Frequency agility
Encyclopedia
Frequency agility is the ability of a radar
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. The term can also be applied to other fields, including laser
s or traditional radio transceiver
s using frequency-division multiplexing
, but it remains most closely associated with the radar field and these other roles generally use the more generic term "frequency hopping".
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
, 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
gathered in Operation Biting
to produce "Window
", 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
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.
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. 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 Type 85 radar in the Linesman/Mediator
network in the United Kingdom
. 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
any information about what signals would be used during a war.
"Brute force" frequency agility, like the Linesman, was common on large early warning radar
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
(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
s and MESFET
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
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
s extremely difficult.
Modern radars like the F-35
's AN/APG-81 use thousands of broadcaster/receiver modules, one for each antenna element.
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 methods to determine the pattern.
Another reason for adding frequency agility has nothing to do with military use; weather radar
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.
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
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. The term can also be applied to other fields, including laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s or traditional radio transceiver
Transceiver
A transceiver is a device comprising both a transmitter and a receiver which are combined and share common circuitry or a single housing. When no circuitry is common between transmit and receive functions, the device is a transmitter-receiver. The term originated in the early 1920s...
s using frequency-division multiplexing
Frequency-division multiplexing
Frequency-division multiplexing is a form of signal multiplexing which involves assigning non-overlapping frequency ranges to different signals or to each "user" of a medium.- Telephone :...
, but it remains most closely associated with the radar field and these other roles generally use the more generic term "frequency hopping".
Jamming
Radar systems generally operate by sending out short pulses of radioRadio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
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
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, 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
Würzburg radar
The Würzburg radar was the primary ground-based gun laying radar for both the Luftwaffe and the German Army during World War II. Initial development took place before the war, entering service in 1940. Eventually over 4,000 Würzburgs of various models were produced...
gathered in Operation Biting
Operation Biting
Operation Biting, also known as the Bruneval Raid, was the codename given to a British Combined Operations raid on a German radar installation in Bruneval, France that occurred between 27–28 February 1942 during World War II...
to produce "Window
Chaff (radar countermeasure)
Chaff, originally called Window by the British, and Düppel by the Second World War era German Luftwaffe , is a radar countermeasure in which aircraft or other targets spread a cloud of small, thin pieces of aluminium, metallized glass fibre or plastic, which either appears as a cluster of secondary...
", 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
Radio direction finder
A radio direction finder is a device for finding the direction to a radio source. Due to low frequency propagation characteristic to travel very long distances and "over the horizon", it makes a particularly good navigation system for ships, small boats, and aircraft that might be some distance...
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.
Agility
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. 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 Type 85 radar in the Linesman/Mediator
Linesman/Mediator
Linesman/Mediator was the dual-purpose civil and military radar network in the United Kingdom. Against a background of the Russian H-bomb and supersonic bombers Linesman was the answer to the Cold War defence policies, replacing the earlier ROTOR and Type 80 Master Radar Stations that were the...
network in the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
. 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
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
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
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 defences the maximum time in which to operate...
s but less common on smaller units where the size of klystrons remained a problem. In the 1960s solid state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...
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
Passive electronically scanned array
A passive electronically scanned array , contrary to its active counterpart AESA, is a phased array which has a central radiofrequency source , sending energy into phase shift modules, which then send energy into the various emitting elements in the front of the antenna...
(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
JFET
The junction gate field-effect transistor is the simplest type of field-effect transistor. It can be used as an electronically-controlled switch or as a voltage-controlled resistance. Electric charge flows through a semiconducting channel between "source" and "drain" terminals...
s and MESFET
MESFET
MESFET stands for metal semiconductor field effect transistor. It is quite similar to a JFET in construction and terminology. The difference is that instead of using a p-n junction for a gate, a Schottky junction is used...
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
Active Electronically Scanned Array
An Active Electronically Scanned Array , also known as active phased array radar is a type of phased array radar whose transmitter and receiver functions are composed of numerous small solid-state transmit/receive modules . AESAs aim their "beam" by broadcasting radio energy that interfere...
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
Radar warning receiver 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. The warning can then be used, manually or automatically, to evade the detected threat...
s extremely difficult.
Modern radars like the F-35
F-35 Lightning II
The Lockheed Martin F-35 Lightning II is a family of single-seat, single-engine, fifth generation multirole fighters under development to perform ground attack, reconnaissance, and air defense missions with stealth capability...
's AN/APG-81 use thousands of broadcaster/receiver modules, one for each antenna element.
Other advantages
The reason that several cell phones can be used at the same time in the same location is due to the use of frequency hopping. 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.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 methods to determine the pattern.
Another reason for adding frequency agility has nothing to do with military use; weather radar
Weather radar
Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...
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.