The E-3 Sentry AWACS Showing Off Its Rotating Dome Housing The Huge AN/APY-1/-2 Radar

This entry is the first installment of our detailed presentation of major upgrades carried out on the Airborne warning And Control System (AWACS) fleet. We have relied on a large number of publicly available documents from Boeing, Northrop Grumman, Department of Defense etc in order to discover the actual stage of systems configuration of each operator’s fleet. Our observations are made in comparison to target configurations as programmed into the funding process. Our analysis will also bring to light the ‘spiral’ development model used for Defense programs. The ‘spiral’ model which allows upgrades to be incremented as newer technology matures, is well suited for software- based upgrades. However we have found it tedious to track down and document accurately. We are covering the Boeing E-3 Sentry AWACS flying with the US Air Force, NATO, United Kingdom, France and Saudi Arabia as well as the Boeing E-767 flying with the Japanese Air Self Defense Force.

The successive upgrades that we have observed have allowed the AWACS fleet to remain the dominant long range radar surveillance and air battle management platform for the past quarter of century. We will see how through various defense programs, the aircraft role is evolving in the era of network centric warfare. ‘Net-centricity’ favors fusion of information from a wide variety of sensors providing real time situational awareness and visual aids to commanders. For rapidly changing tactical situations better informed decisions and more effective use of theater resources are key to winning engagements and decisively concluding campaigns. Through integration of uniquely powerful sensors and advanced data communication networks, the AWACS is likely to remain the primary provider of real time aerial intelligence for joint commanders through the year 2035.

Introduction

The requirement for the Airborne Warning And Control System emerged in the 60’s on account of limitations of radars carried aboard fighter aircraft and inherent weaknesses of coverage from ground radar stations. At that time fighter aircraft were tactically limited by lack of real ‘look-down’ radar capability: while ‘looking down’ a radar is likely to receive reflections of its own electromagnetic pulses bouncing off the ground. Hence the so-called radar ‘ground clutter’ which low flying aircraft can exploit in order to avoid being seen clearly by ‘cluttered’ airborne radar. AWACS Doppler filtering techniques would later solve that issue by processing changes in frequency observed on radar returns of such low flying aircraft. For ground radar stations, enormous gaps in coverage remained thanks to beyond-the-horizon curvature of the earth and also advantageous terrain features in some scenario. Naturally in the AWACS case a large civilian type aircraft could embark heavier more powerful radar equipment while at the same time providing considerable time on station, beyond-the-horizon surveillance capability and strategic mobility. The US Air Force Electronics Division System at Hansom Air Force Base, Massachusetts in charge of the program in the 60’s was actually grappling with two competing proposals involving a McDonnell-Douglas DC8-based airframe and the Boeing 707-320B derivative. By July 1970 the contention centered solely on selecting the appropriate radar: US Secretary of Defense Robert Seamans Jr had already announced Boeing as the winner of the airframe part. Following a series of actual test-flights showcasing both the Westinghouse Electronics Systems radar and a Hughes Aircraft rival system both mounted atop dedicated EC-137D testbed (as was named the 707-derived prototype), the contract was awarded to Westinghouse. The resulting Airborne Warning And Control System solution represented a latest military application for the ever so versatile Boeing 707-320B commercial aircraft. The gigantic dome-housed radar antenna was mounted 11 ft (3.3m ) above the fuselage using two struts. The 30 ft (9.1 m) radius dome was 6 ft (1.8 m) thick at the base and aerodynamically designed to generate enough lift minimizing drag and stress on the AWACS airframe. The powerful Westinghouse-built AN/APY-1 radar and successor AN/APY-2 (Northrop Grumman succeeded Westinghouse Electronics Systems) have given the E-3 Sentry advanced surveillance capabilities at ranges in excess of 200 miles (320 kilometers) for low flying targets and out to 250 miles (400 kilometers) for higher altitude targets. The system can also be employed for detecting stationary and moving ships of various size. The radar dome 360-degree mechanical rotation provides horizontal scan every ten seconds (six rotations per minute) to deliver azimuth (or bearing) data of tracked air vehicle relative to the AWACS position. Simultaneously the radar also employs an electronically shifted radar beam on the vertical plane that determines altitude data of air vehicles. Advanced radar detecting capabilities are enhanced by the AN/APY-1/2 Pulse Repetition Frequency giving the Radar optimal longer range / low altitude combination improved by Doppler signal filtering and advanced signal processing capabilities. Additional identification feature is supported by advanced transponder interrogation features of the Identify Friend or Foe (IFF) suite. The AWACS also gains tremendous value with its Electronic Support Measure (ESM) allowing passive listening mode on the main radar to detect sources of emitted radiation (enemy radar or jamming equipment detection).Under this format the Boeing E-3 Sentry delivers the following operational capabilities; a flight deck crew of 4 supported by 3 technicians and 11 mission crew (or 14 mission crew). The Boeing 707-320B derivative airframe is powered by four TF-33-PW-100A 21,000-lb thrust Pratt & Whitney turbofan engines on typical 11 hour-missions (extended to 72 hours maximum with multiple air refueling) flying at up to 35,000 feet (10,670 meters) and 500 miles/hour (800 kilometers/hour) speed. During Desert Storm/Shield Operation the US Air Force AWACS was to establish itself as the control of the sky overseeing more than 120,000 coalition sorties punctuated by 38 victorious lethal air-to-air engagements.

The Boeing E-3 Sentry

With engineering, test and evaluation phase starting in 1975, the US Air Force took delivery of its first E-3 Sentry in March of 1977. By June 1984 a total of 34 aircraft had been delivered to the US Air Force with assignment to the 552nd Airborne Warning and Control Wing (now 552nd Air Control Wing, Tinker Air Force Base, Oklahoma). Four aircraft are assigned each to the 961st Airborne Air Control Squadron (AACS), Kadena Air Base, Japan, and the 962nd AACS, Elmendorf AFB, Alaska. A special E-3 AWACS aircraft designated AWACS Test System 3 (AT-3) is permanently assigned to Boeing as part of ongoing efforts to test and introduce systems enhancements that are likely be incorporated to the rest of the operational fleet. Following the September 1995 accidental loss of one AWACS aircraft the entire US AWACS force has been left with 32 operational aircraft plus the AT-3.

In October 1980, the NATO Airborne Early Warning Force Command was formed with requirement for 18 E-3A AWACS aircraft being assigned to the Geilenkirchen NATO Air Base in Germany. Under a jointly funded multi-national arrangement three Forward Operating Bases have been established in Turkey, Greece and Italy along with a Forward Operating Location alternative in Norway. The NATO fleet did also suffer one aircraft write-off after a 1998 crash in Greece reducing its AWACS fleet to 17 aircraft.

By February 1987 an order for 6 aircraft by the United Kingdom as part of its AWACS contribution to NATO was initiated, later amended with an extra 7th optional aircraft being ordered. United Kingdom AWACS received the designation E-3D and use the more efficient Snecma/General Electric CFM-56-2 high bypass ratio 24,000 lbs thrust engines. They were delivered between March 1991 and May 1992 and operate out of RAF Waddington as Squadron number 8 and 23 part of NATO. Together with the UK, France a non-NATO US ally would take advantage of Boeing’s 1987 AWACS evaluation program with the United Kingdom to place an initial order for 3 E-3F aircraft also powered with the CFM-56-2, later augmented to 4. The aircraft began to be delivered in May of 1991 until February 1992 and are operated at Avors Air Force base by the 36th EDCA (Escadron de Detection et Control Aerien / squadron of aerial detection and control). The Royal Saudi Air Force has acquired 5 Boeing E-3 Sentry AWACS from the 1981 Peace Sentinel Foreign Military Sales program (along with up to 8 KE-3 refueling tankers a KC-135 Tanker ‘clone’). Deliveries took place from 1986 to 1987. These models also use the CFM-56-2 high bypass ratio turbofan.

JAPAN 4 BOEING E-767 AWACS

The closing of the 707 assembly line by Boeing in 1991 officially ended the career of the 707-320B derivative as a modern airborne military platform solution. The Airborne Warning And Control System AWACS requirement for the Japanese Air Self Defense Force, taking into account the wide maritime areas surrounding the island nation required an early warning radar with surface ship detection capabilities as well as traditional long distance aerial surveillance. Naturally the big AN/APY-2 radar supplied by Northrop Grumman Electronic Systems, could be mounted on top of a newer civilian market-based airframe. The selection of the Boeing 767-200ER was made almost naturally thanks to the aircraft popularity with more than 100 commercial operators around the world, a 50% volume space advantage over the 707 and a certification for Extended Twin-engine Operations. Boeing has argued the modern wide body twin engine offered economical,operational and logistical benefits to military operators relying on 2 General Electric CF6-80C2B6FA high bypass turbofan engines rated at 61,500 pounds thrust. The 4 Boeing E-767 AWACS or E-767 were delivered by Boeing during 1998 and 1999 achieved Initial Operational Capabilities (IOC) with the force in May 2000. Subsequent to the Japan Air Self Defense Force acquisition, the 767-200ER would show wider adoption for military purpose as the KC-767 Tanker ordered by Italy, Japan and more recently the US Air Force under the re-designated KC-46A. The E-767 crew of 21 includes 2 pilots and 19 AWACS specialists involved in missions lasting from 9 to 13 hours. The 5,600 nautical miles range (10,370 km) can be extended with in-flight refueling. The first airframe completed by Boeing in October 1994, undertook its first flight mated to the rotating dome housing the AN/APY-2 radar antenna in August 1996. Major subcontractors on the program were Northrop Grumman (formerly Westinghouse) the supplier of the antenna/dome assembly that houses the AN/APY-2 Radar antenna, General Electric, Rockwell Collins, and Telephonics (formerly Sundstrand). The aircraft operate under the 601st Hikotai (Squadron) 1st Hikohan (Flight) at Misawa and 2nd Hikohan at Hamamatsu according to Air International (July 2003).

The JASDF AN/APY-2 Equipped AWACS Version Derived From The Popular Twin-Engined Wide Body Boeing 767

 

The AN/APY-1/-2 and AWACS Operation

With its normal crew complement of 18 an E-3 Sentry AWACS requires a four-members flight-deck crew, three technicians and an 11 people core AWACS mission crew. For the UK Royal Air Force the mission crew itself comprises a tactical director (mission crew commander), a fighter allocator, three weapons controllers, a surveillance controller, two surveillance operators, a data-link manager, a communications operator and an electronic-support-measures operator. The NATO mission crew consists of a 12 members; a tactical director, a fighter allocator, two weapons controllers, a surveillance controller, three surveillance operators, a passive controller, a communications technician, a radar technician, a system technician depending on the mission.

The AN/APY-2 radar at the heart of the AWACS operation provides multi-mode operational flexibility split between air, maritime and electronic emitters surveillance (Electronic Support Measures). Contrasting key AWACS sub systems capabilities and their interfacing we identify the six main Radar tactical modes as Pulse Doppler Non Elevation Scan (PDNES) mode, Pulse Doppler Elevation Scan (PDES) mode, Beyond-The-Horizon (BTH) mode, Maritime mode, Interleaved mode and Passive mode.

1. Pulse Doppler Non Elevation (PDNES) mode.

This radar mode implements real look-down detection capability. Due to its extreme sensitivity, the radar is likely to ‘see’ both the flying object and the ground ‘clutter’. However the pulse Doppler radar insures that the moving object can be detected as its radar reflection constantly changes frequency. The powerful Doppler ‘filters’ are able to process such variable frequency signals and eliminate ground clutter as a source of confusion for the entire radar-generated tactical picture. In this mode algorithms already assume that the flying objects are operating at low altitude. This allows the processing capabilities to focus entirely on filtering (differentiating) ground clutter from actual threat. The radar operator can extract maximum benefits from a wide variety of signal processing capabilities as well as computer data correlation. Thanks to increases in radar sensitivity detection also applies to object as small as cruise missiles, stealth air vehicles and Unmanned Air Vehicles.

2. Pulse Doppler Elevation Scan (PDES) mode.

This tactical mode also employs the Doppler radar function in order to detect higher flying object. A vertical plane radar beam is also generated using advanced electronic circuitry to scan the sky in front of the AWACS up and down without requiring a mechanically moving antenna. We recognize that scanning of the radar on the horizontal plane is already provided via mechanical rotation of the radar dome. These features gain usefulness when trying to accurately establish a flying object’s azimuth (bearing determined by the rotating antenna dome) and elevation data (altitude) as determined by the vertical electronic beam. This mode can supplement PDNES mode which forgoes precise altitude determination.

3. Beyond The Horizon (BTH) mode.

BTH surveillance mode does not employ Doppler radar function. Instead it uses the raw power of the radar transmitter/antenna/receiver groups to provide extended range surveillance of the sky. This operation relies on the Transmitter Group located inside the AWACS rear lower cargo hold, and the AN/APY-2 antenna Ultra Low Side Lobe design aided by Reflection-less transmit and receive manifold ensuring a very stable noise-free signal processing environment. The AWACS transmitter group large size betrays its ability to generate very high power beams thanks to a 90 kV (90,000 V) power supply and two heavy high power Klystron Power Amplifiers (KPA). The receiver group sensitivity and low noise levels contributes to receiving radar returns at longer range.

4. The Maritime mode.

The AN/APY-2 uses a short pulse for clearer radar return differentiating sea ‘clutter’ from sea vessels. For large stationary ships, background clutter (sea or coastal land) can be problematic as Doppler filtering may not apply. For that reason maritime capability is supported by the presence of a Maritime Cabinet on board of the AWACS cabin housing circuitry including a database of stored maps of geographic land contours. The maps provides additional correlation on the presence of coastal areas in a ship background. The maritime cabinet had been lacking on board early US Air Force AWACS restricting their operational flexibility until later upgrades.

5. Interleaved mode.

The interleaved mode of operation reflects combining two different modes of utilization to achieve tactical gains. In this manner a ‘hybrid’ mode can utilize Pulse Doppler Elevation Scan and Beyond-The-Horizon mode as one option. The other mode combination can be PDNES and Maritime. This ability to combine two radar modes into a single combined operation requires the use of Radar Data Processor interfacing with the Adaptive Signal Processor radar data. We note the high demands being made on the AWACS digital signal processing and data correlation computing facilities.

6. Passive mode.

The passive mode is probably the single most highly valuable asset for the AWACS highlighting the high level of flexibility expected from modern radar systems. Passive mode operation is achieved by turning off the AWACS powerful radar transmitter while allowing the receiver to operate normally in order to capture electromagnetic signals (listening mode) and locate their source of emission. This allows effective operation in high electronic threats environment where enemy radar and electronic jamming systems can be systematically detected and located.

How advanced electronics affect the performance of the AWACS?

We already sensed how advances in digital electronics have benefited the AWACS ability to process a wide variety of electronic signals as well as supporting data cross-correlated computations to provide true, accurate, real time battlefield picture.We can bring more light into critical sub systems and components.

Digital signal processing, advanced electronics and overall stability

The antenna

-The main antenna array composed of 30 waveguides sticks producing beams that are very stable, sharply defined and thus not adversely generating additional clutter to the radar receiving operation provides a stable electromagnetic operation.

-The Transmit manifold that produces very high power Radio Frequencies pulses generated at the transmitter group level.

-The Receive manifold that gathers incoming signals into a main signal channeled towards algorithmic digital signal processors

-a microwave receiver with low noise and high amplification capability is necessary to receive long range signal return

-Beam Steering Phase Shifters control the generation of the electronic vertical beam which scans in a up-and-down motion to determine a flying object elevation

-The antenna rotary coupler also presents a high power channel for transmission along with seven coaxial channels.

The Transmitter Group

The Transmitter Group located inside the AWACS lower aft cargo hold is responsible for generating stable, high power radio wave pulses prior to their being propagated as high power beams by the antenna. Very stable and well defined signals are initially generated locally from very low current using Stable Local Oscillator circuits. The presence of two large, heavy Klystron Power Amplifiers will amplify the well defined but low power signals into the AWACS characteristically high power beam that will be radiated by the antenna.

-The high power transmission to the antenna is allowed through the rotary coupler high power transmission channel (supplemented by seven channel for transmission along with seven coaxial RF channels). With the inclusion of 2 digitally controlled (programmable) attenuators forming the Transmit Angle Control assembly we see that pervading use of digital electronic contribute significantly to overall system stability.

The Analog Cabinet

It is one of 3 function-specific cabinets located in the AWACS cabin (along with the maritime cabinet and Surveillance Radar Computer cabinet). It contains all of the AN/APY-2 main analog electronics systems for signal compression Doppler filtering and digital-to-analog and analog-to-digital signal conversion. Receiving and pre-processing of continuous analog signals is done as they are captured by the microwave receiver. The analog receiver can separate and filter various signal such as ground clutter, Pulse Doppler, Beyond-The-Horizon before providing initial digital signal encoding.

Digital Signal Processing

Adaptive signal Processor (ASP) provides core signal processing and correlation capabilities to various signals ensuring that ‘Clutter’ signals can be filtered out. Algorithms are applied to compress various RF pulse which will control all phases of tactical radar transmit as required by the available modes of operation. Pulse compression allows additional frequency switching in electronic counter measure mode, for continuous operation in electronic warfare environment.

Digital Computers

-Analog to digital converter using Fast Fourier Transform (FFT) algorithm and Spectrum Analyzer (for ECCM) for signal correlation.

-Radar Data Processor receives digitally encoded detection data from the ASP, can be mission-programmed in Ada, has Dual VME bus-based 32-bit architecture with 4 R4400 RISC CPU with 8 MB programmable memory

Radar Control And Maintenance Panel

This function is available to the two surveillance operators who act as radar operators and turn the radar on and off, performing fault isolation on the radar sub systems. Two CRT displays, keyboard and Trackball are available. The spectrum analyzer and FFT are used during ECCM (Electronic Counter Counter Measure) operation to interpret and analyze RF carrier waves using advanced signal processing algorithm. Our approach to understanding the AWACS main radar operation is by connecting concepts of modern electronic and computing systems with their possible application in advanced military systems. As well we have tried to make clear the links between various systems on board the aircraft and their particular mission employment.


End of part 1.

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