The First Airborne Early Warning Aircraft

Skyraider Showing Belly Radome
Facing with Kamikaze threat during 1944 the United States Navy started the development of an airborne radar system in order to expand the radar horizon under which the Fleet was to operate during the series of campaigns through the Philippines and northwards to Japan. Massachusetts Institute of Technology ( MIT ) efforts in developing workable system led in February 1944 under Project Cadillac to a prototype system that was built and flown in August on a modified TBM Avenger torpedo bomber.
US Navy introduced A EW at sea, fitting TBF Avenger aircraft with the AN-APS 20 radar, in 1944. This allowed detection of a bomber sized aircraft out to ranges of 100 miles. Development continued and the AN-APS was next fitted into the Douglas Skyraider, that took service with both the USN and the Royal Navy
Avenger
The Avenger had crew of only a pilot and one radar operator. Radar was controlled on surface ships, transmitting data via a data link which gave the video image and radar antenna angle to giving Combat Information Centre (CIC) a tactical picture. Project Cadillac II started in 1944, with the aim of developing a flying command centre. This led to the development of the PB-1W (below), one of the B-17 Flying Fortress variants using the same AN/APS-20 radar as the TBM-3W, but with several operators on board who could steer defending fighters towards targets via radio. The PB-1W was specifically designed to counter the Kamikaze threat, operating from land bases in support of the Fleet at sea.
Two B-17's were acquired from the Army Air Forces by the Navy during World War II for test work, and designated PB-1. New surplus B-17G's were acquired after the end of the war and modified for reconnaissance and early warning work as PB-1Ws. Extra fuel tanks and search radar were fitted, with no weapons being carried. These were eventually replaced in this role by the WV-2. The Coast Guard also operated PB-1G variants as long-range search and rescue planes, fitted with a parachute-equipped lifeboat carried externally. A total of 31 aircraft were acquired by the U.S. Navy.

more info: http://home.att.net/~jbaugher2/b17_19.html

A US Navy PB-1W with AN/APS-20 Radar
The AN/APS-20 radar as fitted to the TBM-3W and PB-1W became the mainstay of AEW aircraft developments following World War Two. While not designed specifically as an AEW aircraft, the Grumman AF-2W Guardian (below), when fitted with the AN/APS-20 had a secondary capability endowed by this system.
Experience with the Guardian led to the development of an AEW variant of the Douglas Skyraider piston engined attack aircraft. Once again the radar chosen was the AN/APS-20, with a large belly radome being fitted and a crew of three (one pilot and two operators) being carried. The Skyraider was built in three versions, the AD-3W, AD-4W (below) and AD-5W (which was later redesignated the EA-1E). As well as the US Navy, the AD-4W was also supplied to the Royal Navy.
While these aircraft all suffered from technical limitations, experience gained by both the US Navy and Royal Navy led directly to the development of more advanced and specialized AEW aircraft including the Lockheed Warning Star and Fairey Gannet AEW.
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THE PROJECT CADILLAC

The US Navy really wanted this capability. Jerome B. Weisner was put in charge of the effort, which was named "Project Cadillac", after Cadillac Mountain in Maine, where the sun first rises on the United States every morning. The project grew massively, ultimately soaking up 20% of the Rad Lab's staff, as well as 12% of the lab's outside purchases for its entire existence. The Navy also contributed 160 officers and men.
Putting a radar on an aircraft was almost the least of the problem. There was the issue of ensuring proper IFF in the large numbers of air and sea targets on the radarscope, and the extremely difficult issue of considering what to do with the information obtained by the radar. The idea of having a operator on board the aircraft radio the floods of information picked up on the PPI to users on the ground was obviously confusing and impractical.
RCA had been ready to introduce television just before the war broke out, and provided a TV system that was used to relay the PPI display to a surface station. The normal amplitude-modulated TV signal transmission scheme didn't prove adequate, and so Zenith Radio designed a frequency-modulated TV transmitter system. By May 1943, Rad Lab engineers were able to receive useful PPI images sent by an aircraft to a ground station.
Since the PPI display was centered around the aircraft, not the ship that received the image, the relative locations of the aircraft and ship had to be precisely determined using radar beacons. The high-flying aircraft would also serve as a radio relay for command communications; as long as the Navy had a tall tower, they might as well make the best use of it.
The Project Cadillac team was organized into five sections: shipboard system; airborne system; airborne radar; radar transmitter; and beacons and IFF. System elements were in place by early 1945, and the system began flight evaluations in April. By this time, the project's priority had been raised even further. Japanese "kamikaze" suicide aircraft attacks had inflicted major damage on US Navy ships, and Project Cadillac promised to deliver a bigger and tighter screen against the attackers.
The radar element of the system was the "AN/APS-20" X-band radar, which was fitted to Grumman TBM Avenger torpedo-bombers. As mentioned in an earlier chapter, the TBM had carried the first operational American airborne radar, the ASB.
The converted TBM, designated "TBM-3W", was stripped of armament, armor, and bombing gear, then fitted with a big radome between the main landing gear for the AN/APS-20's rotating antenna. The radome gave the aircraft a "pregnant" appearance, and additional "finlets" had to be added to the outboard section of each horizontal tailplane to keep the aircraft flying straight.
The aircraft was littered with various small antennas for other elements of the system, including two VHF radios, IFF interrogator, and the television relay. Although the PPI imagery was relayed to the CIC on board an aircraft carrier or other vessel, it was also monitored by two operators on board the machine.
About 40 TBM-3W conversions were performed, but the system was still in evaluation when the war in the Pacific ended, and did not enter service until 1946. It was still the very first "airborne early warning (AEW)" aircraft.
The Navy was impressed enough by the TBM-3W that they also had the AN/APS-20 fitted to the land-based Boeing B-17 Fortress bomber, which was given the designation "PB-1W". The PB-1W provided greater range and endurance than the TBM-3W and could operate far more autonomously.
The AN/APS-20 radar would have a peculiarly long life. It was fitted to a number of other aircraft in the 1950s, most significantly a variant of the Douglas Skyraider carrier-based attack aircraft, the "AD-3W". The AN/APS-20, with some enhancements, remained in first line service until the early 1960s.
It appears that radar technology reached a sort of evolutionary "plateau" in the postwar period. It was still being improved and used in new applications, but the rate of progress was nothing like the incredible explosion of technology seen during the war. Radar didn't really take another big step forward until the 1960s, when digital circuitry and then computing power was integrated with radar sets, giving them intelligence that allowed them to be much more capable and less magical to operate.
However, the AN/APS-20 remained in service with the British for most of the rest of the century. The British Royal Navy obtained the AD-3W Skyraider for operation off their own carriers. In the late 1950s, as the Skyraiders were being retired from British service, the Royal Navy decided to adapt their Fairey "Gannet" antisubmarine aircraft to the AEW role, modifying the design and fitting them with AN/APS-20 sets scavenged from the British Skyraiders to create the "Gannet AEW.3".
The Gannet AEW.3 served well into the 1960s. When it was retired, the British were in desperate need of an AEW capability, and in 1971 they scavenged the AN/APS-20 sets again, fitting them to existing Avro Shackleton ocean patrol aircraft. The Shackleton was every bit as much an antique, a four-piston engine aircraft that was a derivative of the World War II Avro Lancaster bomber.
The result was the "Shackleton AEW.2". Twelve were converted and served with the RAF in support of the Royal Navy. The Shackleton AEW.2 was an embarrassment, a flying museum piece, and the only good thing that could be said about it was that it was better than nothing.
It was supposed to be a temporary fix while the British developed the advanced "Nimrod AEW" aircraft, but that program proved terminally "snakebitten", leading to seemingly endless delays until it was finally cancelled. The British were finally forced to obtain a US solution, the Boeing "Sentry E-3D Airborne Warning & Control System (AWACS)".
The British got their E-3Ds in the early 1990s and were finally able to retire their exhausted Shackletons. The E-3D was a state-of-the-art machine that was the descendant through several generations of the Project Cadillac Avenger TBM-3W. The RAF jumped through decades of improvement in a single breathtaking step.

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