A geodetic datum or geodetic system is a coordinate system, a set of reference points, used to locate places on the Earth. An approximate definition of sea level is the datum WGS 84, an ellipsoid, whereas a more accurate definition is Earth Gravitational Model 2008, using at least 2,159 spherical harmonics. Other datums are defined at other times. Mars has no oceans and so no sea level, but at least two martian datums have been used to locate places there. Datums are used in geodesy and surveying by cartographers and satellite navigation systems to translate positions indicated on maps to their real position on Earth; each starts with an ellipsoid, defines latitude and altitude coordinates. One or more locations on the Earth's surface are chosen as anchor "base-points"; the difference in co-ordinates between datums is referred to as datum shift. The datum shift between two particular datums can vary from one place to another within one country or region, can be anything from zero to hundreds of meters.
The North Pole, South Pole and Equator will be in different positions on different datums, so True North will be different. Different datums use different interpolations for the precise size of the Earth; because the Earth is an imperfect ellipsoid, localised datums can give a more accurate representation of the area of coverage than WGS 84. OSGB36, for example, is a better approximation to the geoid covering the British Isles than the global WGS 84 ellipsoid. However, as the benefits of a global system outweigh the greater accuracy, the global WGS 84 datum is becoming adopted. Horizontal datums are used for describing a point on the Earth's surface, in latitude and longitude or another coordinate system. Vertical datums measure depths. In surveying and geodesy, a datum is a reference system or an approximation of the Earth's surface against which positional measurements are made for computing locations. Horizontal datums are used for describing a point on the Earth's surface, in latitude and longitude or another coordinate system.
Vertical datums are used to underwater depths. The horizontal datum is the model used to measure positions on the Earth. A specific point on the Earth can have different coordinates, depending on the datum used to make the measurement. There are hundreds of local horizontal datums around the world referenced to some convenient local reference point. Contemporary datums, based on accurate measurements of the shape of the Earth, are intended to cover larger areas; the WGS 84 datum, identical to the NAD83 datum used in North America and the ETRS89 datum used in Europe, is a common standard datum. For example, in Sydney there is a 200 metres difference between GPS coordinates configured in GDA and AGD, an unacceptably large error for some applications, such as surveying or site location for scuba diving. A vertical datum is a reference surface for vertical positions, such as the elevations of Earth features including terrain, water level, man-made structures. In geodetic coordinates, the Earth's surface is approximated by an ellipsoid, locations near the surface are described in terms of latitude and height.
Geodetic latitude, resp. altitude, is different from geocentric latitude, resp. altitude. Geodetic latitude is determined by the angle between the equatorial plane and normal to the ellipsoid, whereas geocentric latitude is determined by the angle between the equatorial plane and line joining the point to the centre of the ellipsoid. Unless otherwise specified, latitude is geodetic latitude; the ellipsoid is parameterised by the semi-major axis a and the flattening f. From a and f it is possible to derive the semi-minor axis b, first eccentricity e and second eccentricity e ′ of the ellipsoid The two main reference ellipsoids used worldwide are the GRS80 and the WGS84. A more comprehensive list of geodetic systems can be found here; the Global Positioning System uses the World Geodetic System 1984 to determine the location of a point near the surface of the Earth. Datum conversion is the process of converting the coordinates of a point from one datum system to another. Datum conversion may be accompanied by a change of grid projection.
A geodetic reference datum is a known and constant surface, used to describe the location of unknown points on the Earth. Since reference datums can have different radii and different center points, a specific point on the Earth can have different coordinates depending on the datum used to make the measurement. There are hundreds of locally developed reference datums around the world referenced to some convenient local reference point. Contemporary datums, based on accurate measurements of the shape of the Earth, are intended to cover larger areas; the mos
Line-of-sight propagation is a characteristic of electromagnetic radiation or acoustic wave propagation which means waves travel in a direct path from the source to the receiver. Electromagnetic transmission includes light emissions traveling in a straight line; the rays or waves may be diffracted, reflected, or absorbed by the atmosphere and obstructions with material and cannot travel over the horizon or behind obstacles. In contrast to line-of-sight propagation, at low frequency due to diffraction, radio waves can travel as ground waves, which follow the contour of the Earth; this enables AM radio stations to transmit beyond the horizon. Additionally, frequencies in the shortwave bands between 1 and 30 MHz, can be reflected back to Earth by the ionosphere, called skywave or "skip" propagation, thus giving radio transmissions in this range a global reach. However, at frequencies above 30 MHz and in lower levels of the atmosphere, neither of these effects are significant. Thus, any obstruction between the transmitting antenna and the receiving antenna will block the signal, just like the light that the eye may sense.
Therefore, since the ability to visually see a transmitting antenna corresponds to the ability to receive a radio signal from it, the propagation characteristic at these frequencies is called "line-of-sight". The farthest possible point of propagation is referred to as the "radio horizon". In practice, the propagation characteristics of these radio waves vary depending on the exact frequency and the strength of the transmitted signal. Broadcast FM radio, at comparatively low frequencies of around 100 MHz, are less affected by the presence of buildings and forests. Low-powered microwave transmitters can be foiled by tree branches, or heavy rain or snow; the presence of objects not in the direct line-of-sight can cause diffraction effects that disrupt radio transmissions. For the best propagation, a volume known as the first Fresnel zone should be free of obstructions. Reflected radiation from the surface of the surrounding ground or salt water can either cancel out or enhance the direct signal.
This effect can be reduced by raising either or both antennas further from the ground: The reduction in loss achieved is known as height gain. See Non-line-of-sight propagation for more on impairments in propagation, it is important to take into account the curvature of the Earth for calculation of line-of-sight paths from maps, when a direct visual fix cannot be made. Designs for microwave used 4⁄3 earth radius to compute clearances along the path. Although the frequencies used by mobile phones are in the line-of-sight range, they still function in cities; this is made possible by a combination of the following effects: 1⁄r 4 propagation over the rooftop landscape diffraction into the "street canyon" below multipath reflection along the street diffraction through windows, attenuated passage through walls, into the building reflection and attenuated passage through internal walls and ceilings within the buildingThe combination of all these effects makes the mobile phone propagation environment complex, with multipath effects and extensive Rayleigh fading.
For mobile phone services, these problems are tackled using: rooftop or hilltop positioning of base stations many base stations. A phone can see at least three, as many as six at any given time. "sectorized" antennas at the base stations. Instead of one antenna with omnidirectional coverage, the station may use as few as 3 or as many as 32 separate antennas, each covering a portion of the circular coverage; this allows the base station to use a directional antenna, pointing at the user, which improves the signal to noise ratio. If the user moves from one antenna sector to another, the base station automatically selects the proper antenna. Rapid handoff between base stations the radio link used by the phones is a digital link with extensive error correction and detection in the digital protocol sufficient operation of mobile phone in tunnels when supported by split cable antennas local repeaters inside complex vehicles or buildingsA Faraday cage is composed of a conductor that surrounds an area on all sides and bottom.
Electromagnetic radiation is blocked. For example, mobile telephone signals are blocked in windowless metal enclosures that approximate a Faraday cage, such as elevator cabins, parts of trains and ships; the same problem can affect signals in buildings with extensive steel reinforcement. The radio horizon is the locus of points at which direct rays from an antenna are tangential to the surface of the Earth. If the Earth were a perfect sphere without an atmosphere, the radio horizon would be a circle; the radio horizon of the transmitting and receiving antennas can be added together to increase the effective communication range. Radio wave propagation is affected by atmospheric conditions, ionospheric absorption, the presence of obstructions, for example mountains or trees. Simple formulas that include the effect of the atmosphere give the range as: h o r i z o n m i l e s ≈ 1.23 ⋅ h e i g h t
A height finder is a ground-based aircraft altitude measuring device. Early height finders were optical range finder devices combined with simple mechanical computers, while systems migrated to radar devices; the unique vertical oscillating motion of height finder radars led to them being known as nodding radar. Devices combining both optics and radar were deployed by the U. S. Military. In World War II, a height finder was an optical rangefinder used to determine the altitude of an aircraft, used to direct anti-aircraft guns. Examples of American and Japanese versions exist. In the Soviet Union it was combined with optical rangefinders. A height finder radar is a type of 2-dimensional radar; the operator slews the antenna toward a desired bearing, identifies a target echo at a desired range on the RHI display bisects the target with a cursor, scaled to indicate the approximate altitude of the target. Such systems complement 2-dimensional radars which find distance and direction. Height finding radars of the 1960s and 70s were distinguished by their antenna being tall, but narrow.
As beam shape is a function of antenna shape, the height finder beam was flat and wide horizontally, but thin vertically, allowing accurate measurement of elevation angle, thus altitude. Modern 3D radar sets find both azimuth and elevation, making separate height finder radars obsolete. Radar Air Ministry Experimental Station A page about a type of height finders found in surface-to-air missile sites
General Services Administration
The General Services Administration, an independent agency of the United States government, was established in 1949 to help manage and support the basic functioning of federal agencies. GSA supplies products and communications for U. S. government offices, provides transportation and office space to federal employees, develops government-wide cost-minimizing policies and other management tasks. GSA employs about 12,000 federal workers and has an annual operating budget of $20.9 billion. GSA oversees $66 billion of procurement annually, it contributes to the management of about $500 billion in U. S. federal property, divided chiefly among 8,700 owned and leased buildings and a 215,000 vehicle motor pool. Among the real estate assets managed by GSA are the Ronald Reagan Building and International Trade Center in Washington, D. C. – the largest U. S. federal building after the Pentagon – and the Hart-Dole-Inouye Federal Center. GSA's business lines include the Federal Acquisition Service and the Public Buildings Service, as well as several Staff Offices including the Office of Government-wide Policy, the Office of Small Business Utilization, the Office of Mission Assurance.
As part of FAS, GSA's Technology Transformation Services helps federal agencies improve delivery of information and services to the public. Key initiatives include FedRAMP, Cloud.gov, the USAGov platform, Data.gov, Performance.gov, Challenge.gov. GSA is a member of the Procurement G6, an informal group leading the use of framework agreements and e-procurement instruments in public procurement. In 1947 President Harry Truman asked former President Herbert Hoover to lead what became known as the Hoover Commission to make recommendations to reorganize the operations of the federal government. One of the recommendations of the commission was the establishment of an "Office of the General Services." This proposed office would combine the responsibilities of the following organizations: U. S. Treasury Department's Bureau of Federal Supply U. S. Treasury Department's Office of Contract Settlement National Archives Establishment All functions of the Federal Works Agency, including the Public Buildings Administration and the Public Roads Administration War Assets AdministrationGSA became an independent agency on July 1, 1949, after the passage of the Federal Property and Administrative Services Act.
General Jess Larson, Administrator of the War Assets Administration, was named GSA's first Administrator. The first job awaiting Administrator Larson and the newly formed GSA was a complete renovation of the White House; the structure had fallen into such a state of disrepair by 1949 that one inspector of the time said the historic structure was standing "purely from habit." Larson explained the nature of the total renovation in depth by saying, "In order to make the White House structurally sound, it was necessary to dismantle, I mean dismantle, everything from the White House except the four walls, which were constructed of stone. Everything, except the four walls without a roof, was stripped down, that's where the work started." GSA worked with President Truman and First Lady Bess Truman to ensure that the new agency's first major project would be a success. GSA completed the renovation in 1952. In 1986 GSA headquarters, U. S. General Services Administration Building, located at Eighteenth and F Streets, NW, was listed on the National Register of Historic Places, at the time serving as Interior Department offices.
In 1960 GSA created the Federal Telecommunications System, a government-wide intercity telephone system. In 1962 the Ad Hoc Committee on Federal Office Space created a new building program to address obsolete office buildings in Washington, D. C. resulting in the construction of many of the offices that now line Independence Avenue. In 1970 the Nixon administration created the Consumer Product Information Coordinating Center, now part of USAGov. In 1974 the Federal Buildings Fund was initiated, allowing GSA to issue rent bills to federal agencies. In 1972 GSA established the Automated Data and Telecommunications Service, which became the Office of Information Resources Management. In 1973 GSA created the Office of Federal Management Policy. GSA's Office of Acquisition Policy centralized procurement policy in 1978. GSA was responsible for emergency preparedness and stockpiling strategic materials to be used in wartime until these functions were transferred to the newly-created Federal Emergency Management Agency in 1979.
In 1984 GSA introduced the federal government to the use of charge cards, known as the GMA SmartPay system. The National Archives and Records Administration was spun off into an independent agency in 1985; the same year, GSA began to provide governmentwide policy oversight and guidance for federal real property management as a result of an Executive Order signed by President Ronald Reagan. In 2003 the Federal Protective Service was moved to the Department of Homeland Security. In 2005 GSA reorganized to merge the Federal Supply Service and Federal Technology Service business lines into the Federal Acquisition Service. On April 3, 2009, President Barack Obama nominated Martha N. Johnson to serve as GSA Administrator. After a nine-month delay, the United States Senate confirmed her nomination on February 4, 2010. On April 2, 2012, Johnson resigned in the wake of a management-deficiency report that detailed improper payments for a 2010 "Western Regions" training conference put on by the Public Buildings Service in Las Vegas.
In July 1991 GSA contractors began the excavation of what is now the Ted Weiss Federal Building in New York City. The planning for that buildin
In navigation, the course of a vessel or aircraft is the cardinal direction in which the craft is to be steered. The course is to be distinguished from the heading, the compass direction in which the craft's bow or nose is pointed; the path that a vessel follows over the ground is called a ground track, course made good or course over the ground. For an aircraft it is its track; the intended track is a route. For ships and aircraft, routes are straight-line segments between waypoints. A navigator determines the bearing of the next waypoint; because water currents or wind can cause a craft to drift off course, a navigator sets a course to steer that compensates for drift. The helmsman or pilot points the craft on a heading. If the predicted drift is correct the craft's track will correspond to the planned course to the next waypoint. Course directions are specified in degrees from north, either magnetic. In aviation, north is expressed as 360°. Navigators used ordinal directions, instead of compass degrees, e.g. "northeast" instead of 45° until the mid-20th century when the use of degrees became prevalent.
Acronyms and abbreviations in avionics Bearing Breton plotter E6B Ground track Navigation Navigation room Rhumb line Pilot's Handbook of Aeronautical Knowledge glossary
United States Department of Defense
The Department of Defense is an executive branch department of the federal government charged with coordinating and supervising all agencies and functions of the government concerned directly with national security and the United States Armed Forces. The department is the largest employer in the world, with nearly 1.3 million active duty servicemen and women as of 2016. Adding to its employees are over 826,000 National Guardsmen and Reservists from the four services, over 732,000 civilians bringing the total to over 2.8 million employees. Headquartered at the Pentagon in Arlington, just outside Washington, D. C. the DoD's stated mission is to provide "the military forces needed to deter war and ensure our nation's security". The Department of Defense is headed by the Secretary of Defense, a cabinet-level head who reports directly to the President of the United States. Beneath the Department of Defense are three subordinate military departments: the United States Department of the Army, the United States Department of the Navy, the United States Department of the Air Force.
In addition, four national intelligence services are subordinate to the Department of Defense: the Defense Intelligence Agency, the National Security Agency, the National Geospatial-Intelligence Agency, the National Reconnaissance Office. Other Defense Agencies include the Defense Advanced Research Projects Agency, the Defense Logistics Agency, the Missile Defense Agency, the Defense Health Agency, Defense Threat Reduction Agency, the Defense Security Service, the Pentagon Force Protection Agency, all of which are under the command of the Secretary of Defense. Additionally, the Defense Contract Management Agency provides acquisition insight that matters, by delivering actionable acquisition intelligence from factory floor to the warfighter. Military operations are managed by ten functional Unified combatant commands; the Department of Defense operates several joint services schools, including the Eisenhower School and the National War College. The history of the defense of the United States started with the Continental Congress in 1775.
The creation of the United States Army was enacted on 14 June 1775. This coincides with the American holiday Flag Day; the Second Continental Congress would charter the United States Navy, on 13 October 1775, create the United States Marine Corps on 10 November 1775. The Preamble of the United States Constitution gave the authority to the federal government to defend its citizens: We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America. Upon the seating of the first Congress on 4 March 1789, legislation to create a military defense force stagnated as they focused on other concerns relevant to setting up the new government. President George Washington went to Congress to remind them of their duty to establish a military twice during this time.
On the last day of the session, 29 September 1789, Congress created the War Department, historic forerunner of the Department of Defense. The War Department handled naval affairs until Congress created the Navy Department in 1798; the secretaries of each of these departments reported directly to the president as cabinet-level advisors until 1949, when all military departments became subordinate to the Secretary of Defense. After the end of World War II, President Harry Truman proposed creation of a unified department of national defense. In a special message to Congress on 19 December 1945, the President cited both wasteful military spending and inter-departmental conflicts. Deliberations in Congress went on for months focusing on the role of the military in society and the threat of granting too much military power to the executive. On 26 July 1947, Truman signed the National Security Act of 1947, which set up a unified military command known as the "National Military Establishment", as well as creating the Central Intelligence Agency, the National Security Council, National Security Resources Board, United States Air Force and the Joint Chiefs of Staff.
The act placed the National Military Establishment under the control of a single Secretary of Defense. The National Military Establishment formally began operations on 18 September, the day after the Senate confirmed James V. Forrestal as the first Secretary of Defense; the National Military Establishment was renamed the "Department of Defense" on 10 August 1949 and absorbed the three cabinet-level military departments, in an amendment to the original 1947 law. Under the Department of Defense Reorganization Act of 1958, channels of authority within the department were streamlined, while still maintaining the ordinary authority of the Military Departments to organize and equip their associated forces; the Act clarified the overall decision-making authority of the Secretary of Defense with respect to these subordinate Military Departments and more defined the operational chain of command over U. S. military forces as running from the president to the Secretary of Defense and to the unified combatant commanders.
Provided in this legislation was a centralized research authority, the Advanced Research Projects Agency known as DARPA. The act was written and promoted by the Eisenhower administration, was signed into law 6 August 1958; the Secretary of Defense, appointed by the president with the advice and consent of the Senate, is by federal law (1
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