Frequency-agile
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Frequency agility is the ability of a
radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...
system to quickly shift its operating frequency to account for atmospheric effects, jamming, mutual interference with friendly sources, or to make it more difficult to locate the radar broadcaster through
radio direction finding Direction finding (DF), radio direction finding (RDF), or radiogoniometry is the use of radio waves to determine the direction to a radio source. The source may be a cooperating radio transmitter or may be an inadvertent source, a natural ...
. The term can also be applied to other fields, including
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
s or traditional
radio transceiver In radio communication, a transceiver is an electronic device which is a combination of a radio ''trans''mitter and a re''ceiver'', hence the name. It can both transmit and receive radio waves using an antenna, for communication purposes. The ...
s using
frequency-division multiplexing In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth (signal processing), bandwidth available in a communication channel, communication medium is divided into a series of non-overlapping freque ...
, but it remains most closely associated with the radar field and these other roles generally use the more generic term "
frequency hopping Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly changing the carrier frequency among many frequencies occupying a large spectral band. The changes are controlled by a code known to both transmitter ...
".


Description


Jamming

Radar systems generally operate by sending out short pulses of
radio Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3  hertz (Hz) and 300  gigahertz (GHz). They are generated by an electronic device called a transmitter connec ...
energy and then turning off the broadcaster and listening for the returning echoes from various objects. Because efficient signal reception requires careful tuning throughout the electronics in the transceiver, each operating frequency required a dedicated transceiver. Due to the size of the tube-based electronics used to construct the transceivers, early radar systems, like those deployed in
World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
, were generally limited to operating on a single frequency. Knowing this operating frequency gives an adversary enormous power to interfere with radar operation or gather further intelligence. The British used the frequency information about the
Würzburg radar The low-UHF band Würzburg radar was the primary ground-based tracking radar for the Wehrmacht's Luftwaffe and Kriegsmarine (German Navy) during World War II. Initial development took place before the war and the apparatus entered service in 194 ...
gathered in
Operation Biting Operation Biting, also known as the Bruneval Raid, was a British Combined Operations (United Kingdom), Combined Operations Raid (military), raid on a German coastal radar installation at Bruneval in northern France, during the Second World War, ...
to produce "
Window A window is an opening in a wall, door, roof, or vehicle that allows the exchange of light and may also allow the passage of sound and sometimes air. Modern windows are usually glazed or covered in some other transparent or translucent ma ...
", aluminum foil strips cut to 1/2 the length of the wavelength of the Würzburg, rendering it almost useless. They also produced jammer units, "Carpet" and "Shivers", that broadcast signals on the Würzburg's frequency, producing confusing displays that were useless for aiming. Post-war calculations estimated these efforts reduced the combat effectiveness of the Würzburg by 75%. These countermeasures forced the Germans to upgrade thousands of units in the field to operate on different frequencies. Knowing the frequency of the Würzburg also helped the British in their attempts to locate the systems using
radio direction finder Direction finding (DF), radio direction finding (RDF), or radiogoniometry is the use of radio waves to determine the direction to a radio source. The source may be a cooperating radio transmitter or may be an inadvertent source, a natural ...
s, allowing aircraft to be routed around the radars, or at least be kept at longer distances from them. It also helped them to find new operating frequencies as they were introduced, by selecting the location of known installations when they disappeared and singling them out for further study.


Agile

A radar system that can operate on several different frequencies makes these countermeasures more difficult to implement. For instance, if a jammer is developed to operate against a known frequency, changing that frequency in some of the in-field sets will render the jammer ineffective against those units. To counter this, the jammer has to listen on both frequencies, and broadcast on the one that particular radar is using. To further frustrate these efforts, a radar can rapidly switch between the two frequencies. No matter how quickly the jammer responds, there will be a delay before it can switch and broadcast on the active frequency. During this period of time the aircraft is unmasked, allowing detection.Galati In its ultimate incarnation, each radar pulse is sent out on a different frequency and therefore renders single-frequency jamming almost impossible. In this case the jammers are forced to broadcast on every possible frequency at the same time, greatly reducing its output on any one channel. With a wide selection of possible frequencies, jamming can be rendered completely ineffective. Additionally, having a wide variety of frequencies makes ELINT much more difficult. If only a certain subset of the possible frequencies are used in normal operation the adversary is denied information on what frequencies might be used in a wartime situation. This was the idea behind the
AMES AMES, short Air Ministry Experimental Station, was the name given to the British Air Ministry's radar development team at Bawdsey Manor (afterwards RAF Bawdsey) in the immediate pre-World War II era. The team was forced to move on three occasion ...
Type 85 radar in the
Linesman/Mediator Linesman/Mediator was a dual-purpose civil and military radar network in the United Kingdom between the 1960s and 1984. The military side (Linesman) was replaced by the Improved United Kingdom Air Defence Ground Environment (IUKADGE), while the ...
network in the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...
. The Type 85 had twelve klystrons that could be mixed to produce sixty output frequencies, but only four of the klystrons were used in peacetime, in order to deny the
Soviet Union The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...
any information about what signals would be used during a war.


Improving electronics

One of the primary reasons that early radars did not use more than one frequency was the size of their tube based electronics. As their size was reduced through improved manufacturing, even early systems were upgraded to offer more frequencies. These, however, were not generally able to be switched on the fly through the electronics itself, but were controlled manually and thus were not really agile in the modern sense. "Brute force" frequency agility, like the Linesman, was common on large
early warning radar An early-warning radar is any radar system used primarily for the long-range detection of its targets, i.e., allowing defences to be alerted as ''early'' as possible before the intruder reaches its target, giving the air defences the maximum tim ...
s but less common on smaller units where the size of klystrons remained a problem. In the 1960s solid state components dramatically decreased the size of the receivers, allowing several solid-state receivers to fit into the space formerly occupied by a single tube-based system. This space could be used for additional broadcasters and offer some agility even on smaller units.
Passive electronically scanned array A passive electronically scanned array (PESA), also known as passive phased array, is an antenna in which the beam of radio waves can be electronically steered to point in different directions (that is, a phased array antenna), in which all the ...
(PESA) radars, introduced in the 1960s, used a single microwave source and a series of delays to drive a large number of antenna elements (the array) and electronically steer the radar beam by changing the delay times slightly. The development of solid-state microwave amplifiers,
JFET The junction field-effect transistor (JFET) is one of the simplest types of field-effect transistor. JFETs are three-terminal semiconductor devices that can be used as electronically controlled switches or resistors, or to build amplifiers. U ...
s and
MESFET A MESFET (metal–semiconductor field-effect transistor) is a field-effect transistor semiconductor device similar to a JFET with a Schottky (metal–semiconductor) junction instead of a p–n junction for a gate. Construction MESFETs are con ...
s, allowed the single klystron to be replaced by a number of separate amplifiers, each one driving a subset of the array but still producing the same amount of total power. Solid-state amplifiers can operate at a wide range of frequencies, unlike a klystron, so solid-state PESAs offered much greater frequency agility, and were much more resistant to jamming. The introduction of
active electronically scanned array An active electronically scanned array (AESA) is a type of phased array antenna, which is a computer-controlled antenna array in which the beam of radio waves can be electronically steered to point in different directions without moving the a ...
s (AESAs) further evolved this process. In a PESA the broadcast signal is a single frequency, although that frequency can be easily changed from pulse to pulse. In the AESA, each element is driven at a different frequency (or at least a wide selection of them) even within a single pulse, so there is no high-power signal at any given frequency. The radar unit knows which frequencies were broadcast, and amplifies and combines only those return signals, thereby reconstructing a single powerful echo on reception. An adversary, unaware of which frequencies are active, has no signal to see, making detection on
radar warning receiver Radar warning receiver (RWR) systems detect the radio emissions of radar systems. Their primary purpose is to issue a warning when a radar signal that might be a threat is detected, like a fighter aircraft's fire control radar. The warning can ...
s extremely difficult. Modern radars like the
F-35 The Lockheed Martin F-35 Lightning II is an American family of single-seat, single-engine, supersonic stealth strike fighters. A multirole combat aircraft designed for both air superiority and strike missions, it also has electronic warf ...
's
AN/APG-81 The AN/APG-81 is an active electronically scanned array (AESA) fire-control radar system designed by Northrop Grumman Electronic Systems (formerly Westinghouse Electronic Systems) for the Lockheed Martin F-35 Lightning II. The AN/APG-81 is a su ...
use thousands of broadcaster/receiver modules, one for each antenna element.


Other advantages

The reason that several
cell phone A mobile phone or cell phone is a portable telephone that allows users to make and receive calls over a radio frequency link while moving within a designated telephone service area, unlike fixed-location phones ( landline phones). This radio ...
s can be used at the same time in the same location is due to the use of
frequency hopping Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly changing the carrier frequency among many frequencies occupying a large spectral band. The changes are controlled by a code known to both transmitter ...
. When the user wishes to place a call, the cell phone uses a negotiation process to find unused frequencies among the many that are available within its operational area. This allows users to join and leave particular cell towers on-the-fly, their frequencies being given up to other users.Marshall Brain, Jeff Tyson and Julia Layton
"How Cell Phones Work"
howstuffworks.com
Frequency agile radars can offer the same advantages. In the case of several aircraft operating in the same location, the radars can select frequencies that are not being used in order to avoid interference. This is not as simple as the case of a cell phone, however, because ideally the radars would change their operating frequencies with every pulse. The algorithms for selecting a set of frequencies for the next pulse cannot be truly random if one wants to avoid all interference with similar systems, but a less-than-random system is subject to
ELINT Signals intelligence (SIGINT) is the act and field of intelligence-gathering by interception of ''signals'', whether communications between people (communications intelligence—abbreviated to COMINT) or from electronic signals not directly u ...
methods to determine the pattern. Another reason for adding frequency agility has nothing to do with military use;
weather radar A weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation (meteorology), precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern w ...
s often have limited agility to allow them to strongly reflect off rain, or alternately, to see through it. By switching the frequencies back and forth, a composite image of the weather can be built up.


See also

*
Variable-frequency oscillator A variable frequency oscillator (VFO) in electronics is an oscillator whose frequency can be tuned (i.e., varied) over some range. It is a necessary component in any tunable radio transmitter and in receivers that work by the superheterodyne pr ...
*
Frequency hopping Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly changing the carrier frequency among many frequencies occupying a large spectral band. The changes are controlled by a code known to both transmitter ...
*
Frequency diversity Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...


References


Footnotes


Bibliography

* Ian Faulconbridge, "Radar Fundamentals", Argos Press, June 2002, * Gaspare Galati, "Advanced radar techniques and systems", IET, 1993, , pp. 481–503 {{refend Radar