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 position, speed / direction and time.
Developed by the U.S. Department of Defense it is officially called NAVSTAR GPS (Contrary to popular belief, NAVSTAR is not an acronym, but simply a name given by Mr. John Walsh, a decision maker key when it came to the budget for the GPS program [1]). The satellite constellation is managed by the United States Air Force 50th Space Wing. The cost of maintaining system is approximately $ 750 million per year [2], including the replacement of aging satellites, and research and development. Despite of these costs, GPS is free for civilian use as a public good.
The GPS has become a tool widely used worldwide for navigation and a useful tool for mapping, surveying, commerce, and scientific uses. GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks.
Method Simplified operation
A GPS receiver calculates its position by measuring the distance between itself and three or more GPS satellites. Measuring the time delay between transmission and reception of each GPS signal microwave gives the distance from each satellite, since the signal travels at a known speed – the speed of light. These signals also carry information on the location of satellites and the general health system (known as almanac and ephemeris). By determining the position of, and distance, at least three satellites, the receiver can calculate its position using trilateration [3]. Receivers have generally not perfectly accurate clocks and therefore track one or more additional satellites, using their atomic clocks to correct an error receiver own clock.
[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 is consists of three main segments. This is 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) in GPS language. The design calls for 24 GPS VS to be distributed equally among six circular orbital planes. [5] The orbital planes are centered on the Earth does not rotate with respect to the distant stars. [6] The six planes have approximately 55 ° inclination (tilt relative to the equator) and are separated by 60 ° rise right of the ascending node (angle along the equator from a reference point at the intersection of the orbit) [2].
Orbiting at an altitude of about 20,200 kilometers (12,600 miles or 10.900 nautical miles; orbital radius of 26.600 km (16,500 km or 14,400 NM)), each SV is complete two orbits every sidereal day, he passes on the same spot on Earth once a day. The orbits are arranged so that at least six satellites are always in line sights from around the surface of the Earth [7].
In September 2007, there are 31 actively broadcasting satellites in the GPS constellation. Additional satellites improve the accuracy of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement has been shown to improve the reliability and availability of the system, compared to a system uniform when several satellites are [8].
[Edit] control segment
The trajectories of the satellites are tracked by the monitoring stations U.S. Air Force in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado, with monitoring stations operated by the National Geospatial-Intelligence Agency (NGA). [9] The tracking information is sent to the station of the Air Force Space Command is the master control at Schriever Air Force Base in Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 PNF) of the United States Air Force (USAF). 2 contacts SOPS each GPS satellite with a regularly updated navigation (using the ground antennas at Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs). The updates synchronize the atomic clocks on board satellites in the microsecond and adjust the satellite ephemeris each internal orbital model. The updates are created by a Kalman filter, which uses inputs from monitoring stations on the ground, the space information weather and other factors [10].
GPS receivers are available in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such as those presented here by the manufacturers Trimble, Garmin and Leica (From left to right).
GPS receivers are available in a variety of formats from embedded devices in cars, phones and watches, specialized equipment, such as those presented here by the manufacturers Trimble, Garmin and Leica (from left to right).
[Edit] User segment
The GPS receiver of the user is the user segment (U.S.) GPS. In general, receptors Consist of a GPS antenna, listening to the frequencies transmitted by the satellites, the 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 it can monitor simultaneously. Initially limited to four or five, which has gradually increased over the years so that, from 2006, receivers typically have between twelve and twenty channels.
A typical OEM GPS receiver module, based on the SiRF Star III, measuring 15 × 17 mm, and used in many products.
A typical OEM GPS receiver module based on the SiRF Star III measuring 15 × 17 mm, and used in many products.
GPS receivers may include an input for differential corrections, using the RTCM SC-104. This is usually in the form of an RS-232 to 4800 bit / s speed. The data is actually sent to a much lower rate, This limits the precision of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external data RTCM. In 2006, even low-cost units commonly include Wide Area Augmentation System (WAAS) receivers.
Many receivers GPS receivers can relay position data to a PC or other device using the NMEA 0183. NMEA 2000 [11] is a new and less widely adopted protocol. The Both are wholly owned and controlled by the U.S. National Marine Electronics Association. References to the NMEA protocols have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols also exist, such as the SiRF and MTK protocols. Receivers can interface with other devices using methods, including a serial connection, USB or Bluetooth.
[Edit] Navigation signals
Main article: GPS signals
broadcast signal GPS
GPS broadcast signal
Each GPS satellite continuously broadcasts a Navigation Message at 50 bit / s giving the time of day, number of GPS Week 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 ephemeris data provides its own precise orbit and came out in 18 seconds, repeating all 30 seconds. The ephemeris is updated every 2 hours and is usually valid for four hours, with provisions for six hours of downtime. The time needed to acquire the ephemeris is becoming an important element of delay in first position, because, as the material becomes able, time to lock onto the satellite signals shrinks, but the ephemeris data requires 30 seconds (worst case) prior be received, because of the speed of data transmission low. The almanac consists of the orbit of big and status information for each satellite of the constellation and takes 12 seconds for each satellite present, with information for a new satellite are transmitted every 30 seconds (15.5 minutes for 31 satellites). The purpose of these data is to assist in the acquisition of satellites at startup, allowing the receiver to generate a list of visible satellites on stored position and time, while the ephemeris of each satellite is needed to calculate the position corrections using the satellite. In older equipment, the lack of an almanac in a new receiver would result in 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 you do not have an almanac is no longer a problem. One important thing to note about navigation data, is that each satellite transmits its ephemeris, but transmits an almanac for all satellites.
Each satellite transmits its navigation message with at least two distinct spread spectrum codes: the Coarse / Acquisition (C / A) code, which is freely accessible to the public, and precision (P) code, which is usually encrypted and reserved for military applications. The C / A code chip is a 1023 pseudo-random (PRN) code at 1.023 million chips / sec, whereas repeats every millisecond. Each satellite's own C / A code so that it can be uniquely identified and received separately from other satellites transmitting on the same frequency. The P-code is a Megachip 10.23 / sec PRN code that merely repeat each week. When anti-spoofing "mode is activated, as it is in normal operation, P code is encrypted Y-code to produce the P (Y) code, which can only be decrypted by units with a valid decryption key. Both the C / A and P (Y) gives the codes for the accuracy of time of day 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 and the L1C news about the future Block III satellite.
* L2 (1227.60 MHz): P (Y) code, and the new L2C code on the Block IIR-M and new satellites.
* L3 (1381.05 MHz): Used by the nuclear explosion (NUDET) Detection System Payload (NDS) to signal detection of nuclear explosions and other high-energy infrared events. Used to enforce nuclear test ban treaties.
* L4 (1379.913 MHz): being studied for additional ionospheric correction.
* L5 (1176.45 MHz): Proposal for use as a civilian backup Life (SoL) signal (see GPS modernization). This frequency falls in a range of international protection for air, promising little or no interference under all circumstances. The first Block IIF satellite that would allow this signal is ready to be launched in 2008.
[Edit Calculation] positions
[Edit] Using the C / A code
To start, the receiver that captures C / A codes to listen by PRN number, based on almanac information it has previously acquired. As it detects the signal from each satellite, it identifies it by its distinct C / A code model, then measures the time for each satellite. To do this, the receiver produces an identical C / a sequence using the number of seeds that the satellite. By aligning the two sequences, the receiver can measure the delay and calculate the distance to the satellite, called the pseudo [12].
pseudo-overlap, represented the curves are modified to yield the probable position
pseudo-overlap, represented as curves, are modified to give probable position
Then, the orbital position data, or ephemeris, the navigation message is then downloaded to calculate the position accurate satellite. A more sensitive receiver will potentially acquire the ephemeris data faster than a 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 of a imaginary sphere centered on that satellite and whose radius is the distance to it. The receivers can replace the altitude of a satellite, the GPS receiver resulting in a pseudo-distance measured from the center of the earth.
Places are calculated not in space in three dimensions, but in space-time four dimensions, which is a measure of the exact duration of the day is very important. The pseudo-distance measurements from four satellites already been determined with the internal clock of the receiver, and thus have an unknown amount of clock error. (The error real or clock time is irrelevant in calculating the initial nickname, because it's based on how much time has elapsed between receipt of each signal. [Clarify] [citation needed]) The four-dimensional point equidistant from the pseudo-distance is calculated as a guess on the location of the receiver, and the factor used to adjust the pseudo-distances to the intersection at that point in four dimensions give a guess as to the receiver clock offset. For each proposal, a geometric dilution of precision (GDOP) vector is calculated on the basis of the relative sky positions of the satellites used. As more satellites are picked up, pseudoranges from several combinations of four satellites can be processed to add more guesses the location and clock offset. The receiver then determines the combinations to use and how to calculate the estimated position by determining the weighted average of these positions and clock offsets. After the final location and time are calculated, position is expressed in a coordinate system specific, eg, latitude / longitude using the WGS 84 datum or local system to a specific country.
[Edit] Using the P (Y) code
Calculating a position with the P (Y) signal is generally similar in concept, assuming one can decrypt it. The encryption is essentially a safety mechanism: if a signal can be decoded successfully, it is reasonable to assume that it is a genuine signal from a GPS satellite. [Edit] By comparison, civil receivers are highly vulnerable to spoofing since correctly formatted C / A signals can easily be generated using generators signal available. RAIM features do not protect against spoofing, since RAIM only checks the signals from a point of view of navigation.
[Edit] Accuracy and error sources
The position calculated by a GPS receiver requires time current position of the satellite and the delay measured from the received signal. The position accuracy depends mainly on the satellite position and signal delay.
To measure the delay, the receiver compares the bit sequence received by the satellite with an internally generated version. By comparing the rising and trailing edges bit transitions, modern electronics can measure signal offset to within about 1% of a bit time, about 10 nanoseconds for the C / A code Since GPS signals propagate nearly at the speed of light, which represents an error of about 3 meters. This is the minimum error possible using only the GPS C / A signal.
Position accuracy can be improved by using better signal chiprate P (Y). Assuming the same accuracy of 1% of the bit time, the higher frequency P (Y) signal results in an accuracy of about 30 cm.
Electronics errors are one of the degrading effects of precision described in the table below. Taken together, fixed GPS devices civilians are generally horizontal position accuracy of approximately 15 meters (50 ft). These effects also reduce the more precise P (Y) the accuracy code.
Effect Sources user equivalent range error (UERE) Source
Ionospheric effects ± 5 meter
Almanac error ± 2.5 m
Satellite clock errors ± 2 m
Multipath distortion of ± 1 meter
tropospheric effects ± 0.5 meter
Numerical errors ± 1 meter
[Edit] Atmospheric effects
Inconsistencies of atmospheric conditions affect the speed of GPS signals as they traverse the Earth's atmosphere and ionosphere. Correcting these errors is a major challenge to improve 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 for a longer time. Once the approximate location of the receiver is known, a mathematical model can be used estimate and compensate for these errors.
Because ionospheric delay affects the speed of microwave signals differently depending on the characteristic frequency of known dispersion two bands can be utilized to help reduce this error. Some military and costly quality survey civilian receivers 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 receivers without decrypting the P (Y) signal carried on L2, following the carrier wave instead of the code modulation. To facilitate this on receivers at low cost, signal a new civil code on L2, called L2C, was added to the Block IIR-M satellite, which first was launched in 2005. It allows a direct comparison of L1 and L2 signals using the coded signal instead of the carrier wave.
The effects of the ionosphere usually develops slowly, and can be averaged over time. The effects for a particular geographic area can be easily calculated by comparing the GPS-measured at a known surveyed location. This correction is also valid for other receivers in the same place. Several systems that send information to the radio or other links to allow L1 only receivers to make ionospheric corrections. Ionospheric data are transmitted by satellite in Satellite Based Augmentation Systems such as WAAS, which transmits the GPS frequency using a special pseudo-random number (PRN) then a single antenna and receiver are required.
Humidity also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the troposphere. The effect is both more localized and changes more quickly than ionospheric effects and is not dependent on frequency. These traits make precise measurement and compensation of humidity errors more difficult than ionospheric effects.
Changes in altitude also change the amount of delay due to the signal passing through less atmosphere at higher altitudes. Since GPS receiver computes its approximate altitude, this error is relatively simple to correct.
[Edit] multipath effects
GPS signals can also be affected by multiple problems, where the radio signals reflect off surrounding terrain, buildings, canyon walls, hard ground, etc. These delayed signals can cause errors. A variety of techniques, including closer spacing correlator, have been developed to mitigate Multiple errors. For long delay multipath, the receiver itself can recognize the wayward signal and discard it. To cope with short-delay multipath from signal reflecting off the ground, specialized antennas may be used to reduce the signal strength received by the antenna. reflections short time are more difficult to filter, because they interfere with the signal true, causing effects almost indistinguishable from routine fluctuations in the delay in the atmosphere.
Multipath effects are much less severe in moving vehicles. When the GPS antenna is in motion, false solutions using reflected signals quickly fail to converge and that the direct signals result in stable solutions.
[Edit] errors ephemeris and clock
The navigation message from a satellite is sent only every 30 seconds. In reality, data contained in these messages tend to be "stale" by a even larger amount. Consider the case where a GPS satellite is boosted Back on a correct orbit, because some time after the operation, the receiver for calculating the satellite position will be incorrect until receives an updated ephemeris. The clocks on board are extremely precise, but they suffer from clock drift. This problem tends to be very low, but can add up to 2 meters (6 ft) of inaccuracy.
This error class is more "stable" than ionospheric problems and tends to change in a few days or weeks rather than minutes. This makes correction fairly simply by sending out an almanac on a separate channel.
[Change] 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 ft) the navigation signal available to the public to confuse, for example, guiding long-range missiles to precise targets. additional details was available in the signal, but in an encrypted form that was available for the U.S. military, its allies and others, government users for most.
SA typically added signal errors by up to about 10 meters (32 feet) horizontally and 30 meters (98 feet) vertically. The vagueness of the civilian signal was deliberately encoded so as not to change very quickly, for example all of the eastern United States might read 30 m off, but 30 m off everywhere and in the same direction. Improving 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 those civilian personnel resulted in a decision of the availability selectively disable. It is ironic, as SA has been introduced specifically for these situations, allowing friendly troops to use the navigation signal Specifically, while at the same time, it deny the enemy. But since SA was also denying the same accuracy to thousands of friendly troops, the turn off or put to an error of zero meters (effectively the same thing) has presented a clear advantage.
In the 1990s, FAA started pressuring the military to permanently disable SA. This would save the FAA millions of dollars each year in the maintenance of their own navigation systems. The military resisted for most of the 1990s, and he finally made a decree SA have withdrawn from the GPS signal. The amount of error added was "zero" [14] at midnight on 1 May 2000, following the announcement by U.S. President Bill Clinton, which allows users to access the L1 signal without error. By the directive, the error induced by SA was amended to add any error on the public signals (C / A code). Selective Availability is still a capacity of GPS system, and the error could in theory, be reintroduced at any time. In practice, given the dangers and costs, which induce the United States and overseas shipping, it is unlikely to be reintroduced, and various government agencies, including the FAA [15], reported that it is not intended to be reintroduced.
The U.S. military has developed the ability to deny local GPS (navigation services and other) hostile forces in a specific area of crisis without affecting the rest of the world or its own military systems, [14].
An interesting side effect Selective Availability hardware is the ability to correct the frequency of cesium and rubidium atomic clocks GPS with an accuracy of about 2 × 10-13 (one in five billion). This represents a significant improvement over the accuracy of clocks first. [Citation needed]
On September 19, 2007, the U.S. Department of Defense announced that they would not get more satellites capable of implementing SA. [16]
[Edit] Relativity
According to the theory of relativity, because of their constant movement and height relative to the Earth-centered inertial frame, the clocks of the satellites are affected by their speed (special relativity) and their potential gravity (general relativity). For GPS satellites, general relativity predicts that atomic clocks at GPS orbital altitudes tick more rapidly, by about 45,900 nanoseconds (Ns) per day, because they are in a gravitational field weaker than the atomic clocks on Earth's surface. Relativity states that the atomic clocks moving at GPS orbital speeds tick more slowly than ground clocks by about 7,200 ns fixed day. When combined, the gap is 38 microseconds per day, a difference of 4.465 parts in 1010. [17]. To reflect this, the frequency standard onboard each satellite is given rate offset prior to launch, making it run a bit slower than the desired frequency on Earth, more specifically, to 10.22999999543 MHz instead of 10.23 MHz [18].
GPS observation processing must also compensate for another relativistic effect, the Sagnac effect. The time scale GPS 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 run-time signal correction having opposite algebraic signs for satellites in Eastern and Western hemispheres heavenly. Ignoring this effect will produce an error-west on the order of several hundreds of nanoseconds, or tens of meters in position [19].
The atomic clocks on board GPS satellites are granted accurately, making the system a practical engineering application of the scientific theory of relativity in a real world environment.
[Edit] GPS jamming and interference
Since GPS signals at terrestrial receivers tend to be relatively low, it is easy for other sources of electromagnetic radiation desensitize the receiver, which makes the acquisition and tracking satellite signals difficult even impossible.
Solar flares are one of those natural emissions and the potential to degrade GPS reception, and their impact can affect the receiving over half of the Earth facing the sun. GPS signals can also be disrupted by natural geomagnetic storms, predominantly found near the poles of the Earth's magnetic field. [20] Another source of problems is the metal embedded in some car windshields to prevent icing, degrading reception just inside the car.
Man-made interference can also disrupt, or jam the signals GPS. In a well-documented case, an entire harbor was unable to receive GPS signals due to unintentional jamming caused by a malfunction TV antenna preamplifier. [21] Intentional jamming is also possible. Generally, stronger signals can interfere with receptor GPS when they are within radio range, or line of sight. In 2002, a detailed description of how to build a short distance GPS L1 C / A jammer was published in the online magazine Phrack [22].
The U.S. government believes that such jammers were used occasionally during the 2001 war Afghanistan and U.S. military seeks to destroy a GPS jammer with a GPS-guided bomb during the war in Iraq. [23] Such a jammer is relatively easy to detect and locate, making it an attractive target for anti-radiation missiles. The British Ministry of Defence has tested a jamming system in the UK West Country on 7 and 8 June 2007. [24]
Some countries allow the use of GPS repeaters to allow reception of signals GPS inside and darkened areas, however, under European law and the United Kingdom, the use of these prohibitions is that the signals can cause interference to other GPS receivers that can receive data from GPS satellites at once and the repeater.
Account Given the potential for both natural and man-made noise, numerous techniques continue to be developed to cope with interference. First is not to rely on GPS as a sole source. According to John Ruley, "IFR pilots should have a backup plan in case of malfunction GPS [25]. Receiver Autonomous Integrity monitoring (RAIM) is a feature now included in some receivers, which is designed to provide a warning to the user if jamming or any other problem is detected. The U.S. military has also deployed their Selective Availability / Anti-Spoofing Module (SAASM) in the Defense Advanced GPS Receiver (DAGR). In demonstration videos, the DAGR is able to detect jamming and maintain the 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 Increase
Increase methods to improve the accuracy based on external information are included in the calculation process. There are many such systems in place and they are generally named or described in how the GPS sensor receives the information. Some systems transmit additional information on sources of error (such as clock drift, ephemeris, or ionospheric delay), others provide direct measurements of how much the signal was off in the past, while a third group provide additional navigational or vehicle information to be included in the calculation process.
Examples of extensions systems include the Wide Area Augmentation System, Differential GPS, Inertial Navigation Systems and Assisted GPS.
[Edit monitoring] states
The accuracy of calculation can also be improved through precise monitoring and measurement of GPS signals in additional resources or replacement.
After SA, which has been extinguished, the largest error in GPS is usually the unpredictable delay through the ionosphere. The satellite broadcast ionospheric model parameters, but errors remain. This is one reason the GPS satellite transmitting on at least two frequencies, L1 and L2. ionospheric delay is defined according to the frequency and the total electron content (TEC) along the path, then measuring the time difference Arrival between the frequencies determines TEC and thus the precise ionospheric delay at each frequency.
Receivers with keys decoding can decode P (Y)-code transmitted on both L1 and L2. However, these keys are reserved for the military and "authorized" agencies and are not accessible to the public. Without keys, it is always possible to use a technique without code to compare the P (Y) codes on L1 and L2 to gain Much of the information itself error. However, this technique is slow, it is currently limited to specialized surveying equipment. In the future, other civil codes should be transmitted on the L2 and L5 frequencies (see GPS modernization, below). Then, all users will able to perform dual frequency measurements and directly compute ionospheric delay errors.
A second form of precise control is called Carrier-Phase Enhancement (CPGPS). The error, which fixes this, is because the pulse transition of the NDP is not instantaneous, and So the correlation (satellite-receiver matching) operation is imperfect. The approach uses CPGPS L1 carrier wave, which has a period 1000 times smaller than that of the C / A bit period, to act as an additional clock signal and resolve the uncertainty. The error difference phase in the normal GPS amounts between 2 and 3 meters (6-10 feet) ambiguity. CPGPS working less than 1% of the perfect transition reduces this error to 3 centimeters (1 inch) of ambiguity. By eliminating this source of error, CPGPS coupled with DGPS normally realizes between 20 and 30 centimeters (8-12 inches) of absolute accuracy.
Related Kinematic Positioning (RKP) is another approach for a precise GPS positioning system. In this approach, determining the signal range can be resolved with an accuracy of less than 10 centimeters (4 in). This is done by adjusting the number cycles in which the signal is transmitted and received by the receiver. This can be accomplished using a combination of differential GPS (DGPS) data correction, to provide GPS signal phase information and techniques for ambiguity resolution by means of statistical tests, perhaps with a real time processing (real time kinematic positioning, RTK).
[Edit] GPS time and date
While most clocks are synchronized Coordinated Universal Time (UTC), the atomic clocks of the satellites are set to GPS time. The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not leap seconds or other corrections which are periodically made to UTC. GPS time was set to correspond to Coordinated Universal Time (UTC) in 1980, but has since diverged. The absence of corrections means that GPS time remains at a constant offset (19 seconds) with International Atomic Time (TAI). periodic corrections are made on the clocks board to correct relativistic effects and keep them synchronized with ground clocks.
The GPS navigation message includes the difference between UTC and GPS which in 2006 was 14 seconds. Receivers subtract this offset from GPS time to calculate the values UTC and specific timezone. New GPS can indicate UTC not correct after receiving the UTC offset message. The offset field can accommodate GPS-UTC leap 255 seconds (eight bits) which, pace current change of Earth rotation, is sufficient to last until the year 2330.
Unlike the year, month, and format day of the Julian calendar, the GPS date is expressed as a number of days per week and the number of the week. The week number is transmitted as a field ten bits in the C / A and P (Y) navigation messages, and it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (0:00:19 TAI) on January 6, 1980 and the week number became zero again for the first time at 11:59:47 p.m. UTC August 21, 1999 (0:00:19 TAI, August 22, 1999). To determine the date of the Gregorian day, a GPS receiver must be provided with the approximate date (within 3584 days) to interpret GPS date signal correctly. To address this concern the modernized GPS navigation messages use a field of 13-bit, which only repeats every the 8192 weeks (157 years), and will not reset until about the year 2137.
[Edit] GPS modernization
Main article: GPS modernization
After reaching the program's requirements for full operational capability (FOC) on July 17, 1995 [27], the GPS completed its original design goals. However, further progress in technology and new demands on the system led to the current effort to modernize the GPS system. Announcements Vice-President and the White House in 1998 initiated these changes, and in 2000, the U.S. Congress authorized the effort, referring to what the GPS III.
The project aims to improve the accuracy and availability for all users and involves new ground stations, new satellites and four additional navigation signals. New civilian signals are called L2C, L5 and L1C; new military code is called M-Code. The initial operational capability (IOC) of the L2C code is expected in 2008 [28]. An objective of 2013 was established for all program, with incentives offered to contractors if they can complete it by 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 military and civilian industry.
[Edit] Military
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The military use GPS for the following purposes:
[Edit] Navigation
GPS allows soldiers to find objectives in the dark or in the unknown territory, and coordinate the movement of troops and supplies.
[Edit] Objective Monitoring
Various military weapons systems use GPS to track potential ground and air targets before they are marked as hostile. These weapon systems pass GPS coordinates of targets precision-guided munitions to allow them to engage targets with precision.
military aircraft, particularly those used in roles of air-ground use GPS to find targets (for example, a video camera canon AH-1 Cobra GPS co-ordinates show that Iraq can be searched in Google Earth).
[Edit] guiding missiles and projectiles
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 to forces of 12,000 G have been developed for use in 155 mm howitzers [29].
[Change] Search and Rescue
Slaughtered pilots can be faster if they have a receiver.
[Edit] Reconnaissance and Map Creation
The military use GPS to high scale mapping and aid recognition.
[Edit]
The GPS satellites also carry detonation detectors Nuclear, which form an important part of the nuclear detonation U.S. detection system [30].
[Edit] Civil
See also: GPS applications
This antenna is mounted on the roof of a hut containing a scientific experiment needing timely.
This antenna is mounted on the roof of a hut containing a scientific experiment needing timely.
Many civilian applications benefit from GPS signals, using one or more of three basic components of GPS absolute location, relative movement, the transfer time.
Capacity to determine the absolute position of the GPS receiver can operate as an investigative tool or 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 exacting standards for the transfer time, which is critical in large communication and observation systems. An example is CDMA digital cellular. Each base station has a GPS receiver to synchronize the timing of its spreading codes of other base stations to facilitate the hand between the cells and non-support hybrid GPS / CDMA positioning of mobiles for emergency calls, and other applications.
Finally, GPS enables researchers to study terrestrial environment, including atmosphere, ionosphere and gravity field. GPS survey equipment has revolutionized tectonics by directly measuring the movement of faults in earthquakes.
To help prevent civilian GPS guidance from being used military purposes of an enemy or improvised weapons, the U.S. government controls the export of civilian receivers. A U.S. manufacturer does can not generally export a GPS receiver unless the receiver contains limits restricting it from functioning when it is both (a) at an altitude of more than 18 km (60,000 ft) and (2) moving faster than 515 m / s (1,000 knots) [31].
[Edit] History
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The design of GPS is based partly on the same ground navigation systems, such as LORAN and the Decca Navigator developed in the early 1940s and used during the Second World War. added inspiration for the GPS system came when the Soviet Union launched the first Sputnik satellite in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. They discovered that, due to the Doppler frequency signal transmitted by Sputnik has been higher than the satellite approached, and lower as it continued away from them. They realized that because they knew their exact location on the globe, they could identify where the satellite was along its orbit by measuring the Doppler distortion.
The first satellite navigation system, Transit, used by U.S. Navy, was first successfully tested in 1960. Using a constellation of five satellites, it could provide a solution Navigation approximately once per hour. In 1967, the U.S. Navy has developed the satellite Timati which proved the possibility of placing accurate clocks in space technology based on GPS. In the 1970s, soil Omega Navigation System, based on the comparison of signal phase, became the first global navigation system.
The first experimental Block-I GPS satellite was launched in February 1978. [28] Satellites GPS was initially manufactured by Rockwell International and are now manufactured by Lockheed Martin. "
[Edit] Chronology
* In 1972, the U.S. Air Force Central Inertial Guidance Test Facility (Holloman AFB) conducted developmental testing of two prototype GPS receivers struggle to White Sands Missile Range in soil using pseudo-satellites.
* In 1978, the first experimental Block-I satellite was launched GPS.
* In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 in restricted Soviet airspace, killing all 269 people aboard, U.S. President Ronald Reagan announced that GPS would be available for civilian use once it has been completed.
* In 1985, ten more experimental Block-I satellites were launched to validate the concept.
* On February 14, 1989, the first modern satellite, Block-II was launched.
* In 1992, the 2nd Space Wing, which originally Management System, has been disabled and replaced by the 50th Space Wing.
* In December 1993, the GPS system achieved initial operational capability [32]
* In January 17, 1994, a full constellation of 24 satellites in orbit has been.
* Full operational capacity was declared by NAVSTAR in April 1995.
* In 1996, recognizing the importance of GPS to civilian users and military users, U.S. President Bill Clinton issued a policy directive [33] GPS telling a dual-use system and establish an inter-GPS of the Executive Council to manage as a national asset.
* In 1998, U.S. Vice President Al Gore has announced plans to upgrade GPS with two new civilian signals for the accuracy of the user and reliability, particularly with regard to aviation safety.
* On May 2, 2000 "Selective Availability" was abandoned following the 1996 Ordinance on management, allowing users to receive a signal not degraded in the world.
* In 2004, the United States government signed a historic agreement with the European Community establishing cooperation related to GPS and the European system Galileo expected.
* In 2004, U.S. President George W. Bush to update the national policy, replacing the Executive Board with the spatial positioning National Navigation and Timing Executive Committee.
* November 2004, Qualcomm announced successful tests of the system, Assisted-GPS phones mobile [3].
* In 2005, the first modernized GPS satellite was launched and began transmitting a second civilian signal (L2C) for performance Improved user.
* The latest launch was November 17, 2006. The oldest GPS satellite still in operation was launched in August 1991.
* On September 14, 2007, the aging mainframe ground segment control system has been transferred to the new Architecture Plan Evolution. [4]
[Edit] number of satellites
Name Launch Period No. of satellites launched, inc. Currently launch failures in service
Block I 1978-1985 11 0
Unit II 1985-1990 9 0
Block IIA 19 15 11 1990-1997
Block IIR 1997-2004 December 12
3 Block IIR-M 2005-3
Total 54 (plus one not running) 30 +1
1One test satellite
[Edit]
Two GPS developers have received the National Academy of Engineering Charles Stark Draper prize year 2003:
* Ivan Getting, president emeritus of the Aerospace Corporation and engineer the Massachusetts Institute of Technology, has created the basis for GPS, improving the system WWII, called terrestrial radio, LORAN (Long-distance radio aids to navigation).
* Bradford Parkinson, professor of Aeronautics and Astronautics University Stanford, designed the current satellite system in the early 1960s and developed in collaboration with the U.S. Air Force.
Developer GPS, Roger L. Easton received the National Medal of Technology on February 13, 2006 at the White House [34].
On February 10, 1993, the National Aeronautic Association selected the Global Positioning System Team as winners of the 1992 Robert J. Collier Trophy, the highest award of aviation in the United States. This team is composed of researchers from the Naval Research Laboratory, 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 and efficient navigation and monitoring of air and spacecraft since the introduction of radio navigation 50 years ago. "
[Edit] Other systems
Main article: Global Navigation Satellite System
other satellite navigation systems in use or various states of development include:
* Beidou – regional system of China that China has proposed to expand into a global system COMPASS appointed.
* Galileo – a proposed global system being developed by the European Union, joined by China, Israel, India, Morocco, Saudi Arabia and South Korea, Ukraine should be operational by 2011-12.
* GLONASS – Global System Russia, which is being restored to full availability in partnership with India.
* Indian Regional Navigation Satellite System (IRNSS) – India proposed regional system.
* QZSS – Japanese proposed regional system, adding better coverage to the Japanese islands.
[Edit] See also
Satellite navigation systems Portal
Nautical Portal
* RAIM
* SIGI
radio Navigation *
* High sensitivity GPS
* Degree Confluence Project uses the GPS to visit integral degrees of latitude and longitude.
* Exif, GPS data transfer.
* Geolocation
* Geocaching
* NaviTraveler.com – a GPS point community sharing.
* GPS Drawing Digital mapping and drawing with GPS tracks.
* GPS Tracking
* GPS / INS
* Assisted GPS
* GPX (XML schema for the exchange waypoints)
* ID Sniper rifle
* OpenStreetMap, maps and photos free content from the street (UTC)
* Telematics: Many telematics devices use GPS to determine location of equipment mobile.
* The American Practical Navigator Chapter 11-satellite navigation "
* Landmark
* System car navigation
* NextGen
[Edit]
1. ^ Parkinson, BW (1996), Global Positioning System: Theory and Applications, chap. 1: Introduction and Heritage of NAVSTAR, the global positioning system. pp. 3-28, American Institute of Aeronautics and Astronautics, Washington, DC
2. Ab ^ GPS Overview from the NAVSTAR Joint Program Office. Accessed December 15, 2006.
3. HowStuffWorks ^. How Receivers GPS work. Accessed May 14, 2006.
4. Globalsecurity.org ^ [1].
5. ^ Dana, Peter H. GPS orbital planes. August 8, 1996.
6. ^ This the Global Positioning System Tells Us about Relativity. Accessed January 2, 2007.
7. ^ USCG Navcen: GPS Frequently Asked Questions. Accessed January 3 2007.
8. Massatt ^, and Paul Brady, Wayne. "Optimizing performance through constellation management, Crosslink, Summer 2002, pages 17-21.
9. ^ United States Coast Guard General GPS News 9-9-05
10. ^ USNO. NAVSTAR Global Positioning System. Accessed 14 May 2006.
11. ^ NMEA NMEA 2000
12. ^ Http: / / gge.unb.ca / Resources / HowDoesGPSWork.html
13. ^ AN02 Assistance Network (HTML). Accessed 10/09/2007.
14. Ab ^ Office of Science and Technology Policy. Statement by the President to stop degrading GPS. 1 May 2000.
15. ^ FAA, the selective availability. Retrieved 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 Nelson A. Robert Via Satellite, November 1999
19. ^ Ashby, Neil Relativity and GPS. Physics Today, May 2002.
20. ^ Space Environment Center. GPS Navigation Systems Page SEC. August 26, 1996.
21. ^ The hunt for an unintentional GPS jammer. GPS World. January 1, 2003.
22. ^ Low Cost and Portable GPS Jammer. Phrack issue 0×3c (60), Article 13]. Published December 28, 2002.
23. ^ Press Service American Forces. 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. Hosted Joint Chiefs website, linked by the USAF GPS Wing DAGR program website. Accessed April 10, 2007
27. ^ U.S. Coast Guard press new. Global Positioning System Fully Operational
28. ^ Journal of AB basin. The evolution of global satellite navigation. Issue # 104 April 2002. Accessed 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 of non-proliferation and Arms Control Technology.
31. ^ Arms Control Association. Control Regime missile technology. Accessed May 17, 2006.
32. ^ U.S. Department of Defense. Announcement of the initial operational capability. December 8, 1993.
33. ^ National Archives and Records Administration. Politics U.S. Global Positioning System. 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
* Site GPS.gov general public education created by the Government of the United States
* National Space-Based PNT Executive Committee-Created in 2004 to oversee management of GPS and GPS increases at national level.
* USCG Navigation Center-condition of the GPS constellation, government policy, and links to other references. Also includes satellite almanac data.
* The GPS Joint Program Office (JPO GPS)-Responsible design and acquisition system on behalf of the Government of the United States.
* U.S. Naval Observatory's GPS constellation status
* U.S. Army Corps of Engineers manual: NAVSTAR HTML and PDF (22.6 MB, 328 pages)
PNT Selective Availability Announcements *
* Specification SPS GPS signal, 2nd Edition-The official Standard Positioning Signal specification.
* Federal Aviation Administration GPS FAQ
Introduction / tutorial links
* How does GPS work? TomTom GPS says, navigation and digital mapping
* Garmin GPS Academy explaing interactive website Web video exactly what GPS is and what it can do for you
* Nicolas is passionate simplified GPS and video about how GPS works.
* Trimble GPS online tutorial designed to introduce you the principles underlying GPS
* GPS and GLONASS Simulation (Java applet vehicle motion) simulation and graphic representation of space, including the calculation of the dilution of precision (DOP)
Technical, historical, and links to related topics
* Dana, Peter H. "Global Positioning System Overview"
* Satellite Navigation: GPS and Galileo (PDF)-16-page paper on the history and working of GPS, Galileo related to the next
* History of GPS, including information on the configuration of each satellite and launch.
* Chadha, Kanwar. "The Global Positioning System: Challenges to put GPS to mainstream consumers" technical article (1998)
* Techniques weapon guidance GPS
* RAND history of the GPS system (PDF)
* Techniques for GPS Anti-Jam Protection
* Crosslink Summer 2002 issuance by the Company Aerospace on satellite navigation.
* Improved weather forecasts from data COSMIC GPS satellite signal occultation.
GPS * s L. David Wilson Accuracy Web Page A thorough analysis of the accuracy of GPS.
* Innovation: Spacecraft Navigator, the GPS stand-alone high Earth orbits Example of GPS receiver designed for high altitude flights.
* Receiver browser GPS Navigator GSFC spaceflight.
* Neil Ashby Relativity in the Global Positioning System
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Operational Flag of the Soviet Union / Flag of Russia GLONASS GPS Drapeau · U.S.
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