Bistatic range
Bistatic range refers to the basic measurement of range made by a radar or sonar system with separated transmitter and receiver. The receiver measures the time difference of arrival of the signal from the transmitter directly, via reflection from the target; this defines an ellipse of constant bistatic range, called an iso-range contour, on which the target lies, with foci centred on the transmitter and receiver. If the target is at range Rrx from the receiver and range Rtx from the transmitter, the receiver and transmitter are a distance L apart the bistatic range is Rrx+Rtx-L. Motion of the target causes a rate of change of bistatic range, which results in bistatic Doppler shift. Speaking, constant bistatic range points draw an ellipsoid with the transmitter and receiver positions as the focal points; the iso-range contours are. When the ground is flat, this intercept forms an ellipse. Note that except when the two platforms have equal altitude, these ellipses are not centered on the specular point.
Bistatic imaging Bistatic radar
Stealth technology
Stealth technology termed low observable technology, is a sub-discipline of military tactics and passive and active electronic countermeasures, which covers a range of techniques used to make personnel, ships, missiles and ground vehicles less visible to radar, infrared and other detection methods. It corresponds to military camouflage for these parts of the electromagnetic spectrum. Development of modern stealth technologies in the United States began in 1958, where earlier attempts in preventing radar tracking of its U-2 spy planes during the Cold War by the Soviet Union had been unsuccessful. Designers turned to developing a particular shape for planes that tended to reduce detection by redirecting electromagnetic waves from radars. Radar-absorbent material was tested and made to reduce or block radar signals that reflect off the surfaces of aircraft; such changes to shape and surface composition comprise stealth technology as used on the Northrop Grumman B-2 Spirit "Stealth Bomber". The concept of stealth is to operate or hide without giving enemy forces any indication as to the presence of friendly forces.
This concept was first explored through camouflage to make an object's appearance blend into the visual background. As the potency of detection and interception technologies have increased, so too has the extent to which the design and operation of military personnel and vehicles have been affected in response; some military uniforms are treated with chemicals to reduce their infrared signature. A modern "stealth" vehicle is designed from the outset to have a chosen spectral signature; the degree of stealth embodied in a particular design is chosen according to the projected threats of detection. The concept of camouflage predates warfare. Hunters have been using vegetation to conceal themselves as long as people have been hunting. In England, irregular units of gamekeepers in the 17th century were the first to adopt drab colours as a form of camouflage, following examples from the continent. During World War I, the Germans experimented with the use of Cellon, a transparent covering material, in an attempt to reduce the visibility of military aircraft.
Single examples of the Fokker E. III Eindecker fighter monoplane, the Albatros C. I two-seat observation biplane, the Linke-Hofmann R. I prototype. In fact, sunlight glinting from the material made the aircraft more visible. Cellon was found to be degraded by both sunlight and in-flight temperature changes, so the attempt to make transparent aircraft was discontinued. In 1916, the British modified a small SS class airship for the purpose of night-time reconnaissance over German lines on the Western Front. Fitted with a silenced engine and a black gas bag, the craft was both invisible and inaudible from the ground but several night-time flights over German-held territory produced little useful intelligence and the idea was dropped. Diffused-lighting camouflage, a shipborne form of counter-illumination camouflage, was trialled by the Royal Canadian Navy from 1941 to 1943; the concept was followed up for aircraft by the Americans and the British: in 1945 a Grumman Avenger with Yehudi lights reached 3,000 yards from a ship before being sighted.
This ability was rendered obsolete by radar. The U-boat U-480 may have been the first stealth submarine, it featured an anechoic tile rubber coating, one layer of which contained circular air pockets to defeat ASDIC sonar. Radar absorbent rubber/semiconductor composite paints and materials were used by the Kriegsmarine on submarines in World War II. Tests showed they were effective in reducing radar signatures at both long wavelengths. In 1956 the CIA began attempts to reduce the radar cross-section of the U-2 spyplane. Three systems were developed, Trapeze, a series of wires and ferrite beads around the planform of the aircraft, a covering material with pcb circuitry embedded in it, radar absorbent paint; these were deployed in the field on the so-called'dirty birds' but results were disappointing, the weight/drag increase was not worth any reduction in detection rates. More successful was the application of camouflage to the bare metal aircraft; the weight of this cost 250 ft in max altitude but made the aircraft harder for interceptors to spot.
In 1958, the U. S. Central Intelligence Agency requested funding for a reconnaissance aircraft to replace the existing U-2 spy planes, Lockheed secured contractual rights to produce it. "Kelly" Johnson and his team at Lockheed's Skunk Works were assigned to produce the A-12, which operated at high altitude of 70,000 to 80,000 ft and speed of Mach 3.2 to avoid radar detection. Various plane shapes designed to reduce radar detection were developed in earlier prototypes, named A-1 to A-11; the A-12 included a number of stealthy features including special fuel to reduce the signature of the exhaust plume, canted vertical stabilizers, the use of composite materials in key locations, the overall finish in radar absorbing paint. In 1960, the USAF reduced the radar-cross-section of a Ryan Q-2C Firebee drone; this was achieved through specially designed screens over the air intake, radar-absorbent material on the fuselage and a special radar-absorbing paint. During the 1970s the U. S. Department of Defense launched project Lockheed Have Blue, with the aim of developing a stealth fighter.
There was fierce bidding between Lockheed and Northrop to secure the multibillion-dollar cont
Radar
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, spacecraft, guided missiles, motor vehicles, weather formations, 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. Radio waves from the transmitter reflect off the object and return to the receiver, giving information about the object's location and speed. Radar was developed secretly for military use by several nations in the period before and during World War II. A key development was the cavity magnetron in the UK, which allowed the creation of small systems with sub-meter resolution; the term RADAR was coined in 1940 by the United States Navy as an acronym for RAdio Detection And Ranging The term radar has since entered English and other languages as a common noun, losing all capitalization.
The modern uses of radar are diverse, including air and terrestrial traffic control, radar astronomy, air-defense systems, antimissile systems, marine radars to locate landmarks and other ships, aircraft anticollision systems, ocean surveillance systems, outer space surveillance and rendezvous systems, meteorological precipitation monitoring and flight control systems, guided missile target locating systems, ground-penetrating radar for geological observations, range-controlled radar for public health surveillance. High tech radar systems are associated with digital signal processing, machine learning and are capable of extracting useful information from high noise levels. Radar is a key technology that the self-driving systems are designed to use, along with sonar and other sensors. Other systems similar to radar make use of other parts of the electromagnetic spectrum. One example is "lidar". With the emergence of driverless vehicles, Radar is expected to assist the automated platform to monitor its environment, thus preventing unwanted incidents.
As early as 1886, German physicist Heinrich Hertz showed that radio waves could be reflected from solid objects. In 1895, Alexander Popov, a physics instructor at the Imperial Russian Navy school in Kronstadt, developed an apparatus using a coherer tube for detecting distant lightning strikes; the next year, he added a spark-gap transmitter. In 1897, while testing this equipment for communicating between two ships in the Baltic Sea, he took note of an interference beat caused by the passage of a third vessel. In his report, Popov wrote that this phenomenon might be used for detecting objects, but he did nothing more with this observation; the German inventor Christian Hülsmeyer was the first to use radio waves to detect "the presence of distant metallic objects". In 1904, he demonstrated the feasibility of detecting a ship in dense fog, but not its distance from the transmitter, he obtained a patent for his detection device in April 1904 and a patent for a related amendment for estimating the distance to the ship.
He got a British patent on September 23, 1904 for a full radar system, that he called a telemobiloscope. It operated on a 50 cm wavelength and the pulsed radar signal was created via a spark-gap, his system used the classic antenna setup of horn antenna with parabolic reflector and was presented to German military officials in practical tests in Cologne and Rotterdam harbour but was rejected. In 1915, Robert Watson-Watt used radio technology to provide advance warning to airmen and during the 1920s went on to lead the U. K. research establishment to make many advances using radio techniques, including the probing of the ionosphere and the detection of lightning at long distances. Through his lightning experiments, Watson-Watt became an expert on the use of radio direction finding before turning his inquiry to shortwave transmission. Requiring a suitable receiver for such studies, he told the "new boy" Arnold Frederic Wilkins to conduct an extensive review of available shortwave units. Wilkins would select a General Post Office model after noting its manual's description of a "fading" effect when aircraft flew overhead.
Across the Atlantic in 1922, after placing a transmitter and receiver on opposite sides of the Potomac River, U. S. Navy researchers A. Hoyt Taylor and Leo C. Young discovered that ships passing through the beam path caused the received signal to fade in and out. Taylor submitted a report, suggesting that this phenomenon might be used to detect the presence of ships in low visibility, but the Navy did not continue the work. Eight years Lawrence A. Hyland at the Naval Research Laboratory observed similar fading effects from passing aircraft. Before the Second World War, researchers in the United Kingdom, Germany, Japan, the Netherlands, the Soviet Union, the United States, independently and in great secrecy, developed technologies that led to the modern version of radar. Australia, New Zealand, South Africa followed prewar Great Britain's radar development, Hungary generated its radar technology during the war. In France in 1934, following systematic studies on the split-anode magnetron, the research branch of the Compagnie Générale de Télégraphie Sans Fil headed by Maurice Ponte with Henri Gutton, Sylvain Berline and M. Hugon, began developing an obstacle-locatin
EISCAT
EISCAT operates three incoherent scatter radar systems, at 224 MHz, 931 MHz in Northern Scandinavia and one at 500 MHz on Svalbard, used to study the interaction between the Sun and the Earth as revealed by disturbances in the ionosphere and magnetosphere. At the Ramfjordmoen facility, it operates an ionospheric heater facility, similar to HAARP. Additional receiver stations are located in Sodankylä, Kiruna, Sweden; the EISCAT Svalbard radar is located in Norway. The EISCAT Headquarters are located in Kiruna. EISCAT is funded and operated by research institutes and research councils of Norway, Finland, Japan and the United Kingdom. Institutes in other countries contribute to operations, including Russia, Ukraine and South Korea; the system was tested for space debris tracking and the radars were proven to be capable of statistical observations of Low-Earth orbit debris down to 2 cm in size. Since these measurements are insufficient to determine complete orbits, the radar has only limited space surveillance value.
Because the space debris tracking change is only a dedicated back-end computer system, the primary EISCAT observations are not compromised. As a result of that, the EISCAT radars allow continuous monitoring of the LEO debris in a beam park mode, functioning as a space surveillance system part of the European Space Agency's Space Situational Awareness Programme. In 1973, the EISCAT proposal —, planned for France and the three Nordic countries — seemed moribund. Welsh physicist Granville Beynon became involved and by 1975, the agreement was signed, with the UK as a member; the proposal for UK membership had been turned down by the appropriate SRC committee. Beynon, persuaded the Board to reverse the decision of the committee and as a result of his efforts, hundreds of European scientists have had the opportunity to use the world's most advanced ionospheric radar. In 2008, Doritos embarked upon an "out-of-this-world" advertising campaign beaming a 30-second advertisement for Doritos brand tortilla chips into a solar system 42 light years away.
This project is in collaboration with EISCAT Space Centre in Svalbard. The "You Make It, We'll Play It" contest chose the winning advertisement, transmitted on June 12, 2008; the ad was beamed towards a distant star, within the Ursa Major constellation, orbited by planets which may harbor life. EISCAT operates several facilities north of the Scandinavian arctic circle. At Ramfjordmoen, near Tromsø, Norway the EISCAT facility has: a Ionospheric heater with HF radar capabilities. From the start in 1981 the UHF radar was a steerable tristatic system, but due to interference from telecommunications in the 930 MHZ band, the remote receivers were converted to receive the VHF signal during 2012. At Kiruna, EISCAT operates a 32 m parabolic dish antenna receiver, part of the tristatic UHF system but was converted to receive the VHF frequency during 2012. At Sodankylä, Finland, EISCAT operates a 32 m VHF-band parabolic dish antenna receiver working at 224 MHz. At Longyearbyen, on the Norwegian Svalbard archipelago, EISCAT operates the EISCAT Svalbard radar.
It consists of a 32 m steerable parabolic dish antenna and a 42 m field aligned antenna, operating at 500 MHz. The Tromsø VHF transmitter, together with the Kiruna and Sodankylä VHF receivers, form a multistatic radar system. Additionally, the Kilpisjärvi Atmospheric Imaging Receiver Array, near Kilpisjärvi, Finland can serve as a VHF receiver in conjunction with the Tromsø transmitter. EISCAT is planning on building a next generation radar capable of providing 3D monitoring of the atmosphere and ionosphere; the new system is called EISCAT_3D. In 2008, the European Strategy Forum on Research Infrastructures selected EISCAT_3D for its "Roadmap 2008 for Large-Scale European Research Infrastructures for the next 20–30 years."EISCAT_3D will be a multistatic radar composed of five phased-array antenna fields. Each field will have around 10,000 crossed dipole antenna elements. All five sites will act with a single core site transmitting at 233 MHz; the sites will be spread over Finland and Sweden. Each site will have a central array, surrounded by a set of smaller arrays, providing a high spatial resolution via aperture synthesis.
During the summer of 2017, EISCAT will build a 91-element subarray at the site at Ramfjordmoen for hardware testing purposes and the full system is expected to be operational around 2021. The KAIRA system is a pathfinder for the development of EISCAT_3D. Publication List About EISCAT EISCAT_3D website University Courses on Svalbard
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power expressed in decibels. A ratio higher than 1:1 indicates more signal than noise. While SNR is quoted for electrical signals, it can be applied to any form of signal, for example isotope levels in an ice core, biochemical signaling between cells, or financial trading signals. Signal-to-noise ratio is sometimes used metaphorically to refer to the ratio of useful information to false or irrelevant data in a conversation or exchange. For example, in online discussion forums and other online communities, off-topic posts and spam are regarded as "noise" that interferes with the "signal" of appropriate discussion; the signal-to-noise ratio, the bandwidth, the channel capacity of a communication channel are connected by the Shannon–Hartley theorem. Signal-to-noise ratio is defined as the ratio of the power of a signal to the power of background noise: S N R = P s i g n a l P n o i s e, where P is average power.
Both signal and noise power must be measured at the same or equivalent points in a system, within the same system bandwidth. Depending on whether the signal is a constant or a random variable, the signal to noise ratio for random noise N with expected value of zero becomes: S N R = s 2 σ N 2 or S N R = E σ N 2 where E refers to the expected value, i.e. in this case the mean of S 2. If the signal and the noise are measured across the same impedance, the SNR can be obtained by calculating the square of the amplitude ratio: S N R = P s i g n a l P n o i s e = 2, where A is root mean square amplitude; because many signals have a wide dynamic range, signals are expressed using the logarithmic decibel scale. Based upon the definition of decibel and noise may be expressed in decibels as P s i g n a l, d B = 10 log 10 and P n o i s e, d B = 10 log 10 . In a similar manner, SNR may be expressed in decibels as S N R d B = 10 log 10 . Using the definition of SNR S N R d B = 10 log 10 . Using the quotient rule for logarithms 10 log 10 = 10
Antenna diversity
Antenna diversity known as space diversity or spatial diversity, is any one of several wireless diversity schemes that uses two or more antennas to improve the quality and reliability of a wireless link. In urban and indoor environments, there is no clear line-of-sight between transmitter and receiver. Instead the signal is reflected along multiple paths before being received; each of these bounces can introduce phase shifts, time delays and distortions that can destructively interfere with one another at the aperture of the receiving antenna. Antenna diversity is effective at mitigating these multipath situations; this is. Each antenna will experience a different interference environment. Thus, if one antenna is experiencing a deep fade, it is that another has a sufficient signal. Collectively such a system can provide a robust link. While this is seen in receiving systems, the analog has proven valuable for transmitting systems as well. Inherently an antenna diversity scheme requires additional hardware and integration versus a single antenna system but due to the commonality of the signal paths a fair amount of circuitry can be shared.
With the multiple signals there is a greater processing demand placed on the receiver, which can lead to tighter design requirements. However, signal reliability is paramount and using multiple antennas is an effective way to decrease the number of drop-outs and lost connections. Antenna diversity can be realized in several ways. Depending on the environment and the expected interference, designers can employ one or more of these methods to improve signal quality. In fact multiple methods are used to further increase reliability. Spatial diversity employs multiple antennas with the same characteristics, that are physically separated from one another. Depending upon the expected incidence of the incoming signal, sometimes a space on the order of a wavelength is sufficient. Other times much larger distances are needed. Cellularization or sectorization, for example, is a spatial diversity scheme that can have antennas or base stations miles apart; this is beneficial for the mobile communication industry since it allows multiple users to share a limited communication spectrum and avoid co-channel interference.
Pattern diversity consists of two or more co-located antennas with different radiation patterns. This type of diversity makes use of directional antennas that are physically separated by some distance. Collectively they are capable of discriminating a large portion of angle space and can provide a higher gain versus a single omnidirectional radiator. Polarization diversity combines pairs of antennas with orthogonal polarizations. Reflected signals can undergo polarization changes depending on the medium through which they are traveling. A polarization difference of 90° will result in an attenuation factor of up to 34 dB in signal strength. By pairing two complementary polarizations, this scheme can immunize a system from polarization mismatches that would otherwise cause signal fade. Additionally, such diversity has proven valuable at radio and mobile communication base stations since it is less susceptible to the near random orientations of transmitting antennas. Transmit/Receive diversity receive functions.
Such a configuration eliminates the need for a duplexer and can protect sensitive receiver components from the high power used in transmit. Adaptive arrays can be a single antenna with active elements or an array of similar antennas with ability to change their combined radiation pattern as different conditions persist. Active electronically scanned arrays manipulate phase shifters and attenuators at the face of each radiating site to provide a near instantaneous scan ability as well as pattern and polarization control; this is beneficial for radar applications since it affords a single antenna the ability to switch among several different modes such as searching, tracking and jamming countermeasures. All of the above techniques require some sort of post processing to recover the desired message. Among these techniques are: Switching: In a switching receiver, the signal from only one antenna is fed to the receiver for as long as the quality of that signal remains above some prescribed threshold.
If and when the signal degrades, another antenna is switched in. Switching is the easiest and least power consuming of the antenna diversity processing techniques but periods of fading and desynchronization may occur while the quality of one antenna degrades and another antenna link is established. Selecting: As with switching, selection processing presents only one antenna’s signal to the receiver at any given time; the antenna chosen, however, is based on the best signal-to-noise ratio among the received signals. This requires that a pre-measurement take place and that all antennas have established connections leading to a higher power requirement; the actual selection process can take place in between received packets of information. This ensures. Switching can take place on a packet-by-packet basis if necessary. Combining: In combining, all antennas maintain established connections at all times; the signals are combined and presented to the receiver. Depending on the sophistication of the system, the signals can be added directly or weighted and added coherently (ma
Passive radar
Passive radar systems encompass a class of radar systems that detect and track objects by processing reflections from non-cooperative sources of illumination in the environment, such as commercial broadcast and communications signals. It is a specific case of bistatic radar, the latter including the exploitation of cooperative and non-cooperative radar transmitters. Conventional radar systems comprise a collocated transmitter and receiver, which share a common antenna to transmit and receive. A pulsed signal is transmitted and the time taken for the pulse to travel to the object and back allows the range of the object to be determined. In a passive radar system, there is no dedicated transmitter. Instead, the receiver uses third-party transmitters in the environment, measures the time difference of arrival between the signal arriving directly from the transmitter and the signal arriving via reflection from the object; this allows the bistatic range of the object to be determined. In addition to bistatic range, a passive radar will also measure the bistatic Doppler shift of the echo and its direction of arrival.
These allow the location and speed of the object to be calculated. In some cases, multiple transmitters and/or receivers can be employed to make several independent measurements of bistatic range and bearing and hence improve the final track accuracy; the term "passive radar" is sometimes used incorrectly to describe those passive sensors that detect and track aircraft by their RF emissions. However, these systems do not exploit reflected energy and hence are more described as ESM systems. Well known examples include the Ukrainian Kolchuga system; the concept of passive radar detection using reflected ambient radio signals emanating from a distant transmitter is not new. The first radar experiments in the United Kingdom in 1935 by Robert Watson-Watt demonstrated the principle of radar by detecting a Handley Page Heyford bomber at a distance of 12 km using the BBC shortwave transmitter at Daventry. Early radars were all bistatic because the technology to enable an antenna to be switched from transmit to receive mode had not been developed.
Thus many countries were using bistatic systems in air defence networks during the early 1930s. For example, the British deployed the CHAIN HOME system; the Germans used a passive bistatic system during World War II. This system, called the Klein Heidelberg Parasit or Heidelberg-Gerät, was deployed at seven sites and operated as bistatic receivers, using the British Chain Home radars as non-cooperative illuminators, to detect aircraft over the southern part of the North Sea. Bistatic radar systems gave way to monostatic systems with the development of the synchronizer in 1936; the monostatic systems were much easier to implement since they eliminated the geometric complexities introduced by the separate transmitter and receiver sites. In addition and shipborne applications became possible as smaller components were developed. In the early 1950s, bistatic systems were considered again when some interesting properties of the scattered radar energy were discovered, indeed the term "bistatic" was first used by Siegel in 1955 in his report describing these properties.
One of the largest and most complex passive radar systems was the UK's RX12874, or "Winkle". Winkle was deployed in the 1960s in response to the introduction of the carcinotron, a radar jammer, so powerful it appeared to render long-distance radars useless. Winkle was able to home in on carcinotron broadcasts with the same accuracy as a conventional radar, allowing the jammer aircraft to be tracked and attacked at hundreds of miles range. Additionally, by indicating the location of the jammer, other radars in the Linesman/Mediator network could reduce the sensitivity of their receivers when pointed in that direction, thereby reducing the amount of jamming received when pointed near the jammer's location; the rise of cheap computing power and digital receiver technology in the 1980s led to a resurgence of interest in passive radar technology. For the first time, these allowed designers to apply digital signal processing techniques to exploit a variety of broadcast signals and to use cross-correlation techniques to achieve sufficient signal processing gain to detect targets and estimate their bistatic range and Doppler shift.
Classified programmes existed in several nations, but the first announcement of a commercial system was by Lockheed-Martin Mission Systems in 1998, with the commercial launch of the Silent Sentry system, that exploited FM radio and analogue television transmitters. Passive radar systems have been developed that exploit the following sources of illumination: Analog television signals FM radio signals Cellular phone base stations Digital audio broadcasting Digital video broadcasting Terrestrial High-definition television transmitters in North America GPS satellites. Satellite signals have been found to be inadequate for passive radar use, either because the powers are too low or because the orbits of the satellites are such that illumination is too infrequent; the possible exception to this is the exploitation of satellite-based radar and satellite radio systems. In 2011, researchers Barott and Butka from Embry-Riddle Aeronautical University announced results claiming success
