subject: Active Electronically Scanned Array [print this page] Basic concept Basic concept
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Radar systems generally work by connecting an antenna to a powerful radio transmitter to broadcast a short pulse of signal. The transmitter is then disconnected and the antenna is attached to a sensitive receiver which amplifies any echos from target objects and then sends the resulting output to a display of some sort. The transmitter elements were typically klystron tubes, which are suitable for amplifying a small range of frequencies. In order to scan a portion of the sky, the radar antenna has to be physically moved to point in different directions.
Starting in the 1960s new solid-state delays were introduced that led to the first practical large-scale passive electronically scanned array (PESA), or simply phased array radar. PESAs took a signal from a single source, split it up into hundreds of paths, selectively delayed some of them, and sent them to individual antennas. The resulting broadcasts overlapped in space, and the interference patterns between the individual signals was selected in order to reinforce the signal at certain angles, and mute it down in all others. The delays could be easily controlled electronically, allowing the beam to be steered without the antenna having to move. A PESA can scan a volume of space much more quickly than a traditional mechanical system. Additionally, as the electronics improved, PESAs added the ability to produce several active beams, allowing them to continue scanning the sky while at the same time focusing smaller beams on certain targets for tracking or guiding semi-active radar homing missiles. PESAs quickly became widespread on ships and large fixed emplacements in the 1960s, followed by airborne sensors as the electronics shrank.
AESAs are the result of further developments in solid-state electronics. In earlier systems the broadcast signal was originally created in a klystron tube or similar device, which are relatively large. Receiver electronics were also large due to the high frequencies that they worked with. The introduction of gallium arsenide microelectronics through the 1980s served to greatly reduce the size of the receiver elements, until effective ones could be built at sizes similar to those of handheld radios, only a few centimeters in volume. The introduction of JFETs and MESFETs did the same to the transmitter side of the systems as well. Now an entire radar, the transmitter, receiver and antenna, could be shrunk into a single "transmitter-receiver module" (TRM) about the size of a carton of milk.
The primary advantage of a AESA over a PESA is that the different modules can operate on different frequencies. Unlike the PESA, where the signal was generated at single frequencies by a small number of transmitters, in the AESA each module broadcasts its own independent signal. This allows the AESA to produce numerous "sub-beams" and actively "paint" a much larger number of targets. Additionally, the solid-state transmitters are able to broadcast effectively at a much wider range of frequencies, giving AESAs the ability to change their operating frequency with every pulse sent out. AESAs can also produce beams that consist of many different frequencies at once, using post-processing of the combined signal from a number of TRMs to re-create a display as if there was a single powerful beam being sent.
Advantages
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In addition to the advantages offered by PESAs, notably the lack of mechanical steering and the ability to form multiple beams, AESAs add many capabilities of their own. Among these are the ability to use some of the TRMs for "other purposes", like radar detection, and more importantly, the difficulties that AESAs cause for radar detectors.
Low Probability of Intercept
See also Low Probability of Intercept Radar
Radar systems work by sending out a signal and then listening for its echo off distant objects. Each of these paths, to and from the target, is subject to the inverse square law of propagation. That means that a radar's received energy drops with the fourth power of distance, which is why radar systems require high powers, often in the megawatt range, in order to be effective at long range.
The radar signal being sent out is a simple radio signal, and can be received with a simple radio receiver. It is common to use such a receiver in the targets, normally aircraft, to detect radar broadcasts. Unlike the radar unit, which has to send the pulse out and then receive its reflection, the target's receiver does not need the reflection and thus the signal drops off only as the square of distance. This means that the receiver is always at an advantage over the radar in terms of range - it will always be able to detect the signal long before the radar can see the target's echo. Since the position of the radar is extremely useful information in an attack on that platform, this means that radars generally have to be turned off for lengthy periods if they are subject to attack; this is common on ships, for instance.
Turning that received signal into a useful display is the purpose of the "radar warning receiver" (RWR). Unlike the radar, which knows which direction it is sending its signal, the receiver simply gets a pulse of energy and has to interpret it. Since the radio spectrum is filled with noise, the receiver's signal is integrated over a short period of time, making periodic sources like a radar add up and stand out over the random background. Typically RWRs store the detected pulses for a short period of time, and compare their broadcast frequency and pulse repetition frequency against a database of known radars. The rough direction can be calculated using a rotating antenna, or similar passive array, and combined with symbology indicating the likely purpose of the radar - airborne early warning, surface to air missile, etc.
This technique is much less useful against AESA radars. Since the AESA can change its frequency with every pulse, and generally does so using a pseudo-random sequence, integrating over time does not help pull the signal out of the background noise. Nor does the AESA have any sort of fixed pulse repetition frequency, which can also be varied and thus hide any periodic brightening across the entire spectrum. Traditional RWRs are essentially useless against AESA radars.
High jamming resistance
Jamming is likewise much more difficult against an AESA. Traditionally, jammers have operated by determining the operating frequency of the radar and then broadcasting a signal on it to confuse the receiver as to which is the "real" pulse and which is the jammer's. This technique works as long as the radar system cannot easily change its operating frequency. When the transmitters were based on klystron tubes this was generally true, and radars, especially airborne ones, had only a few frequencies to chose among. A jammer could listen to those possible frequencies and select the one being used to jam.
Since an AESA changes its operating frequency with every pulse, and spreads the frequencies across a wide band even in a single pulse, jammers are much less effective. Although it is possible to send out broadband white noise against all the possible frequencies, this means the amount of energy being sent at any one frequency is much lower, reducing its effectiveness. Moreover, AESAs can be switched to a receive-only mode, and use the jamming signals as a powerful source to track its source, something that required a separate receiver in older platforms.
AESAs are so much more difficult to detect, and so much more useful in receiving signals from the targets, that they can broadcast continually and still have a very low chance of being detected. This allows the radar system to generate far more data than if it is being used only periodically, greatly improving overall system effectiveness.
Other advantages
Since each element in a AESA is a powerful radio receiver, active arrays have many roles besides traditional radar. One use is to dedicate several of the elements to reception of common radar signals, eliminating the need for a separate radar warning receiver. The same basic concept can be used to provide traditional radio support, and with some elements also broadcasting, form a very high bandwidth data link. The F-35 uses this mechanism to send sensor data between aircraft in order to provide a synthetic picture of higher resolution and range than any one radar could generate.
AESAs are also much more reliable than either a PESA or older designs. Since each module operates independently of the others, single failures have little effect on the operation of the system as a whole. Additionally, the modules individually operate at low powers, perhaps 40 to 60 watts, so the need for a large high-voltage power supply is eliminated.
Replacing a mechanically scanned array with a fixed AESA mount (such as on the F/A-18E/F Super Hornet) can help reduce an aircraft's overall radar cross-section (RCS), but some designs (such as the Eurofighter Typhoon) forgo this advantage in order to add the limits of mechanically scanning to the limits of electronic scanning and provide a larger angle of coverage.
List of existing systems
US based manufacturers of the AESA radars used in the F22 and Super Hornet include Northrop Grumman and Raytheon. These companies also design, develop and manufacture the transmit/receive modules which comprise the 'building blocks' of an AESA radar. The requisite electronics technology was developed in-house via Department of Defense research programs such as MIMIC Program.
Airborne systems
Northrop Grumman/Raytheon AN/APG-77, for the F-22 Raptor
Northrop Grumman AN/APG-80, for the F-16E/F Block 60 Fighting Falcon
Northrop Grumman AN/APG-81, for the F-35 Joint Strike Fighter
Northrop Grumman Multi-role AESA, for the Boeing Wedgetail (AEW&C)
Northrop Grumman APY-9, for the E-2D Advanced Hawkeye
Northrop Grumman SABR, for F-16 Fighting Falcon upgrades
Raytheon AN/APG-63(V)2 and AN/APG-63(V)3, for the F-15C Eagle and Republic of Singapore's F-15SG
Raytheon APG-79, for the F/A-18E/F Super Hornet and EA-18G Growler
Raytheon AN/APQ-181 (AESA upgrade currently in development), for the B-2 Spirit bomber
AMSAR, research from the European GTDAR consortium, for Eurofighter and Rafale fighter Radar
Captor-E CAESAR (CAPTOR Active Electronically Scanning Array Radar)
RBE2-AA Radar Balayage Electronique 2 - Active Array
SELEX Seaspray 7000E, for helicopters
SELEX Vixen 500E
Mitsubishi Electric Corporation J/APG-1, AESA for the Mitsubishi F-2 fighter
Ericsson Erieye AEW&C
Ericsson PS-05/A MK-5 for JAS 39 Gripen. Will be available by 2012.
Phazotron NIIR Zhuk-AE, for MiG-35
Tikhomirov NIIP Epaulet-A
Elta EL/M-2083 aerostat-mounted air search radar
Elta EL/M-2052, for fighters. Interim candidate for HAL Tejas. Also, suitable for F-15, MiG-29 & Mirage 2000
Elta EL/M-2075 radar for the IAI Phalcon AEW&C system
NRIET-designed (Nanjing Research Institute of Electronic Technology) radar mounted on the KJ-2000 AEW&C system
Toshiba HPS-106, air & surface search radar, for the Kawasaki P-1 maritime patrol aircraft, four antenna arrays.
Mitsubishi Electric Corporation HPS-104, for the Mitsubishi SH-60
Ground and sea-based systems
APAR Thales multi-function radar, primary sensor of Dutch De Zeven Provincin and German Sachsen class frigates. The T/R modules for these radar systems are manufactured by the Canadian company BreconRidge Corp located in Ottawa, Ontario
Active Electronically Steered Arrays A Maturing Technology
FLUG REVUE December 1998: Modern fighter radar technology
Phased Arrays and Radars Past, Present and Future
Northrop Grumman white paper
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