Global Positioning System
The Global Positioning System Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Air Force. It is a global navigation satellite system that provides geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. Obstacles such as mountains and buildings block the weak GPS signals; the GPS does not require the user to transmit any data, it operates independently of any telephonic or internet reception, though these technologies can enhance the usefulness of the GPS positioning information. The GPS provides critical positioning capabilities to military and commercial users around the world; the United States government created the system, maintains it, makes it accessible to anyone with a GPS receiver. The GPS project was launched by the U. S. Department of Defense in 1973 for use by the United States military and became operational in 1995.
It was allowed for civilian use in the 1980s. Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS and implement the next generation of GPS Block IIIA satellites and Next Generation Operational Control System. Announcements from Vice President Al Gore and the White House in 1998 initiated these changes. In 2000, the U. S. Congress authorized the modernization effort, GPS III. During the 1990s, GPS quality was degraded by the United States government in a program called "Selective Availability"; the GPS system is provided by the United States government, which can selectively deny access to the system, as happened to the Indian military in 1999 during the Kargil War, or degrade the service at any time. As a result, several countries have developed or are in the process of setting up other global or regional satellite navigation systems; the Russian Global Navigation Satellite System was developed contemporaneously with GPS, but suffered from incomplete coverage of the globe until the mid-2000s.
GLONASS can be added to GPS devices, making more satellites available and enabling positions to be fixed more and to within two meters. China's BeiDou Navigation Satellite System is due to achieve global reach in 2020. There are the European Union Galileo positioning system, India's NAVIC. Japan's Quasi-Zenith Satellite System is a GPS satellite-based augmentation system to enhance GPS's accuracy; when selective availability was lifted in 2000, GPS had about a five-meter accuracy. The latest stage of accuracy enhancement uses the L5 band and is now deployed. GPS receivers released in 2018 that use the L5 band can have much higher accuracy, pinpointing to within 30 centimetres or 11.8 inches. The GPS project was launched in the United States in 1973 to overcome the limitations of previous navigation systems, integrating ideas from several predecessors, including classified engineering design studies from the 1960s; the U. S. Department of Defense developed the system, which used 24 satellites, it was developed for use by the United States military and became operational in 1995.
Civilian use was allowed from the 1980s. Roger L. Easton of the Naval Research Laboratory, Ivan A. Getting of The Aerospace Corporation, Bradford Parkinson of the Applied Physics Laboratory are credited with inventing it; the work of Gladys West is credited as instrumental in the development of computational techniques for detecting satellite positions with the precision needed for GPS. The design of GPS is based on similar ground-based radio-navigation systems, such as LORAN and the Decca Navigator, developed in the early 1940s. Friedwardt Winterberg proposed a test of general relativity – detecting time slowing in a strong gravitational field using accurate atomic clocks placed in orbit inside artificial satellites. Special and general relativity predict that the clocks on the GPS satellites would be seen by the Earth's observers to run 38 microseconds faster per day than the clocks on the Earth; the GPS calculated positions would drift into error, accumulating to 10 kilometers per day. This was corrected for in the design of GPS.
Winterberg, Friedwardt. “Relativistische Zeitdilatation eines künstlichen Satelliten ” When the Soviet Union launched the first artificial satellite in 1957, two American physicists, William Guier and George Weiffenbach, at Johns Hopkins University's Applied Physics Laboratory decided to monitor its radio transmissions. Within hours they realized that, because of the Doppler effect, they could pinpoint where the satellite was along its orbit; the Director of the APL gave them access to their UNIVAC to do the heavy calculations required. Early the next year, Frank McClure, the deputy director of the APL, asked Guier and Weiffenbach to investigate the inverse problem—pinpointing the user's location, given that of the satellite; this led them and APL to develop the TRANSIT system. In 1959, ARPA played a role in TRANSIT. TRANSIT was first tested in 1960, it used a constellation of five satellites and could provide a navigational fix once per hour. In 1967, the U. S. Navy developed the Timation satellite, which proved the feasibility of placing accurate clocks in space, a technology required for GPS.
In the 1970s, the ground-based OMEGA navigation system, based on phase comparison of signal transmission from pairs of stations
In navigation, bearing is the horizontal angle between the direction of an object and another object, or between it and that of true north. Absolute bearing refers to the angle between an object. For example, an object to the East would have an absolute bearing of 90 degrees. Relative bearing refers to the angle between the craft's forward direction and the location of another object. For example, an object relative bearing of 0 degrees would be dead ahead. Bearings can be measured in degrees; the US Army defines the bearing from Point A to Point B as the angle between a ray in the direction of north or south, whose origin is Point A, Ray AB, the ray whose origin is Point A and which contains Point B. The bearing consists of 2 characters and 1 number: first, the character is either N or S. Next is the angle value. Third, the character representing the direction of the angle away from the reference ray - thus, either E, or W; the angle value will always be less than 90 degrees. For example, if Point B is located southeast of Point A, the bearing from Point A to Point B is S 45° E.
The US Army defines the azimuth between Point A and Point B as the angle, measured in the clockwise direction, between the north reference ray and Ray AB. For example, if the bearing between Point A and Point B is S 45° E, the azimuth between Point A and Point B is 135°; the angle value in a bearing can be specified in the units of mils, or grad. An azimuth is specified in the same angle units. A bearing can be taken on another vessel to aid piloting. If the two vessels are travelling towards each other and the relative bearing remains the same over time, there is likelihood of collision and action needs to be taken by one or both vessels to prevent this from happening. A bearing can be taken to a fixed or moving object; this is used by ground troops when planning on using an air-strike on the target. A bearing can be taken to a vessel in distress in order to go to their aid. Types of bearings include: compass bearings grid bearings magnetic bearings relative bearings true bearingsA true bearing is measured in relation to the fixed horizontal reference plane of true north, that is, using the direction toward the geographic north pole as a reference point, while a magnetic bearing is measured in relation to magnetic north, in relation to the north direction of the Earth's magnetic field lines at the given location.
The latter is not the same as the direction toward the magnetic north pole due to magnetic anomalies. A grid bearing is measured in relation to the fixed horizontal reference plane of grid north, that is, using the direction northwards along the grid lines of the map projection as a reference point, while a compass bearing, as in vehicle or marine navigation, is measured in relation to the magnetic compass of the navigator's vehicle or vessel, it should be close to the magnetic bearing. The difference between a magnetic bearing and a compass bearing is the deviation caused to the compass by ferrous metals and local magnetic fields generated by any variety of vehicle or shipboard sources A relative bearing is one in which the reference direction is straight ahead, where the bearing is measured relative to the direction the navigator is facing or in relation to the vessel's bow. There are several methods used to measure navigation bearings including: In land navigation, a'bearing' is ordinarily calculated in a clockwise direction starting from a reference direction of 0° and increasing to 359.9 degrees.
Measured in this way, a bearing is referred to as an azimuth by the US Army but not by armies in other English speaking nations, which use the term bearing. If the reference direction is north, the bearing is termed an absolute bearing. In a contemporary land navigation context, true and grid bearings are always measured in this way, with true north, magnetic north, or grid north being 0° in a 360-degree system. In aircraft navigation, an angle is measured from the aircraft's track or heading, in a clockwise direction. If the aircraft encounters a target, not ahead of the aircraft and not on an identical track the angular bearing to that target is called a relative bearing. In marine navigation, starboard bearings are'green' and port bearings are'red'. Thus, in ship navigation, a target directly off the starboard side would be'Green090' or'G090'; this method is only used for a relative bearing. A navigator on watch does not always have a corrected compass available with which to give an accurate bearing.
If available, the bearing might not be numerate. Therefore, every forty-five degrees of direction from north on the compass was divided into four'points'. Thus, 32 points of 11.25° each makes a circle of 360°. An object at 022.5° relative would be'two points off the starboard bow', an object at 101.25° relative would be'one point abaft the starboard beam' and an object at 213.75° relative would be'three points on the port quarter'. This method is only used for a relative bearing. An informal method of measuring a relative bearing is by using the'clock method'. In this method, the direction a vessel, aircraft or object is measured as if a clock face is laid over the vessel or aircraft, with the number twelve pointing forward. Something straight ahead is at'twelve o'clock', while something directly off to the right is at'three o'clock'; this method is only used for a relative bearing. In land surveying, a bearing is the clockwise or counterclockwise angle between no
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