BOEING E-3C SENTRY AIRBORNE WARNING AND CONTROL SYSTEM
Ever since our simian ancestors learned to climb trees, we have known instinctively that the higher you climb, the farther you can see. Later, many ancient cultures devoted considerable labor to building hilltop watchtowers. Spotting an approaching enemy even a few minutes sooner can make the critical difference between victory and defeat. The development of radar in the 1930s provided proof that nature is consistent. Generally speaking, radar works much like light — it travels in straight lines and usually cannot bend to peer over the local horizon. While a mountaintop is a good spot for a radar station, a mountain is rarely located where you need it, and it's hard to move. However, if you could put a big radar antenna on a high-flying airplane, your radar horizon could theoretically reach out to two or three hundred miles. Also, if you put an air battle control staff on the same airplane and provide them with powerful computers, situation displays, and secure communications, you have what is called an Airborne Warning and Control System (AWACS) — the king on the chessboard of the modern air battle. Its status also makes it the most prized target in the sky, making the Sentry the sort of high-value airborne asset that will normally be protected by a hefty escort of fighters.
AWACS aircraft had their start at the end of World War II, when the U.S. Navy was desperately trying to fight off the hordes of Japanese Kamikaze suicide aircraft that were trying to stop the invasion and battle fleets of the Americans. The Navy's solution to the relative vulnerability of their surface ships was to convert TBM Avenger torpedo bombers into primitive AWACS aircraft. These early AWACS aircraft would have been available for the invasion of Japan in late 1945, had it taken place. Later, purpose-built AWACS aircraft were built by both the Air Force and Navy to their specific needs, usually on transport or airliner airframes. For many years, the USAF birds were based upon the classic Lockheed C-121 Super Constellation airliner /transport. Called the EC-121 Warning Star, it served in the AWACS mission for over twenty years before being replaced by the current AWACS aircraft, the E-3 Sentry, in the later 1970s.
The Boeing E-3C Sentry AWACS looks like a large jet airliner being attacked by a small flying saucer. The airliner is the old reliable Boeing 707- 320B airframe, with a flight deck crew of four (pilot, copilot, navigator, and flight engineer) and a 'mission crew' of thirteen to eighteen controllers, supervisors, and technicians back in the main cabin. Using an airframe similar to the venerable KC-135 and all the other Boeing Model 320 derivatives has proven quite popular with the U.S. military, and quite practical for the taxpayers. The saucer, or 'rotodome,' is 30 feet/9.1 meters in diameter, 6 feet/1.8 meters thick at the center, and is supported 11 feet/3.35 meters above the fuselage on two streamlined struts just aft of the trailing edge of the wings. It is designed to generate enough aerodynamic lift to support itself, and does not place any stress, other than drag, on the wings or airframe. Mounted back-to-back with the main APY-2 radar antenna (upgraded from the original APY-1 version) inside the rotodome is an antenna array for the APX-103 IFF/ Tactical Digital Data Link (IFF/TADIL–C) system. This is a highly sophisticated IFF system, capable of interrogating virtually any IFF transponder in the world within 200 nm./365.7 km. (It reportedly has some sort of NCTR capabilities as well.) When transmitting, the rotodome, powered by hydraulic motors, makes one complete revolution every ten seconds. When it is not transmitting, it makes one revolution every four minutes, to keep the bearings lubricated. Considering the flight stresses it has to support and the complex of wave guides, power cables, and signal lines that must pass through it, the saucer's rotary slip joint is a marvel of mechanical engineering. The radar transmitters and their elaborate power supplies and cooling equipment are located under the floor of the aft cabin, where conventional 707s stow the passengers' luggage.
All of this is packaged inside a standard Boeing Model 707-320B/VC-137 airframe with four Pratt & Whitney JT3D/TF33 turbofan engines. It is also quite expensive, having originally cost something like $270 million a copy.
Getting into an E-3 is roughly the same as a KC-135, through a normal passenger hatch on the left side of the aircraft, where the cargo door is on the tanker. The first thing that strikes you is that the interior is much more comfortably appointed than the -135s. The interior is covered with the same kinds of sound-deadening walls as a conventional airliner, mainly to ensure the comfort of the mission crew. Along with all the display consoles and other electronic gear, they are crammed into the cabin for missions that can last most of a day (though twelve to sixteen hours is normal). The tops of all the consoles in the main cabin are covered with blue indoor/outdoor carpet, which is actually quite nice to lean on! The flight deck is roughly similar to that of the Stratotanker, though some of the controls and displays are a bit more modern than the 1960s-vintage instruments on the -135.
As you move back through the main cabin, there are any number of large cabinets and consoles scattered throughout, which can make moving about somewhat tight. These include the main computers for the radar system, as well as the symbology/display generator systems for the controller consoles. Towards the mid-cabin area over the wings are the radar control consoles. There are fourteen of these in back-to-back rows, with a flight seat (complete with shoulder harnesses and seat belts) in front of each position. Each console is configurable by the user, and can be set up for a controller, supervisor, or mission commander. Everyone is linked by a thirteen-channel intercom system, which feeds into a bank of secure Have Quick II radios, as well as other sets capable of UHF, VHF, and HF communications. In addition, the E-3 is equipped with a JTIDS data link terminal, which does much to reduce the burden on the radio channels.
Obviously, the primary reason for this aircraft's existence is the radar system it is designed to carry. The original AWACS radar system, designated APY-1, was designed by Westinghouse after a 1972 competition with Hughes. The AWACS radar operates in the E/F band, meaning that it generates radar waves in the 2-to-4-Gigahertz (GHz) range, with a wavelength of from 7.5 to 15 cm./2.95 to 5.9 in. The radar uses the pulse-Doppler principle, relying on precise measurement of the tiny frequency shift in energy reflected from a moving target to distinguish flying aircraft from background ground clutter. This gives the radar the ability to 'look down' and detect low-flying targets, as long as they are moving faster than 80 knots/148 kph.
The normal E-3 mission crew consists of separate surveillance and control sections, each typically commanded by a senior captain. In the surveillance section, three to five technicians monitor the air traffic in a huge volume of airspace and pass on information to the control section. This is composed of two to five weapons controllers sitting at multi-purpose consoles, guiding friendly aircraft to intercept enemy or unidentified contacts. Depending on its particular mission, an AWACS also may carry senior staff officers, radar technicians, radio operators, a communications technician, and a computer technician.
While the E-3 displays are a great improvement over the 'bogey dope' screens of the old EC-121s, which required almost mystical powers to interpret, they are rapidly becoming dated. The symbology is somewhat hard to interpret, and the screens can easily become cluttered. On the positive side, the trackball 'mouse' used to select or 'hook' targets on the screens is quite easy to use, and once you get used to the idea that a small symbol with a track number is an aircraft, you do quite well.
Aft from the console area are more electronics cabinets, as well as an area reserved for passengers and off-duty personnel. While the seats are not terribly comfortable, they are an improvement over the maximum-stress environment of being 'on the scope.' There is also a tiny galley, as well as several small bunks which are usually reserved for spare flight crew personnel (pilots, navigators, etc.). Combat AWACS missions in excess of eighteen hours during Desert Storm were not uncommon, and spare personnel were often necessary. At the very back of the cabin is a rack of parachutes, and there's a bail-out door in the floor of the forward cabin. This, fortunately, has never had to be used, since no E-3 Sentry has ever been lost.
The key to making this system work is the need for steady, consistent flying. Sentry pilots are trained to fly a precise, wide oval racetrack course, straight and level, avoiding any sharp banking turns that might disrupt the radar beam's normal sweep. Typical cruising altitude for an operation mission is 29,000 feet/8,840 meters (just about the height of Mt. Everest), at a maximum cruising speed of 443 knots/510 mph./860 kph. Unrefueled, the E-3
