How To Remove Guess Watch Band Links

How To Remove Guess Watch Band Links

About GPS

Global Positioning System

The Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). Using a constellation of at least 24 medium Earth orbit satellites that transmit precise microwave signals, the system allows a GPS receiver to determine its location, speed and direction, and time.

Developed by the U.S. Defense Department is officially named NAVSTAR GPS (contrary to popular belief, NAVSTAR is not an acronym, but simply a name given by Mr. John Walsh, a key decision maker when it comes to budget for the program GPS [1]). The satellite constellation is managed by the U.S. Air Force 50th Space Wing. The cost of maintaining the system is approximately $ 750,000,000 U.S. year [2], including the replacement of aging satellites, and research and development. Despite these costs, GPS is free for civilian use as a public good.

GPS has become a widely used aid worldwide for navigation, and a useful tool for mapping, surveying, commerce and scientific applications. GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks.

Simplified method of operation

A GPS receiver calculates its position by measuring the distance between itself and three or more GPS satellites. Measure the time between transmission and reception of each GPS microwave signal gives the distance to each satellite, since the signal travels at a known speed - the speed of light. These signals also have information on the location of satellites and general health system (known as almanac and ephemeris data). In determining the position and the distance, at least three satellites, the receiver can calculate its position using trilateration [3]. Recipients are generally not very accurate clocks and therefore track one or more additional satellites, using their atomic clocks to correct the receiver's own clock error.

[Edit] Technical description

Unlaunched GPS satellite on display at the San Diego Aerospace Museum

Unlaunched GPS satellite on display at the San Diego Aerospace Museum

[Edit] System segmentation

The current GPS consists of three main segments. These are the space segment (SS), a control segment (CS), and a user segment (U.S.) [4].

[Edit] Space segment

The space segment (SS) consists of GPS satellites in orbit or space vehicles (SV) GPS in language. The GPS design calls for 24 SVS will be distributed equally among six circular orbital planes. [5] The orbital planes are centered on Earth does not rotate with respect to distance stars. [6] The six planes have approximately 55 ° tilt (inclination to the Earth Ecuador) and are separated by 60 ° right ascension of the ascending node (angle along the Ecuador from a reference point to the intersection of the orbit) [2].

Orbiting at an altitude of approximately 20,200 kilometers (12,600 nautical miles or 10,900 miles; orbital radius of 26,600 kilometers (16,500 miles or 14,400 NM)), each SV for two complete orbits each sidereal day, so it passes over the same spot on Earth once a day. The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface [7].

A From September 2007 there are 31 actively broadcasting satellites in the GPS constellation. The additional satellites improve the precision of the calculations GPS receiver, providing redundant measurements. With the increasing number of satellites, the constellation was changed to a nonuniform arrangement. This agreement has been demonstrated which improves system reliability and availability in relation to a uniform system, when multiple satellites no [8].

[Edit] Segment Control

The flight paths of the satellites are controlled by the monitoring stations of the U.S. Air Force in Hawaii, Kwajalein, Ascension Island, Diego Garcia and Colorado Springs, Colorado, along with monitor stations operated by the National Geospatial-Intelligence Agency (NGA). [9] The tracking information is sent to the station Air Force Space Command's master control at Schriever Air Force Base in Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 SOP) United States Air Force (USAF). 2 contacts each GPS satellite PNT regularly with a navigational update (with the antennae of land on the island of Ascension, Diego Garcia, Kwajalein and Colorado Springs). These updates synchronize the atomic clocks on board satellites with an accuracy of one microsecond and adjust the ephemeris of the internal model of the orbit of each satellite. The updates are created by a Kalman filter that uses input from ground stations monitoring, space weather, and several other inputs. [10]

GPS receivers come in a variety of formats, from devices integrated into vehicles, phones and watches, to dedicated devices such as those shown here from manufacturers Trimble, Garmin and Leica (from left to right).

GPS receivers come in a variety of formats, from devices integrated into cars, phones and watches, specialized devices such as this shown manufacturers Trimble, Garmin and Leica (from left to right).

[Edit] user segment

The user's GPS receiver is the segment Users (U.S.) GPS system. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a very stable clock (often a crystal oscillator). They may also include a screen to provide the location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites can be monitored simultaneously. Originally limited to four or five, this has gradually increased during the recent years so that, from 2006, receivers typically have between twelve and twenty channels.

A typical OEM GPS receiver module based on SiRF chipset Star III, which measures 15 x 17 mm, and is used in many products.

A typical OEM GPS receiver module based on SiRF Star III chipset, which measures 15 x 17 mm, and is used in many products.

GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of an RS-232-4800 bits / s of speed. The data is sent to a much slower rate, which limits the accuracy of the signal sent using RTCM. Receivers Receivers Internal DGPS users can overcome external RTCM data. From 2006, including low-cost units often include Wide Area Augmentation System (WAAS) receivers.

Many GPS receivers can transmit position data to a PC or other device using the NMEA 0183 protocol. NMEA 2000 [11] is a new and less widely adopted protocol. Both are owned and controlled by the U.S. National Marine Electronics Association. References to the NMEA protocols have been compiled from public records, allowing that open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols also exist, such as the SiRF and protocols MTK. Receptors may interact with other devices using methods including a serial connection, USB or Bluetooth.

[Edit] Navigation signals

Main article: GPS signals

GPS broadcast signal

GPS broadcast signal

Each GPS satellite continuously transmits a navigation message at 50 bit / s giving the time of day, GPS week number and satellite health information (all transmitted in the first part of the message), an ephemeris (transmitted in the second part of the message) and an almanac (later part of the message). The satellite ephemeris data gives very precise orbit and the output is more than 18 seconds, repeating every 30 seconds. The ephemeris is updated every two hours and is usually valid for four hours, with provisions for 6 hours of standby time. The time needed to acquire the ephemeris is becoming a significant element of the delay to fix the position first, because, as hardware becomes more capable, the time to ensure that signals from the satellite is reduced, but the ephemeris data requires 30 seconds (worst case) before it is received, due to low data transmission rate. The almanac consists of coarse orbit and status information for each satellite in the constellation and takes 12 seconds for each satellite present, with a new satellite data that are transmitted every 30 seconds (15.5 minutes for 31 satellites). The purpose data is to assist in satellite acquisition during power to allow the receiver to generate a list of visible satellites by storing position and time, while an ephemeris from each satellite is needed to compute position fixes using that satellite. On older hardware, lack of an almanac in a new receiver would cause long delays before providing a valid position, because the search for each satellite was a slow process. Advances in hardware have made the acquisition process much faster, so do not have a calendar is no longer a problem. An important thing to note about navigation data is that each satellite transmits only its own ephemeris, but transmits an almanac for all satellites.

Each satellite transmits its navigation message at least two different spread spectrum codes: the coarse / acquisition (C / A) code, which is available free to the public, and precise (P) code, which is usually encrypted and reserved for military applications. The C / A code is a 1,023 chip pseudo-random (PRN) Code 1.023 million in chips / sec so that it repeats every millisecond. Each satellite has its own C / A code can only be identified and received separately from other satellites transmitting on the same frequency. The P-code is a 10.23 megachip / sec PRN code that repeats only every week. When the "anti-spoofing" is on the way as it is in normal operation, the P code is encrypted Y-code to produce the P (Y) code, which can only be decrypted by units with a key valid decryption. Both the C / A and P (Y) codes provide the precise time of day for the user. The frequencies used by GPS include

* L1 (1575.42 MHz): Mix of Navigation Message, coarse acquisition (C / A) code and encrypted precision P (Y) code, plus the new L1C on future Block III satellites.

* L2 (1227.60 MHz): P (Y) code, plus the new L2C code on the Block IIR-M and the new satellites.

* L3 (1381.05 MHz): Used by the nuclear detonation (NUDET) Detection System Payload (NDS) to signal the detection of nuclear detonations and other high-energy events-infrared. Used to enforce nuclear test ban treaties.

* L4 (1379.913 MHz): study for the correction additional ionospheric.

* L5 (1176.45 MHz): Proposal for use as a civilian security of Life (SoL) signal (see GPS modernization). This often falls in a range of international protection for air navigation, promising little or no interference under all circumstances. The first Block IIF satellite provide this signal is set to be released in 2008.

[Edit Calculation of positions]

[Edit] Using C / A code

To begin with, the receiver that picks up C / A codes to listen to PRN number, based on the almanac information above has been obtained. As it detects each satellite signal, identifying it by its distinct C / A code pattern, then measures the time delay for each satellite. To this end, the receiver produces an identical C / A sequence using the same seed number for the satellite. Aligning the two sequences, the receiver can measure the delay and calculate the distance to the satellite, called pseudorange [12].

Overlapping pseudoranges, represented as curves, are modified to produce the probable position

Overlapping pseudoranges, represented as curves, are modified to produce the probable position

Then, the orbital position data, or ephemeris, from the message Navigation is downloaded to calculate the exact position of the satellite. A more sensitive receiver potentially acquire the ephemeris data faster than less sensitive receiver, especially in a noisy environment. [13] Knowing the position and distance of a satellite indicates that the receiver is somewhere on the surface an imaginary sphere centered on that satellite and whose radius is the distance to it. Receivers can substitute for a satellite altitude, the GPS receiver results in a pseudorange measured from the center of the earth.

Locations are not calculated in three dimensional space, but in the spacetime of four dimensions, which means a measure the exact time of day is very important. The measure pseudoranges from four satellites have been determined with the internal clock of the receiver, and therefore have a number unknown clock error. (The error of real time clock or not important in calculating the initial pseudorange, since it is based on how much time has passed between the receipt each of the signals. [Clarify] [citation needed]) The four dimensions of pseudoranges midpoint is calculated as a guess as to the location of receptor, and the factor used to adjust the pseudoranges to cross at that point in four dimensions gives a guess as to the receiver clock offset. With each attempt to guess geometric dilution of precision (GDOP) vector is calculated based on the relative sky positions of the satellites used. As more satellites are collected, pseudoranges are more combinations of four satellites can be processed to add more chances to guess the location and clock offset. The receiver determines which combinations to use and how to calculate the estimated position by determining the weighted average of these positions and clock movements. After final location and time of calculation, the location is expressed in a specific coordinate system, for example, latitude and longitude, using the geodetic datum WGS 84 or a specific local system of a country.

[Edit] Using the P (Y) code

Calculating a position with P (Y) signal is generally similar in concept, assuming one can decipher. The encryption is essentially a safety mechanism: if a signal can be decoded successfully, it is reasonable to assume that is a real signal being sent by a GPS satellite. [Citation needed] In comparison, civil receivers are highly vulnerable to theft as the correct format C / A signals can be generated using readily available signal generators. RAIM features do not protect against spoofing identity, since RAIM only checks the signals from the perspective of navigation.

[Edit] Accuracy and error sources

The position calculated by a GPS receiver requires the current time, satellite position and measures the delay of the received signal. The position accuracy depends mainly on the position the satellite and the delay of the signal.

To measure the delay, the receiver compares the bit sequence received from the satellite with an internally generated version. By comparing the rising and trailing edges of the bit transitions, modern electronics can measure signal offset to within 1% of a bit time, or about 10 nanoseconds for C / A code Since GPS signals propagate at nearly the speed of light, this represents an error of about 3 meters. This is the minimum error possible using only GPS C / A signal.

Position accuracy can be improved by using high chiprate signal P (Y). Assuming the same 1% accuracy Soon the high frequency P (Y) signal results in an accuracy of about 30 centimeters.

Electronics errors are one of several degrading effects accuracy, shown in the table below. When taken together, autonomous civilian GPS solutions are typically horizontal position accuracy of about 15 meters (50 feet). These effects also reduce the more precise P (Y) code's accuracy.

User error sources equivalent Range (Uere) Source Effect

Ionospheric effects ± 5 meters

Ephemeris errors ± 2.5 meters

Satellite clock errors ± 2 meters

Multipath distortion of ± 1 meters

tropospheric effects ± 0.5 meters

Numerical errors ± 1 meters

[Edit] Atmospheric effects

Inconsistencies of atmospheric conditions affect the speed of GPS signals to pass through the Earth's atmosphere and ionosphere. Correcting these errors is a major challenge to improve the accuracy of GPS position. These effects are smaller when the satellite is directly overhead and become greater for satellites nearer the horizon since the signal is affected by a longer time. Once the receiver approximate location is known, a mathematical model can be used to calculate and compensate for these errors.

Because ionospheric delay affects the speed of microwave signals differently depending on the frequency-a characteristic known as dispersion, both frequency bands can be used to help reduce this error. Some military and civilian recipients expensive survey grade compare the different delays in the L1 and L2 frequencies to measure atmospheric dispersion, and apply a more precise correction. This can be done in civilian life receptors without decrypting the P (Y) signal carried on L2, by tracking the carrier wave modulated instead of the code. To facilitate this on low-cost receivers, a new civilian code signal on L2, called L2C, was added to the Block IIR-M satellite, which was first launched in 2005. It allows a direct comparison of L1 and L2 signals in the coded signal instead of the carrier wave.

The effects of the ionosphere generally change slowly, and can be calculated over time. The effects of any given geographical area can be easily calculated by comparing the GPS position, measured at a known surveyed location. This correction is also valid for other receivers in the same general location. Several systems send this information via radio or other links to allow that L1 only receivers to make ionospheric corrections. Data is transmitted via satellite in the ionosphere of satellite-based augmentation systems like WAAS, transmitted in the GPS frequency using a special pseudo-random number (PRN), so that only one antenna and receiver are required.

Humidity also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the troposphere. This effect is more localized and more rapid changes ionospheric effects and is not dependent on the frequency. These traits make precise measurement and compensation of humidity errors more difficult than the effects ionospheric.

Changes in altitude also change the amount of delay due to the signal passing through less atmosphere at high altitudes. Because the receiver GPS calculates its approximate altitude, this error is relatively easy to correct.

[Edit] The effects of multipath

GPS signals can also be affected by multiple issues, where radio signals are reflected in the surrounding terrain, buildings, canyon walls, hard ground, etc. These signals can cause exact delay. A variety of techniques, including more space narrow correlator have been developed to mitigate multipath errors. For multipath delay the receiver can recognize the wayward signal and discard. To address shorter delay multipath signals reflected by the ground, specialized antennas can be used to reduce the power of the signal received by the antenna. short-term considerations are more difficult to filter by interfering with the real signal, causing effects almost indistinguishable from routine fluctuations in atmospheric delay.

Multipath effects are much less severe in moving vehicles. When GPS antenna is moving, the false solutions using reflected signals quickly fail to converge and only the direct signals result in stable solutions.

[Edit] ephemeris and clock errors

The navigation message is sent from a satellite every 30 seconds only. In fact, the data in these messages tend to be "outdated" by an even greater amount. Consider the case when a GPS satellite is driven back into a proper orbit, as some time after the operation, the receiver for calculating satellite position is wrong until it receives another ephemeris update. The clocks on board are highly accurate, but suffer from some clock drift. This problem tends to be very small, but can add up to 2 meters (6 feet) of inaccuracy.

This kind of error is more "stable" than ionospheric problems and tends to change over days or weeks rather than minutes. This makes correction fairly simple by sending a more accurate almanac on a separate channel.

[Edit selective availability]

The GPS includes a feature called Selective Availability (SA) that introduces intentional, slowly changing random errors of up to one hundred meters (328 feet) navigation signal available to the public to confuse, for example, guiding long-range missiles to precise targets. additional precision is available in the signal, but in a coded form was only available to the U.S. military, its allies and a few others, mostly government users.

SA typically added signal errors of up to 10 meters (32 feet) horizontally and 30 meters (98 feet) vertically. The inaccuracy of civilians deliberately encoded signal does not change very rapidly, eg throughout the eastern U.S. might read 30 m free, but 30 m off everywhere and in the same direction. To enhance the usefulness of GPS for civilian navigation, Differential GPS was used by many civilian GPS receivers to greatly improve accuracy.

During the Gulf War, the shortage of military GPS units and the wide availability of civilians among the staff led to a decision to disable Selective Availability. This was ironic, as SA had been introduced specifically for these situations, allowing friendly troops to use the signal for accurate navigation and at the same time, denying the enemy. But SA also rejects the same accuracy to thousands of friendly troops, turning it off or set it to an error of zero meters (actually the same thing) presented a clear benefit.

In the 1990s, FAA started pressuring the military to turn SA permanently. This would save the FAA millions of dollars each year to maintain their own navigation systems. The military resisted for most of the 1990s and eventually took an executive order that SA have withdrawn from the GPS signal. The amount of error terms "zero" [14] at midnight on May 1, 2000 following an announcement by U.S. President Bill Clinton, allowing users to Access to error-free L1 signal. In the directive, the induced error of SA was changed to add no error to the public signals (C / A code). Selective Availability is still a GPS system capacity, and error could, in theory, be reintroduced at any time. In practice, given of the dangers and costs that this leads to the U.S. and other sending countries, is unlikely to be reintroduced, and various government agencies including the FAA, [15] have stated that it is not intended to be reintroduced.

The U.S. Army has developed the ability to deny GPS locally (and other navigation services) hostile forces in a specific area of crisis without affecting the rest of the world or its own military system [14].

An interesting side effect of hardware Selective Availability is the ability to correct the frequency of the GPS cesium and rubidium atomic clocks to an accuracy of about 2 x 10-13 (one in five billion dollars). This represented a significant improvement over the raw accuracy of the clocks. [Citation needed]

On September 19, 2007, the United States Department of Defense announced that it would acquire satellites capable of implementing SA. [16]

[Edit] Relativity

According to the theory of relativity due to his constant movement and height relative to the earth-centered inertial reference system, the clocks of the satellites are affected by their speed (special relativity) and its potential gravity (general relativity). For GPS satellites, general relativity predicts that atomic clocks on GPS orbital altitudes make faster, at about 45 900 nanoseconds (ns) per day, because they are in a weaker gravitational field than atomic clocks on Earth's surface. Special relativity predicts that atomic clocks moving at GPS orbital speeds tick more slowly than stationary ground clocks by about 7,200 ns per day. When combined, the difference is 38 microseconds per day, a difference of 4.465 parts in 1010. [17]. To account for this, the frequency standard onboard each satellite is given a displacement rate before its release, making it run a little slower than the desired frequency on Earth, specifically to 10.22999999543 MHz instead of 10.23 MHz [18].

GPS observation processing must also compensate another relativistic effect, the Sagnac effect. The GPS time scale is defined in an inertial system but observations are processed in an Earth-centered, earth-fixed (co-rotating) system, a system in which simultaneity is not uniquely defined. The Lorentz transformation between the two systems modifies the runtime of the signal, a correction having opposite algebraic signs for satellites in the Eastern and Western celestial hemispheres. Ignoring this effect fails from east to west in the order of hundreds of nanoseconds, or tens of meters in position [19].

The atomic clocks on board the GPS satellites are adjusted accurately, making the system a practical engineering application of the scientific theory of relativity in a real environment.

[Edit] GPS interference and blocking

Since GPS signals at terrestrial receivers tend to be relatively weak, it is easy other sources of electromagnetic radiation to desensitize the receptor, so the acquiring and tracking satellite signals difficult or impossible.

Solar flares are a natural example of issue and the potential to degrade GPS reception, and their impact can affect reception in the front half of the land the sun. GPS signals can also be interfered with by naturally occurring geomagnetic storms, especially near the poles of the Earth's magnetic field. [20] Another source of problems is the metal embedded in the windshields of several vehicles to prevent ice formation, degrading reception just inside the car.

Artificial can also create interference problems, or jam, GPS signals. In a well documented case, a port of any failure to receive GPS signals, due to unintentional interference caused by a malfunctioning TV antenna preamplifier. [21] of intentional interference is also possible. In general, signals stronger may interfere with GPS receivers when they are in the range of radio, or line of sight. In 2002, a detailed description of how to build a Short GPS L1 C / gag was published in Phrack magazine online [22].

The U.S. government believes that such jammers were used occasionally during the 2001 war in Afghanistan and the U.S. military states to destroy a GPS jammer with a GPS-guided bomb during the war in Iraq. [23] For example, a jammer is relatively easy to detect and locate, making it an attractive target for anti-radiation missiles. The British Defense Ministry tested a system interference in the country west of the UK 7 and June 8, 2007. [24]

Some countries allow the use of GPS repeaters to allow reception of GPS signals indoors and in obscured locations, however, under the laws of the EU and the United Kingdom, prohibited use of these signals can cause interference GPS receivers that can receive data from both GPS satellites and the repeater.

Due to the possibility that both natural and man-made noise, numerous techniques continue to be developed to deal with interference. The first is to not rely on GPS as a single source. According to John Ruley, "IFR pilots should have an alternative plan in case of a GPS malfunction "[25]. Receiver Autonomous Integrity Monitoring (RAIM) is a feature now included in some receivers, that is designed to provide a warning to the user in case of interference or other problems that are discovered. U.S. military has also deployed their Selective Availability / Anti-Spoofing Module (SAASM) at the Defense Advanced GPS Receiver (DAGR). In demonstration videos, the DAGR is able to detect bottlenecks and maintain its lock on the encrypted GPS signals during interference which causes civilian receivers to lose lock [26].

[Edit] Techniques to improve accuracy

[Edit] Increase

Main article: GNSS Augmentation

Enhanced methods for improving the accuracy depends on external information being integrated in the calculation process. There are many such systems in place and are named in general or on the basis of how we describe the GPS sensor receives the information. Some systems transmission of information about the sources of error (such as clock drift, ephemeris, or ionospheric delay), other direct measurements of the amount signal was out in the past, while a third group provide additional navigational or vehicle to be integrated into the calculation process.

Examples of augmentation systems include the Wide Area Augmentation System, Differential GPS, Inertial Navigation Systems and Assisted GPS.

[Edit track] states

The accuracy of a calculation can also be improved through monitoring and accurate measurement of the existing GPS signals in a complementary or alternative.

After SA, which has been turned off, the biggest mistake of GPS is usually the unpredictable delay through the ionosphere. The spacecraft broadcast ionospheric parameters model, but still errors. This is one reason the GPS spacecraft transmit on at least two frequencies, L1 and L2. Ionospheric delay is a well-defined function frequency and total electron content (TEC) along the way, to measure the arrival time difference between the frequencies determines TEC and thus the exact delay each frequency in the ionosphere.

Receivers with decryption keys can decode the P (Y)-code transmitted on both L1 and L2. However, these keys are reserved for the military and "authorized" agencies and are not available to the public. Without the keys, it is still possible to use a technique without code to compare the P (Y) codes on L1 and L2 to obtain all the information the same error. However, this technique is slow, so is currently limited specialized teams of measurement. In the future, civil codes are expected to be transmitted on the L2 and L5 frequencies (see GPS modernization, below). Then, all users will be able to perform dual-frequency measurements and directly compute ionospheric delay errors.

A second form to precisely control carrier is called the phase enhancement (CPGPS). The error, which it corrected, because the pulse transition of the PRN is not instantaneous, and Thus the correlation (satellite-receiver sequence matching) operation is imperfect. CPGPS approach uses the L1 carrier wave, which has a period of 1000 times smaller than that of the C / A bit period, to act as an additional clock signal and resolve the uncertainty. The phase difference error in the GPS for quantities normal 2 to 3 meters (6-10 feet) of ambiguity. CPGPS work within 1% of the perfect transition reduces this error to 3 centimeters (1 inch) of ambiguity. By eliminating this source of error, along with CPGPS DGPS normally realizes between 20 and 30 centimeters (8-12 inches) of absolute accuracy.

Relative Kinematics position (RKP) is another approach to a precision positioning system based on GPS. In this approach, determining the signal spectrum can be resolved with an accuracy of less than 10 centimeters (4 inches). This is done to resolve the number of cycles in which the signal is transmitted and received by the receiver. This can be achieved through a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and resolution techniques of ambiguity through statistical tests, possibly with processing in real time (real time kinematic positioning, RTK).

[Edit] GPS time and date

While most clocks are synchronized to Coordinated Universal Time (UTC), the atomic clocks of the satellites is configured to GPS time. The difference is that GPS time is not corrected to match the rotation of the Earth, so it contains no leap seconds or other corrections to be added UTC regularly. GPS time is set to coincide with the Coordinated Universal Time (UTC) in 1980, but has changed since then. The lack of corrections means GPS that time remains in a constant offset (19 seconds) with International Atomic Time (TAI). perform periodic corrections in the clocks on board to correct relativistic effects and keep them synchronized with ground clocks.

The GPS navigation message includes the difference between GPS and UTC time, which from 2006 is 14 seconds. Receivers subtract the calibration of the GPS time to calculate the UTC and specific timezone values. New GPS units may not show the correct UTC until after receiving the UTC offset message. field_offset The GPS-UTC leap accommodate 255 seconds (eight bits) which, at current rates of change of Earth rotation is sufficient to last until 2330.

Unlike the year, month, date and format of the Julian calendar, the GPS date is expressed as a number of week and one day, the number of weeks. The week number is transmitted as a field of ten bits in the C / A and P (Y) navigation messages, so it becomes zero again every 1024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6, 1980 and the week number became zero again for the first time at 23:59:47 UTC on August 21, 1999 (00:00:19 ITF on August 22, 1999). To determine the current Gregorian date, a GPS receiver must have the date approximate (to within 3584 days) to translate correctly the GPS date signal. To address this concern the modernization of the navigation messages GPS use a 13-bit field, which only repeats every 8192 weeks (157 years), and will not return to zero until about 2137.

[Edit] GPS modernization

Main article: GPS modernization

Having met the requirements of the program of full operational capability (FOC) on 17 July, 1995 [27], the GPS completed its original design goals. However, further advances in technology and new demands on the existing system led to efforts modernization of GPS. Classifieds Vice President and the White House in 1998 initiated these changes, and in 2000 the U.S. Congress authorized the effort, referring to it as GPS III.

The project aims to improve the accuracy and availability of all users and involves new ground stations, new satellites, and four additional navigation signals. New civilian signals are called L2C, L5 and L1C; the new military code is called M-Code. Initial Operational Capability (IOC) L2C Act is expected in 2008 [28]. One goal of 2013 has been set for the entire program, with incentives offered to contractors if they can complete in 2011.

[Edit] Applications

The Global Positioning System, while originally a military project, is considered a dual-use technology, which means it has important applications for both military and civilian industry.

[Edit] Military

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The army's use of GPS for the following purposes:

[Edit] Navigation

GPS allows soldiers to find objectives in the dark or in unfamiliar territory, and coordinate the movement of troops and supplies.

[Edit] target tracking

Various military weapons systems use GPS to track potential ground and air targets before they are flagged as hostile. These weapon systems pass GPS coordinates of the objectives of precision guided munitions to participate objectives accurately.

Military aircraft, particularly those used in air-ground roles use GPS to find targets (for example, gun video chamber AH-1 Cobra in Iraq show GPS coordinates that can be searched in Google Earth).

[Edit] Missiles and missile guidance

GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles and precision guided munitions.

Artillery projectiles with embedded GPS receivers able to withstand the forces of 12,000 G have been developed for use in 155-mm [29].

[Edit] Search and Rescue

Fallen pilots can be located faster if they have a GPS receiver.

[Edit] Surveying and mapping

The military use GPS extensively to aid mapping and reconnaissance.

[Edit] Other

GPS satellites also carry nuclear detonation detectors, which form an important part of U.S. nuclear detonation detection system [30].

[Edit] Civil

See also: GPS applications

This antenna is mounted on the roof of a hut containing a scientific experiment needing precise moment.

This antenna is mounted on the roof of a hut containing a scientific experiment needing precise moment.

Many civilian applications benefit from GPS signals, using one or more of the three basic components of the GPS, favorable location relative motion, the transfer time.

The ability to determine the absolute location of the receiver allows GPS receivers to function as a surveying tool or as an aid to navigation. The ability to determine relative movement enables a receiver to calculate local velocity and orientation, useful in vessels or observations of the Earth. Being able to synchronize clocks to meet the standards allows for a transfer time, which is essential in the communication of large and observation systems. An example is CDMA digital cellular. Each base station has a GPS receiver to synchronize the time of spreading codes with other base stations to facilitate the exchange between the cell and outside support hybrid GPS / CDMA positioning of mobiles for emergency calls and other applications.

Finally, GPS enables researchers to explore the Earth environment including the atmosphere, ionosphere and the gravity field. GPS survey equipment has revolutionized tectonics by directly measuring the movement of faults in earthquakes.

To help prevent civilian GPS guidance to be used in military of an enemy or improvised weapons, the U.S. government controls the export of civilian receivers. A US-based manufacturer usually can not export a GPS receiver unless the receiver contains limits restricting it to function when it is both (a) at an altitude of more than 18 kilometers (60,000 feet) and (2) traveling more than 515 m / s (1,000 knots) [31].

[Edit] History

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The design GPS is based partly on navigation systems like terrestrial radio, such as LORAN and the Decca Navigator developed in the 1940s, and used for World War II. Additional inspiration for the GPS system came when the Soviet Union launched Sputnik in 1957. A team of U.S. scientists addressed by Dr. Richard B. Kershner were monitoring radio transmissions from Sputnik. They found that due to the Doppler effect, the frequency of the signal transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that since they knew their exact location in the world could determine where the satellite was in orbit by measuring the Doppler distortion.

The first satellite navigation system, Transit, used by the U.S. Navy, was first successfully tested in 1960. With a constellation of five satellites, could provide a navigation solution approximately once per hour. In 1967, the U.S. Navy Timation satellite developed that demonstrated the ability to place accurate clocks in space, a technology system GPS is based on. In the 1970s, the plea of Omega Navigation System, based on comparison of the phase signal, became a radio navigation system first in the world.

The first experimental Block-I GPS satellite was launched in February 1978 [28]. GPS satellites were initially manufactured by Rockwell International and are manufactured by Lockheed Martin.

[Edit] Timeline

* In 1972, U.S. Air Force Inertial Central Installation Orientation Test (Holloman AFB) conducted developmental tests the struggle of two prototype GPS receivers over White Sands Missile Range, ground-based pseudo-satellites.

* In 1978, the first experimental Block-I GPS satellite was launched.

* In 1983, after Soviet interceptor aircraft shot down the KAL 007 airliner restricted Soviet airspace, killing all 269 people on board, U.S. President Ronald Reagan announced that GPS will available for civilian use, once it was completed.

* In 1985, ten more experimental Block-satellites had been launched to validate the concept.

* On February 14, 1989, the first modern Block-II satellite was launched.

* In 1992, the second Space Wing, originally the management system, was deactivated and replaced by the 50th Space Wing.

* In December 1993 the GPS system achieved initial operational capability [32]

* As of 17 January 1994 a complete constellation of 24 satellites in orbit was.

* Full operational capability was declared by NAVSTAR in April 1995.

* In 1996, recognizing the importance of GPS to users both civilian and military users, U.S. President Bill Clinton issued a policy directive [33] declaring GPS as a dual-use system and establishing an Interagency GPS Executive Board for its management as a national asset.

* In 1998, USA Vice-president Al Gore announced plans to upgrade GPS with two new civilian signals to the user accuracy and reliability improved, particularly with respect to aviation safety.

* On May 2, 2000 "Selective Availability" was discontinued as a result of Executive Order 1996, which allows users to receive a degraded signal worldwide.

* In 2004, the United Kingdom States Government signed an historic agreement with the European Community establishing cooperation related GPS and planned European Galileo system.

* In 2004, USA President George W. Bush updated the national policy, replacing the executive board National Institute for Space Positioning, Navigation and Timing Executive Committee.

* November 2004, Qualcomm announced tests success of Assisted-GPS system for mobile phones [3].

* In 2005, the first modernized GPS satellite was launched and began transmitting a second civil signal (L2C) for enhanced user performance.

* The most recent release was on November 17, 2006. The oldest GPS satellite still operation was launched in August 1991.

* The September 14, 2007, the aging mainframe ground segment control system was the transition Plan the new Architecture Evolution. [4]

[Edit] satellite numbers

Name Number of previous launch satellites launched, inc. Currently launch failures in service

Block I 1978-1985 11 0

Block II 1985-1990 9 0

Block II 1990-1997 19 November 15

Block IIR 1997-2004 December 1912

Block IIR-M 2005-3 3

Total 54 (plus one unreleased) January 30

1One satellite test

[Edit]

Two GPS developers have received from the National Academy of Engineering Charles Stark Draper prize year 2003:

* Ivan Getting, president emeritus of the Aerospace Corporation and engineer at the Massachusetts Institute of Technology, established the basis for GPS, improved radio system War II Ground-based World called LORAN (Long Distance Radio Aids to Navigation).

* Bradford Parkinson, professor of aeronautics and astronautics at the University Stanford, conceived the present satellite-based system in early 1960 and developed jointly with the U.S. Air Force.

A developer GPS, Roger L. Easton received the National Medal of Technology on February 13, 2006 at the White House [34].

February 10, 1993, the National Association Aviation selected the Global Positioning System Team as winners of the 1992 Robert J. Collier Trophy, the most prestigious award in aviation in the United States. This team consists of researchers from the Naval Research Laboratory, the U.S. Air Force, the Aerospace Corporation, Rockwell International Corporation, and IBM Federal Systems Company. The citation accompanying the presentation of the trophy honors the GPS Team "for the most significant development for safe navigation and efficient monitoring of air and spacecraft since the introduction of radio navigation 50 years ago. "

[Edit] Other systems

Main article: System Global Navigation Satellite

Other satellite navigation systems in use or various states of development include:

* Beidou - China's regional system that China has proposed to extend to a global system named COMPASS.

* Galileo - a proposed global system is being developed by European Union, along with China, Israel, India, Morocco, Saudi Arabia and South Korea, Ukraine planned to be operational by 2011-12.

* GLONASS - Russia's global system being restored to full availability in partnership with India.

* Indian Regional Navigation Satellite System (IRNSS) - proposed system Regional India.

* QZSS - Japan proposed regional system, adding better coverage to the Japanese islands.

[Edit] See also

The navigation satellite systems portal

Nautical Portal

* RAIM

* SIGI

* Radionavigation

* High sensitivity GPS

* Degree Confluence Project Use GPS to visit integral degrees of latitude and longitude.

* Exif, GPS transfer data.

* Geotagging

* Geocaching

* NaviTraveler.com - a GPS point sharing community.

* GPS Drawing Digital mapping and drawing with GPS tracks.

* GPS Location

* GPS / INS

* Assisted GPS

* GPX (XML schema for exchanging Points of Interest)

* Identification sniper rifle

* OpenStreetMap, free content maps and street pictures (GFDL)

* Telematics: Many telematics devices use GPS to determine the location of mobile equipment.

* The American Practical Navigator, the Chapter 11 "Satellite Navigation

* Point of Interest

* Car Navigation System

* NextGen

[Edit]

1. ^ Parkinson, BW (1996), Global Positioning System: Theory and Applications, chap. 1: Introduction and Heritage the NAVSTAR Global Positioning System. pp. 3-28, American Institute of Aeronautics and Astronautics, Washington, DC

2. ^ Ab GPS General's Office NAVSTAR Joint Program. Retrieved on December 15, 2006.

3. HowStuffWorks ^. How GPS Receivers Work. Retrieved on May 14, 2006.

4. Globalsecurity.org ^ [1].

May. ^ Dana, Peter H. GPS orbital planes. August 8, 1996.

6. ^ What the Global Positioning System tells us about Relativity. Retrieved on January 2, 2007.

7. ^ NAVCO USCG: Frequently Asked Questions GPS. Retrieved on January 3, 2007.

In August. ^ Massatt, Paul and Brady, Wayne. "Optimizing performance through constellation management", Crosslink, Summer 2002, pages 17-21.

9. ^ U.S. Coast Guard General GPS News 09/09/1905

October. ^ USNO. NAVSTAR Global Positioning System. Retrieved on May 14, 2006.

11. ^ NMEA NMEA 2000

12. ^ Http: / / gge.unb.ca / Resources / HowDoesGPSWork.html

13. ^ AN02 Assistance Network (HTML). Retrieved on 2007-09-10.

14. Ab ^ Office of Science and Technology Policy. Statement by the President to stop degrading GPS. May 1, 2000.

15. ^ FAA, Selective Availability. Retrieved on January 6, 2007.

16. ^ Http: / / www.defenselink.mil/releases/release.aspx?releaseid=11335

17. ^ Rizos, Chris. University of New South Wales. GPS Satellite Signals. 1999.

18. ^ The Global Positioning System by Robert A. Nelson Via Sat, November 1999

19. ^ Ashby, Neil Relativity and GPS. Physics Today, May 2002.

20. ^ Space Environment Center. GPS Navigation Systems SEC Page. August 26, 1996.

21. ^ The search for an unintentional GPS jammer. GPS World. January 1, 2003.

22. ^ Low Cost and Portable GPS Jammer. Phrack 0x3c Topic (60), article 13]. Published December 28, 2002.

23. ^ Forces Press Service. CENTCOM charts progress. March 25, 2003.

24. ^ [2]

25. ^ Ruley, John. AVweb. GPS jamming. February 12, 2003.

26. ^ Commercial GPS Receivers: Facts for the Warfighter. Housed in the site Joint Chiefs website, linked by the USAF's GPS Wing DAGR program website. Retrieved on April 10, 2007

27. ^ U.S. Coast Guard press release. Global Positioning System fully operational

28. ^ Ab Hydrographic Society Journal. The evolution of global navigation satellite. Issue # 104 April 2002. Retrieved on April 5, 2007.

29. ^ XM982 Excalibur Precision Guided Extended Range Artillery Projectile. GlobalSecurity.org (29/05/2007). Retrieved on 2007-09-26.

30. ^ Sandia National Laboratory programs and Non-Proliferation of weapons control technology.

31. ^ Armas association control. Missile Technology Control Regime. Retrieved on May 17, 2006.

32. ^ U.S. Department of Defense. Notice of Availability operational. December 8, 1993.

33. ^ National Archives and Records Administration. U.S. GLOBAL POSITIONING SYSTEM POLICY. March 29, 1996.

34. ^ U.S. Naval Research Laboratory. National Medal of Technology for GPS. November 21, 2005

[Edit]

Wikimedia Commons has media related to:

Global Positioning System

Government links

* GPS.gov General public education website created by the U.S. Government

* National Space-based PNT Executive Committee in 2004 to oversee management of GPS and augmentation systems GPS national level.

* USCG Navigation Center-Status of the GPS constellation, government policy, and links to other references. It also includes almanac data from satellites.

* Joint Program Office GPS (GPS JPO)-Responsible for designing and acquiring the system on behalf of the Government of U.S..

* U.S. Naval Observatory GPS Constellation Status

* U.S. Army Corps of Engineers manual: NAVSTAR HTML and PDF (22.6 MB, 328 pages)

* PNT Selective Availability Announcements

* MSF GPS Signal Specification, 2nd edition, "the official Standard Positioning Signal specification.

* Federal Aviation Administration GPS FAQ

Introduction / tutorial links

* How does GPS work? Says TomTom GPS navigation and digital map

* Garmin GPS Academy interactive web site highlighting videos what exactly GPS and what it can do for you

* Explanation HowStuffWorks' Simplified GPS and video about how GPS works.

* Trimble GPS Online Tutorial Tutorial designed to introduce the principles behind GPS

* GPS and GLONASS Simulation (Java applet) Simulation and representation graphic space vehicle motion including the calculation of the dilution of precision (DOP)

Technical, historical and links to more ancillary items

* Dana, Peter H. "Vision Global Positioning System"

* Satellite Navigation: GPS and Galileo (PDF)-document 16 pages on the history and working of GPS, Galileo play in the next

* History of GPS, including information about the configuration of each satellite and launch.

* Chadha, Kanwar. "The Global Positioning System: Challenges in Bringing GPS to mainstream consumers" Technical Article (1 998)

* GPS Guidance Techniques Weapons

* RAND history of the GPS system (PDF)

* GPS Anti-Jam Protection Techniques

* Summer 2002 issue by The Aerospace Corporation Crosslink satellite navigation.

* Improved weather predictions from of data from COSMIC GPS satellite signal occultation.

* David L. Wilson 'S GPS Accuracy Web Page A thorough analysis of GPS accuracy.

* Innovation: Spacecraft Navigator, Autonomous GPS positioning at high Earth orbits of GPS receiver designed Example for spaceflight high.

* Receiver Navigator browser spaceflight GPS Receiver GSFC.

* Neil Ashby of Relativity in the Global System Positioning

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