Today (September 20th, 2022) marks 80 years since the end of the Battle of Britain, in which radar played a critical role in warfare for the first time.
It’s also an opportune moment to recall that British radar development stemmed from an Air Ministry prize of £1,000 to anyone who could build a ray that could kill a sheep at 100 yards. The idea of a “death ray” was not new, H. G. Wells included them in the War of the Worlds, but it was given a sudden impetus in 1934 when Nikola Tesla claimed to have invented such a “weapon to end all weapons”.
This, and reports that the Germans were ahead in “death ray” development caused alarm at the British Air Ministry. The Scottish physicist Robert Watson-Watt was asked to investigate these reports in 1934. Watson-Watt had spent the best part of two decades working on the problem of detecting lightning strikes from a distance to warn aircraft pilots of the danger of approaching thunderstorms, so this was in his particular area of expertise.
He turned the problem of whether there was any merit in the “death ray” idea over to his assistant Arnold Wilkins, who quickly proved that the theory was demonstrably impossible, but also concluded that the principle might be used to detect aircraft at a distance.
Watson-Watt and Watkins worked quickly on this theory and within a matter of months built a prototype set that was used to detect a Handley-Page Heyford bomber flying at 6,000 feet from 7 miles away.
The transmitter used was simply an existing BBC shortwave radio broadcast station. They set their recievers up and detected the civilian radio broadcasts bouncing back towards them off of the target. Wilkins made this inspired suggestion, avoiding the massive cost of constructing a powerful transmitter, as he had been playing about with commercial radio sets and a model from the General Post Office mentioned in the user manual that end users could expect interference in their listening experience if aircraft were flying overhead
Wilkins and Watson-Watt are credited with the British invention of radar and this first success test against an aircraft. A number of nations at this time were making independent leaps and bounds in “radar” so it’s not possible to credit a single, overarching inventor. As a result of their work, the British “Chain Home” air defence radar network was built in great secrecy and at great speed. Technically it was relatively simple but this diagram shows just how wide the coverage was in 1939 then 1940.
What proved to be much more important than the level of technological development of the Chain Home radar was its almost complete coverage and the sound strategic understanding of its strengths and weaknesses (it was useless over land, so faced out to sea, handing over to ground-based spotters inland). Most importantly though was its integration into the “Dowding System”, the worlds first wide-area, ground-controlled interception network. Put simply; it was a system to collect, transmit, process, filter and understand the huge volumes of data being collected by radar stations and ground observers and to allow the ground commanders to quickly make sound tactical air interception decisions based on it.
If there’s a moral to this story, it is don’t try and kill sheep, don’t believe the hype, that how you use something is usually more important than what it can actually do, and always (always!) read the instruction manual.
One war-winning contribution of this magnitude would be an achievement for most, but Watson-Watt was also the father of “High Frequency Direction Finding” (HF/DF or Huffduff), the principles of which he had first come up with back when he was trying to track far off lightning strikes. HF/DF allowed distant radio transmissions to be accurately located, and when the system was miniaturised into a shipborne version was instrumental (alongside more advanced radar) in helping win the “Battle of the Atlantic”.
The German U-boat system was so terribly effective in the Atlantic war because it was centrally controlled – ironically much like Britain’s air defences during the summer of 1940. Central control relied on receiving regular status updates from U-boats of their positions, target sightings etc., in order that High Command could order multiple submarines to converge on convoys and attack it far more effectively in a coordinated manner, overwhelming its defences. Such long distance control was therefore entirely dependent on intense two-way radio traffic between shore and submarines. The German Navy was confident its codes were unbreakable so could transmit great detail too and fro at will, there was no reason to cut back on radio traffic. They also had such a confidence that fast and accurate radio direction finding at sea was impossible that they never changed this behaviour.
The beauty of HF/DF from the users point of view is that it’s totally passive – you don’t need to transmit a signal yourself – therefore you don’t potentially give away your presence, it can’t be jammed and also you don’t need to know what’s being transmitted so the Herculean efforts of cryptanalysis are not required. At its simplest, HF/DF is basically just get a moving dial that says “there be dragons”. If you get two or three sets on different ships working together, you can very quickly triangulate a precise location of the signal you are tracking. It also worked at greater ranges than ship-board radar and allows a good operator to guess from the characteristics of the received signal an approximate range and even if the transmitting antenna is wet or not (i.e. is it a ship? an aircraft? or a freshly surfaced submarine? )
It was the work of an exiled Polish engineer, Wacław Struszyński, that made a miniaturised and accurate shipborne set possible. Previously the system used land-based receivers on a baseline of hundreds of miles form eachother, and perhaps thousands from the target, when even tiny detection angle errors could move the target tens of miles from its true position. Struszyński’s development of a miniaturised antennae and also a method to cancel out the disruption caused by the ships own superstructure is described as “a breakthrough of transcendent importance“.
Under Struszyński’s leadership, the HF/DF sets were continually improved. There were soon models that automatically scanned likely frequencies, alerting the operator by an alarm when a likely signal was found so that they could take over. Ironically, the Germans had plenty of photos of the HF/DF antennae on allied ships and they knew that allied ships and aircraft could quite reliably and conveniently find themselves in the right place at the right time to attack their U-boats. They just did not and could not believe this could be down to passive direction finding and by the time they started doing something about it, it was much too late.
It is estimated that “without shipborne high frequency direction finding, Allied convoy losses in early 1943 would have been 25 to 50 percent higher, with U-boat kills being reduced by one-third“.
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