In geodesy, a reference ellipsoid is a mathematically defined surface that approximates the geoid, the truer figure of the Earth, or other planetary body. Current practice uses the word alone in preference to the full term oblate ellipsoid of revolution or the older term oblate spheroid. In the rare instances where a more general shape is required as a model the term used is triaxial ellipsoid. A great many ellipsoids have been used with various sizes and centres, the shape of an ellipsoid is determined by the shape parameters of that ellipse which generates the ellipsoid when it is rotated about its minor axis. The semi-major axis of the ellipse, a, is identified as the radius of the ellipsoid. For the Earth, f is around 1/300 corresponding to a difference of the major and minor semi-axes of approximately 21 km, some precise values are given in the table below and in Figure of the Earth. A great many other parameters are used in geodesy but they can all be related to one or two of the set a, b and f, a primary use of reference ellipsoids is to serve as a basis for a coordinate system of latitude and elevation.
For this purpose it is necessary to identify a zero meridian, for other bodies a fixed surface feature is usually referenced, which for Mars is the meridian passing through the crater Airy-0. It is possible for many different coordinate systems to be defined upon the same reference ellipsoid, the longitude measures the rotational angle between the zero meridian and the measured point. By convention for the Earth and Sun it is expressed in degrees ranging from −180° to +180° For other bodies a range of 0° to 360° is used. The latitude measures how close to the poles or equator a point is along a meridian, and is represented as an angle from −90° to +90°, the common or geodetic latitude is the angle between the equatorial plane and a line that is normal to the reference ellipsoid. Depending on the flattening, it may be different from the geocentric latitude. For non-Earth bodies the terms planetographic and planetocentric are used instead, see geodetic system for more detail. If these coordinates, i. e. N is the radius of curvature in the prime vertical, in contrast, extracting φ, λ and h from the rectangular coordinates usually requires iteration. A straightforward method is given in an OSGB publication and in web notes, more sophisticated methods are outlined in geodetic system.
Currently the most common reference used, and that used in the context of the Global Positioning System, is the one defined by WGS84. Traditional reference ellipsoids or geodetic datums are defined regionally and therefore non-geocentric, modern geodetic datums are established with the aid of GPS and will therefore be geocentric, e. g. WGS84. Reference ellipsoids are useful for mapping of other planetary bodies including planets, their satellites, asteroids
Ordnance Survey is a non-ministerial government department which acts as the national mapping agency for Great Britain and is one of the worlds largest producers of maps. Since 1 April 2015 it has operated as Ordnance Survey Ltd, the Ordnance Survey Board remain accountable to the Secretary of State for Business and Industrial Strategy. It is a member of the Public Data Group, the agencys name indicates its original military purpose, mapping Scotland in the wake of the Jacobite rebellion in 1745. There was a general and nationwide need in light of the potential threat of invasion during the Napoleonic Wars. Ordnance Survey mapping is usually classified as either large-scale or small-scale, the Surveys large-scale mapping comprises maps at six inches to the mile or more and was available as sheets until the 1980s, when it was digitised. Small-scale mapping comprises maps at less than six inches to the mile, such as the one inch to the mile leisure maps. These are still available in sheet form.
Ordnance Survey maps remain in copyright for fifty years after their publication, some of the Copyright Libraries hold complete or near-complete collections of pre-digital OS mapping. The origins of the Ordnance Survey lie in the aftermath of the last Jacobite rising which was defeated by forces loyal to the government at the Battle of Culloden in 1746. In 1747, Lieutenant-colonel David Watson proposed the compilation of a map of the Highlands to facilitate the subjugation of clans, in response, King George II charged Watson with making a military survey of the Highlands under the command of the Duke of Cumberland. Among Watsons assistants were William Roy, Paul Sandby and John Manson, the survey was produced at a scale of 1 inch to 1000 yards and included the Duke of Cumberlands Map now held in the British Library. This work was the point of the Principal Triangulation of Great Britain. Roys technical skills and leadership set the standard for which Ordnance Survey became known. Work was begun in earnest in 1790 under Roys supervision, when the Board of Ordnance began a military survey starting with the south coast of England.
A set of stamps, featuring maps of the Kentish village of Hamstreet, was issued in 1991 to mark the bicentenary. In 1801, the first one-inch-to-the-mile map was published, detailing the county of Kent, during the next twenty years, roughly a third of England and Wales was mapped at the same scale under the direction of William Mudge, as other military matters took precedence. It took until 1823 to re-establish a relationship with the French survey made by Roy in 1787, by 1810, one inch to the mile maps of most of the south of England were completed, but were withdrawn from sale between 1811 and 1816 because of security fears. It was gruelling work, major Thomas Colby, the longest serving director general of Ordnance Survey, in 1824, Colby and most of his staff moved to Ireland to work on a six-inches-to-the-mile valuation survey
North American Datum
The North American Datum is the datum now used to define the geodetic network in North America. A datum is a description of the shape of the Earth along with an anchor point for the coordinate system. In surveying and land-use planning, two North American Datums are in use, the North American Datum of 1927 and the North American Datum of 1983, both are geodetic reference systems based on slightly different assumptions and measurements. In 1901 the United States Coast and Geodetic Survey adopted a national horizontal datum called the United States Standard Datum and it was fitted to data previously collected for regional datums, which by that time had begun to overlap. In 1913, Canada and Mexico adopted that datum, so it was renamed the North American Datum. As more data were gathered, discrepancies appeared, so the datum was recomputed in 1927, using the same spheroid, the datum declares the Meades Ranch Triangulation Station to be 39°13′26. 686″ north latitude, 98°32′30. 506″ west longitude.
NAD27 is oriented by declaring the azimuth from Meades Ranch to Waldo to be 255°28′14. 52″ from north. These are the dimensions for NAD27, but Clarke actually defined his 1866 spheroid as a =20,926,062 British feet. The conversion to meters uses Clarkes 1865 inch-meter ratio of 39.370432, because Earth deviates significantly from a perfect ellipsoid, the ellipsoid that best approximates its shape varies region by region across the world. Clarke 1866, and North American Datum of 1927 with it, were surveyed to best suit North America as a whole, historically, most regions of the world used ellipsoids measured locally to best suit the vagaries of Earths shape in their respective locales. While ensuring the most accuracy locally, this practice makes integrating and disseminating information across regions troublesome and this is because satellites naturally deal with Earth as a monolithic body. Therefore, the GRS80 ellipsoid was developed for best approximating the Earth as a whole, the North American Datum of 1983 is based on a newer defined spheroid, it is an Earth-centered datum having no initial point or initial direction. NOAA provides a converter between the two systems, the initial definition of NAD83 was intended to match GRS80 and WGS84, and many older publications indicate no difference.
Subsequent more accurate measurements found a difference typically on the order of a meter over much of the United States, each datum has undergone refinements with more accurate and measurements. Thus there is a change over time as to the difference between the systems, for much of the United States the relative rate is on the order of 1 to 2 cm per year. 1994 SBE National Convention and World Media Expo
Quasi-Zenith Satellite System
The first satellite Michibiki was launched on 11 September 2010. Full operational status was expected by 2013, in March 2013, Japans Cabinet Office announced the expansion of the Quasi-Zenith Satellite System from three satellites to four. The $526 million contract with Mitsubishi Electric for the construction of three satellites is slated for launch before the end of 2017, the basic four-satellite system is planned to be operational in 2018. The work was taken over by the Satellite Positioning Research and Application Center, QZSS is targeted at mobile applications, to provide communications-based services and positioning information. With regards to its service, QZSS can only provide limited accuracy on its own and is not currently required in its specifications to work in a stand-alone mode. As such, it is viewed as a GNSS Augmentation service. S, federal Aviation Administrations Wide Area Augmentation System. QZSS uses three satellites, each 120° apart, in highly inclined, slightly elliptical, geosynchronous orbits, because of this inclination, they are not geostationary, they do not remain in the same place in the sky.
Instead, their ground traces are asymmetrical figure-8 patterns, designed to ensure one is almost directly overhead over Japan at all times. The nominal orbital elements are, The primary purpose of QZSS is to increase the availability of GPS in Japans numerous urban canyons, a secondary function is performance enhancement, increasing the accuracy and reliability of GPS derived navigation solutions. The Quasi-Zenith Satellites transmit signals compatible with the GPS L1C/A signal, as well as the modernized GPS L1C, L2C signal and this minimizes changes to existing GPS receivers. It improves reliability by means of monitoring and system health data notifications. QZSS provides other support data to users to improve GPS satellite acquisition, according to its original plan, QZSS was to carry two types of space-borne atomic clocks, a hydrogen maser and a rubidium atomic clock. The development of a hydrogen maser for QZSS was abandoned in 2006. The positioning signal will be generated by a Rb clock and a similar to the GPS timekeeping system will be employed.
Although the ﬁrst generation QZSS timekeeping system will be based on the Rb clock, during the ﬁrst half of the two year in-orbit test phase, preliminary tests will investigate the feasibility of the atomic clock-less technology which might be employed in the second generation QZSS. This allows the system to operate optimally when satellites are in contact with the ground station. Low satellite mass and low satellite manufacturing and launch cost are significant advantages of this system
BeiDou Navigation Satellite System
The BeiDou Navigation Satellite System is a Chinese satellite navigation system. It consists of two separate satellite constellations – a limited test system that has been operating since 2000, the first BeiDou system, officially called the BeiDou Satellite Navigation Experimental System and known as BeiDou-1, consists of three satellites and offers limited coverage and applications. It has been offering services, mainly for customers in China and neighboring regions. It became operational in China in December 2011, with 10 satellites in use and it is planned to begin serving global customers upon its completion in 2020. In-mid 2015, China started the build-up of the third generation BeiDou system in the global coverage constellation, the first BDS-3 satellite was launched 30 September 2015. As of March 2016,4 BDS-3 in-orbit validation satellites have been launched, the official English name of the system is BeiDou Navigation Satellite System. It is named after the Big Dipper constellation, which is known in Chinese as Běidǒu, the name literally means Northern Dipper, the name given by ancient Chinese astronomers to the seven brightest stars of the Ursa Major constellation.
Historically, this set of stars was used in navigation to locate the North Star Polaris, as such, the name BeiDou serves as a metaphor for the purpose of the satellite navigation system. The original idea of a Chinese satellite navigation system was conceived by Chen Fangyun, the third satellite, BeiDou-1C, was put into orbit on 25 May 2003. The successful launch of BeiDou-1C meant the establishment of the BeiDou-1 navigation system. On 2 November 2006, China announced that from 2008 BeiDou would offer a service with an accuracy of 10 meters, timing of 0.2 microseconds. In February 2007, the fourth and last satellite of the BeiDou-1 system and it was reported that the satellite had suffered from a control system malfunction but was fully restored. In April 2007, the first satellite of BeiDou-2, namely Compass-M1 was successfully put into its working orbit, the second BeiDou-2 constellation satellite Compass-G2 was launched on 15 April 2009. On 2 June 2010, the satellite was launched successfully into orbit.
The fifth orbiter was launched into space from Xichang Satellite Launch Center by an LM-3I carrier rocket on 1 August 2010, three months later, on 1 November 2010, the sixth satellite was sent into orbit by LM-3C. Another satellite, the Beidou-2/Compass IGSO-5 satellite, was launched from the Xichang Satellite Launch Center by a Long March-3A on 1 December 2011. In September 2003, China intended to join the European Galileo positioning system project and was to invest €230 million in Galileo over the few years. At the time, it was believed that Chinas BeiDou navigation system would only be used by its armed forces
Ordnance Survey Ireland
Ordnance Survey Ireland is the national mapping agency of Ireland. It was established in 2002 as a body corporate and it is the successor to the former Ordnance Survey of Ireland. It and the Ordnance Survey of Northern Ireland are the successors to the Irish operations of the British Ordnance Survey. OSI is part of the Irish public service, OSI has made modern and historic maps of the state free to view on its website. OSI is headquartered at Mountjoy House in the Phoenix Park in Dublin, Mountjoy House was the headquarters, until 1922, of the Irish section of the British Ordnance Survey. Under the Ordnance Survey Ireland Act 2001, the Ordnance Survey of Ireland was dissolved and it employs 235 staff in the Phoenix Park and in six regional offices in Cork, Kilkenny, Longford and Tuam. OSI had sales of €13.3 million in 2012, the board publishes a series of 1,50000 maps of the entire island known as the Discovery Series and a series of 1,25000 maps of places of interest and the Geology of Ireland.
Thomas Colby, the long-serving Director-General of the Ordnance Survey in Great Britain, was the first to suggest that the Ordnance Survey be used to map Ireland. In 1824, a committee was established under the direction of Thomas Spring Rice, MP for Limerick, to oversee the foundation of an Irish Ordnance Survey. Spring Rice believed in the importance of Irish involvement in the process, but was overruled by the Duke of Wellington. Instead, the Irish Ordnance Survey was initially staffed entirely by members of the British Army, the resulting maps portrayed the country in a degree of detail never attempted before, and when the survey of the whole country was completed in 1846, it was a world first. The concrete triangulation posts built on the summits of many Irish mountains can still be seen to this day, the Royal Engineer officers in charge of the operation were Thomas Colby and Lieutenant Thomas Larcom. They were assisted by George Petrie, who headed the Surveys Topographical Department which employed the likes of John ODonovan, captain J. E.
Portlock compiled extensive information on agricultural produce and natural history, particularly geology. The total cost of the Irish Survey was £860,000, the original survey was revisited and revised maps issued on a number of occasions. All of these maps are in the public domain and while the originals can be hard to find. The British Ordnance Survey ceased to map Ireland just before the creation of the Irish Free State in 1922. The new Ordnance Survey of Northern Ireland officially came into existence on 1 January 1922, while the new Ordnance Survey of Ireland came into being slightly later, the OSI was initially part of the Irish Army under the Department of Defence. All staff employed were military personnel until the 1970s, when the first civilian employees were recruited, in more recent times, the Ordnance Survey of Ireland replaced traditional ground surveying with mapping based primarily on aerial photography
Geodesists study geodynamical phenomena such as crustal motion and polar motion. For this they design global and national networks, using space and terrestrial techniques while relying on datums. Geodesy — from the Ancient Greek word γεωδαισία geodaisia — is primarily concerned with positioning within the temporally varying gravity field, such geodetic operations are applied to other astronomical bodies in the solar system. It is the science of measuring and understanding the earths geometric shape, orientation in space and this applies to the solid surface, the liquid surface and the Earths atmosphere. For this reason, the study of the Earths gravity field is called physical geodesy by some, the geoid is essentially the figure of the Earth abstracted from its topographical features. It is an idealized surface of sea water, the mean sea level surface in the absence of currents, air pressure variations etc. The geoid, unlike the ellipsoid, is irregular and too complicated to serve as the computational surface on which to solve geometrical problems like point positioning.
The geometrical separation between the geoid and the ellipsoid is called the geoidal undulation. It varies globally between ±110 m, when referred to the GRS80 ellipsoid, a reference ellipsoid, customarily chosen to be the same size as the geoid, is described by its semi-major axis a and flattening f. The quantity f = a − b/a, where b is the axis, is a purely geometrical one. The mechanical ellipticity of the Earth can be determined to high precision by observation of satellite orbit perturbations and its relationship with the geometrical flattening is indirect. The relationship depends on the density distribution, or, in simplest terms. The 1980 Geodetic Reference System posited a 6,378,137 m semi-major axis and this system was adopted at the XVII General Assembly of the International Union of Geodesy and Geophysics. It is essentially the basis for geodetic positioning by the Global Positioning System and is in widespread use outside the geodetic community. The locations of points in space are most conveniently described by three cartesian or rectangular coordinates, X, Y and Z.
Since the advent of satellite positioning, such systems are typically geocentric. The X-axis lies within the Greenwich observatorys meridian plane, the coordinate transformation between these two systems is described to good approximation by sidereal time, which takes into account variations in the Earths axial rotation. A more accurate description takes polar motion into account, a closely monitored by geodesists
It belongs to the broader field of space geodesy. Traditional astronomical geodesy is not commonly considered a part of satellite geodesy, satellite geodesy relies heavily on orbital mechanics. Satellite geodesy began shortly after the launch of Sputnik in 1957, observations of Explorer 1 and Sputnik 2 in 1958 allowed for an accurate determination of Earths flattening. The 1960s saw the launch of the Doppler satellite Transit-1B and the balloon satellites Echo 1, Echo 2, the first dedicated geodetic satellite was ANNA-1B, a collaborative effort between NASA, the DoD, and other civilian agencies. ANNA-1B carried the first of the US Armys SECOR instruments and these missions led to the accurate determination of the leading spherical harmonic coefficients of the geopotential, the general shape of the geoid, and linked the worlds geodetic datums. Soviet military satellites undertook geodesic missions to assist in ICBM targeting in the late 1960s, the Transit satellite system was used extensively for Doppler surveying and positioning.
Observations of satellites in the 1970s by worldwide triangulation networks allowed for the establishment of the World Geodetic System, the development of GPS by the United States in the 1980s allowed for precise navigation and positioning and soon became a standard tool in surveying. In the 1980s and 1990s satellite geodesy began to be used for monitoring of geodynamic phenomena, such as motion, Earth rotation. The 1990s were focused on the development of permanent geodetic networks, dedicated satellites were launched to measure Earths gravity field in the 2000s, such as CHAMP, GRACE, and GOCE. Global navigation satellite systems are dedicated radio positioning services, which can locate a receiver to within a few meters, the most prominent system, GPS, consists of a constellation of 31 satellites in high, 12-hour circular orbits, distributed in six planes with 55° inclinations. The principle of location is based on trilateration, each satellite transmits a precise ephemeris with information on its own position and a message containing the exact time of transmission.
The receiver compares this time of transmission with its own clock at the time of reception, four pseudoranges are needed to obtain the precise time and the receivers position within a few meters. More sophisticated methods, such as real-time kinematic can yield positions to within a few millimeters, in geodesy, GNSS is used as an economical tool for surveying and time transfer. It is used for monitoring Earths rotation, polar motion, the presence of the GPS signal in space makes it suitable for orbit determination and satellite-to-satellite tracking. Satellite laser ranging is a proven geodetic technique with significant potential for important contributions to studies of the Earth/Atmosphere/Oceans system. Example, LAGEOS Doppler positioning involves recording the Doppler shift of a signal of stable frequency emitted from a satellite as the satellite approaches and recedes from the observer. The observed frequency depends on the velocity of the satellite relative to the observer. If the observer knows the orbit of the satellite, the recording the Doppler profile determines the observers position, conversely, if the observers position is precisely known, the orbit of the satellite can be determined and used to study the Earths gravity
A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows small electronic receivers to determine their location to high precision using time signals transmitted along a line of sight by radio from satellites, the system can be used for providing position, navigation or for tracking the position of something fitted with a receiver. The signals allow the receiver to calculate the current local time to high precision. Satnav systems operate independently of any telephonic or internet reception, though these technologies can enhance the usefulness of the information generated. A satellite navigation system with global coverage may be termed a global satellite system. As of December 2016 only the United States NAVSTAR Global Positioning System, the Russian GLONASS, the European Unions Galileo GNSS is scheduled to be fully operational by 2020. China is in the process of expanding its regional BeiDou Navigation Satellite System into the global BeiDou-2 GNSS by 2020, India currently has satellite-based augmentation system, GPS Aided GEO Augmented Navigation, which enhances the accuracy of NAVSTAR GPS and GLONASS positions.
India has already launched the IRNSS, with an operational name NAVIC and it is expected to be fully operational by June 2016. France and Japan are in the process of developing regional navigation systems as well, Global coverage for each system is generally achieved by a satellite constellation of 18–30 medium Earth orbit satellites spread between several orbital planes. The actual systems vary, but use orbital inclinations of >50°, Ground based augmentation is provided by systems like the Local Area Augmentation System. GNSS-2 is the generation of systems that independently provides a full civilian satellite navigation system. These systems will provide the accuracy and integrity monitoring necessary for civil navigation and this system consists of L1 and L2 frequencies for civil use and L5 for system integrity. Development is in progress to provide GPS with civil use L2 and L5 frequencies, making it a GNSS-2 system. ¹ Core Satellite navigation systems, currently GPS, GLONASS, Global Satellite Based Augmentation Systems such as Omnistar and StarFire.
Regional SBAS including WAAS, EGNOS, MSAS and GAGAN, Regional Satellite Navigation Systems such as Chinas Beidou, Indias NAVIC, and Japans proposed QZSS. Continental scale Ground Based Augmentation Systems for example the Australian GRAS, Regional scale GBAS such as CORS networks. Local GBAS typified by a single GPS reference station operating Real Time Kinematic corrections, early predecessors were the ground based DECCA, LORAN, GEE and Omega radio navigation systems, which used terrestrial longwave radio transmitters instead of satellites. These positioning systems broadcast a radio pulse from a known master location, the delay between the reception of the master signal and the slave signals allowed the receiver to deduce the distance to each of the slaves, providing a fix. The first satellite system was Transit, a system deployed by the US military in the 1960s
Earth radius is the distance from the Earths center to its surface, about 6,371 km. This length is used as a unit of distance, especially in astronomy and geology. This article deals primarily with spherical and ellipsoidal models of the Earth, see Figure of the Earth for a more complete discussion of the models. The Earth is only approximately spherical, so no single value serves as its natural radius, distances from points on the surface to the center range from 6,353 km to 6,384 km. Several different ways of modeling the Earth as a sphere each yield a mean radius of 6,371 km. It can mean some kind of average of such distances, writing in On the Heavens around 350 BC, reports that the mathematicians guess the circumference of the Earth to be 400,000 stadia. Due to uncertainty about which stadion variant Aristotle meant, scholars have interpreted Aristotles figure to be anywhere from highly accurate to almost double the true value, the first known scientific measurement and calculation of the radius of the Earth was performed by Eratosthenes about 240 BC.
Estimates of the accuracy of Eratosthenes’s measurement range from within 0. 5% to within 17%, as with Aristotles report, uncertainty in the accuracy of his measurement is due to modern uncertainty over which stadion definition he used. Earths rotation, internal density variations, and external tidal forces cause its shape to deviate systematically from a perfect sphere, local topography increases the variance, resulting in a surface of profound complexity. Our descriptions of the Earths surface must be simpler than reality in order to be tractable, hence, we create models to approximate characteristics of the Earths surface, generally relying on the simplest model that suits the need. Each of the models in use involve some notion of the geometric radius. Strictly speaking, spheres are the solids to have radii. In the case of the geoid and ellipsoids, the distance from any point on the model to the specified center is called a radius of the Earth or the radius of the Earth at that point. It is common to refer to any mean radius of a model as the radius of the earth.
When considering the Earths real surface, on the hand, it is uncommon to refer to a radius. Rather, elevation above or below sea level is useful, regardless of the model, any radius falls between the polar minimum of about 6,357 km and the equatorial maximum of about 6,378 km. Hence, the Earth deviates from a sphere by only a third of a percent. While specific values differ, the concepts in this article generalize to any major planet
Global Positioning System
The Global Positioning System is a space-based radionavigation system owned by the United States government and operated by the United States Air Force. The GPS system operates independently of any telephonic or internet reception, the GPS system provides critical positioning capabilities to military and commercial users around the world. The United States government created the system, maintains it, the US government can selectively deny access to the system, as happened to the Indian military in 1999 during the Kargil War. The U. S. Department of Defense developed the system and it became fully operational in 1995. Roger L. Easton of the Naval Research Laboratory, Ivan A, getting of The Aerospace Corporation, and Bradford Parkinson of the Applied Physics Laboratory are credited with inventing it. 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. In addition to GPS, other systems are in use or under development, mainly because of a denial of access.
The Russian Global Navigation Satellite System was developed contemporaneously with GPS, GLONASS can be added to GPS devices, making more satellites available and enabling positions to be fixed more quickly and accurately, to within two meters. There are the European Union Galileo positioning system and Chinas BeiDou Navigation Satellite System and general relativity predict that the clocks on the GPS satellites would be seen by the Earths observers to run 38 microseconds faster per day than the clocks on the Earth. The GPS calculated positions would quickly drift into error, accumulating to 10 kilometers per day, the relativistic time effect of the GPS clocks running faster than the clocks on earth was corrected for in the design of GPS. The Soviet Union launched the first man-made satellite, Sputnik 1, two American physicists, William Guier and George Weiffenbach, at Johns Hopkinss Applied Physics Laboratory, decided to monitor Sputniks radio transmissions. Within hours they realized that, because of the Doppler effect, the Director of the APL gave them access to their UNIVAC to do the heavy calculations required.
The next spring, Frank McClure, the deputy director of the APL, asked Guier and Weiffenbach to investigate the inverse problem — pinpointing the users location and this led them and APL to develop the TRANSIT system. In 1959, ARPA played a role in TRANSIT, the first satellite navigation system, TRANSIT, used by the United States Navy, was first successfully tested in 1960. It used a constellation of five satellites and could provide a navigational fix approximately 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 by GPS. In the 1970s, the ground-based OMEGA navigation system, based on comparison of signal transmission from pairs of stations. Limitations of these systems drove the need for a more universal navigation solution with greater accuracy, during the Cold War arms race, the nuclear threat to the existence of the United States was the one need that did justify this cost in the view of the United States Congress.
This deterrent effect is why GPS was funded and it is the reason for the ultra secrecy at that time