Top of the list of subjects to film tends to be Battle of Britain radar – we rightly dreaded 2020, the Battle’s 80th anniversary – and apart from the usual ‘Are any of the original scientists or operators still alive?’, a close second question is often ‘We’d like to do a recreation of the Battle of Britain radars, can you help us?’, or ‘Is there an angle that no-one else has tried to show in a TV documentary yet?’.

The answers, of course, are ‘Yes, but very, very, few’, ‘Yes’ and ‘Yes’, but the result might not be what the viewers want. This is because, as Newcomen members may know, Chain Home (CH) radar of Battle of Britain vintage is not at all the concept of radar which viewers, and indeed TV companies, have in mind – an image typified by the Plan Position Indicator (PPI), a map-like style of presentation with the radial time-base sweeping round the screen clockwise.

Explaining that the Battle of Britain radar display gave only range at first, and if you were skilled, some vague idea of number, by interpreting a jumbled A-scope display – and then going on to explain the operation of the ‘gonio’, first to obtain bearing, and then with a bit of help, height, results in horrified expressions of shock from the researcher. Going further, to explain that British fighters were tracked with the aid of the radio navigation system Pip-squeak and good old direction finding (D/F), and finally that, once over land, Chain Home gave way to the Observer Corps, with all three information feeds going into the Filter Room, one can tell that the TV companies’ mental circuits are in overload!

This is sad, because the full story would allow a number of unsung heroes to be given their rightful due. The GPO engineers, the Observer Corps, and the Filter Room staff, to name but three – plus the operators and mechanics at the Chain Home sites have never really received proper praise for their skill in interpreting the traces on their screens.

But even before those operators could exercise their skill, another almost completely-forgotten group had practised their own dark magic. These were the calibration staff, without whose efforts the radar stations could give wildly misleading results.

It seems to us, easy for a TV company to explain that each of the Chain Home stations had different characteristics, often due to aerial problems, for no doubt many TV viewers twiddle their aerials, wondering why theirs has to be a little different to next-door’s to get a good signal. This (we say to TV researchers) is similar to the dark art of calibration, where each of the radar stations separately had to take readings on a target flying at a known distance, bearing and height, and use the results to modify future readings to find the true target position.

Unpublished National Archives memoranda show that on 1 August 1940, Ventnor, Poling and Pevensey (half the South Coast radars) could not give accurate bearings, while Ventnor, Poling and the key Dover radar could not give accurate height. To define “inaccurate”, bearings could be as much as 20 degrees wrong, or as much as 17 miles at a 50 mile range!

One big operational problem was that while calibration was being performed, the radar had to be taken out of action, so a gap appeared in Britain’s defences; calibration was always performed under pressure.

Another problem was how to fly targets of known distances, heights and bearings from each radar station without using up every trained pilot in Fighter Command!

One solution was to try balloons, with small transmitters on their anchoring cables; since the Chain Home stations looked seawards, the balloons had to fly from boats. Bawdsey scientists chartered the boats “Ialine” and the “Recovery of Leith” as tenders on whose balloons the Chain Home stations might take bearings, and calibrate their equipment. However, coastal winds often blew the balloons out of position, and Fighter Command HQ pronounced them as slow, cumbersome and seriously inaccurate; plus the fact that manning static boats in the Channel during the Battle of Britain would hardly have been a sought-after duty!

Enter the autogyro, one of the strangest weapons in the RAF’s Battle of Britain armoury!

The autogyro’s ability to keep position at different heights made it ideal for carrying out calibration, and skilled fighter pilots were not required – indeed, as one of the popular songs of time implies, autogyro pilots were regarded so little as not to wear “wings” at all:

“Oh, I fly an auto-gyro, but I may not have my wings,
It’s a pity – it’s a pity, for they’re complicated things,
Like a tipsy daddy-long-legs, or a May Bug, rather drunk,
Though I know their wayward nature, and have learnt there’s naught to funk,
And I teeter madly skyward, caring nix for what Fortune brings,
For I love my auto-gyro, though I may not have my wings”.

The Avro Rota carried out azimuth calibration of the Chain Home stations, typically by flying in a tight circle or flying very slowly into a head wind (strictly speak they could not hover), in turn over each of 12 chosen locations. There are several UK survivors at Duxford, the Science Museum, and as components, with the Helicopter Museum in Weston-super-Mare.

An Air Ministry Conference in November 1939 resulted in a six-month contract for Cierva to provide the civilian Reggie Brie’s services to set up and run radar calibration by autogyro. Three Cierva C.30s were ferried to Hendon and on 1^st December 1939, attached to 24 Squadron. Under scientist Dr Kinsey, special equipment was installed, and the Dover Chain Home radar calibrated. The concept proven, within two months Reggie Brie personally calibrated seven Chain Home radars from Ventnor, Isle of Wight to Netherbutton, Orkneys. By January 1941, the force had become the nine-strong Rota Calibration Flight, Duxford, Brie himself joining the RAF as an Acting Flying Officer on 5^th May 1940 but rising to Acting Squadron Leader by 7^th July! Alan Marsh was to take over the Duxford calibration flight after Brie.

An interview with Reggie Brie can be listened to at www.aerosociety.com

The autogyros were sorely needed, for calibration was not a “once-for-all-time” exercise. Newcomen members might guess that it needed to be repeated every time frequent adjustments were made to the equipment, or to the aerial system and its feeders. Add to this, the scientists wanting to try new circuitry almost every day, and maintenance by inexperienced technicians – each time demanding yet more calibration – and the burden placed on Brie’s small team can be imagined.

By 8^th August 1940, as the Battle of Britain was raging, calibration had become such a headache that Dowding personally chaired a review. Because a station being re-calibrated left a gap in coverage, Dowding insisted that his HQ in future authorise each one. He also authorised more calibration aircraft, decided that some had to be armed, and forbade autogyro use where they were exposed to enemy aircraft. This was as well – an autogyro had to make itself scarce when German bombers attacked the Ventnor radar!

Someday, a TV company might like to make the true story of Battle of Britain radar and its crop of unsung heroes, including the autogyros; we live in hope . . .

With thanks to the Defence Electronics History Society (DEHS) for allowing us to publish this item.