[2.0] Longwave Radar At War / Early American Radar Efforts

v2.0.6 / chapter 2 of 12 / 01 jan 15 / greg goebel

* By the beginning of World War II in 1939, radar had evolved to the level where it could be effectively used in combat. The British had given a great deal of serious consideration to just how radar could be used effectively, and this foresight proved extremely valuable in the dark days of 1940.

In the meantime, the Americans were independently conducting their own research into radar, and developing longwave radar technologies comparable to those of the British. However, the Americans had not yet fully realized what radar could do for them, and for the moment failed to give the technology the priority it deserved.

SCR-270 radar



* While work on radar continued in Britain, tensions with Hitler's Germany were increasing. The Munich Crisis of September 1938 made it clear that war was coming soon, and work on the Chain Home network accelerated. By the spring of 1939, there were 20 stations in operation, providing a mostly unbroken electronic barrier from Ventnor near Portsmouth in the south, to Netherbutton in the Orkneys to the north.

It was an impressive achievement, and it was completed just in time. On 1 September 1939, Germany invaded Poland, and Britain declared war on Germany. World War II had begun. The day after war was declared, the order came down to evacuate Bawdsey Manor. The powers-that-be were certain the Germans knew something was going on there, and felt Bawdsey was likely to be bombed. The reason for this fear was that a few months before, on 31 May 1939, the CH stations picked up a huge echo moving up the English channel.

It was the German rigid airship GRAF ZEPPELIN. Colonel Wolfgang Martini, then head of the Luftwaffe (German air force) signals intelligence organization, wanted to obtain intelligence on British electronic systems, and had decided that the airship's ability to lift heavy cargoes and fly slowly made it ideal for that purpose. The British not only tracked the big airship on radar -- it was hard to miss -- but intercepted radio transmissions from it as well. When a German crewman radioed back that the GRAF ZEPPELIN was over the North Sea off the coast of Yorkshire, the British tracking it laughed since it was over the town of Hull, about 32 kilometers (20 miles) inland and hidden by cloud cover. One Briton said later: "We were sorely tempted to radio a correction message to the airship."

The GRAF ZEPPELIN came back for another surveillance mission on 2:3 August 1939. The British felt certain that the GRAF ZEPPELIN had been on electronic intelligence missions, and that the Germans had now identified the Chain Home system. The first belief was correct, the second was not. For reasons that are now unclear, the airship didn't pick up the radar stations. The most plausible explanation is that the Germans weren't looking on the right frequencies.

As discussed later, the Germans had radars of their own at the time, which were in fact in many respects superior to British radars, and they worked at much shorter wavelengths than Chain Home; the Germans may have not thought to search the longer wavelengths. In addition, the Chain Home radars looked nothing like German radar stations -- in fact, it's difficult now to tell that the Chain Home tower array was for a radar system just by looking at a picture of it, and that may have misled the Germans as well. Whatever the details, after the war Martini was startled to find out that the airship had been tracked.

Watson-Watt had earlier come to an agreement with the University of Dundee in Scotland to house the radar organization if an emergency arose, and most of the Bawdsey research staff, now up to a headcount of 250, packed up and moved out as fast as possible, with some of the staff dispersing to other installations. Unfortunately, when the researchers got to Dundee, they found out the university had completely forgotten the agreement, and such facilities as were scrounged up were inadequate. Obviously, other new facilities had to be found.

In short, the British radar effort was caught up in the fog and confusion of war, with everyone running off in all directions at once. Getting things straightened out was a nightmare, since the government and the military were trying to deal with an endless list of other "do it yesterday" action items that the outbreak of war had dumped in their lap. The organization, such as it was, was no longer associated with Bawdsey Manor and so acquired the formal name of the "Air Ministry Research Establishment (AMRE)".

The radar researchers did what they could to keep on working. They knew the British military needed AI and ASV immediately, and they were also busy with a major refinement that would make radar much more powerful, the "plan position indicator (PPI)" display, sometimes referred to as the "polar plot indicator" display.

Early radar used the simple A-scope display. The radar antenna was fixed in position, or could be steered from target to target by an operator, much like a searchlight, with the direction to the target given by the antenna position and the range by the length of the trace on the A-scope. The scheme was clearly only workable for a single target, or a group of targets considered as a single target. In contrast, in its most straightforward form, PPI used a CRT to display echoes from a rotating antenna. A moving sweep on the display, tracking the rotating antenna, painted a "picture" of what the radar unit saw. The CRT had a high-persistence phosphor so the sweep would leave an image on the display that would fade slowly, and be updated by the sweep on the next pass. This gave the operator a much better idea of what was going on around the radar, and in particular allowed observation of multiple targets with a single radar. The TRE got the first PPI working in the spring of 1940.

Eddie Bowen found himself trying to develop AI with primitive facilities at various remote locations, finally ending up at RAF Saint Athan on the south coast of Wales. Although Bowen had a good team, including Hanbury Brown and another bright young physicist named Bernard Lovell, getting anything done under the circumstances was very difficult.

In May 1940 the radar organization, with the new and deliberately misleading name of the "Telecommunications Research Establishment (TRE)", was finally rebuilt more or less in full at Swanage on the Dorset coast, across the English Channel from Cherbourg. Unsurprisingly, the fire drill over the previous months had led to a great deal of friction and antagonism. In particular, Eddie Bowen had quarreled with the bureaucracy and earned the hostility of A.P. Rowe.

* Although the first months of the war were chaotic for the radar researchers, Britain was under no direct military pressure for that time. Since the outbreak of the war, the French and the British expeditionary force had been engaged in a staring contest with the Germans across their border defenses, resulting in an idle "Sitzkrieg (Sitting War)". Incidentally, the British brought a few of their MRUs, the "portable" Chain Home system, along with them, but the radars were unable to provide height information when surrounded by terrain instead of staring out to sea, and setting up a communications system to link them to fighter defense units proved troublesome. Radar contributed little to the defense of France.

The Sitting War came to an abrupt end in the spring of 1940 when the German Army rolled like a freight train across the Low Countries and into France. Military disaster followed, leading to the evacuation of British forces from the beaches at Dunkirk in early June 1940. Martini's technical experts were able to examine the remains of British MRU and GL sets that had been wrecked and abandoned. The Germans were not very impressed with the British radar, feeling with some good reason that it was inferior to their own gear. The captured gear didn't give them clues that the British knew a few tricks that the Germans hadn't really thought of yet.

Hitler had hoped to make peace with Britain. The new prime minister, Winston Churchill, would have none of it, vowing to fight every step of the way: "We shall never surrender." Now Hitler ordered Reichsmarshal Hermann Goering, boss of the Luftwaffe, to send his bombers to pound England. The Germans began daylight air raids on England in earnest on 10 July 1940, beginning what would be known as the "Battle of Britain". The Luftwaffe greatly outnumbered the RAF, and Chain Home was one of the few tools the British had to counter German air power. The network had been extended though the integration of 30 Chain Home Low radars beginning in late 1939.

The radar network still had limitations. It allowed the British to detect the bearing and distance of aircraft, but it could not easily determine the number of aircraft in a formation, and it also did not provide much coverage over the interior of Britain. Once past the electronic barrier, intruders had to be tracked by a network of ground spotters, known as the "Royal Observer Corps (ROC)".

From the beginning of the Chain Home effort, Air Marshall Dowding had understood that radar was useless without a command and control system to absorb and act on the information provided, and had driven the development of such a system. Although British radar gear was nothing special, the British had thought out the implications of the technology much better than anyone else, and made far better use of it. The RAF had worked with British Postal Service engineers to come up with a scheme where observations from radar and ROC stations were collected at a central command center, where they were used to obtain a precise plot of the location of intruders and even some measure of their number. The command center was known as a "filter room", since it filtered out information from many different sources.

The RAF had established the first experimental filter room at Bawdsey Manor in August 1937. In the fall of 1938, the RAF had established a central underground operational filter room in the London suburbs, on the site of a 12th-century monastery named "Bently Priory". The Germans might have had a low opinion of what they had seen of British radar, but for the present the Germans had nothing to compete with the filter room system.

As the Luftwaffe hammered Britain, the Bently Priory filter room became the central operations center for air defense. Reports of intruder formations tracked by radar and ROC networks were relayed into the filter room on dedicated military phone lines. Officers logged the reports and assigned a "battle number" to each formation. In the center of the filter room was a huge table, with a map of the British Isles and neighboring coastal regions of Europe. Staffers moved markers across the map with tee sticks to track the movements of the intruders. A controller observed the movements and determined which fighter squadrons to scramble. Alerts were sent to the four Fighter Command groups by telephone or teletype. The system was clumsy by modern standards, but a clever and impressive feat of organization for the time.

Most of the "worker bees" in the network were females, members of the "Women's Auxiliary Air Force (WAAFs)", organized in 1937 when the RAF realized the service faced a manpower shortage in the coming war. After the network had been in full operation for some time, the general belief arose that women were more patient and attentive than men, and were better suited to the work of tracking air battles. The WAAFs also displayed an entirely British coolness under pressure.

* With the filter system, the RAF was able to selectively focus its fighter forces on Luftwaffe bomber formations, providing an edge to balance the Luftwaffe's numerical superiority. The Germans could have seriously disrupted the system by attacking the radar stations, but they never seemed to really understand their importance. Exactly why this was so is another mystery. Over time, the Germans were building up a picture of Britain's defense network, but for whatever reason the CH stations were not disturbed for over a month into the air campaign against Britain. One issue may have been that Martini was a gentle and very unassertive man and didn't make much of a fuss about British radar to Hermann Goering.

By the beginning of August, the British had been able to shoot down 270 of the attackers to a loss of 145 of their own aircraft, and of course many of the RAF pilots were able to bail out and fight again. Germans who parachuted out of their crippled aircraft over England became prisoners.

By that time, the Germans were aware that the CH stations were probably radar stations. Five were attacked on 12 August, but damage was limited, with the stations repaired and MRUs filling the gap in coverage where necessary. The stations were back on the air so quickly that the Germans decided it wasn't worth their bother to hit them again, and never did so again in earnest.

The Luftwaffe was focusing mostly on airfields for the moment. The RAF was sorely pressed, until Luftwaffe bombers hit London by accident on the night of 24 August. Churchill ordered RAF bombers to perform night attacks on Berlin in retaliation. Hitler redirected the Luftwaffe's attacks to English cities, particularly London. Heavy Luftwaffe raids on the cities began on 7 September 1940. This change in strategy inflicted greater suffering on civilians, but it gave the RAF the chance to survive and strike back, and the RAF inflicted unacceptable losses on the Luftwaffe.

On 15 September 1940, the Germans mounted their last major air raid of the Battle of Britain. British fighter squadrons attacked the intruders in relays, ultimately destroying 60 German aircraft for the loss of 26 of their own. As a reward for his service in protecting Britain, Stuffy Dowding was put on the retired list in November. Dowding's fate remains controversial, but his grasp of technical matters was not matched by his ability to manage his people. He allowed quarrels and disputes to disrupt his command, and his superiors ran out of patience with him. He was supposed to have retired a year earlier anyway, and eventually he would be given a peerage to become Lord Dowding.

The Luftwaffe's raids on the cities did not end, but now they would take place at night. The air war over Britain now entered a second phase, the "Blitz", in which radar would be even more important and the challenge that much more desperate.



* With the end of the Battle of Britain, the British radar design community was faced with new deadly challenges. The Luftwaffe's shift to a campaign of night raids on England's cities had rendered the RAF helpless. Schemes using searchlights to pin down bombers for attack by fighters did not work well. Obsolescent Bristol Blenheim light bombers were fitted with belly machine gun packs and new AI radar, but the Blenheims were too slow and too poorly armed to be very effective.

The early 1.5 meter (200 MHz) longwave AI radars were unreliable and primitive even when they worked. The initial "AI Mark I" had been a disaster, generally unworkable in field conditions, and "AI Mark II" wasn't that much better. Of course, the primitive development of the radar itself was reflected by primitive operator and service documentation and training, compounded by the fact that since radar was an entirely new technology, nobody had, or could have had, a clear idea of how to use it. Night-fighter crews learned to distrust the radar boffins and their marvels, which made the job of the boffins all the more difficult.

"AI Mark III" did clear up many of the bugs, but it was still not quite satisfactory. The radar had a maximum range of about 2,750 meters (9,000 feet) under normal conditions, and a minimum range of about 330 meters (1,080 feet), the minimum range being a consequence of the pulse width. Generating very short, predictable pulses strained the electronics technology of the time. The short maximum range meant that a night fighter had to be guided precisely to the target by ground controllers to have any chance of finding the intruder, and the long minimum range meant that the target could easily escape by taking evasive action as the night fighter closed in through the "blind range". Worse, the early AI sets had a tendency to jam the fighter's communications gear.

The Blenheims were configured with an arrowhead-shaped transmit antenna in the nose, and a pair of dipole antennas placed well outboard on the leading edge of each wing. Targeting was through lobe switching, with the AI operator directing the pilot to turn in the direction of the strongest echo. Getting useful results out of such equipment seemed more art than science. RAF night fighter pilot John Cunningham expressed his frustration at AI and its handlers when he saw a Luftwaffe bomber he was stalking drop its bombs on target. Cunningham looked back at the radar operator:


The magician was still kneeling on his prayer mat of blankets muttering to himself, the green glow from the CRT flickered on his face. A witch doctor, I thought, a witch doctor and black magic -- and just about as useful.


* Fortunately, Bowen's tinkerings with EMI television receiver technology as the basis for his early AI work had apparently led EMI to wonder what was going on. Whatever happened to get them curious, they soon began radar work on their own, and approached the British government in 1939 with a design of their own for a gun-laying radar. In December, the company was put to work on AI instead.

The EMI engineering team was led by Alan Dower Blumlein, Britain's most prominent electrical engineer. Bringing in a top professional was a great help to the often ingenious but sometimes amateurish physicists, and Blumlein was able to develop a fast pulse switching scheme that reduced the radar pulse width and cut the minimum range of AI to 130 meters (426 feet). The result was "AI Mark IV", which was a major step forward from its predecessors. Some sources also mention a "Mark V", described as a Mark IV for single-seat fighters, and a "Mark VI", which was an improved Mark V -- but the RAF decided single-seat night fighters weren't a good idea, and so that line of investigation was a dead end.

* Although progress was being made on AI, however painfully, the indifferent attitude of the Army towards radar ensured that the GL radars were not only inadequate in themselves, but were not put to any intelligent use in directing British anti-aircraft guns against the German night radars.

The situation became so desperate that the Army finally instituted a comprehensive program to get people trained on using and maintaining the GL sets, and hopefully even figure out a way of using them effectively. Unfortunately, the effort failed to make much of a difference. Most people regarded the Army's anti-aircraft guns as doing little more than providing reassuring sound effects for the public.

* Work on ASV was moving along a little more smoothly than AI, though not by much. The 1.5 meter (200 MHz) "ASV Mark I" set was available in early 1940, and was installed the aircraft of three RAF Coastal Command ocean patrol squadrons. ASV.I was another unreliable piece of junk, but it did work well enough to allow patrol aircraft to keep track of convoys in foul weather, which was common in the North Sea patrol area.

A clever RAF officer also managed to improvise a "radar beacon" that was set up on an airfield and emitted a signal in response to an ASV pulse. This helped patrol aircraft find their way back home in socked-in conditions and proved particularly useful. Improved radar beacons would be used for the rest of the war. However, ASV.I was not very effective at picking up German submarines, or "U-boats". Something better was needed.

In February 1940, the chaos into which the British radar research had been thrown by the outbreak of war was beginning to resolve itself, and ASV work was transferred from the pathetic facilities at Saint Athan to a much better site, the Royal Aircraft Establishment at Farnborough. Hanbury Brown, who had been in the field helping Coastal Command get ASV.I working, went along with the move, and helped another researcher, Gerald Touch of the Farnsworth company, get started on an improved ASV set.

Touch's "ASV Mark II" was still not everything that was needed, but was a big step forward. On patrol, beams were shot out in the direction of the wingtips to scan a track 40 kilometers (25 miles) wide. If the radar operator picked up a positive contact off a wingtip, the pilot pivoted the aircraft in that direction, with the radar beams sent forward, using lobe switching to pin down the target. An order was placed for 4,000 ASV.II sets in the spring of 1940, but AI was the production priority, and ASV deliveries were delayed.

* ASV.II also led to a ground-based transportable lightweight air-warning radar for British troops in the field. The "Light Weight (LW)" or "AMES Type 6" radar was actually a pretty good piece of gear, not particularly accurate but adequate for detecting intruders from about 40 kilometers (25 miles) away, and much less cumbersome than the MRU. The LW was built in a number of versions, with some set up in a tent and others mounted on a truck or jeep. Early LWs had a distinctive antenna, with four Yagis held using a crossbrace structure, though some later sets had dipole arrays.



* While the British were building longwave radar systems, the Americans were pursuing similar technology in parallel, if in a much more leisurely fashion. The Americans were generally focused on short wavelengths at first, but the difficulty of generating signals of adequate power at such frequencies gradually forced them to longer wavelengths.

Robert Page's demonstration of a pulsed radar system in December 1934 had been just that, a demonstration, and though it had successfully picked up an echo from an aircraft target, that was about all it had done. Page realized he had his work cut out for him, with the task made all the more difficult by the limited funding available to the Depression-era military.

Page managed to struggle into early 1936 by robbing another NRL program for funds, but that couldn't be done indefinitely, so he lobbied with the House Naval Appropriations Committee and got a grant of $100,000 USD for his work. Page was aided by the fact that in the spring of 1935, Rear Admiral Harold G. Bowen (no relation to Eddie) had been appointed to head the Navy's Bureau of Engineering, which oversaw the NRL. Admiral Bowen was a believer in NRL research and very interested in the lab's radar work.

Page was joined in his radar development effort by Robert Guthrie, with Page working on the receiver system and Guthrie working on the transmitter. By April 1936, they had developed a workable pulsed radar system that could pick up aircraft targets at a range of 8 kilometers (5 miles), and they quickly increased the range to 27 kilometers (17 miles).

* Admiral Bowen was impressed with the demonstration and assigned the radar effort top priority. Taylor then told Page that he needed to design the radar so that it used the same antenna for both transmitter and receiver, and insisted that a shipboard demonstration be conducted as quickly as possible.

The ability of a radar system to focus on a target is partly a function of antenna size: the bigger the antenna, the tighter the focus, and this is true for both transmit and receive. Instead of having two small antennas, one for transmit and receive, it made more sense to have a single larger antenna for both functions. The problem with this configuration was that a radar receiver has to be sensitive to pick up faint echoes from a distant target, and if a single antenna was used, the powerful transmit pulse was fed directly back into the receiver, possibly damaging it. Page developed a device that he named a "duplexer", a clever arrangement of transmission lines that provided a high resistance against the transmit pulse to block it from being fed back to the receiver, while providing a low resistance to the return echo.

A prototype naval radar operating at 1.5 meters (200 MHz) was demonstrated on the destroyer USS LEARY in April 1937. The radar used a Yagi antenna mounted to a deck gun for steering. The radar worked well enough, but it lacked power and range, and Page and Guthrie went back to the drawing board, working to get an operational prototype radar, codenamed "XAF", ready for demonstration by September 1938.

RCA and Bell Laboratories were both given a demonstration of the Navy radar effort in mid-July 1937. RCA was familiar with radar concepts, since an RCA engineer named Irving Wolff had been tinkering with radar technology through the 1930s. Wolff had demonstrated a continuous-wave system in 1934 and a pulsed system in 1937. RCA wanted to build XAF for the Navy, but also wanted to build their own design, the "CXZ", and include it in the trials. The Bell Labs people were too unfamiliar with the technology to promise any help, but they were given much to think about.

The trials began in the Caribbean in January 1939, with XAF mounted on the battleship USS NEW YORK and CXZ mounted on the USS TEXAS. The naval officers involved with the trials were tremendously impressed, with aircraft spotted at a range of 77 kilometers (48 miles) and vessels at 16 kilometers (10 miles). Night destroyer attacks were thwarted, shells could be tracked in flight, and the radar was even used for navigation by ranging peaks on nearby islands. Since CXZ proved unreliable under operational conditions while XAF was sturdy, the Navy ordered RCA to build 20 XAF sets to NRL specifications. These sets were put into operational use as "CXAM" on battleships and carriers.

CXAM proved extremely useful. It included a switch to allow it to change its PRF, allowing an operator to detect ghost echoes. Switching the PRF did not change a return from a true target, but it did cause a ghost target to jump to a new position on the display. Later on, this feature also allowed CXAM to be used for secure communications: the switch was changed to a telegraph key, and two CXAMs could be used to trade Morse code messages over line-of-sight distances, with the narrow beam and relatively high frequencies making eavesdropping difficult.

CXAM would be refined into the excellent "SK" set by the addition of a rotating antenna and PPI, instead of an antenna directed by the operator onto targets for ranging by an A-scope. The SK, nicknamed the "Flying Bedspring" for the appearance of its antenna, was the US Navy's standard early-warning radar through World War II. The antenna featured a 6-by-6 square array of dipoles, 4.6 meters (15 feet) on a side. Peak power was 330 kW, an order of magnitude greater than its XAF ancestor, with a pulse width of 5 microseconds.

The Navy also built similar 1.5 meter (200 MHz) sets. The "SC" used much the same electronics, but was fitted with a smaller antenna featuring a 6-by-2 array of dipoles, 4.6 meters (15 feet) tall and 1.8 meters (5 feet) wide, giving it a narrow horizontal beam. It was for use on destroyers. The "SA", with an even smaller antenna, was used on destroyer escorts and other small vessels, such as minesweepers. The SK family proved surprising reliable for such a new system, and would remain in general service all through the coming conflict.



* The US Army Signal Corps was also working on longwave radar in parallel with the US Navy effort. The "father" of the Army effort was William R. Blair, director of the Signal Corps Laboratories, who had become interested in radar technology in the late 1920s.

Although the Army, along with a number of other military services around the world, remained focused on infrared detection schemes through much of the 1930s, Blair kept his interest in radar schemes alive, and he acquired a powerful convert in the form of Lieutenant Colonel Roger B. Colton, a headstrong sort who was chief of the lab's research and engineering department.

The ignition point of the Signal Corps effort took place in December 1935 when, at Colton's request, one of Blair's engineers, William D. Hershberger, paid a visit to the NRL to chat with Page and inspect the Navy's radar work. Hershberger was highly impressed, suggesting on his return that the Army drop their work on crude microwave detection systems and follow in the Navy's path.

Blair passed on a copy of Hershberger's report to the Major General James B. Allison, the Chief Signal Officer, and strongly recommended that the Army get into radar in earnest. Allison was behind the effort, but the Army had just as much trouble getting money as the Navy, and just as with the Navy, Blair had to rob funds from a different program to keep on working. Blair and his people put their backs in it, and by December 1936 they had a working prototype. In the spring of 1937 they demonstrated an improved version of the set, picking up a bomber at night.

The brass wanted the set put into production immediately, but the Signal Corps dug in their heels since all they had was a lashup prototype that was totally inappropriate for field use. However, Colton, now Blair's commander, was able to push through a request for $250,000 USD for development of a practical radar system, to be designated "Signal Corps Radio 268 (SCR-268)".

Getting the SCR-268 ready for production proved difficult. Blair's health went bad and he was forced to retire, so Colton took over complete control of the program. By November 1938, a crude prototype of the SCR-268 was ready for demonstration, with a number of officials, including the head of the Army Air Corps, General Henry H. "Hap" Arnold, in attendance. The radar was to pick up a Martin B-10 bomber flying overhead, but nothing was found until the radar operators searched around and found the bomber had been blown out over the sea by strong crosswinds. The radar operators had a better idea of where the bomber was than the plane's crew did, and Hap Arnold was suitably impressed.

* The prototype used in this test was still far from an operational system. The whole kit had to be refined to the point where it could be used as a mobile system by troops in the field with acceptable reliability. A production prototype was finally ready by the middle of 1940, with a production contract then awarded to Western Electric. Initial deliveries of the SCR-268 were in February 1941.

The SCR-268 was a clumsy-looking device. Since Colton had dictated that it use both vertical and horizontal lobe switching, it had three antennas, including a transmit antenna, a vertical receive antenna, and a horizontal receive antenna, fitted horizontally on a truss that was held by an altitude-azimuth gun-style mount. The transmitter antenna consisted of 4-by-4 square array of dipoles. The vertical receive antenna consisted of a 2-by-6 rectangular array of dipoles, mounted long axis vertical on the right. The horizontal receive antenna was a 6-by-4 array of dipoles, mounted short axis vertical on the left.

SCR-268 radar

Despite appearances, the SCR-268 was a very effective piece of gear. It operated at 1.5 meters (200 MHz), had a peak power of 50 kW, a pulse width of 7 to 15 microseconds, and a PRF of 4,098 Hz. The SCR-268 had a beam width of 2 degrees in both the horizontal and vertical directions, and a maximum range of 36 kilometers (23 miles). It would remain in first-line service for gun laying and searchlight direction up to 1944.

In the meantime, the Signal Corps was working on an early warning radar that emphasized range at the expense of accuracy. Two similar versions were planned, including a mobile set, the "SCR-270", and a fixed-site set, the "SCR-271", both operating at 3 meters (100 MHz), with a peak power of 100 kW, a pulse width from 10 to 25 microseconds, and a PRF of 621 Hz. They lacked lobe switching.

Evaluation of an engineering model of the early warning radar in June 1939 showed it had a range on the order of 120 to 240 kilometers (75 to 150 miles). The Signal Corps awarded a contract to Westinghouse for production of the early-warning radars in August 1940, and over a hundred had been delivered by the end of 1941. The SCR-270 would remain the Army's standard early-warning set to the end of the war.

* The Signal Corps also investigated "radar altimeters" during this time, which were simple radars that were focused downward from an aircraft to see how far away the ground was. RCA was awarded a development contract for a radar altimeter in 1937, and in the spring of 1940 an operational set, designated the "SCR-518", was put into production. The initial version weighed 40 kilograms (88 pounds), but the weight was gradually reduced to 12 kilograms (26 pounds). The SCR-518 could measure heights accurately up to an altitude of 12 kilometers (40,000 feet).