Frequency agility

Last updated April 02, 2019

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 lasers or traditional radio transceivers 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".

Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna and a receiver and processor to determine properties of the object(s). Radio waves from the transmitter reflect off the object and return to the receiver, giving information about the object's location and speed.

Radar jamming and deception is the intentional emission of radio frequency signals to interfere with the operation of a radar by saturating its receiver with noise or false information. There are two types of radar jamming: Mechanical and Electronic jamming.

Laser device which emits light via optical amplification

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term "laser" originated as an acronym for "Light Amplification by Stimulated Emission of Radiation". The first laser was built in 1960 by Theodore H. Maiman at Hughes Research Laboratories, based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow.

Description

Jamming

Radar systems generally operate by sending out short pulses of radio 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.

Radio is the technology of signalling or communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connected to an antenna which radiates the waves, and received by a radio receiver connected to another antenna. Radio is very widely used in modern technology, in radio communication, radar, radio navigation, remote control, remote sensing and other applications. In radio communication, used in radio and television broadcasting, cell phones, two-way radios, wireless networking and satellite communication among numerous other uses, radio waves are used to carry information across space from a transmitter to a receiver, by modulating the radio signal in the transmitter. In radar, used to locate and track objects like aircraft, ships, spacecraft and missiles, a beam of radio waves emitted by a radar transmitter reflects off the target object, and the reflected waves reveal the object's location. In radio navigation systems such as GPS and VOR, a mobile receiver receives radio signals from navigational radio beacons whose position is known, and by precisely measuring the arrival time of the radio waves the receiver can calculate its position on Earth. In wireless remote control devices like drones, garage door openers, and keyless entry systems, radio signals transmitted from a controller device control the actions of a remote device.

World War II, also known as the Second World War, was a global war that lasted from 1939 to 1945. The vast majority of the world's countries—including all the great powers—eventually formed two opposing military alliances: the Allies and the Axis. A state of total war emerged, directly involving more than 100 million people from over 30 countries. The major participants threw their entire economic, industrial, and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. World War II was the deadliest conflict in human history, marked by 50 to 85 million fatalities, most of whom were civilians in the Soviet Union and China. It included massacres, the genocide of the Holocaust, strategic bombing, premeditated death from starvation and disease, and the only use of nuclear weapons in war.

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.^[1] Post-war calculations estimated these efforts reduced the combat effectiveness of the Würzburg by 75%.^[2] These countermeasures forced the Germans to upgrade thousands of units in the field to operate on different frequencies.

The low-UHF band Würzburg radar was the primary ground-based gun laying radar for the Luftwaffe and the Wehrmacht Heer during World War II. Initial development took place before the war and the apparatus entered service in 1940. Eventually over 4,000 Würzburgs of various models were produced. It took its name from the city of Würzburg.

Operation Biting, also known as the Bruneval Raid, was a British Combined Operations raid on a German coastal radar installation at Bruneval in northern France during the Second World War, on the night of 27–28 February 1942.

Knowing the frequency of the Würzburg also helped the British in their attempts to locate the systems using radio direction finders, 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.

A radio direction finder (RDF) is a device for finding the direction, or bearing, to a radio source. The act of measuring the direction is known as radio direction finding or sometimes simply direction finding (DF). Using two or more measurements from different locations, the location of an unknown transmitter can be determined; alternately, using two or more measurements of known transmitters, the location of a vehicle can be determined. RDF is widely used as a radio navigation system, especially with boats and aircraft.

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.^[3] 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.^[3]

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 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.^[4]

AMES, short Air Ministry Experimental Station, was the name given to the British Air Ministry's radar development team at Bawdsey Manor in the immediate pre-World War II era. The team was forced to move on three occasions, changing names as part of these moves, so the AMES name applies only to the period between 1936 and 1939.

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 civilian side (Mediator) became the modern public-private National Air Traffic Services (NATS).

The United Kingdom (UK), officially the United Kingdom of Great Britain and Northern Ireland, informally as Britain, is a sovereign country lying off the north-western coast of the European mainland. The United Kingdom includes the island of Great Britain, the north-eastern part of the island of Ireland and many smaller islands. Northern Ireland is the only part of the United Kingdom that shares a land border with another sovereign state, the Republic of Ireland. Apart from this land border, the United Kingdom is surrounded by the Atlantic Ocean, with the North Sea to the east, the English Channel to the south and the Celtic Sea to the south-west, giving it the 12th-longest coastline in the world. The Irish Sea lies between Great Britain and Ireland. With an area of 242,500 square kilometres (93,600 sq mi), the United Kingdom is the 78th-largest sovereign state in the world. It is also the 22nd-most populous country, with an estimated 66.0 million inhabitants in 2017.

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 radars 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, JFETs and MESFETs, 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 arrays (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.^[3] An adversary, unaware of which frequencies are active, has no signal to see, making detection on radar warning receivers extremely difficult.

Modern radars like the F-35's AN/APG-81 use thousands of broadcaster/receiver modules, one for each antenna element.^[5]

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.^[6]

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 radars 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.

Related Research Articles

Barrage jamming is an electronic warfare technique that attempts to blind radar systems by filling the display with noise, rendering the broadcaster's blip invisible on the display, and often those in the nearby area as well. "Barrage" refers to systems that send signals in many bands of frequencies compared to the bandwidth of any single radar. This allows the jammer to jam multiple radars at once, and reduces or eliminates the need for adjustments to respond to any single radar.

In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving the antennas. In an array antenna, the radio frequency current from the transmitter is fed to the individual antennas with the correct phase relationship so that the radio waves from the separate antennas add together to increase the radiation in a desired direction, while cancelling to suppress radiation in undesired directions. In a phased array, the power from the transmitter is fed to the antennas through devices called phase shifters, controlled by a computer system, which can alter the phase electronically, thus steering the beam of radio waves to a different direction. Since the array must consist of many small antennas to achieve high gain, phased arrays are mainly practical at the high frequency end of the radio spectrum, in the UHF and microwave bands, in which the antenna elements are conveniently small.

A klystron is a specialized linear-beam vacuum tube, invented in 1937 by American electrical engineers Russell and Sigurd Varian, which is used as an amplifier for high radio frequencies, from UHF up into the microwave range. Low-power klystrons are used as oscillators in terrestrial microwave relay communications links, while high-power klystrons are used as output tubes in UHF television transmitters, satellite communication, radar transmitters, and to generate the drive power for modern particle accelerators.

An active electronically scanned array (AESA) is a type of phased array antenna, which is a computer-controlled array antenna in which the beam of radio waves can be electronically steered to point in different directions without moving the antenna. In the AESA, each antenna element is connected to a small solid-state transmit/receive module (TRM) under the control of a computer, which performs the functions of a transmitter and/or receiver for the antenna. This contrasts with a passive electronically scanned array (PESA), in which all the antenna elements are connected to a single transmitter and/or receiver through phase shifters under the control of the computer. AESA's main use is in radar, and these are known as active phased array radar (APAR).

Electronic counter-countermeasures (ECCM) is a part of electronic warfare which includes a variety of practices which attempt to reduce or eliminate the effect of electronic countermeasures (ECM) on electronic sensors aboard vehicles, ships and aircraft and weapons such as missiles. ECCM is also known as electronic protective measures (EPM), chiefly in Europe. In practice, EPM often means resistance to jamming.

A low-probability-of-intercept radar (LPIR) is a radar employing measures to avoid detection by passive radar detection equipment while it is searching for a target or engaged in target tracking. This characteristic is desirable in a radar because it allows finding and tracking an opponent without alerting them to the radar's presence. This also protects the radar installation from anti-radiation missiles (ARM).

Gee-H, sometimes written G-H or GEE-H, was a radio navigation system developed by Britain during World War II to aid RAF Bomber Command. The name refers to the system's use of the earlier Gee equipment, as well as its use of the "H principle" or "twin-range principle" of location determination. Its official name was AMES Type 100.

A backward wave oscillator (BWO), also called carcinotron or backward wave tube, is a vacuum tube that is used to generate microwaves up to the terahertz range. Belonging to the traveling-wave tube family, it is an oscillator with a wide electronic tuning range.

A passive electronically scanned array (PESA), also known as passive phased array, is a phased array antenna, that is an antenna in which the beam of radio waves can be electronically steered to point in different directions, in which all the antenna elements are connected to a single transmitter and/or receiver. This contrasts with an active electronically scanned array (AESA) antenna, which has a separate transmitter and/or receiver unit for each antenna element, all controlled by a computer. AESA is a more advanced, sophisticated versatile second-generation version of the original PESA phased array technology.

Radar engineering details are technical details pertaining to the components of a radar and their ability to detect the return energy from moving scatterers — determining an object's position or obstruction in the environment. This includes field of view in terms of solid angle and maximum unambiguous range and velocity, as well as angular, range and velocity resolution. Radar sensors are classified by application, architecture, radar mode, platform, and propagation window.

During World War II, the German Luftwaffe relied on an increasingly diverse array of electronic communications, IFF and RDF equipment as avionics in its aircraft and also on the ground. Most of this equipment received the generic prefix FuG for Funkgerät, meaning "radio equipment". Most of the aircraft-mounted Radar equipment also used the FuG prefix. This article is a list and a description of the radio, IFF and RDF equipment.

A Microwave Power Module (MPM) is a microwave device used to amplify radio frequency signals to high power levels. It is a hybrid combination of solid-state and vacuum tube electronics, which encloses a solid-state driver amplifier (SSPA), traveling wave tube amplifier (TWTA) and electronic power conditioning (EPC) modules into a single unit. Their average output power capability falls between that of solid-state power amplifiers (SSPAs) and dedicated Traveling Wave Tube (TWT) amplifiers. They may be applied wherever high power microwave amplification is required, and space is at a premium. They are available in various frequency ranges, from S band up to W band. Typical output power at K_u band ranges from 20W to 1 kW.

FuG 25a Erstling was an identification friend or foe (IFF) transponder installed in Luftwaffe aircraft starting in 1941 in order to allow German radar stations to identify them as friendly. Later, the FuG 25a became a key component of the EGON night fighter guidance procedure.

German Luftwaffe and Navy Kriegsmarine Radar Equipment during World War II, relied on an increasingly diverse array of communications, IFF and RDF equipment for its function. Most of this equipment received the generic prefix FuG, meaning "radio equipment". During the war, Germany renumbered their radars. From using the year of introduction as their number they moved to a different numbering scheme.

RX12874, also known as the Passive Detection System (PDS) and by its nickname "Winkle", was a radar detector system used as part of the Royal Air Force's Linesman/Mediator radar network until the early 1980s. Winkle passed out of service along with the rest of the Linesman system as the IUKADGE network replaced it.

The AMES Type 85, also known by its rainbow code Blue Yeoman, was an extremely powerful early warning (EW) and fighter direction (GCI) radar used by the Royal Air Force (RAF) as part of the Linesman/Mediator radar network. First proposed in early 1958, it was eleven years before they became operational in late 1968, by which time they were already considered obsolete. The Type 85 remained the RAF's primary EW and GCI radar until it was replaced by Marconi Martello sets in the late-1980s as part of the new IUKADG network.

References

Footnotes

↑ Alan Levine, "The Strategic Bombing of Germany", Greenwood Publishing Group, 1992, pg. 61
↑ "Radar Countermeasures", Electronics, January 1946, pg. 92-97
1 2 3 Galati
↑ Dick Barrett, "Linesman/Mediator system, Radar Type 85", 4 April 2004
↑ Visual inspection of the antenna shows about 1600 elements.
↑ Marshall Brain, Jeff Tyson and Julia Layton, "How Cell Phones Work", howstuffworks.com

Bibliography

Ian Faulconbridge, "Radar Fundamentals", Argos Press, June 2002, ISBN 0-9580238-1-6
Gaspare Galati, "Advanced radar techniques and systems", IET, 1993, ISBN 0-86341-172-X, pp. 481–503

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[1] Alan Levine, "The Strategic Bombing of Germany", Greenwood Publishing Group, 1992, pg. 61

[2] "Radar Countermeasures", Electronics, January 1946, pg. 92-97

[galati-3] 1 2 3 Galati

[4] Dick Barrett, "Linesman/Mediator system, Radar Type 85", 4 April 2004

[5] Visual inspection of the antenna shows about 1600 elements.

[6] Marshall Brain, Jeff Tyson and Julia Layton, "How Cell Phones Work", howstuffworks.com