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112-442: 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 aircraft , ships , spacecraft , guided missiles , motor vehicles , map weather formations , and terrain . A radar system consists of a transmitter producing electromagnetic waves in

224-421: A coherer receiver with a cylindrical parabolic antenna that could rotate 360 degrees. While the transmitted signal had a broad coverage, the receiving antenna was narrowly focused. When a reflected signal reached the receiver, a relay was actuated and, in turn, rang an electric bell. The basic patent description is as follows: Hertzian-wave Projecting and Receiving Apparatus Adapted to Indicate or Give Warning of

336-470: A fractal surface, such as rocks or soil, and are used by navigation radars. A radar beam follows a linear path in vacuum but follows a somewhat curved path in atmosphere due to variation in the refractive index of air, which is called the radar horizon . Even when the beam is emitted parallel to the ground, the beam rises above the ground as the curvature of the Earth sinks below the horizon. Furthermore,

448-412: A microphone , a video signal representing moving images from a video camera , or a digital signal representing data from a computer . In the transmitter, an electronic oscillator generates an alternating current oscillating at a radio frequency , called the carrier wave because it creates the radio waves that "carry" the information through the air. The information signal is used to modulate

560-418: A resonator , similarly to a tuning fork . The tuned circuit has a natural resonant frequency at which it oscillates. The resonant frequency is set equal to the frequency of the desired radio station. The oscillating radio signal from the desired station causes the tuned circuit to oscillate in sympathy, and it passes the signal on to the rest of the receiver. Radio signals at other frequencies are blocked by

672-552: A space heater or wood fire. The oscillating electric field of the wave causes polar molecules to vibrate back and forth, increasing the temperature; this is how a microwave oven cooks food. Radio waves have been applied to the body for 100 years in the medical therapy of diathermy for deep heating of body tissue, to promote increased blood flow and healing. More recently they have been used to create higher temperatures in hyperthermia therapy and to kill cancer cells. However, unlike infrared waves, which are mainly absorbed at

784-424: A transmitter that emits radio waves known as radar signals in predetermined directions. When these signals contact an object they are usually reflected or scattered in many directions, although some of them will be absorbed and penetrate into the target. Radar signals are reflected especially well by materials of considerable electrical conductivity —such as most metals, seawater , and wet ground. This makes

896-626: A Consortium for commercializing the invention would no longer be applicable. In 1904, while still heavily engaged with the Telemobiloscope, Hülsmeyer filed for a patent (DE180009) on a machine for diameter reduction of metallic rods and tubes, and in 1906, established a firm supplying equipment for producing incandescent lamps. This was followed in 1907 by his forming the company Kessel-und Apparatebau Christian Hülsmeyer (Boilers and Apparatus Construction) in Düsseldorf; in 1910, he bought

1008-646: A daughter named Annelise Hülsmeyer-Hecker, maintained collection of documents related to her father, and was a source of much that has been written concerning him. She was also instrumental in collecting items, including components of the Telemobiloscope, that are now displayed in the Deutsches Museum . Upon his death in Ahrweiler on 31 Jan. 1957, Christian Hülsmeyer was buried in the North Cemetery at Düsseldorf. On 19 October 2019, 115 years after

1120-483: A device (called a Telephonogram ) that telegraphed sounds; an electro-optical system for turning a truck into a mobile, multi-faced billboard; and a wireless apparatus for remotely igniting explosives. Within a year, he filed several patent applications on these and other inventions. In developing the wireless apparatus, Hülsmeyer read of Heinrich Hertz 's discovery that electromagnetic waves were reflected from metallic surfaces. He then turned his full attention to using

1232-482: A different dielectric constant or diamagnetic constant from the first, the waves will reflect or scatter from the boundary between the materials. This means that a solid object in air or in a vacuum , or a significant change in atomic density between the object and what is surrounding it, will usually scatter radar (radio) waves from its surface. This is particularly true for electrically conductive materials such as metal and carbon fibre, making radar well-suited to

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1344-401: A factory site at Düsseldorf-Flingern for the firm. For many years, this company built steam and water apparatus, high-pressure gauges, and anti-rust-filters (trade named "Rostex"). The company continued to operate until 1953. Altogether in his career, Hülsmeyer developed and patented some 180 inventions; these and his various businesses ultimately brought him financial success. Although there

1456-537: 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 already 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 of thunderstorms to airmen and during

1568-461: A human user. The radio waves from many transmitters pass through the air simultaneously without interfering with each other. They can be separated in the receiver because each transmitter's radio waves oscillate at a different rate, in other words each transmitter has a different frequency , measured in kilohertz (kHz), megahertz (MHz) or gigahertz (GHz). The bandpass filter in the receiver consists of one or more tuned circuits which act like

1680-517: A leader of radar technology development in Great Britain, and received a patent on the system in 1935). In a discussion with Hülsmeyer as to who was the rightful inventor of this technology, it is said that Watson-Watt ended the discussion by remarking, "I am the father of radar, whereas you are its grandfather." On 29 October 1910, Christian Hülsmeyer married Luise Petersen of Bremen . Between 1911 and 1924, they had six children. One of these,

1792-403: A left-hand sense. Right circularly polarized radio waves consist of photons spinning in a right hand sense. Left circularly polarized radio waves consist of photons spinning in a left hand sense. Plane polarized radio waves consist of photons in a quantum superposition of right and left hand spin states. The electric field consists of a superposition of right and left rotating fields, resulting in

1904-531: A method of using two vertical measurements and trigonometry to calculate approximate range. A relatively detailed description of the Telemobiloscope system, including a copy of the patent, is contained in a paper by Bauer. The first public demonstration of the Telemobiloscope was in the courtyard of the Dom Hotel in Cologne on 17 May 1904. The metal gate to the courtyard was the target, and the transmission path

2016-748: 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

2128-605: A plane oscillation. Radio waves are more widely used for communication than other electromagnetic waves mainly because of their desirable propagation properties, stemming from their large wavelength . Radio waves have the ability to pass through the atmosphere in any weather, foliage, and through most building materials. By diffraction , longer wavelengths can bend around obstructions, and unlike other electromagnetic waves they tend to be scattered rather than absorbed by objects larger than their wavelength. The study of radio propagation , how radio waves move in free space and over

2240-402: A plane perpendicular to the direction of motion. In a horizontally polarized radio wave the electric field oscillates in a horizontal direction. In a vertically polarized wave the electric field oscillates in a vertical direction. In a circularly polarized wave the electric field at any point rotates about the direction of travel, once per cycle. A right circularly polarized wave rotates in

2352-496: A proposal for further intensive research on radio-echo signals from moving targets to take place at NRL, where Taylor and Young were based at the time. Similarly, in the UK, L. S. Alder took out a secret provisional patent for Naval radar in 1928. W.A.S. Butement and P. E. Pollard developed a breadboard test unit, operating at 50 cm (600 MHz) and using pulsed modulation which gave successful laboratory results. In January 1931,

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2464-730: A pulsed system, and the first such elementary apparatus was demonstrated in December 1934 by the American Robert M. Page , working at the Naval Research Laboratory . The following year, the United States Army successfully tested a primitive surface-to-surface radar to aim coastal battery searchlights at night. This design was followed by a pulsed system demonstrated in May 1935 by Rudolf Kühnhold and

2576-442: A rescue. For similar reasons, objects intended to avoid detection will not have inside corners or surfaces and edges perpendicular to likely detection directions, which leads to "odd" looking stealth aircraft . These precautions do not totally eliminate reflection because of diffraction , especially at longer wavelengths. Half wavelength long wires or strips of conducting material, such as chaff , are very reflective but do not direct

2688-404: A right-hand sense about the direction of travel, while a left circularly polarized wave rotates in the opposite sense. The wave's magnetic field is perpendicular to the electric field, and the electric and magnetic field are oriented in a right-hand sense with respect to the direction of radiation. An antenna emits polarized radio waves, with the polarization determined by the direction of

2800-676: A system might do, Wilkins recalled the earlier report about aircraft causing radio interference. This revelation led to the Daventry Experiment of 26 February 1935, using a powerful BBC shortwave transmitter as the source and their GPO receiver setup in a field while a bomber flew around the site. When the plane was clearly detected, Hugh Dowding , the Air Member for Supply and Research , was very impressed with their system's potential and funds were immediately provided for further operational development. Watson-Watt's team patented

2912-512: A wide region and direct fighter aircraft towards targets. Marine radars are used to measure the bearing and distance of ships to prevent collision with other ships, to navigate, and to fix their position at sea when within range of shore or other fixed references such as islands, buoys, and lightships. In port or in harbour, vessel traffic service radar systems are used to monitor and regulate ship movements in busy waters. Meteorologists use radar to monitor precipitation and wind. It has become

3024-841: A writeup on the apparatus was entered in the Inventions Book maintained by the Royal Engineers. This is the first official record in Great Britain of the technology that was used in coastal defence and was incorporated into Chain Home as Chain Home (low) . Before the Second World War , researchers in the United Kingdom, France , Germany , Italy , Japan , the Netherlands , the Soviet Union , and

3136-452: Is a simplification for transmission in a vacuum without interference. The propagation factor accounts for the effects of multipath and shadowing and depends on the details of the environment. In a real-world situation, pathloss effects are also considered. Frequency shift is caused by motion that changes the number of wavelengths between the reflector and the radar. This can degrade or enhance radar performance depending upon how it affects

3248-404: Is absorbed within a few meters, so the atmosphere is effectively opaque. In radio communication systems, information is transported across space using radio waves. At the sending end, the information to be sent, in the form of a time-varying electrical signal, is applied to a radio transmitter . The information, called the modulation signal , can be an audio signal representing sound from

3360-450: Is as follows, where F D {\displaystyle F_{D}} is Doppler frequency, F T {\displaystyle F_{T}} is transmit frequency, V R {\displaystyle V_{R}} is radial velocity, and C {\displaystyle C} is the speed of light: Passive radar is applicable to electronic countermeasures and radio astronomy as follows: Only

3472-689: Is controversy about his inventing radar, Christian Hülsmeyer is still held in high esteem in Germany. In January 1982, Professor K. Mauel gave a lecture at the Organization of German Engineers Center in Düsseldorf on Radar History, celebrating the centenary of Hülsmeyer's birth. At the 2002 EUSAR Conference in Cologne, the keynote speech was "Hülsmeyer – The Inventor of Radar." During a radar conference held in Frankfurt in 1953, Hülsmeyer and Robert Watson-Watt were honored guests. (Watson-Watt had been

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3584-442: Is extremely small, from 10 to 10   joules . So the antenna of even a very low power transmitter emits an enormous number of photons every second. Therefore, except for certain molecular electron transition processes such as atoms in a maser emitting microwave photons, radio wave emission and absorption is usually regarded as a continuous classical process, governed by Maxwell's equations . Radio waves in vacuum travel at

3696-567: Is intended. Radar relies on its own transmissions rather than light from the Sun or the Moon, or from electromagnetic waves emitted by the target objects themselves, such as infrared radiation (heat). This process of directing artificial radio waves towards objects is called illumination , although radio waves are invisible to the human eye as well as optical cameras. If electromagnetic waves travelling through one material meet another material, having

3808-608: Is strictly regulated by law, coordinated by an international body called the International Telecommunication Union (ITU), which defines radio waves as " electromagnetic waves of frequencies arbitrarily lower than 3000  GHz , propagated in space without artificial guide". The radio spectrum is divided into a number of radio bands on the basis of frequency, allocated to different uses. Higher-frequency, shorter-wavelength radio waves are called microwaves . Radio waves were first predicted by

3920-417: Is the range. This yields: This shows that the received power declines as the fourth power of the range, which means that the received power from distant targets is relatively very small. Additional filtering and pulse integration modifies the radar equation slightly for pulse-Doppler radar performance , which can be used to increase detection range and reduce transmit power. The equation above with F = 1

4032-426: Is the wavelength of a 1  hertz radio signal. A 1  megahertz radio wave (mid- AM band ) has a wavelength of 299.79 meters (983.6 ft). Like other electromagnetic waves, a radio wave has a property called polarization , which is defined as the direction of the wave's oscillating electric field perpendicular to the direction of motion. A plane-polarized radio wave has an electric field that oscillates in

4144-400: Is weak mechanistic evidence of cancer risk via personal exposure to RF-EMF from mobile telephones. Radio waves can be shielded against by a conductive metal sheet or screen, an enclosure of sheet or screen is called a Faraday cage . A metal screen shields against radio waves as well as a solid sheet as long as the holes in the screen are smaller than about 1 ⁄ 20 of wavelength of

4256-601: The Holland-Amerika Lijn (HAL) invited Telemobiloskop-Gesellschaft to provide a demonstration of their apparatus during the conference. This demonstration took place on 9 June during a tour through the harbor at Rotterdam aboard the ship-tender Columbus . The Minutes of the conference (contained in the HAL Archives in the Municipal Archives of Rotterdam) include the following description of

4368-539: The Lehrerseminare (Teacher Training College) in Bremen . At the school, his major interest was in physics , and, after classroom hours, he was allowed to use the physics laboratory for his own experimenting. His interest was more with the potential applications of physics than with the academic side. In June 1900, Hülsmeyer left college without completing his studies and obtained a job as an electrical trainee in

4480-628: The Nyquist frequency , since the returned frequency otherwise cannot be distinguished from shifting of a harmonic frequency above or below, thus requiring: Or when substituting with F D {\displaystyle F_{D}} : As an example, a Doppler weather radar with a pulse rate of 2 kHz and transmit frequency of 1 GHz can reliably measure weather speed up to at most 150 m/s (340 mph), thus cannot reliably determine radial velocity of aircraft moving 1,000 m/s (2,200 mph). In all electromagnetic radiation ,

4592-715: The RAF's Pathfinder . The information provided by radar includes the bearing and range (and therefore position) of the object from the radar scanner. It is thus used in many different fields where the need for such positioning is crucial. The first use of radar was for military purposes: to locate air, ground and sea targets. This evolved in the civilian field into applications for aircraft, ships, and automobiles. In aviation , aircraft can be equipped with radar devices that warn of aircraft or other obstacles in or approaching their path, display weather information, and give accurate altitude readings. The first commercial device fitted to aircraft

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4704-565: The Siemens & Halske factory in Bremen . There he learned how concepts of devices were turned into commercial applications, intensifying his inventive nature. In April 1902, he left employment with Siemens to live with his brother Wilhelm in Düsseldorf and pursue his ideas for electrical and optical products. His brother initially funded him in setting up a shop where, a number of ideas were quickly turned into working items. These included

4816-511: The Telemobiloskop–Gesellschaft firm. On 12 August 1904, rights to the system were sold to Trading Company Z.H. Gumpel daselbst of Hannover. The sales agreement, to which Heinrich Mannheim was a signer, said that Hülsmeyer would be given up to 5,000 Marks for future research, and 45 percent of net profits from future sales. It noted that the previous agreement with Mannheim was then obsolete, and after Hülsmeyer has provided proof of

4928-439: The electromagnetic spectrum . One example is lidar , which uses predominantly infrared light from lasers rather than radio waves. 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 ,

5040-460: The radio or microwaves domain, a transmitting antenna , a receiving antenna (often the same antenna is used for transmitting and receiving) and a receiver and processor to determine properties of the objects. Radio waves (pulsed or continuous) from the transmitter reflect off the objects and return to the receiver, giving information about the objects' locations and speeds. Radar was developed secretly for military use by several countries in

5152-407: The reflective surfaces . A corner reflector consists of three flat surfaces meeting like the inside corner of a cube. The structure will reflect waves entering its opening directly back to the source. They are commonly used as radar reflectors to make otherwise difficult-to-detect objects easier to detect. Corner reflectors on boats, for example, make them more detectable to avoid collision or during

5264-415: The speed of light c {\displaystyle c} . When passing through a material medium, they are slowed depending on the medium's permeability and permittivity . Air is tenuous enough that in the Earth's atmosphere radio waves travel at very nearly the speed of light. The wavelength λ {\displaystyle \lambda } is the distance from one peak (crest) of

5376-482: The "near field" zone, the physical relationships between the electric and magnetic components of the field can be complex, and it is best to use the field strength units discussed above. Power density is measured in terms of power per unit area, for example, with the unit milliwatt per square centimeter (mW/cm ). When speaking of frequencies in the microwave range and higher, power density is usually used to express intensity since exposures that might occur would likely be in

5488-534: 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 (the common term for interference at the time) when aircraft flew overhead. By placing a transmitter and receiver on opposite sides of the Potomac River in 1922, U.S. Navy researchers A. Hoyt Taylor and Leo C. Young discovered that ships passing through

5600-467: The 1909 Nobel Prize in physics for his radio work. Radio communication began to be used commercially around 1900. The modern term " radio wave " replaced the original name " Hertzian wave " around 1912. Radio waves are radiated by charged particles when they are accelerated . Natural sources of radio waves include radio noise produced by lightning and other natural processes in the Earth's atmosphere, and astronomical radio sources in space such as

5712-413: 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

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5824-551: The Hertz phenomenon in a system for preventing collisions between ships. Giving the name Telemobiloskop (Telemobiloscope) to the system, he made a patent application on 21 November 1903, and also advertised for a financial backer. Henry Mannheim, a leather merchant in Cologne, responded, and in March 1904, invested 2,000 Marks for 20 percent of future profits from the apparatus. The firm Telemobiloskop–Gesellschaft Hülsmeyer & Mannheim

5936-510: The Presence of a Metallic Body, such as Ships or Train, in the Line of Projecting of such Waves. The system included a mechanism synchronizing the aiming direction of the receiving antenna with a compass-like indicator; it also included a means of rejecting false signals. Although the Telemobiloscope could not directly indicate range, a separate patent (DE 169154) was granted 2 April 1906, showing

6048-414: The Sun, galaxies and nebulas. All warm objects radiate high frequency radio waves ( microwaves ) as part of their black body radiation . Radio waves are produced artificially by time-varying electric currents , consisting of electrons flowing back and forth in a specially shaped metal conductor called an antenna . An electronic device called a radio transmitter applies oscillating electric current to

6160-485: The Telemobiloscope operation. As to competition, Marconi's Wireless Telegraph Company dominated Europe and had agreements with essentially all shipping firms prohibiting their use of anything except Marconi equipment. The Official Registry in Cologne shows that the Telemobiloskop–Gesellschaft Hülsmeyer & Mannheim firm was dissolved 5 October 1905. Also, the agreement with Gumpel to establish

6272-786: The United States, independently and in great secrecy, developed technologies that led to the modern version of radar. Australia, Canada, New Zealand, and South Africa followed prewar Great Britain's radar development, Hungary and Sweden 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 la télégraphie sans fil (CSF) headed by Maurice Ponte with Henri Gutton, Sylvain Berline and M. Hugon, began developing an obstacle-locating radio apparatus, aspects of which were installed on

6384-540: The antenna, and the antenna radiates the power as radio waves. Radio waves are received by another antenna attached to a radio receiver . When radio waves strike the receiving antenna they push the electrons in the metal back and forth, creating tiny oscillating currents which are detected by the receiver. From quantum mechanics , like other electromagnetic radiation such as light, radio waves can alternatively be regarded as streams of uncharged elementary particles called photons . In an antenna transmitting radio waves,

6496-536: The arrest of Oshchepkov and his subsequent gulag sentence. In total, only 607 Redut stations were produced during the war. The first Russian airborne radar, Gneiss-2 , entered into service in June 1943 on Pe-2 dive bombers. More than 230 Gneiss-2 stations were produced by the end of 1944. The French and Soviet systems, however, featured continuous-wave operation that did not provide the full performance ultimately synonymous with modern radar systems. Full radar evolved as

6608-478: 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 immediately continue the work. Eight years later, Lawrence A. Hyland at the Naval Research Laboratory (NRL) observed similar fading effects from passing aircraft; this revelation led to a patent application as well as

6720-410: The carrier, altering some aspect of it, encoding the information on the carrier. The modulated carrier is amplified and applied to an antenna . The oscillating current pushes the electrons in the antenna back and forth, creating oscillating electric and magnetic fields , which radiate the energy away from the antenna as radio waves. The radio waves carry the information to the receiver location. At

6832-403: The contour of the Earth ( ground waves ), shorter waves can reflect off the ionosphere and return to Earth beyond the horizon ( skywaves ), while much shorter wavelengths bend or diffract very little and travel on a line of sight , so their propagation distances are limited to the visual horizon. To prevent interference between different users, the artificial generation and use of radio waves

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6944-400: The demonstration: Newspapers carried articles about the demonstration, all giving praise to the new maritime safety invention. One of these closed the article with the following: "Because, above and under water metal objects reflect waves, this invention might have significance for future warfare." The building of the Telemobiloscope and its demonstrations had depleted the initial funds of

7056-408: The detection of aircraft and ships. Radar absorbing material , containing resistive and sometimes magnetic substances, is used on military vehicles to reduce radar reflection . This is the radio equivalent of painting something a dark colour so that it cannot be seen by the eye at night. Radar waves scatter in a variety of ways depending on the size (wavelength) of the radio wave and the shape of

7168-476: The detection process. As an example, moving target indication can interact with Doppler to produce signal cancellation at certain radial velocities, which degrades performance. Sea-based radar systems, semi-active radar homing , active radar homing , weather radar , military aircraft, and radar astronomy rely on the Doppler effect to enhance performance. This produces information about target velocity during

7280-411: The detection process. This also allows small objects to be detected in an environment containing much larger nearby slow moving objects. Doppler shift depends upon whether the radar configuration is active or passive. Active radar transmits a signal that is reflected back to the receiver. Passive radar depends upon the object sending a signal to the receiver. The Doppler frequency shift for active radar

7392-623: The device in patent GB593017. Development of radar greatly expanded on 1 September 1936, when Watson-Watt became superintendent of a new establishment under the British Air Ministry , Bawdsey Research Station located in Bawdsey Manor , near Felixstowe, Suffolk. Work there resulted in the design and installation of aircraft detection and tracking stations called " Chain Home " along the East and South coasts of England in time for

7504-538: The electric field is perpendicular to the direction of propagation, and the electric field direction is the polarization of the wave. For a transmitted radar signal, the polarization can be controlled to yield different effects. Radars use horizontal, vertical, linear, and circular polarization to detect different types of reflections. For example, circular polarization is used to minimize the interference caused by rain. Linear polarization returns usually indicate metal surfaces. Random polarization returns usually indicate

7616-421: The electrons in the antenna emit the energy in discrete packets called radio photons, while in a receiving antenna the electrons absorb the energy as radio photons. An antenna is a coherent emitter of photons, like a laser , so the radio photons are all in phase . However, from Planck's relation E = h ν {\displaystyle E=h\nu } , the energy of individual radio photons

7728-438: The energy is deposited. For example, the 2.45 GHz radio waves (microwaves) in a microwave oven penetrate most foods approximately 2.5 to 3.8 cm . Looking into a source of radio waves at close range, such as the waveguide of a working radio transmitter, can cause damage to the lens of the eye by heating. A strong enough beam of radio waves can penetrate the eye and heat the lens enough to cause cataracts . Since

7840-473: The entire area in front of it, and then used one of Watson-Watt's own radio direction finders to determine the direction of the returned echoes. This fact meant CH transmitters had to be much more powerful and have better antennas than competing systems but allowed its rapid introduction using existing technologies. A key development was the cavity magnetron in the UK, which allowed the creation of relatively small systems with sub-meter resolution. Britain shared

7952-730: The far field zone. ELF 3 Hz/100 Mm 30 Hz/10 Mm SLF 30 Hz/10 Mm 300 Hz/1 Mm ULF 300 Hz/1 Mm 3 kHz/100 km VLF 3 kHz/100 km 30 kHz/10 km LF 30 kHz/10 km 300 kHz/1 km MF 300 kHz/1 km 3 MHz/100 m HF 3 MHz/100 m 30 MHz/10 m VHF 30 MHz/10 m 300 MHz/1 m UHF 300 MHz/1 m 3 GHz/100 mm SHF 3 GHz/100 mm 30 GHz/10 mm EHF 30 GHz/10 mm 300 GHz/1 mm THF 300 GHz/1 mm 3 THz/0.1 mm Christian H%C3%BClsmeyer Christian Hülsmeyer (Huelsmeyer) (25 December 1881 – 31 January 1957)

8064-411: The firm GEMA  [ de ] in Germany and then another in June 1935 by an Air Ministry team led by Robert Watson-Watt in Great Britain. In 1935, Watson-Watt was asked to judge recent reports of a German radio-based death ray and turned the request over to Wilkins. Wilkins returned a set of calculations demonstrating the system was basically impossible. When Watson-Watt then asked what such

8176-637: The following: With the distribution of these Minutes in the European shipping community, the prospect of the Telemobiloscope as a viable product ended. There have been many explanations as to the failure; these mainly cite either poor equipment design or the competition of Marconi. The Telemobiloscope design used wireless technology of the late 1890s, and did not include tuning circuits for frequency selection. By 1904, there were many wireless sets aboard ships and at shore stations, and, without tuning capability, these could not be rejected and thus interfered with

8288-527: The heating effect is in principle no different from other sources of heat, most research into possible health hazards of exposure to radio waves has focused on "nonthermal" effects; whether radio waves have any effect on tissues besides that caused by heating. Radiofrequency electromagnetic fields have been classified by the International Agency for Research on Cancer (IARC) as having "limited evidence" for its effects on humans and animals. There

8400-467: The levels of electric and magnetic field strength at a measurement location. Another commonly used unit for characterizing an RF electromagnetic field is power density . Power density is most accurately used when the point of measurement is far enough away from the RF emitter to be located in what is referred to as the far field zone of the radiation pattern. In closer proximity to the transmitter, i.e., in

8512-434: The metal antenna elements. For example, a dipole antenna consists of two collinear metal rods. If the rods are horizontal, it radiates horizontally polarized radio waves, while if the rods are vertical, it radiates vertically polarized waves. An antenna receiving the radio waves must have the same polarization as the transmitting antenna, or it will suffer a severe loss of reception. Many natural sources of radio waves, such as

8624-505: The ocean liner Normandie in 1935. During the same period, Soviet military engineer P.K. Oshchepkov , in collaboration with the Leningrad Electrotechnical Institute , produced an experimental apparatus, RAPID, capable of detecting an aircraft within 3 km of a receiver. The Soviets produced their first mass production radars RUS-1 and RUS-2 Redut in 1939 but further development was slowed following

8736-430: The other radio signals picked up by the antenna, then amplifies the signal so it is stronger, then finally extracts the information-bearing modulation signal in a demodulator . The recovered signal is sent to a loudspeaker or earphone to produce sound, or a television display screen to produce a visible image, or other devices. A digital data signal is applied to a computer or microprocessor , which interacts with

8848-531: The outbreak of World War II in 1939. This system provided the vital advance information that helped the Royal Air Force win the Battle of Britain ; without it, significant numbers of fighter aircraft, which Great Britain did not have available, would always have needed to be in the air to respond quickly. The radar formed part of the " Dowding system " for collecting reports of enemy aircraft and coordinating

8960-1365: The period before and during World War II . A key development was the cavity magnetron in the United Kingdom , which allowed the creation of relatively 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 an anacronym , a common noun, losing all capitalization . The modern uses of radar are highly diverse, including air and terrestrial traffic control, radar astronomy , air-defense systems , anti-missile systems , marine radars to locate landmarks and other ships, aircraft anti-collision systems, ocean surveillance systems, outer space surveillance and rendezvous systems, meteorological precipitation monitoring, radar remote sensing , altimetry and flight control systems , guided missile target locating systems, self-driving cars , and ground-penetrating radar for geological observations. Modern high tech radar systems use digital signal processing and machine learning and are capable of extracting useful information from very high noise levels. Other systems which are similar to radar make use of other parts of

9072-706: The primary tool for short-term weather forecasting and watching for severe weather such as thunderstorms , tornadoes , winter storms , precipitation types, etc. Geologists use specialized ground-penetrating radars to map the composition of Earth's crust . Police forces use radar guns to monitor vehicle speeds on the roads. Automotive radars are used for adaptive cruise control and emergency breaking on vehicles by ignoring stationary roadside objects that could cause incorrect brake application and instead measuring moving objects to prevent collision with other vehicles. As part of Intelligent Transport Systems , fixed-position stopped vehicle detection (SVD) radars are mounted on

9184-432: The radial component of the velocity is relevant. When the reflector is moving at right angle to the radar beam, it has no relative velocity. Objects moving parallel to the radar beam produce the maximum Doppler frequency shift. When the transmit frequency ( F T {\displaystyle F_{T}} ) is pulsed, using a pulse repeat frequency of F R {\displaystyle F_{R}} ,

9296-401: The range of practical radio communication systems decreases with increasing frequency. Below about 20 GHz atmospheric attenuation is mainly due to water vapor. Above 20 GHz, in the millimeter wave band, other atmospheric gases begin to absorb the waves, limiting practical transmission distances to a kilometer or less. Above 300 GHz, in the terahertz band , virtually all the power

9408-421: The reality of Maxwell's electromagnetic waves by experimentally generating electromagnetic waves lower in frequency than light, radio waves, in his laboratory, showing that they exhibited the same wave properties as light: standing waves , refraction , diffraction , and polarization . Italian inventor Guglielmo Marconi developed the first practical radio transmitters and receivers around 1894–1895. He received

9520-437: The received signal. Radio waves are very widely used in modern technology for fixed and mobile radio communication , broadcasting , radar and radio navigation systems, communications satellites , wireless computer networks and many other applications. Different frequencies of radio waves have different propagation characteristics in the Earth's atmosphere; long waves can diffract around obstacles like mountains and follow

9632-446: The receiver, the oscillating electric and magnetic fields of the incoming radio wave push the electrons in the receiving antenna back and forth, creating a tiny oscillating voltage which is a weaker replica of the current in the transmitting antenna. This voltage is applied to the radio receiver , which extracts the information signal. The receiver first uses a bandpass filter to separate the desired radio station's radio signal from all

9744-414: The response. Given all required funding and development support, the team produced working radar systems in 1935 and began deployment. By 1936, the first five Chain Home (CH) systems were operational and by 1940 stretched across the entire UK including Northern Ireland. Even by standards of the era, CH was crude; instead of broadcasting and receiving from an aimed antenna, CH broadcast a signal floodlighting

9856-410: The resulting frequency spectrum will contain harmonic frequencies above and below F T {\displaystyle F_{T}} with a distance of F R {\displaystyle F_{R}} . As a result, the Doppler measurement is only non-ambiguous if the Doppler frequency shift is less than half of F R {\displaystyle F_{R}} , called

9968-424: The roadside to detect stranded vehicles, obstructions and debris by inverting the automotive radar approach and ignoring moving objects. Smaller radar systems are used to detect human movement . Examples are breathing pattern detection for sleep monitoring and hand and finger gesture detection for computer interaction. Automatic door opening, light activation and intruder sensing are also common. A radar system has

10080-406: The scattered energy back toward the source. The extent to which an object reflects or scatters radio waves is called its radar cross-section . The power P r returning to the receiving antenna is given by the equation: where In the common case where the transmitter and the receiver are at the same location, R t = R r and the term R t ² R r ² can be replaced by R , where R

10192-419: The signal is attenuated by the medium the beam crosses, and the beam disperses. The maximum range of conventional radar can be limited by a number of factors: Radio wave Radio waves are generated artificially by an electronic device called a transmitter , which is connected to an antenna , which radiates the waves. They are received by another antenna connected to a radio receiver , which processes

10304-416: The sun, stars and blackbody radiation from warm objects, emit unpolarized waves, consisting of incoherent short wave trains in an equal mixture of polarization states. The polarization of radio waves is determined by a quantum mechanical property of the photons called their spin . A photon can have one of two possible values of spin; it can spin in a right-hand sense about its direction of motion, or in

10416-412: The surface of objects and cause surface heating, radio waves are able to penetrate the surface and deposit their energy inside materials and biological tissues. The depth to which radio waves penetrate decreases with their frequency, and also depends on the material's resistivity and permittivity ; it is given by a parameter called the skin depth of the material, which is the depth within which 63% of

10528-588: The surface of the Earth, is vitally important in the design of practical radio systems. Radio waves passing through different environments experience reflection , refraction , polarization , diffraction , and absorption . Different frequencies experience different combinations of these phenomena in the Earth's atmosphere, making certain radio bands more useful for specific purposes than others. Practical radio systems mainly use three different techniques of radio propagation to communicate: At microwave frequencies, atmospheric gases begin absorbing radio waves, so

10640-491: The target. If the wavelength is much shorter than the target's size, the wave will bounce off in a way similar to the way light is reflected by a mirror . If the wavelength is much longer than the size of the target, the target may not be visible because of poor reflection. Low-frequency radar technology is dependent on resonances for detection, but not identification, of targets. This is described by Rayleigh scattering , an effect that creates Earth's blue sky and red sunsets. When

10752-581: The technology with the U.S. during the 1940 Tizard Mission . In April 1940, Popular Science showed an example of a radar unit using the Watson-Watt patent in an article on air defence. Also, in late 1941 Popular Mechanics had an article in which a U.S. scientist speculated about the British early warning system on the English east coast and came close to what it was and how it worked. Watson-Watt

10864-437: The theory of electromagnetism that was proposed in 1867 by Scottish mathematical physicist James Clerk Maxwell . His mathematical theory, now called Maxwell's equations , predicted that a coupled electric and magnetic field could travel through space as an " electromagnetic wave ". Maxwell proposed that light consisted of electromagnetic waves of very short wavelength. In 1887, German physicist Heinrich Hertz demonstrated

10976-879: The transmitter. The reflected radar signals captured by the receiving antenna are usually very weak. They can be strengthened by electronic amplifiers . More sophisticated methods of signal processing are also used in order to recover useful radar signals. The weak absorption of radio waves by the medium through which they pass is what enables radar sets to detect objects at relatively long ranges—ranges at which other electromagnetic wavelengths, such as visible light , infrared light , and ultraviolet light , are too strongly attenuated. Weather phenomena, such as fog, clouds, rain, falling snow, and sleet, that block visible light are usually transparent to radio waves. Certain radio frequencies that are absorbed or scattered by water vapour, raindrops, or atmospheric gases (especially oxygen) are avoided when designing radars, except when their detection

11088-411: The tuned circuit and not passed on. Radio waves are non-ionizing radiation , which means they do not have enough energy to separate electrons from atoms or molecules , ionizing them, or break chemical bonds , causing chemical reactions or DNA damage . The main effect of absorption of radio waves by materials is to heat them, similarly to the infrared waves radiated by sources of heat such as

11200-487: The two length scales are comparable, there may be resonances . Early radars used very long wavelengths that were larger than the targets and thus received a vague signal, whereas many modern systems use shorter wavelengths (a few centimetres or less) that can image objects as small as a loaf of bread. Short radio waves reflect from curves and corners in a way similar to glint from a rounded piece of glass. The most reflective targets for short wavelengths have 90° angles between

11312-472: The use of radar altimeters possible in certain cases. The radar signals that are reflected back towards the radar receiver are the desirable ones that make radar detection work. If the object is moving either toward or away from the transmitter, there will be a slight change in the frequency of the radio waves due to the Doppler effect . Radar receivers are usually, but not always, in the same location as

11424-517: The usefulness of his invention, the Gumpel Company would establish a Consortium to commercialize the invention. It also noted that If the Telemobiloscope rights were in turn sold by Gumpel, the sales price would have to exceed 1,000,000 Marks. Improvements were made on the equipment, particularly in extending the operational distance. Patent applications on the Telemobiloscope were filed in a number of countries. The application in Great Britain

11536-420: The wave's electric field to the next, and is inversely proportional to the frequency f {\displaystyle f} of the wave. The relation of frequency and wavelength in a radio wave traveling in vacuum or air is where Equivalently, c {\displaystyle c} , the distance that a radio wave travels in vacuum in one second, is 299,792,458 meters (983,571,056 ft), which

11648-442: The waves. Since radio frequency radiation has both an electric and a magnetic component, it is often convenient to express intensity of radiation field in terms of units specific to each component. The unit volt per meter (V/m) is used for the electric component, and the unit ampere per meter (A/m) is used for the magnetic component. One can speak of an electromagnetic field , and these units are used to provide information about

11760-607: Was a 1938 Bell Lab unit on some United Air Lines aircraft. Aircraft can land in fog at airports equipped with radar-assisted ground-controlled approach systems in which the plane's position is observed on precision approach radar screens by operators who thereby give radio landing instructions to the pilot, maintaining the aircraft on a defined approach path to the runway. Military fighter aircraft are usually fitted with air-to-air targeting radars, to detect and target enemy aircraft. In addition, larger specialized military aircraft carry powerful airborne radars to observe air traffic over

11872-477: Was a German inventor, physicist and entrepreneur. He is credited with the invention of radar , although his apparatus, called the "Telemobiloscope," could not directly measure distance to a target. The Telemobiloscope was, however, the first patented device using radio waves for detecting the presence of distant objects like ships. Hülsmeyer was born at Eydelstedt , a village in Lower Saxony , Germany . He

11984-599: Was granted in only 10 weeks, but most of the others were either withdrawn, rejected, or not processed because fees were not paid. A request was made to Holland-America to allow a demonstration at sea; this was apparently turned down but another demonstration was given near the Hook of Holland in the autumn of 1904. A second conference of the shipping firms was held in London in June 1905; the Conference Minutes include

12096-510: Was opened the following May and officially registered in Cologne on 7 July 1904. Hülsmeyer's initial patent application for the Telemobiloscope was rejected, but a refiling, dated 30 April 1904, was accepted, resulting in Patent Publication DE 165546. An article on the system was published in a British technical magazine. The Telemobiloscope was primarily a spark-gap transmitter connected to an array of dipole antennas , and

12208-746: Was sent to the U.S. in 1941 to advise on air defense after Japan's attack on Pearl Harbor . Alfred Lee Loomis organized the secret MIT Radiation Laboratory at Massachusetts Institute of Technology , Cambridge, Massachusetts which developed microwave radar technology in the years 1941–45. Later, in 1943, Page greatly improved radar with the monopulse technique that was used for many years in most radar applications. The war precipitated research to find better resolution, more portability, and more features for radar, including small, lightweight sets to equip night fighters ( aircraft interception radar ) and maritime patrol aircraft ( air-to-surface-vessel radar ), and complementary navigation systems like Oboe used by

12320-459: 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 later a patent for a related amendment for estimating the distance to the ship. He also obtained a British patent on 23 September 1904 for

12432-419: Was the youngest of five children of Johann Heinrich Ernst Meyer and Elisabeth Wilhelmine Brenning. His birth name was Johann Christel , but after early childhood the name Christian was used. Following completion of the local Volksschule (elementary school), he attended Grundschule (primary school) in nearby Donstorf. A teacher there recognized his capabilities and, in 1896, assisted him in gaining admission to

12544-408: Was through a curtain – showing that the apparatus could work when the target could not be seen. The demonstration was reported widely in newspapers, one giving a detailed description. A conference was held in June 1904, at Scheveningen , the Netherlands , involving the major shipping firms of the region; ship safety was a major topic. After learning of the demonstration at the Dom Hotel, the head of

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