In the second half of the 60s, it became clear that the modernization potential of the EC-121 Warning Star AWACS aircraft was practically exhausted. The leaky cabin and piston engines did not allow high-altitude patrols and the full potential of on-board radars. The use of two radars of different types for viewing the lower and upper hemispheres significantly reduced the aerodynamic quality of the aircraft and increased the weight of the equipment. In addition, for servicing different stations, their own operators were required, so, on the latest modifications of the Warning Star, the number of crew members reached 26 people, and most of them were just engaged in servicing radar and communication equipment. Although in the 60s, attempts were made to transfer the element base of equipment from electrovacuum devices to semiconductor elements, radar stations created in the 40-50s contained a significant number of electronic tubes, which made them very cumbersome, energy-intensive and not very reliable.
In the early 70s, advances in aircraft construction and solid-state electronics made it possible to create a heavy AWACS aircraft capable of long-term patrolling at an altitude of 7-9 km and optimally use the capabilities of a surveillance radar. Calculations showed that the radar at an altitude of 9000 m will have a viewing range of up to 400 km. As already mentioned in the second part, in the 60s, the EC-121L AWACS aircraft with AN / APS-82 radar, which had a rotating antenna in a disc-shaped fairing, were tested in the USA. For a number of reasons, this version was not built in series, but even then it became clear that the "air radar picket" with one rotating antenna above the fuselage had great prospects.
Due to the fact that by the 70s nuclear-missile parity had been achieved between the two superpowers, Western strategists were no longer afraid of Soviet long-range bombers, whose role faded into the background, but of a breakthrough by tank and motorized rifle divisions of the Internal Affairs Directorate of NATO's defense in Europe. The superiority of the USSR and the Warsaw Pact countries in conventional weapons was to fend off tactical nuclear weapons and fighter-bombers. It is clear that to deliver airstrikes against Soviet tanks rushing to the English Channel and smash communications without having air superiority. it was, to put it mildly, difficult. The Americans and their allies needed an AWACS aircraft with a powerful radar, capable of carrying out long patrols at high altitudes and timely notifying of the approach of enemy aircraft and directing the actions of their combat aviation. At the same time, the same attention was paid to the possibilities of using the aircraft as an air command post, as to the characteristics of the radar complex.
As already mentioned, the EU-121 Warning Star is hopelessly outdated, and the E-2 Hawkeye used by the American fleet for the scale of the European theater and air defense of North America had insufficient range and flight altitude. In addition, the first Hokai modifications had serious problems with the reliability of avionics, and the experience of operating the E-2A with the AN / APS-96 radar in Southeast Asia demonstrated the inability to detect targets against the background of the earth's surface.
In the second half of the 60s, the United States launched the Overland Radar Technology (ORT) program to develop radars for detecting air targets against the background of the earth. Within the framework of this program, a pulse-Doppler radar was created, operating on the principle of comparing the repetition rate of the pulses of the emitted signal with the frequency of the reflected echo signal. In other words, the Doppler frequency was extracted from a moving target against the background of signals reflected from the ground.
The creation of radars capable of effectively working on low-altitude targets at a great distance went with great difficulties. The first relatively workable sample of the Westinghouse AN / APY-1 radar had a lot of shortcomings. In addition to quite predictable problems with low reliability, the station gave a lot of false serifs from objects on the ground. For example, in windy weather, swaying tree crowns were perceived as low-altitude targets. To eliminate this drawback, it was necessary to use a very powerful computer by the standards of the 70s, capable of selecting targets and displaying only genuine air objects and their real coordinates on the screens of operators.
Determination of the target azimuth is carried out as a result of several scans and comparison of the results obtained from different positions of the target in time and space. This mode allows you to get the maximum amount of information, but the range is minimal. When the detection range of distant targets is more important than information about their flight altitude, it switches to the pulse-Doppler scanning mode without determining the elevation angle, and no vertical scanning occurs. The station can also operate in a passive electronic reconnaissance mode, receiving signals emitted by radars from other aircraft.
Initially, for the new heavy aircraft AWACS (Airborne Warning And Control System), by analogy with the deck E-2 Hawkeye, it was planned to create a new specialized platform with 8 General Electric TF34 turbofan aircraft engines, grouped in pairs. These motors were installed on the A-10 Thunderbolt II attack aircraft and the S-3 Viking anti-submarine aircraft launched in the early 70s in the series. However, this route was considered too costly, calculations showed that the equipment, operators and an external radar antenna can be placed on existing models of military transport aircraft or long-range passenger airliners. The Boeing 707-320, widely used at that time, with native Pratt & Whitney TF33-P-100 / 100A (JT3D) engines was chosen as the base. By that time, the US Air Force was already operating tanker aircraft, reconnaissance aircraft, air command posts and transport and passenger vehicles based on the Boeing 707.
With a maximum takeoff weight of about 157,300 kg, the aircraft is capable of staying in the air without refueling for 11 hours. The maximum speed reaches 855 km / h. The ceiling is 12,000 meters. The tactical range is 1600 km. Patrolling is usually carried out at an altitude of 8000-10000 meters at a speed of 750 km / h.
The first two prototypes built are known as the EC-137D. Serial AWACS aircraft received the E-3A Sentry index (English Sentry). The construction of aircraft of the AWACS system began in 1975. In just 8 years, 34 machines of the E-3A modification were built.
E-3A Sentry
The first aircraft in 1977 entered the operational 552nd Airborne Early Warning Wing at Tinker Air Force Base in Oklahoma. Twenty-seven AWACS aircraft were assigned to Tinker. Four of them on a shift basis patrolled the Far East and were stationed at Kadena airbase in Japan, two more aircraft at Elmendorf airbase in Alaska. After the start of deliveries of the E-3A, integrated with the air defense system of the United States and Canada, a massive write-off of obsolete E-121 AWACS aircraft began. Despite the initially low reliability of the radar and problems with linking to the centralized air defense system of North America, the new early warning and control aircraft initially demonstrated a high potential for detecting Soviet bombers and aiming fighter-interceptors at them.
In addition to the US Air Force, AWACS of the first modification were supplied to the NATO allies; in total, 18 E-3A were sent to Europe. 1984 to 1990 five E-3A with truncated communications and radar equipment were sold to Saudi Arabia. Iran in the late 70s also ordered 10 AWACS, but after the overthrow of the Shah, this order could not be fulfilled. Total from 1977 to 1992 68 aircraft of the E-3 Sentry family were produced.
In 1982, aircraft intended for operations in the European theater of operations were equipped with an operational system for transmitting tactical information JITIDS, which makes it possible to exchange not only voice information, but also transmit visually displayed symbolic information at a distance of up to 600 km. The use of this equipment greatly simplified interaction with fighter aircraft and made it possible to control the actions of several dozen interceptors.
The most noticeable part of the AWACS aircraft was a rotating disc-shaped plastic radio-transparent radar fairing mounted on two 3.5-meter supports above the fuselage. Inside a plastic disk weighing about 1.5 tons, 9.1 meters in diameter and 1.8 meters thick, in addition to a passive antenna array with electronic scanning, antennas of the friend-or-foe recognition system and communication equipment are installed. The antenna could complete a complete revolution in 10 seconds. Cooling of the main antenna of the radar and other equipment was carried out by the oncoming air flow through special holes. Radio and communication equipment, computing complex and information display facilities consumed electricity several times more than the equipment of the base Boeing 707-320. In this regard, the power of the generators on the E-3A was increased to 600 kW.
Half radar fairing
Although the aircraft was created mainly for operations outside the United States, the equipment included the SAGE and BUIC systems designed for the automated guidance of interceptors over the territory of North America. The data processing subsystem of the first 23 aircraft, built on the basis of an IBM CC-1 computer with a data processing speed of 740,000 operations per second, provides stable tracking of up to 100 targets simultaneously. Target information was displayed on 9 monitors. The IBM CC-2 computer installed on the twenty-fourth production aircraft has a main memory of 665,360 words. This aircraft has also introduced an integrated system of covert exchange of tactical information between AWACS aircraft, fighters and ground control points. It provides fast and secure communication channels for thousands of users.
Operator workplaces of the British Sentry AEW.1
The workstations of the radar and communication operators are located in three rows across the cabin immediately behind the cockpit and the avionics compartment. Behind them is the control officer's workplace and the flight engineer's compartment. At the rear there is a kitchen and seating areas. The number of the crew can be 23 people, of which four are flight personnel, the rest are operators and technical personnel.
But even with a powerful radar and modern computer systems at that time, the ability of the first E-3A to see low-flying targets against the background of the earth was low. Therefore, the on-board equipment of AWACS aircraft underwent revision. The task of effectively arming air targets against the background of the earth's surface was solved after installing an improved AN / APY-2 10-cm range radar on the aircraft. On the modernized AWACS aircraft, in addition to increasing the energy potential of the radar, the power of computers has increased. The mass of digital signal processing units was almost 25% of the weight of the radar itself - more than 800 kg. The total weight of the radar equipment was approximately 3.5 tons. The AN / APY-2 radar has high noise immunity due to the low level of the rear and side lobes of the antenna directional pattern.
The AN / APY-2 radar can operate in several modes:
1. Pulse-Doppler without scanning the beam in the vertical plane.
2. Pulse-Doppler with beam scanning in elevation to estimate the flight altitude of air targets.
3. Over-horizon search, with cut-off of signals below the horizon line without Doppler selection.
4. Survey of the water surface with short pulses (to suppress reflections from the sea surface).
5. Passive direction finding of interference sources in the frequency range of the AN / APY-2 radar.
It is also possible to combine all of the above modes in any combination.
The modernized version, designated E-3B, has been under construction since 1984. 24 E-3A aircraft were converted into this modification. Simultaneously with the radar, passive detection means were developed, recording the operation of on-board radars and other aviation radio-technical systems.
The aircraft, upgraded to the AWACS Block 30/35 level, received an AB / AYR-1 electronic reconnaissance station. Visually, they differ from earlier modifications by lateral antennas (on the right and left sides), approximately 4x1 meters in size, which protrude about 0.5 meters beyond the fuselage contours. There are also antennas in the nose and tail of the aircraft. The station consists of 23 modules with a total weight of 850 kg. After the installation of the RTR station on board the aircraft, it was necessary to equip a workplace for another operator. In addition to the US Air Force aircraft, NATO AWACS aircraft underwent a similar revision.
The station is based on two digital receivers united by a processor unit. Which, in addition to instantaneous frequency measurement, carry out amplitude direction finding and parametric recognition of the type of intercepted radiation source. According to data published in open sources, the AB / AYR-1 recognition system is capable of identifying more than 500 types of ground and airborne radars. The station, operating in the frequency range 2-18 GHz, provides circular scanning in a sector of 360 degrees and direction finding of radio emission sources with an error of no more than 3 degrees at a distance of 250 km. Its performance is approximately 100 recognition of radiation sources in 10 s. The maximum operating range of the AB / AYR-1 reconnaissance radio equipment over powerful signal sources exceeds 500 km.
Following the E-3B variant, the E-3C appeared, featuring improved avionics. On this model, in addition to new, more high-performance computers, the APS-133 navigation radar and the AIL APX-103 IFF / TADIL-J digital communication equipment were installed. On this modification, the equipment for displaying radar information was also updated. All cathode ray tube monitors have been replaced with plasma or LCD panels.
British AWACS aircraft Sentry AEW.1, accompanied by interceptors Tornado F.3
The modification with CFM International CFM56-2A engines for the British Air Force received the designation E-3D (Sentry AEW.1). The first aircraft was handed over to the RAF in March 1991; in total, the United Kingdom ordered 7 aircraft. Four AWACS E-3F aircraft with the same engines but different avionics were bought by France.
Modernization of the E-3 Sentry at Tinker airbase
In 2003, the United States allocated $ 2.2 billion to modernize the existing Sentry fleet. In 2007, practical work on modifying Block 40/45 began at Tinker airbase. The first US Air Force E-3G reached full combat readiness in 2015. It is planned to re-equip all American aircraft of the AWACS system with a sufficient flight resource into this version.
