On 17 June 1935 the first successful experiment at Orfordness detected a Supermarine Scapa seaplane at a range of 17 miles. This was a remarkable achievement, but nowhere near good enough. Simply detecting enemy aircraft approaching was pointless; the information had to be available in time to do something useful with it - in other words, to let RAF fighter pilots know where they could find the enemy. The main Luftwaffe bomber until the Battle of Britain was the Heinkel He 111, which had a cruising speed of around 165mph. At that speed it would cover 17 miles in six minutes - nowhere near enough time to arrange any effective opposition.
By 24 July a range of 40 miles was possible and a mere four months later, in October, detection was possible at 80 miles - giving half an hour's warning of the approach of German bombers. This was good enough for the government, and in December the first five operational Chain Home stations were authorised: they were commissioned the following year. The speed with which Watson-Watt and his team worked is quite remarkable. From the initial, tentative proof of concept at Daventry to a functioning long range radar station took only six months.
Chain Home was a relatively crude system and complicated to operate - in fact, much of its success derived from the skill of the (mainly women) operators who could even, on occasion. detect and track targets well beyond the theoretical range limit of 120 miles or so. Nevertheless it was effective and continued in use, with some improvements, throughout the war as the principal early warning system for the UK. As Cold War tensions grew, 28 Chain Home sites were rebuilt as part of the successor ROTOR system, and the last Chain Home stations were not decommissioned until 1955.
ROTOR was just a stop-gap, however. The Cold War brought a new threat, of fast-moving missiles launched from hundreds or even thousands of miles away. Simple radar systems like Chain Home work on line of sight only, so their range is limited not only by the electronics but by the curve of the earth getting in the way. From the 1950s many countries started developing over-the-horizon (OTH) radar to get round this problem. The main technique tried was to bounces the signals, in both directions, off the ionosphere.
The problems of using the ionosphere for radar
The ionosphere is the region of the atmosphere, starting at about 85km above ground, where incoming solar radiation ionises some of the atmospheric gases. The resulting ions (molecules with missing electrons) conduct electricity and also reflect radio waves, particularly at high frequencies - this is why the BBC World Service broadcasts on short wave. Robert Watson-Watt is credited with inventing the term 'ionosphere' as part of his atmospheric physics research work before radar, and indeed had considered it as a means of extending the range of the systems developed at Orfordness. That idea was soon dropped, though, because as OTH radar developers discovered after the war, using the ionosphere for radar is very difficult. Radio waves bounce off different levels of the ionosphere, and can make bounces off the earth as well, so establishing the range to the target is highly problematic.
Cobra Mist antenna system at Orfordness
The American government used Orfordness to built Cobra Mist, an OTH radar system which was intended to keep watch over the whole of Europe. It cost over $100m and when the project was abandoned in 1974, had never successfully worked. The remains of the Cobra Mist antenna system, a huge fan-like array of aerials, can still be seen at Orfordness.
Long range radar today
Nowadays over-the-horizon radar has been abandoned in favour of airborne early warning systems which are more flexible, can get nearer the areas they are monitoring, and avoid the complication of ionospheric bounces. After the disastrous failure of the Nimrod AEW3 project (£1bn spent/wasted) the UK uses Boeing E-3 Sentry aircraft.
Not all long range use of radar is military. The Arecibo radio telescope in Puerto Rico is equipped with a 1MW 12.6cm wavelength radar system with which it survey extraterrestrial objects.
The huge dish, 300m in diameter, can pick up minute reflections from distant bodies: Arecibo has mapped the surface of Mercury, around 50 million miles away from the earth and studied the rings of Saturn, over 800 millions miles away. That's quite an achievement for a technology which was originally developed to spot an aeroplane a mere hundred miles away.
This article is part of the Radar: What happened next? collection, which looks at different aspects of radar technology since the introduction of Chain Home, the first functioning radar defence system developed during the Second World War. This collection was inspired by the OU/BBC drama Castles in the Sky, featuring Eddie Izzard as Robert Watson-Watt, the father of radar.