VHF omnidirectional range
High Frequency Omni-Directional Range is a type of short-range radio navigation system for aircraft, enabling aircraft with a receiving unit to determine its position and stay on course by receiving radio signals transmitted by a network of fixed ground radio beacons. It uses frequencies in the high frequency band from 108.00 to 117.95 MHz. Developed in the United States beginning in 1937 and deployed by 1946, VOR is the standard air navigational system in the world, used by both commercial and general aviation. By 2000 there were about 3,000 VOR stations around the world including 1,033 in the US, reduced to 967 by 2013 with more stations being decommissioned with the widespread adoption of GPS. A VOR ground station sends out an omnidirectional master signal, a directional second signal is propagated by a phased antenna array and rotates clockwise in space 30 times a second; this signal is timed so that its phase varies as the secondary signal rotates, this phase difference is the same as the angular direction of the'spinning' signal.
By comparing the phase of the secondary signal with the master, the angle to the aircraft from the station can be determined. This line of position is called the "radial" from the VOR; the intersection of radials from two different VOR stations can be used to fix the position of the aircraft, as in earlier radio direction finding systems. VOR stations are short range: the signals are line of sight between transmitter and receiver and are useful for up to 200 miles; each station broadcasts a VHF radio composite signal including the navigation signal, station's identifier and voice, if so equipped. The navigation signal allows the airborne receiving equipment to determine a bearing from the station to the aircraft; the station's identifier is a three-letter string in Morse code. The voice signal, if used, is the station name, in-flight recorded advisories, or live flight service broadcasts. At some locations, this voice signal is a continuous recorded broadcast of Hazardous Inflight Weather Advisory Service or HIWAS.
Developed from earlier Visual Aural Radio Range systems, the VOR was designed to provide 360 courses to and from the station, selectable by the pilot. Early vacuum tube transmitters with mechanically-rotated antennas were installed in the 1950s, began to be replaced with solid-state units in the early 1960s, they became the major radio navigation system in the 1960s, when they took over from the older radio beacon and four-course system. Some of the older range stations survived, with the four-course directional features removed, as non-directional low or medium frequency radiobeacons. A worldwide land-based network of "air highways", known in the US as Victor airways and "jetways", was set up linking VORs. An aircraft can follow a specific path from station to station by tuning into the successive stations on the VOR receiver, either following the desired course on a Radio Magnetic Indicator, or setting it on a course deviation indicator or a horizontal situation indicator and keeping a course pointer centred on the display.
As of 2005, due to advances in technology, many airports are replacing VOR and NDB approaches with RNAV approach procedures. VOR signals provide greater accuracy and reliability than NDBs due to a combination of factors. Most significant is that VOR provides a bearing from the station to the aircraft which does not vary with wind or orientation of the aircraft. VHF radio is less vulnerable to diffraction around terrain coastlines. Phase encoding suffers less interference from thunderstorms. VOR signals offer a predictable accuracy of 90 m, 2 sigma at 2 NM from a pair of VOR beacons. VOR stations rely on "line of sight" because they operate in the VHF band—if the transmitting antenna cannot be seen on a clear day from the receiving antenna, a useful signal cannot be received; this limits VOR range to the horizon -- or closer. Although the modern solid state transmitting equipment requires much less maintenance than the older units, an extensive network of stations, needed to provide reasonable coverage along main air routes, is a significant cost in operating current airway systems.
VORs are assigned radio channels between 117.95 MHz. The first 4 MHz is shared with the instrument landing system band. To leave channels for ILS, in the range 108.0 to 111.95 MHz, the 100 kHz digit is always so 108.00, 108.05, 108.20, 108.25, so on are VOR frequencies but 108.10, 108.15, 108.30, 108.35 and so on, are reserved for ILS in the US. The VOR encodes azimuth as the phase relationship between a variable signal; the omnidirectional signal contains a modulated continuous wave 7 wpm Morse code station identifier, contains an amplitude modulated voice channel. The conventional 30 Hz reference signal is frequency modulated on a 9,960 Hz subcarrier; the variable amplitude modulated signal is conven
A spacecraft is a vehicle or machine designed to fly in outer space. Spacecraft are used for a variety of purposes, including communications, earth observation, navigation, space colonization, planetary exploration, transportation of humans and cargo. All spacecraft except single-stage-to-orbit vehicles cannot get into space on their own, require a launch vehicle. On a sub-orbital spaceflight, a space vehicle enters space and returns to the surface, without having gained sufficient energy or velocity to make a full orbit of the Earth. For orbital spaceflights, spacecraft enter closed orbits around the Earth or around other celestial bodies. Spacecraft used for human spaceflight carry people on board as crew or passengers from start or on orbit only, whereas those used for robotic space missions operate either autonomously or telerobotically. Robotic spacecraft used to support scientific research are space probes. Robotic spacecraft that remain in orbit around a planetary body are artificial satellites.
To date, only a handful of interstellar probes, such as Pioneer 10 and 11, Voyager 1 and 2, New Horizons, are on trajectories that leave the Solar System. Orbital spacecraft may be recoverable or not. Most are not. Recoverable spacecraft may be subdivided by method of reentry to Earth into non-winged space capsules and winged spaceplanes. Humanity has achieved space flight but only a few nations have the technology for orbital launches: Russia, the United States, the member states of the European Space Agency, China, Taiwan (National Chung-Shan Institute of Science and Technology, Taiwan National Space Organization, Israel and North Korea. A German V-2 became the first spacecraft when it reached an altitude of 189 km in June 1944 in Peenemünde, Germany. Sputnik 1 was the first artificial satellite, it was launched into an elliptical low Earth orbit by the Soviet Union on 4 October 1957. The launch ushered in new political, military and scientific developments. Apart from its value as a technological first, Sputnik 1 helped to identify the upper atmospheric layer's density, through measuring the satellite's orbital changes.
It provided data on radio-signal distribution in the ionosphere. Pressurized nitrogen in the satellite's false body provided the first opportunity for meteoroid detection. Sputnik 1 was launched during the International Geophysical Year from Site No.1/5, at the 5th Tyuratam range, in Kazakh SSR. The satellite travelled at 29,000 kilometers per hour, taking 96.2 minutes to complete an orbit, emitted radio signals at 20.005 and 40.002 MHz While Sputnik 1 was the first spacecraft to orbit the Earth, other man-made objects had reached an altitude of 100 km, the height required by the international organization Fédération Aéronautique Internationale to count as a spaceflight. This altitude is called the Kármán line. In particular, in the 1940s there were several test launches of the V-2 rocket, some of which reached altitudes well over 100 km; as of 2016, only three nations have flown crewed spacecraft: USSR/Russia, USA, China. The first crewed spacecraft was Vostok 1, which carried Soviet cosmonaut Yuri Gagarin into space in 1961, completed a full Earth orbit.
There were five other crewed missions. The second crewed spacecraft was named Freedom 7, it performed a sub-orbital spaceflight in 1961 carrying American astronaut Alan Shepard to an altitude of just over 187 kilometers. There were five other crewed missions using Mercury spacecraft. Other Soviet crewed spacecraft include the Voskhod, flown uncrewed as Zond/L1, L3, TKS, the Salyut and Mir crewed space stations. Other American crewed spacecraft include the Gemini spacecraft, Apollo spacecraft, the Skylab space station, the Space Shuttle with undetached European Spacelab and private US Spacehab space stations-modules. China developed, but did not fly Shuguang, is using Shenzhou. Except for the Space Shuttle, all of the recoverable crewed orbital spacecraft were space capsules. Crewed space capsules The International Space Station, crewed since November 2000, is a joint venture between Russia, the United States and several other countries; some reusable vehicles have been designed only for crewed spaceflight, these are called spaceplanes.
The first example of such was the North American X-15 spaceplane, which conducted two crewed flights which reached an altitude of over 100 km in the 1960s. The first reusable spacecraft, the X-15, was air-launched on a suborbital trajectory on July 19, 1963; the first reusable orbital spacecraft, a winged non-capsule, the Space Shuttle, was launched by the USA on the 20th anniversary of Yuri Gagarin's flight, on April 12, 1981. During the Shuttle era, six orbiters were built, all of which have flown in the atmosphere and five of which have flown in space. Enterprise was used only for approach and landing tests, launching from the back of a Boeing 747 SCA and gliding to deadstick landings at Edwards AFB, California; the first Space Shuttle to fly into space was Columbia, followed by Challenger, Discovery and Endeavour. Endeavour was built to replace Challenger when it was lost in January 1986. Columbia broke up during reentry in February 2003; the first automatic reusable spacecraft was the Buran-class shuttle, launched by the USSR on November 15, 1988, although it made only one flight and this was uncrewed.
This spaceplane was designed for a crew and resembled the U
An aircraft is a machine, able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines. Common examples of aircraft include airplanes, airships and hot air balloons; the human activity that surrounds aircraft is called aviation. The science of aviation, including designing and building aircraft, is called aeronautics. Crewed aircraft are flown by an onboard pilot, but unmanned aerial vehicles may be remotely controlled or self-controlled by onboard computers. Aircraft may be classified by different criteria, such as lift type, aircraft propulsion and others. Flying model craft and stories of manned flight go back many centuries, however the first manned ascent – and safe descent – in modern times took place by larger hot-air balloons developed in the 18th century; each of the two World Wars led to great technical advances. The history of aircraft can be divided into five eras: Pioneers of flight, from the earliest experiments to 1914.
First World War, 1914 to 1918. Aviation between the World Wars, 1918 to 1939. Second World War, 1939 to 1945. Postwar era called the jet age, 1945 to the present day. Aerostats use buoyancy to float in the air in much the same way, they are characterized by one or more large gasbags or canopies, filled with a low-density gas such as helium, hydrogen, or hot air, less dense than the surrounding air. When the weight of this is added to the weight of the aircraft structure, it adds up to the same weight as the air that the craft displaces. Small hot-air balloons called sky lanterns were first invented in ancient China prior to the 3rd century BC and used in cultural celebrations, were only the second type of aircraft to fly, the first being kites which were first invented in ancient China over two thousand years ago. A balloon was any aerostat, while the term airship was used for large, powered aircraft designs – fixed-wing. In 1919 Frederick Handley Page was reported as referring to "ships of the air," with smaller passenger types as "Air yachts."
In the 1930s, large intercontinental flying boats were sometimes referred to as "ships of the air" or "flying-ships". – though none had yet been built. The advent of powered balloons, called dirigible balloons, of rigid hulls allowing a great increase in size, began to change the way these words were used. Huge powered aerostats, characterized by a rigid outer framework and separate aerodynamic skin surrounding the gas bags, were produced, the Zeppelins being the largest and most famous. There were still no fixed-wing aircraft or non-rigid balloons large enough to be called airships, so "airship" came to be synonymous with these aircraft. Several accidents, such as the Hindenburg disaster in 1937, led to the demise of these airships. Nowadays a "balloon" is an unpowered aerostat and an "airship" is a powered one. A powered, steerable aerostat is called a dirigible. Sometimes this term is applied only to non-rigid balloons, sometimes dirigible balloon is regarded as the definition of an airship.
Non-rigid dirigibles are characterized by a moderately aerodynamic gasbag with stabilizing fins at the back. These soon became known as blimps. During the Second World War, this shape was adopted for tethered balloons; the nickname blimp was adopted along with the shape. In modern times, any small dirigible or airship is called a blimp, though a blimp may be unpowered as well as powered. Heavier-than-air aircraft, such as airplanes, must find some way to push air or gas downwards, so that a reaction occurs to push the aircraft upwards; this dynamic movement through the air is the origin of the term aerodyne. There are two ways to produce dynamic upthrust: aerodynamic lift, powered lift in the form of engine thrust. Aerodynamic lift involving wings is the most common, with fixed-wing aircraft being kept in the air by the forward movement of wings, rotorcraft by spinning wing-shaped rotors sometimes called rotary wings. A wing is a flat, horizontal surface shaped in cross-section as an aerofoil. To fly, air must generate lift.
A flexible wing is a wing made of fabric or thin sheet material stretched over a rigid frame. A kite is tethered to the ground and relies on the speed of the wind over its wings, which may be flexible or rigid, fixed, or rotary. With powered lift, the aircraft directs its engine thrust vertically downward. V/STOL aircraft, such as the Harrier Jump Jet and F-35B take off and land vertically using powered lift and transfer to aerodynamic lift in steady flight. A pure rocket is not regarded as an aerodyne, because it does not depend on the air for its lift. Rocket-powered missiles that obtain aerodynamic lift at high speed due to airflow over their bodies are a marginal case; the forerunner of the fixed-wing aircraft is the kite. Whereas a fixed-wing aircraft relies on its forward speed to create airflow over the wings, a kite is tethered to the ground and relies on the wind blowing over its wings to provide lift. Kites were the first kind of aircraft to fly, were invented in China around 500 BC.
Much aerodynamic research was done with kites before test aircraft, wind tunnels, computer modelling programs became available. The first heavier-than-air craft capable of controlled free-flight were gliders. A glider designed by Geo
Cessna Citation I
The Cessna 500 Citation I is a small business jet announced by Cessna in October 1968 and is the basis of the Citation family. The Fanjet 500 prototype first flew on September 15, 1969 and it was certified as the Citation on September 9, 1971, upgraded in 1976 as the Citation I and a single pilot variant, production ended in 1985 after 689 deliveries. Powered by JT15D turbofans, the straight wing aircraft was developed into the Citation II/Bravo. In October 1968 Cessna announced an eight place business jet capable of operating from light/medium twins airfields; the Fanjet 500 prototype first flew on September 15, 1969. The renamed Citation had a long development program with a longer forward fuselage, repositioned engine nacelles, a larger tail and more dihedral to the horizontal tail, it was FAA certified on September 9, 1971. In early 1976, its wing span grew from 43.9 to 47.1 ft. It gained thrust reversers and higher gross weights; the enhanced Citation I was introduced in 1976 with higher weights, JT15D-1A engines and an increased span wing.
The 501 Citation I/SP, certificated for single pilot operations, was delivered in early 1977. Production ended in 1985, it was developed into the Citation II/Bravo and the Citation V/Ultra/Encore. Over 690 Citations, Citation Is and I/SPs were built between 1971 and 1985. By 2018, used 1970s model 500s were valued at $300,000, Citation ISPs at $695,000 to $1.25 million with the Eagle II package. The aircraft was powered by two Pratt & Whitney Canada JT15D-1 turbofan engines after Cessna's experience with the T-37 Tweet twinjet trainer. Turbofan rather than turbojets and straight wings rather than swept wings made it cruise compared to other business jets and Learjet salesmen mocked it as the “Nearjet” vulnerable to “bird strikes from the rear”. NorwayFlyDirect AngolaNational Air Force of Angola ArgentinaArgentine Army People's Republic of ChinaPeople's Liberation Army Air Force EcuadorEcuadorian Navy MexicoMexican Air Force VenezuelaVenezuelan Air Force New York Yankees catcher Thurman Munson was killed in his Citation I/SP on August 2, 1979 while practicing touch-and-go landings.
Data from Jane's Civil and Military Aircraft Upgrades 1994-95 General characteristics Crew: Two Capacity: 5 passengers Length: 43 ft 6 in Wingspan: 47 ft 1 in Height: 14 ft 4 in Wing area: 278.5 sq ft Aspect ratio: 7.83:1 Empty weight: 6,631 lb Max takeoff weight: 11,850 lb Fuel capacity: 564 US gal usable fuel Powerplant: 2 × Pratt & Whitney Canada JT15D-1B turbofans, 2,200 lbf thrust eachPerformance Maximum speed: Mach 0.705 Cruise speed: 357 kn at 35,000 ft Stall speed: 82 kn Range: 1,328 nmi at 41,000 ft Service ceiling: 41,000 ft Rate of climb: 2,719 ft/min Related development Cessna Citation series Cessna Citation II
General Aviation represents the'private transport' and recreational flying component of aviation. General aviation is the name or term given to all civil aviation aircraft operations with the exception of commercial air transport or aerial work, they are flight activities not involving commercial air transportation of passengers, cargo or mail for remuneration or hire, or an aerial work operation such as agriculture, photography, surveying and patrol, search and rescue, aerial advertisement, etc. It covers certain commercial and private flights that can be carried out under both visual flight and instrument flight rules, such as light aircraft and private jets or helicopters. General aviation thus represents the'private transport' component of aviation; the International Civil Aviation Organization defines civil aviation aircraft operations in three categories: General Aviation, Aerial Work and Commercial Air Transport. The International Council of Aircraft Owner and Pilot Associations includes the following definitions for General Aviation aircraft activities: Corporate Aviation: Company own-use flight operations Fractional Ownership Operations: aircraft operated by a specialized company on behalf of two or more co-owners Business Aviation: self-flown for business purposes Personal/Private Travel: travel for personal reasons/personal transport Air Tourism: self-flown incoming/outgoing tourism Recreational Flying: powered/powerless leisure flying activities Air Sports: Aerobatics, Air Races, Rallies etc.
In 2003 the European Aviation Safety Agency was established as the central EU regulator, taking over responsibility for legislating airworthiness and environmental regulation from the national authorities. Of the 21,000 civil aircraft registered in the UK, 96 percent are engaged in GA operations, annually the GA fleet accounts for between 1.25 and 1.35 million hours flown. There are 28,000 Private Pilot Licence holders, 10,000 certified glider pilots; some of the 19,000 pilots who hold professional licences are engaged in GA activities. GA operates from more than 1,800 airports and landing sites or aerodromes, ranging in size from large regional airports to farm strips. GA is regulated by the Civil Aviation Authority, although regulatory powers are being transferred to the European Aviation Safety Agency; the main focus is on standards of airworthiness and pilot licensing, the objective is to promote high standards of safety. General aviation is popular in North America, with over 6,300 airports available for public use by pilots of general aviation aircraft.
In comparison, scheduled flights operate from around 560 airports in the U. S. According to the U. S. Aircraft Owners and Pilots Association, general aviation provides more than one percent of the United States' GDP, accounting for 1.3 million jobs in professional services and manufacturing. Most countries have authorities that oversee all civil aviation, including general aviation, adhering to the standardized codes of the International Civil Aviation Organization. Examples include the Federal Aviation Administration in the United States, the Civil Aviation Authority in the United Kingdom, Civil Aviation Authority of Zimbabwe in Zimbabwe, the Luftfahrt-Bundesamt in Germany, the Bundesamt für Zivilluftfahrt in Switzerland, Transport Canada in Canada, the Civil Aviation Safety Authority in Australia, the Directorate General of Civil Aviation in India and Iran Civil Aviation Organization in Iran. Aviation accident rate statistics are estimates. According to the U. S. National Transportation Safety Board, in 2005 general aviation in the United States suffered 1.31 fatal accidents for every 100,000 hours of flying in that country, compared to 0.016 for scheduled airline flights.
In Canada, recreational flying accounted for 0.7 fatal accidents for every 1000 aircraft, while air taxi accounted for 1.1 fatal accidents for every 100,000 hours. More experienced GA pilots appear safer, although the relations between flight hours, accident frequency, accident rates are complex and difficult to assess. Environmental impact of aviation List of current production certified light aircraftAssociationsAircraft Owners and Pilots Association Canadian Owners and Pilots Association Experimental Aircraft Association General Aviation Manufacturers Association National Business Aviation Association International Aircraft Owners and Pilots Associations European General Aviation Safety Team "No Plane No Gain" website about business aviation Save-GA.org website concerned with General Aviation in the United States "GA price index". Flight International. 13 Oct 1979
The cathode-ray tube is a vacuum tube that contains one or more electron guns and a phosphorescent screen, is used to display images. It modulates and deflects electron beam onto the screen to create the images; the images may represent electrical waveforms, radar targets, or other phenomena. CRTs have been used as memory devices, in which case the visible light emitted from the fluorescent material is not intended to have significant meaning to a visual observer. In television sets and computer monitors, the entire front area of the tube is scanned repetitively and systematically in a fixed pattern called a raster. An image is produced by controlling the intensity of each of the three electron beams, one for each additive primary color with a video signal as a reference. In all modern CRT monitors and televisions, the beams are bent by magnetic deflection, a varying magnetic field generated by coils and driven by electronic circuits around the neck of the tube, although electrostatic deflection is used in oscilloscopes, a type of electronic test instrument.
A CRT is constructed from a glass envelope, large, deep heavy, fragile. The interior of a CRT is evacuated to 0.01 pascals to 133 nanopascals, evacuation being necessary to facilitate the free flight of electrons from the gun to the tube's face. The fact that it is evacuated makes handling an intact CRT dangerous due to the risk of breaking the tube and causing a violent implosion that can hurl shards of glass at great velocity; as a matter of safety, the face is made of thick lead glass so as to be shatter-resistant and to block most X-ray emissions if the CRT is used in a consumer product. Since the late 2000s, CRTs have been superseded by newer "flat panel" display technologies such as LCD, plasma display, OLED displays, which in the case of LCD and OLED displays have lower manufacturing costs and power consumption, as well as less weight and bulk. Flat panel displays can be made in large sizes. Cathode rays were discovered by Johann Wilhelm Hittorf in 1869 in primitive Crookes tubes, he observed that some unknown rays were emitted from the cathode which could cast shadows on the glowing wall of the tube, indicating the rays were traveling in straight lines.
In 1890, Arthur Schuster demonstrated cathode rays could be deflected by electric fields, William Crookes showed they could be deflected by magnetic fields. In 1897, J. J. Thomson succeeded in measuring the mass of cathode rays, showing that they consisted of negatively charged particles smaller than atoms, the first "subatomic particles", which were named electrons; the earliest version of the CRT was known as the "Braun tube", invented by the German physicist Ferdinand Braun in 1897. It was a modification of the Crookes tube with a phosphor-coated screen; the first cathode-ray tube to use a hot cathode was developed by John B. Johnson and Harry Weiner Weinhart of Western Electric, became a commercial product in 1922. In 1925, Kenjiro Takayanagi demonstrated a CRT television that received images with a 40-line resolution. By 1927, he improved the resolution to 100 lines, unrivaled until 1931. By 1928, he was the first to transmit human faces in half-tones on a CRT display. By 1935, he had invented an early all-electronic CRT television.
It was named in 1929 by inventor Vladimir K. Zworykin, influenced by Takayanagi's earlier work. RCA was granted a trademark for the term in 1932; the first commercially made electronic television sets with cathode-ray tubes were manufactured by Telefunken in Germany in 1934. Flat panel displays dropped in price and started displacing cathode-ray tubes in the 2000s, with LCD screens exceeding CRTs in 2008; the last known manufacturer of CRTs ceased in 2015. In oscilloscope CRTs, electrostatic deflection is used, rather than the magnetic deflection used with television and other large CRTs; the beam is deflected horizontally by applying an electric field between a pair of plates to its left and right, vertically by applying an electric field to plates above and below. Televisions use magnetic rather than electrostatic deflection because the deflection plates obstruct the beam when the deflection angle is as large as is required for tubes that are short for their size. Various phosphors are available depending upon the needs of the display application.
The brightness and persistence of the illumination depends upon the type of phosphor used on the CRT screen. Phosphors are available with persistences ranging from less than one microsecond to several seconds. For visual observation of brief transient events, a long persistence phosphor may be desirable. For events which are fast and repetitive, or high frequency, a short-persistence phosphor is preferable; when displaying fast one-shot events, the electron beam must deflect quickly, with few electrons impinging on the screen, leading to a faint or invisible image on the display. Oscilloscope CRTs designed for fast signals can give a brighter display by passing the electron beam through a micro-channel plate just before it reaches
Radio navigation or radionavigation is the application of radio frequencies to determine a position of an object on the Earth. Like radiolocation, it is a type of radiodetermination; the basic principles are measurements from/to electric beacons Angular directions, e.g. by bearing, radio phases or interferometry, Distances, e.g. ranging by measurement of time of flight between one transmitter and multiple receivers or vice versa, Distance differences by measurement of times of arrival of signals from one transmitter to multiple receivers or vice versa Partly velocity, e.g. by means of radio Doppler shift. Combinations of these measurement principles are important—e.g. Many radars measure azimuth of a target; these systems used some form of directional radio antenna to determine the location of a broadcast station on the ground. Conventional navigation techniques are used to take a radio fix; these were introduced prior to World War I, remain in use today. The first system of radio navigation was the Radio Direction Finder, or RDF.
By tuning in a radio station and using a directional antenna, one could determine the direction to the broadcasting antenna. A second measurement using another station was taken. Using triangulation, the two directions can be plotted on a map where their intersection reveals the location of the navigator. Commercial AM radio stations can be used for this task due to their long range and high power, but strings of low-power radio beacons were set up for this task near airports and harbours. Early RDF systems used a loop antenna, a small loop of metal wire, mounted so it can be rotated around a vertical axis. At most angles the loop has a flat reception pattern, but when it is aligned perpendicular to the station the signal received on one side of the loop cancels the signal in the other, producing a sharp drop in reception known as the "null". By rotating the loop and looking for the angle of the null, the relative bearing of the station can be determined. Loop antennas can be seen on most pre-1950s aircraft and ships.
The main problem with RDF is that it required a special antenna on the vehicle, which may not be easy to mount on smaller vehicles or single-crew aircraft. A smaller problem is that the accuracy of the system is based to a degree on the size of the antenna, but larger antennas would make the installation more difficult. During the era between World War I and World War II, a number of systems were introduced that placed the rotating antenna on the ground; as the antenna rotated through a fixed position due north, the antenna was keyed with the morse code signal of the station's identification letters so the receiver could ensure they were listening to the right station. They waited for the signal to either peak or disappear as the antenna pointed in their direction. By timing the delay between the morse signal and the peak/null dividing by the known rotational rate of the station, the bearing of the station could be calculated; the first such system was the German Telefunken Kompass Sender, which began operations in 1907 and was used operationally by the Zeppelin fleet until 1918.
An improved version was introduced by the UK as the Orfordness Beacon in 1929 and used until the mid-1930s. A number of improved versions followed, replacing the mechanical motion of the antennas with phasing techniques that produced the same output pattern with no moving parts. One of the longest lasting examples was Sonne, which went into operation just before World War II and was used operationally under the name Consol until 1991; the modern VOR system is based on the same principles. A great advance in the RDF technique was introduced in the form of phase comparisons of a signal as measured on two or more small antennas, or a single directional solenoid; these receivers were smaller, more accurate, simpler to operate. Combined with the introduction of the transistor and integrated circuit, RDF systems were so reduced in size and complexity that they once again became quite common during the 1960s, were known by the new name, automatic direction finder, or ADF; this led to a revival in the operation of simple radio beacons for use with these RDF systems, now referred to as non-directional beacons.
As the LF/MF signals used by NDBs can follow the curvature of earth, NDB has a much greater range than VOR which travels only in line of sight. NDB can be categorized as short range depending on their power; the frequency band allotted to non-directional beacons is 190–1750 kHz, but the same system can be used with any common AM-band commercial station. VHF omnidirectional range, or VOR, is an implementation of the reverse-RDF system, but one, more accurate and able to be automated. Instead of a single signal, the VOR transmitter sends out three signals – one is a simple voice channel that sends morse code to identify the station, another is a continuous signal sent in all directions, the last is a signal, rotated at 30 RPM. Like the Orfordness concept, the bearing of the station is measured by finding the rotating signal's peak or null, but instead of timing the signal, the rotating signal is changed in phase in synchronicity with its rotation, such that it is in-phase when pointed north, 90 degrees off when it points east, so forth.
By comparing the phase of the received signal with the one being broadcast omnidirectionally, the angle can be determined using simple electronics. This angle is displayed in the cockpit of the aircraft, can be used to take a fix just like the earlier RDF systems, although it is easier to use; as VOR required two VHF receivers as well as a conventional radio for station identification, the system did not becom