Active_electronically_scanned_array

# Active electronically scanned array An **active electronically scanned array** (AESA) is a [[Phased_array|phased array]] in which every antenna element carries its own small solid-state transmit/receive module, rather than sharing one transmitter and one receiver among all the elements through a passive network of phase shifters. A three.js companion sketch elsewhere on the site renders this same many-element, closely spaced array steering across a wide scan, in three dimensions. The distinction matters because a passive electronically scanned array (PESA), the technology AESA succeeded, can only radiate one beam at one frequency at a time, since every element still draws on the same single transmitter; an AESA, with each element generating or receiving its own signal under independent computer control, can radiate several beams at several frequencies simultaneously, and keeps working, if with reduced gain, when a handful of its many modules fail rather than going dark the instant its one shared transmitter does. AESA radar, sometimes called active phased-array radar, is by far the technology's leading application, and it feeds forward into other radar functions, including [[Radar_jamming_and_deception|jamming and deception]] countermeasures that an agile, multi-beam array is well placed to support. ## History Active arrays followed passive ones by necessity rather than by choice: a solid-state transmit/receive module capable of generating enough power, handling enough heat and costing little enough to be built by the thousand did not exist until [[Semiconductor_device|semiconductor]] and [[Integrated_circuit|monolithic microwave integrated circuit]] fabrication matured, decades after the first passive phased-array radars using [[Cavity_magnetron|magnetron]] or klystron transmitters had already entered service.[^mmic-cost-cn] Soviet and later Russian designers are generally credited with fielding some of the earliest active-array fighter radars, built around gallium-arsenide transmit/receive modules, around the same period Western manufacturers were still refining passive designs for their own front-line aircraft.[^pesa-aesa-history-cn] Western air forces followed through the late 1990s and 2000s as the cost of each transmit/receive module fell enough to make an active array practical for a fighter-sized radar rather than only for the largest, most expensive ground and shipboard systems built during the [[Cold_War|Cold War]]. Since then, active arrays have displaced passive ones as the default choice for any new phased-array radar with the budget to afford them, extending from fighter and airborne early-warning radar into ground-based air-defence and shipboard radar, largely because each successive generation of transmit/receive modules has continued to fall in cost and rise in reliability rather than because the underlying array theory changed. ## Basic concept Each element of an active array sits behind its own transmit/receive module: on transmit, a small solid-state power amplifier generates that element's share of the outgoing signal at whatever phase, and potentially whatever frequency, the beam-steering computer commands; on receive, a low-noise amplifier boosts the element's own echo before it is combined with every other element's contribution. Because amplification happens right at each element rather than after a lossy shared feed network, the same distributed placement that lets an AESA form its beam also improves the receiver's overall noise figure, since a low-noise gain stage placed ahead of a loss suppresses that loss's contribution to the total noise far more effectively than the same gain placed behind it.[^friis-noise] Digitally controlling thousands of independent modules also lets an AESA do things a single-transmitter phased array cannot: form several simultaneous beams, each with its own phase pattern computed independently, and transmit or receive on several different frequencies within the array's bandwidth at once, using one physical aperture for tasks, such as search, track and jamming, that an older radar would have needed separate hardware or separate time slots to perform. The same [[Beamforming|beamforming]] mathematics an ordinary phased array uses still governs each individual beam; what changes is that nothing stops the array from computing several such beams in parallel. ## Advantages Distributing transmit and receive electronics across every element, rather than concentrating them in one shared chain, buys an active array several advantages beyond raw beam agility. ### Low probability of intercept An AESA can spread its transmitted energy thinly, hopping frequency from pulse to pulse or coding its waveform much as a spread-spectrum communication link does, so that the power present at any single frequency an eavesdropping receiver happens to be tuned to stays far below what a simple intercept receiver needs to notice, while the radar's own matched filter recovers the full signal by working across the whole coded bandwidth at once.[^lpi-spread] A radar built this way is described as having a low probability of intercept, related in spirit to the passive side of [[Stealth_technology|stealth technology]], since an adversary's warning receiver may fail to detect or classify an emission that the radar itself receives without difficulty. ### High jamming resistance Because every element carries its own module, an AESA can null a [[Radar_jamming_and_deception|jammer]] adaptively, weighting each element's contribution so that the many individual beams add against the jammer's bearing while still adding constructively toward the target, and can hop frequency or split attention across several channels faster than a jammer built to follow a single, slowly retuned transmitter can react. Losing individual elements to a jammer's own countermeasures, or to ordinary battle damage, degrades an active array's performance gradually rather than disabling it outright, unlike a design that depends on one shared transmitter surviving intact. ### Other advantages An active array's thousands of small, individually replaceable modules also make the whole radar far more tolerant of ordinary component failure than a design built around one large transmitter tube: losing a small percentage of modules trims gain and raises sidelobes slightly rather than silencing the set, a graceful degradation with a direct parallel in the fault tolerance an aperiodic [[Antenna_array|antenna array]] gains from its own element redundancy. The same per-element flexibility also gives an active array wider instantaneous bandwidth and faster beam agility than a passive design built around one narrowband transmitter and a single, comparatively slow-switching feed network, letting one radar cover search, track and communication roles that older designs split across separate antennas entirely. ## Limitations An active array's advantages come at a price. Building thousands of individually capable transmit/receive modules costs far more than building one shared transmitter behind a passive phase-shifter network, and that cost scales roughly with the number of elements rather than dropping much with an array's overall size the way a single large transmitter's cost might. Every module also turns some of its input power into heat rather than radiated energy, so cooling an active array, with its amplifiers spread across the whole aperture rather than concentrated somewhere a designer can plumb coolant to directly, is a harder thermal engineering problem than cooling one large transmitter tube. Calibrating thousands of individually manufactured modules, each with its own small variation in gain and phase, so that the array's computed beams behave as the theory predicts, adds a manufacturing and maintenance burden a passive design, with far fewer active components to calibrate, does not carry. None of this changes the physical limits every [[Phased_array|phased array]] shares regardless of how its elements are driven: steering far off broadside still costs effective aperture and gain, and an active array's practical scan volume is bounded by the same geometry as a passive one. ## List of existing systems Active electronically scanned arrays have been fielded widely enough, across enough air forces and navies, that only a representative sample is given here rather than an exhaustive list; a new fighter, destroyer or ground-based air-defence radar programme starting today would be unusual if it chose a passive array over an active one, so the roll of active-array systems now entering service is long and still growing. ### Airborne systems Fighter and airborne early-warning radars built around active arrays include the United States' AN/APG-77, carried by the F-22 Raptor, and AN/APG-81, carried by the F-35 Lightning II; France's RBE2-AA, carried by the Rafale; the Eurofighter Typhoon's Captor-E; Sweden's Raven ES-05, carried by the Gripen E; and various Zhuk-AE family radars fitted to later-model Russian fighters.[^airborne-systems-cn] ### Surface systems (land, maritime) Land- and sea-based active arrays include long-range missile-defence radars such as the AN/TPY-2, naval air-defence radars such as the AN/SPY-6 fitted to newer United States Navy destroyers, the Royal Navy's SAMPSON, the multinational APAR radar carried by several European navies' frigates, and land-based air-defence systems such as the United States Army's LTAMDS, developed to replace older passive-array Patriot radars.[^surface-systems-cn] ## Microsims This article carries no p5.js sketch of its own. A three.js companion, built as a variant of the Antenna_array sketch, instead renders an array with many elements at close spacing scanning across a wide angle, the configuration an active array's many small transmit/receive modules make practical, in contrast with a passive array's single feed and correspondingly gentler tolerance for close element spacing across a large aperture. *Try:* in the [[Antenna_array]] sketch, raise the element count toward its maximum and watch the main lobe narrow while the sidelobe structure fills in around it; an active array can afford exactly this many elements because each one's transmit/receive module is cheap enough to build by the thousand. *Try:* in the [[Radar]] sketch, picture the single sweeping beam replaced by several beams at once, one holding a search pattern and others revisiting already-detected targets; that simultaneous multi-beam operation, impossible for the single-transmitter design the sketch itself animates, is what an active array's independent per-element modules make possible. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Active_electronically_scanned_array) : [Wikitube](https://en.wikitube.io/wiki/Active_electronically_scanned_array) Skeleton mirrored at revision 1371403430. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Phased_array]] - [[Antenna_array]] - [[Beamforming]] - [[Radar]] - [[Radar_jamming_and_deception]] - [[Radar_cross_section]] - [[Stealth_technology]] ## References The relations governing beam steering, aperture size and scan-angle limits are the same array-factor antenna theory described under [[Phased_array]] and are not separately footnoted here, per the Wikitube style guide's §6.1. This topic's brief names no specific sub-manual section; the two page-cited claims below instead draw on the genuinely on-topic noise-figure and spread-spectrum material in sub-manual 06. [^mmic-cost-cn]: Citation needed: a manufacturing history giving the date and process by which monolithic-microwave-integrated-circuit transmit/receive modules became cheap enough to build a large array from would confirm the timeline claimed here. [^pesa-aesa-history-cn]: Citation needed: the specific Soviet or Russian fighter radar programme generally credited as an early operational active electronically scanned array, with its designer and in-service date. [^friis-noise]: Ellingson, S. *Radio Systems Engineering, Revised First Edition*. 2023, pp. 102-103 (PDF pages): the Friis noise-figure cascade, showing that a low-noise gain stage placed ahead of a lossy stage suppresses that loss's noise contribution far more than the same gain placed behind it. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. [^lpi-spread]: Ellingson, S. *Radio Systems Engineering, Revised First Edition*. 2023, pp. 179-180 (PDF pages): spread-spectrum processing gain, showing how coding a signal across a wide bandwidth lowers the power present at any single frequency well below what a receiver not matched to the code would need to detect it. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. [^airborne-systems-cn]: Citation needed: manufacturer or government sources confirming each named airborne system's designation, carrier aircraft and fielding status. [^surface-systems-cn]: Citation needed: manufacturer or government sources confirming each named surface system's designation, platform and fielding status. ## Bibliography - Steven Ellingson. *Radio Systems Engineering, Revised First Edition*. 2023. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. - Christian Tiberius; Max Mulder. *Engineering Signal Analysis: From Fourier to filtering: Theory*. 2026. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/engineering-signal-analysis-from-fourier-to-filtering-theory . CC BY. - Don Johnson. *Fundamentals of Electrical Engineering I*. 2014. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/fundamentals-of-electrical-engineering-1 . CC BY. - Michael Stiber; Bilin Stiber; Eric Larson. *Signal Computing: Digital Signals in the Software Domain*. 2020. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/signal-computing-digital-signals-in-the-software-domain . CC BY-SA. - Allen Downey. *Think DSP: Digital Signal Processing in Python*. 2012. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/think-dsp-digital-signal-processing-in-python . CC BY-NC. - John Dyer; Chad Davis. *Measurement and Instrumentation: An Introduction to Concepts and Methods, 1st Edition*. 2020. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/measurement-and-instrumentation-an-introduction-to-concepts-and-methods . CC BY-NC-SA. ## External links This article's Microsims section points outward to sketches carried by neighbouring articles; it carries none of its own to list here.