy.

Towards the end of the month, however, the outsidepressure was still further increased and, on 3rd July 1942,the Prime Minister reviewed the position and ordered anall-out effort to be made to fit two squadrons of heavieswith H2S by October 1942. Quite apart from the insupera-ble production and fitting difficulties, the actual develop-ment was in a most parlous condition. It is true that aHalifax was now flying with apparatus which could beregarded as a functional prototype; and the replacementfor the ill-fated V9977 was also ready, but, even whenusing the forbidden magnetron as a transmitter, the resultswere very unsatisfactory. It was soon evident that the insis-tence on the use of the klystron would make it impossibleto attain even the minimum Air Staff requirements, and,on 15th July 1942, the Secretary of State ruled that devel-opment work on the klystron for H2S should cease, thatthe two H2S Squadrons should be equipped with mag-netrons, but that a decision as to their use over enemyterritory would depend on the war situation at the time in

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question. Efforts to make a satisfactory destructive devicefor the magnetron were then redoubled, but the results ofthe most successful trial were that a 10 ft (3 m) hole wasblown in the side of a JU88; and an expert was able to

reconstruct the magnetron dimensions from its fragments!After that, the destructive device was confined to a smalldetonator so placed as to prohibit the immediate use of thevalve should it fall into enemy hands.

Fig. 3 Underview of a Lancaster atDefford showing large radome usedby the end of the warCrown Copyright reserved

Fig. 4 Sir Bernard Lovell (at left) and some members of his teamFacing Sir Bernard, from left to right, are EX. Killip (RAF), J. Richards (RAF), Dr.F.C. Thompson, P.J. Hillman (RAF), N.Z. Alcock and D.N.C. Ramsay (RAF)Crown Copyright reserved

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