Fountain

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. This is gin." And it was gin. China and France are also developing other satellite navigation systems. On a Tiffany card in a corner was the note: "This is not New York's precious water. The European Union is developing Galileo as an alternative to GPS, planned to be in operation by 2010. During New York City's first drought emergency in modern times, Gene Moore, window designer for Tiffany's, created sparkling fountains hung with diamonds. There are plans to restore GLONASS to full operation by 2008.

(Today it is enclosed behind glass.). Russia operates an independent system called GLONASS (global navigation system), although with only twelve active satellites as of 2004, the system is of limited usefulness. For the Barcelona exhibition of 1937 Alexander Calder created in remembrance the miners who were killed at the mercury mines at Almadén a memorial fountain, which uses mercury instead of water. There were also incidents of unintentional jamming, traced back to malfunctioning TV antenna preamplifiers. If the water is unsafe, fences must be designed to keep people far enough away, so that they cannot touch the water, otherwise children get water on their hands, and put their fingers into their mouths, and end up getting sick, thus subjecting owners and operators to legal liability. According to the reference below, "IFR pilots should have a fallback plan in case of a GPS malfunction". In the United States fountain operators and owners are legally liable for failure to either fence-in fountains, or to properly filter, chlorinate or otherwise treat the water, if the fountains are not fenced in. And there has been at least one well-documented case of unintentional jamming; if similar, but stronger, signals were generated on purpose, they could interfere with aviation GPS receivers at a range of 50 km.

It was therefore necessary to put a fence around the fountain to keep people away. A detailed description of how to build a GPS jammer was posted on a hackers' site by an anonymous author. Children played in fountains and swallowed water, and spurted the water out of their mouths to mimic the way nozzles in the fountain spurted the water. Air Force conducted GPS jamming exercises in 2003. In July 1997, an outbreak of Cryptosporidiosis was connected to an ornamental fountain at the Minnesota Zoo, which did not have proper filtration and water treatment. The U.S. (Pool and Spa News Online). In either case, the jammers are attractive targets for anti-radiation missiles.

In Disneyland, for example, people have been reported to change their babies' diapers and then wash their hands in the water fountain (thus adding unexpected bacteria and organics into the water). Some officials believe that jammers could be used to attract the precision-guided munitions towards noncombatant infrastructure, other officials believe that the jammers are completely ineffective. In reality, however, people will interact with ornamental water fountains in the most surprising ways. invasion of Afghanistan. In theory, a free-standing water feature shouldn't have a bather load, and subsequently, many builders would not choose to install filters or sanitation devices. government believes that such jammers were used occasionally during the U.S. Guidelines have been developed for control of legionella in ornamental fountains (Legionella Risk Management-Guidelines). The U.S.

Therefore, minimum water quality standards are necessary, regardless of intended use. GPS jammers are available, from Russia, and are about the size of a cigarette box. Additionally, fountain spray can contain legionella bacteria and has been linked to legionnaires' disease outbreaks. A large part of modern munitions, the so-called "smart bombs" or precision-guided munitions, use GPS. Regardless of the fact that some fountains are designed and built not as bathing fountains, but are rather used simply as architectural decor, people will often drink from, bathe or wash their hands in any fountain. The increased accuracy comes mostly from being able to use both the L1 and L2 frequencies and thus better compensate for the varying signal delay in the ionosphere (see above). There is a need for good water quality in contemporary fountains, regardless of their avowed intended use. Military (and selected civilian) users still enjoy some technical advantages which can give quicker satellite lock and increased accuracy.

However, the fountain is central to the University community, and Fountain Day celebrates that centrality. Authorized military units are still able to decrypt the corrected signals using a tamper-resistant hardware module called an SAASM, Selective Availability / Anti-Spoofing Module. As a response the University moved the event to a weekend, incresed security, and limited admittance to the fountain area itself to only UAlbany students. The original SA system could only limit the accuracy of GPS signals world-wide, or not at all. In 2004 Fountain Day was a media nightmare with drunk students and multiple injuries. The US military maintains the ability to use a more advanced version of Selective Availability, called "Selective Deniability", to reduce the accuracy of civilian GPS units in a specific area without affecting the rest of the world. Fountain day is a school sponsored event, initiated by the Human Awareness Program (HAP), in 1979, so students and faculty could break down the barriers that separate them. [9].

This brings huge crowds celebrating together in the fountains, and creates something close in spirit to an urban beach. The military resisted for most of the 1990s, but SA was eventually turned off in 2000 following an announcement by then US President Bill Clinton, allowing all users to enjoy nearly the same level of access. University of Albany has a "Fountain Day" every year which is a day in which the university can come together to celebrate the coming of spring, and the near-end of the semester. This would save the FAA millions of dollars every year in maintenance of their own, less accurate, radio navigation systems. The pumps, filter, electrical switch box and plumbing controls are often housed in a "plant room". In the 1990s the FAA started pressuring the military to turn off SA permanently. Lighting is often submerged and must be suitably designed. During the Gulf War, the shortage of military GPS units and the wide availability of civilian ones among personnel resulted in disabling the Selective Availability.

Low voltage lighting, typically 12 volt Direct Current, is used to minimise electrical hazards. In order to improve the usefulness of GPS for civilian navigation, Differential GPS was used by many civilian GPS receivers to greatly improve accuracy. The water may need chlorination or anti-algal treatment; however, it may also utilise biological methods to filter and clean water. The inaccuracy of the civilian signal was deliberately encoded so as not to change very quickly, for instance the entire eastern US area might read 30 m off, but 30 m off everywhere and in the same direction. A water filter, typically a media filter, removes particles from the water -- this filter requires its own pump to force water through it and plumbing to remove the water from the pool to the filter and then back to the pool. SA typically added signal errors of up to about 10 m horizontally and 30 m vertically. "Static head" is useful to quantify this pressure, see Head_(hydraulic). Additional accuracy was available in the signal, but in an encrypted form that was only available to the United States military, its allies and a few others, mostly government users.

The pressure that causes water to move through the fountain may be produced instead by a motor-driven (often electric) pump. When it was first deployed, GPS included a feature called Selective Availability (or SA) that introduced intentional errors of up to a hundred meters into the publicly available navigation signals, making it difficult to use for guiding long range missiles to precise targets. Allowance must also be made to handle overflow in the case of heavy rain. The accuracy of GPS can be improved in a number of ways:. This closed, recirculating system must still be filled at the start from the local water supply system and also topped up through its life to offset the effects of evaporation. The citation accompanying the presentation of the trophy honors the GPS Team "for the most significant development for safe and efficient navigation and surveillance of air and spacecraft since the introduction of radio navigation 50 years ago.". In many circumstances fountains obtain their water from an internally recycling system. Air Force, the Aerospace Corporation, Rockwell International Corporation, and IBM Federal Systems Company.

In modern fountains the traditional gravitational pressure from an unseen reservoir at a higher level is not always practical. This team consists of researchers from the Naval Research Laboratory, the U.S. Changerooms are located in level P1 of the public parking. Collier Trophy, the most prestigious aviation award in the United States. The special texture on the slabs ensures that they are not slippery when wet. On February 10, 1993, the National Aeronautic Association selected the Global Positioning System Team as winners of the 1992 Robert J. The entire surface of Dundas Square is made of special nonslip square granite slabs that match the size of the metal grilles. Two GPS developers have received the National Academy of Engineering Charles Stark Draper prize year 2003:.

Both the architects and the designers have confirmed that these were intended for waterplay, and the facility operators have confirmed that the water is treated to pool water quality standards, and that the water quality is tested, by the health department, at least once a day. Bartolomé Coll has recently developed the basic notions necessary for a fully relativistic theory of Positioning Systems [7]. Twenty such grilles are arranged in two rows of 10, right in the middle of the main walkway through Dundas Square. Whether relativity must be considered as a mere correction to a Newtonian GPS theory, or, rather, as the necessary foundation of a cleaner (and more fundamental) GPS theory, is currently under debate. Each group of 30 nozzles is located beneath a stainless steel grille. Neil Ashby presented a good account of how these relativistic corrections are applied, why, and their orders of magnitude, in Physics Today (May 2002) [6]. It consists of 600 ground nozzles arranged in groups of 30 (3 rows of 10 nozzles). This offset is a practical demonstration of the theory of relativity in a real-world system; it is exactly that predicted by the theory, within the limits of accuracy of measurement.

A recent example of a public splash fountain, intended for waterplay, is the one located in Toronto's Dundas Square. This amounts to a discrepancy of around 38 microseconds per day, which is corrected by electronics on each satellite. These splash pads are often located in public pools, public parks, or public playgrounds (known as spraygrounds). The clocks on the satellites are also affected by both special and general relativity, which causes them to run at a slightly faster rate than do clocks on the Earth's surface. These have zero standing water, to eliminate possible drowning hazards, so that no lifeguards or supervision is required. Several frequencies make up the GPS electromagnetic spectrum:. More recently, splash fountains have begun to appear. Shorter multipath signals from ground reflections can often be very close to the direct signals, and can greatly reduce precision.

In other situations, fountains are designed to allow easy access, and feature nonslip surfaces, so that people can safely use them to cool off in on hot summer days. For shorter delay multipath signals that result from reflections from the ground, special antenna features may be used such as a ground plane, or a choke ring antenna. Some fountains are fenced in, or have raised edges as a barricade to keep people out. A variety of receiver techniques, most notably Narrow Correlator spacing, have been developed to mitigate multipath errors. Although many fountains were not designed as bathing fountains, children of all ages often use them for that purpose. For long delay multipath signals, the receiver itself can filter the signals out. A splash fountain or bathing fountain is a fountain intended for people to cool off in. GPS signals can also be affected by multipath reflections of the radio signals off the ground and/or surrounding structures (buildings, canyon walls, etc).

Modern indoor drinking fountains may incorporate filters to remove impurities from the water and chillers to reduce its temperature. Newer GPS receivers can compare the phase difference between the L1 and L2 frequencies to actually measure the atmospheric effects on the signals and apply precise corrections.^{[citation needed]}. A water fountain is designed to provide drinking water and has a basin arrangement with either continuously running water or a tap. Because ionospheric delay affects the speed of radio waves differently based on their frequencies, a second frequency band was added to help eliminate this type of error. In Islam a fountain is the name of the place in the Mosque where worshippers can wash before Prayer. Once the receiver's rough location is known, an internal mathematical model can be used to estimate and correct for the error. A musical fountain is a type of fountain that uses laser and harmonics principles to create three-dimensional images that look like holograms. The effect is minimized when the satellite is directly overhead and becomes greater toward the horizon, as the satellite signals must travel through the greater "thickness" of the ionosphere as the angle increases.

For example, the such fountains can spit up one ball of water which then explodes, showering people with a fine mist. One of biggest problems for GPS accuracy is that changing atmospheric conditions change the speed of the GPS signals unpredictably as they pass through the ionosphere. Animated fountains often use laminar jets that provide water that moves like ping pong balls in animation, so that it breaks up, as the height varies, and the behaviour of each jet operates independently with up to 5Hz modulation frequency 1/5sec, so that the water packets collide with themselves. The receiver is able to determine exactly when the signals were received by adjusting its internal clock (and therefore the spheres' radii) so that the spheres intersect near one point. The practical Romans marked the delivery end of aqueducts with a public fountain, a practice that was revived in Rome in the 15th century, when the restored Aqua Felice once more delivered a symbolic presentation of its waters to Rome in the original Trevi Fountain, since replaced by the familiar Baroque fusion of water, architecture and sculpture. Fortunately, even the relatively simple clock within the receiver provides an accurate comparison of the timing of the signals from the different satellites. From the Fountain of Youth one can drink to gain immortality, or to regain ones youth. One complication is that GPS receivers do not have atomic clocks, so the precise time is not known when the signals arrive.

An offshoot of the Fountain of Life was the legend of the Fountain of Youth, which Juan Ponce de León sought in Florida. In practice, GPS calculations are more complex for several reasons. The Fountain of Life appears in Christian illuminated manuscripts of Late Antiquity, and elaborate Gothic fountains formed centerpieces for exclosed gardens. Once the location and distance of each satellite is known, the receiver should theoretically be located at the intersection of four imaginary spheres, one around each satellite, with a radius equal to the time delay between the satellite and the receiver multiplied by the speed of the radio signals. Christian allegory made much use of the concept of the fountain, specifically the Fountain of Life, associated with the rebirth that was intended to be experienced at the Baptismal font. Each satellite uses a different sequence, which lets them share the same radio frequencies, using Code Division Multiple Access, while still allowing receivers to identify each satellite.
. In order to measure the delay, the satellite sends a repeating 1,023 bit long pseudo random sequence; the receiver knows the seed of the sequence, constructs an identical sequence and shifts it until the two sequences match.

Early Modern English employed fountain to refer to a natural spring water or source, which the 16th century garden fountain might consciously imitate in a grotto. The receiver calculates the orbit of each satellite based on information encoded in their radio signals, and measures the distance to each satellite, called a pseudorange, based on the time delay from when the satellite signals were sent until they were received. In the 16th century elaborate fountain displays were garden features of Mannerist gardens of Central Italy and the Mughal gardens of India. GPS receivers calculate their current position (latitude, longitude, elevation), and the precise time, using the process of trilateration after measuring the distance to at least four satellites by comparing the satellites' coded time signal transmissions. Other early fountains were geometrically regularized springs, developed in the classic Persian garden. Each satellite repeatedly broadcasts its own orbital elements, and a precise time-code. Hellenistic hydraulic engineers employed great originality in designing fountains, where the water pressure might be employed to animate automata and water organs. They regularly synchronize the atomic clocks onboard each satellite, and send updates to the satellites of their observed position in orbit.

Early fountains depended on the natural gravity flow of water, from a spring or from an aqueduct supplied from a distant and higher source of water, which provided hydraulic head. Ground-based observatories around the world monitor the flight paths of the GPS satellites. . [5]. A famous example of such a modern fountain rises from the surface of Lake Geneva (below, left). The orbits are designed so at least four satellites are always within line of sight from almost any place on earth. One of the most common features of a fountain, if there is enough pressure, is a jet or multiple jets, where water is forced into the air under pressure to some height. Each satellite circles the Earth twice each day at an altitude of 20,200 kilometres (12,600 miles).

Many fountains are located in small, artificial ornamental ponds, basins and formal garden pools, and often they include sculpture. The GPS system is made up of a satellite constellation of 24 working satellites and three spares in intermediate circular orbits, in 6 orbital planes. Basins may overflow from one into another, or the overflow may imitate a natural cascade. The oldest GPS satellite still in operation was launched in February 1989. In fountains sheets of water may flow over varied surfaces of stone, concrete or metal. The most recent launch was in September 2005. Fountains may be wall fountains or free-standing. [4].

A traditional fountain is an arrangement where water issues from a source (Latin fons), fills a basin of some kind, and is drained away. The first modern Block-II satellite was launched in February 1989, and a complete constellation of 24 satellites was in orbit by 1993. The Fountain Of Wealth is the world's largest fountain found at Suntec City, Singapore. By 1985, ten more experimental Block-I satellites had been launched to validate the concept. The fountain at Point State Park in Pittsburgh, Pennsylvania. In 1983, after Soviet jet interceptors shot down the civilian airliner KAL 007 in restricted Soviet airspace, killing all 269 people on board, Ronald Reagan announced that the GPS system would be made available for civilian uses once it was completed. Tubes and nozzles radiating from a hollow sphere. The GPS satellites were initially manufactured by Rockwell and now manufactured by Lockheed Martin.

The El Alamein Memorial Fountain in Fitzroy Gardens, Kings Cross, Sydney, Australia. The first experimental Block-I GPS satellite was launched in February 1978 [3]. The waterfall cascade at Paley Park, New York City. It was only a small leap of logic to realize that the converse was also true; if the satellite's position was known then they could identify their own position on Earth. Tyler Davidson Fountain at Fountain Square in downtown Cincinnati, Ohio. They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion. The world's tallest fountain, located in the town of Fountain Hills, Arizona. They discovered that, due to the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them.

Fountain of Wealth (largest fountain in the world since 1998) In Singapore. Kershner were monitoring Sputnik's radio transmissions. Vancouver's Harbour Green urban park with pavement waterworks fountains that kids can frolic in. Richard B. Urban Oases. scientists led by Dr. Centennial Olympic Park (computer animated frolicking fountain with 251 ground nozzles that shoot 12 to 35 feet in the air). A team of U.S.

Dundas Square fountains designed by artist Dan Euser as both an architectural landmark as well as for frolicking (waterplay). The inspiration for the GPS system came when the Soviets launched the first Sputnik in 1957. Splash Fountains at Christian Science Plaza, Boston, MA. However, this is usually corrected on the display within 15 minutes once the UTC offset message is received for the first time. Fountains in front of Brooklyn Museum (frequent frolicking, no standing water; less chance of drowning). New GPS units will initially show the incorrect time after achieving a GPS lock for the first time. Along the far side there is zero-depth entry (no barrier, and the plane of the water matches the plane of the floor). Receivers thus apply a clock-correction offset (which is periodically transmitted along with the other data) in order to display UTC correctly, and optionally adjust for a local time zone.

to clearstream product) which fill a giant bowl that sprays 20 nozzles up, one central nozzle shoots up approximately 10 meters, 3 times in succession, then the process repeats. Today, GPS time is 14 seconds ahead [2] of UTC, because it does not follow leap seconds. equiv. The atomic clocks on the satellites are set to "GPS time", which is the number of seconds since 00:00:00 UTC, January 6, 1980. There are 44 outer nozzles (each 3/8in dia. For instance, when deploying sensors (for seismology or other monitoring application), GPS may be used to provide each recording apparatus with some precise time source, so that the time of events may be recorded accurately. Toronto Eaton Centre in Toronto, Canada. Many synchronization systems use GPS as a source of accurate time, hence one of the most common applications of this use is that of GPS as a reference clock for time code generators or NTP clocks.

Triton Fountain in Italy. This allows the data to be reported in real-time, using either web browser based tools or customized software. Swann Memorial Fountain in Philadelphia. The recorded data can be stored within the tracking unit, or it may be transmitted to a central location, or internet-connected computer, using a cellular modem, 2-way radio, or satellite. Various fountains, including the "Fountain of Lions" at the Alhambra, Granada, Spain. A GPS tracking system uses GPS to determine the location of a vehicle, person, or pet and to record the position at regular intervals in order to create a track file or log of activities. The Wallace fountains of Paris. On the other extreme, some airlines integrate GPS tracking of the aircraft into their aircraft's seat-back television entertainment systems, available even during takeoff and landing to all passengers.

Fontana di Trevi in Rome. Additionally, some airline companies disallow use of hand-held receivers for security reasons, such as unwillingness to let ordinary passengers track the flight route. The "Fountains of Bellagio" at the Bellagio casino in Las Vegas. Most airlines allow private use of ordinary GPS units on their flights, except during landing and take-off, like all other electronic devices. The staircase and Atlas fountains at Peterhoff, the summer palace of Peter the Great. Geocaching often includes walking or hiking to natural locations, and is popular with both children and adults. Buckingham Fountain in Chicago. Geocaching involves using a hand-held GPS unit to travel to a specific longitude and lattitude to search for objects deliberately hidden there by other Geocachers.

The garden fountains of the Palace of Versailles near Paris. The availability of hand-held GPS receivers for a cost of about $90 and up (as of March 2005) has led to recreational applications including Geocaching. The Trafalgar Square fountains in London. For information about navigation systems for the visually impaired, including MoBIC, Drishti, Brunel Navigation System for the Blind, NOPPA, BrailleNote GPS, and Trekker, refer to the main article GPS for the visually impaired. The Jet d'Eau in Lake Geneva. More costly and precise receivers are used by land surveyors to locate boundaries, structures, and survey markers, and for road construction. My Time at Tiffany's. Low cost GPS receivers are often combined in a bundle with a PDA, car computer, or vehicle tracking system.

Gene Moore, with Jay Hyams, 1990. Glider pilots use the logged signal to verify their arrival at turnpoints in competitions. http://www.bluffton.edu/~sullivanm/spain/barcelona/fundmiro/calder.html. Hand-held GPS receivers can be used by mountain climbers and hikers. [1]. The system can also be used by computer controlled harvesters, mine trucks and other vehicles. GPS is used by people around the world as a navigation aid in cars, airplanes, and ships.

Commercial civilian GPS receivers are required to have limits on the velocities and altitudes at which they will report coordinates; this is to prevent them from being used to create improvised missiles. The satellites also carry nuclear detonation detectors, which form a major portion of the United States Nuclear Detonation Detection System. GPS allows accurate targeting of cruise missiles and precision-guided munitions (or "smart bombs"), as well as improved command and control of forces through improved locational awareness. .

Although the cost of maintaining the system is approximately US$400 million per year, including the replacement of aging satellites, GPS is available for free use in civilian applications as a public good. The satellite constellation is managed daily by the 2d Space Operations Squadron at Schriever Air Force Base. United States Department of Defense developed the system, officially named NAVSTAR GPS (Navigation Signal Timing and Ranging Global Positioning System). GPS accuracy can be improved further, to about 1 cm (half an inch) over short distances, using techniques such as Differential GPS (DGPS).

The Wide-Area Augmentation System (WAAS), available since August 2000, increases the accuracy of GPS signals to within 2 meters (6 ft) [1] for compatible receivers. GPS also provides an extremely precise time reference, required for some scientific research, including the study of earthquakes. Since GPS was declared fully operational in 1993, it has become a vital global utility, indispensible for modern navigation on land, sea, and air around the world, as well as an important tool for map-making, and land surveying. A constellation of more than two dozen GPS satellites broadcasts precise timing signals by radio to electronic GPS receivers which allow them to accurately determine their location (longitude, latitude, and altitude) in real time, day or night, in any weather.

The Global Positioning System, usually called GPS, is the Earth's only fully-functional satellite navigation system. GPS Anti-Jamming Protection. The hunt for an unintentional GPS jammer. GPS jamming.

noaa.gov Selective Availability Factsheet (pdf) or [10]. This is similar in principle to the combination of GPS and inertial navigation used in ships and aircraft, but less accurate and less expensive because it only fills in for short periods. Many automobile GPS systems combine the GPS unit with a gyroscope and speedometer pickup, allowing the computer to maintain a continuous navigation solution by dead reckoning when buildings, terrain, or tunnels block the satellite signals. This can be accomplished by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests—possibly with processing in real-time (real-time kinematic positioning, RTK).

This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver. In this approach, accurate determination of range signal can be resolved to an accuracy of less than 10 centimetres. Relative Kinematic Positioning (RKP) is another approach for a precise GPS-based positioning system. Wide Area GPS Enhancement (WAGE) is an attempt to improve GPS accuracy by providing more accurate satellite clock and ephemeris (orbital) data to specially-equipped receivers.

CPGPS working to within 1% of perfect transition matching can achieve 3 mm ambiguity; in reality, CPGPS coupled with DGPS normally realizes 20-30 cm accuracy. The phase difference error in the normal GPS amounts to a 2-3 m ambiguity. CPGPS solves this problem by using the L1 carrier, which has a period 1/1000 that of the C/A bit width, to define the transition point instead. A successful correlation could be defined in a number of various places along the rising/falling edge of the pulse, which imparts timing errors.

The problem arises from the fact that the transition from 0-1 or 1-0 on the C/A signal is not instantaneous, it takes a non-zero amount of time, and thus the correlation (satellite-receiver sequence matching) operation is imperfect. This technique utilizes the 1.575 GHz L1 carrier wave to act as a sort of clock signal, resolving ambiguity caused by variations in the location of the pulse transition (logic 1-0 or 0-1) of the C/A PRN signal. A Carrier-Phase Enhancement (CPGPS). Exploitation of DGPS for Guidance Enhancement (EDGE) is an effort to integrate DGPS into precision guided munitions such as the Joint Direct Attack Munition (JDAM).

These correction data are typically useful for only about a thirty to fifty kilometer radius around the transmitter. But in this case, the correction data are transmitted from a local source, typically at an airport or another location where accurate positioning is needed. This is similar to WAAS, in that similar correction data are used. A Local Area Augmentation System (LAAS).

However, variants of the WAAS system are being developed in Europe (EGNOS, the Euro Geostationary Navigation Overlay Service), and Japan (MSAS, the Multi-Functional Satellite Augmentation System), which are virtually identical to WAAS. The current WAAS system only works for North America (where the reference stations are located), and due to the satellite location the system is only generally usable in the eastern and western coastal regions. Although only a few WAAS satellites are currently available as of 2004, it is hoped that eventually WAAS will provide sufficient reliability and accuracy that it can be used for critical applications such as GPS-based instrument approaches in aviation (landing an airplane in conditions of little or no visibility). This uses a series of ground reference stations to calculate GPS correction messages, which are uploaded to a series of additional satellites in geosynchronous orbit for transmission to GPS receivers, including information on ionospheric delays, individual satellite clock drift, and suchlike.

The Wide Area Augmentation System (WAAS). The "difference" is broadcast as a local FM signal, allowing many civilian GPS receivers to "fix" the signal for greatly improved accuracy. Differential GPS (DGPS) can improve the normal GPS accuracy of 4-20 meters to 1-3 meters.[8] DGPS uses a network of stationary GPS receivers to calculate the difference between their actual known position and the position as calculated by their received GPS signal. Bradford Parkinson, teacher of aeronautics and astronautics at Stanford University developed the system.

Ivan Getting, emeritus president of The Aerospace Corporation and engineer at the Massachusetts Institute of Technology established the basis for GPS, improving on the World War II land-based radio system called LORAN (Long-range Radio Aid to Navigation). The first Block IIF satellite that would provide this signal is set to be launched in 2007. This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. L5 (1176.45 MHz):
Proposed for use as a civilian safety-of-life signal.

L4 (1841.40 MHz):
Being studied for additional ionospheric correction. L3 (1381.05 MHz):
Carries the signal for the GPS constellation's alternative role of detecting missile/rocket launches (supplementing Defense Support Program satellites), nuclear detonations, and other high-energy infrared events. Recognizing the civilian need for increased accuracy, "modernized" IIR-M (IIR-14 (M) and later) satellites carry a civilian signal interleaved with an improved military signal on both the L1 and L2 frequencies. In spite of not having the P(Y) code encryption key, several high-end GPS receiver manufacturers have developed techniques for utilizing this signal (in a round-about manner) to increase accuracy and remove error caused by the ionosphere.

The keys are changed on a daily basis. government and are generally provided only for military use. The encryption keys required to directly use the P(Y) code are tightly controlled by the U.S. L2 (1227.60 MHz):
Usually carries only the P(Y) code.

L1 (1575.42 MHz):
Carries a publicly usable coarse-acquisition (C/A) code as well as an encrypted precision P(Y) code. Precise time reference. GPS tracking. GPS on airplanes.

Geocaching. GPS for the visually impaired. Surveying. Navigation.

Military Applications.