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 microwave signals accurate, the system enables a GPS position, speed and direction determine and time.
Developed by the U.S. Department of Defense, it 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 policy makers for budget GPS Program [1]). The satellite constellation is determined by the United States Air Force 50th Space Wing manages. The cost of system maintenance is about $ 750 million per year [2], including the replacement of aging satellites, and research and development. Despite this cost GPS is free for civilian use as a public good.
GPS has become a tool widely used for navigation around the world and a useful tool for mapping, surveying, commercial and scientific purposes. GPS provides precise time reference used in many applications including scientific study will be used by earthquakes, and synchronization of telecommunications networks.
Simplified procedures for the operation
A GPS receiver calculates its position by measuring the distance between itself and three or more GPS satellites. The extent of the delay between sending and receiving a microwave signal GPS provides distance from each satellite, since the signal at a known speed – the speed of light. These signals lead information on the location of satellites and the general health system (such as almanac and ephemeris data “). The determination of position and distance of at least three satellites, the receiver can calculate its position using trilateration. [3] receiver is usually not perfectly accurate clocks and therefore track one or more additional satellites, with their own clocks atomic clock receiver to correct errors.
[Edit] Technical description
Unlaunched GPS satellite display in the Museum of the San Diego Aerospace
Unlaunched GPS satellite display in the Museum of the San Diego Aerospace
[Edit] segmentation system
The current GPS consists of three segments. This is the space component (SS), a control segment (CS), and a user segment (U.S.). [4]
[Edit] Space Segment
The space segment (SS) GPS satellites are in orbit or space vehicles (SV) Using GPS together. The GPS will design calls for 24-SVS equitably distributed among six circular orbits. [5] The orbits are centered on the earth does not rotate relative to distant stars. [6] The six planes have approximately 55 ° inclination (tilt relative) at the equator of the earth and a rise of 60 ° to the right of the ascending node (angle along the equator from a point reference until it is isolated at the junction of the railway). [2]
Orbit at an altitude of about 20,200 kilometers (12,600 miles or 10.900 nautical miles, the orbital radius of 26,600 kilometers (16,500 miles or 14,400 NM)) done every two complete orbits each sidereal day SV, then it goes to the same place on the Earth every day. The orbits are arranged so that at least six satellites always in the crosshairs of everywhere on Earth. [7]
Since September 2007, there are 31 actively broadcasting satellites in the GPS constellation. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increasing number of satellites, the constellation was changed to an irregular arrangement. Such an arrangement has been shown that reliability and system availability based on a single system, if not improve, several satellites. [8]
[Edit] control segment
The flight paths are monitored by satellite stations Force U. S. air monitoring in Hawaii, Kwajalein, Ascension Island Iceland, Diego Garcia, and Colorado Springs, Colo., will operate in conjunction with the monitoring stations of the National Geospatial-Intelligence Agency (NGA). [9] The tracking information is captain of the Air Force Space Command Control Station at Schriever Air Force Base, Colorado Springs, which is operated by the 2D-Space Operations Squadron (2 IPO) of the United States Air Force (USAF) sent. 2 SOPS contacts of GPS satellites with a regularly updated navigation (using the ground antennas at Ascension Island, Diego Garcia, Kwajalein and Colorado Springs). These updates synchronize the atomic clocks on board satellites in a microsecond and adjust the ephemeris of the internal model of the orbit of each satellite. The updates are used by a Kalman filter that input from ground stations, information space weather and other factors of production created. [10]
GPS receivers come in a variety of formats, from devices in cars that integrates mobile phones and watches, dedicated devices, as illustrated here by the manufacturers Trimble, Garmin and Leica (from left to right).
GPS receivers come in a variety of formats, from devices in cars that integrates mobile phones and watches, dedicated devices, as illustrated here by the manufacturers Trimble, Garmin and Leica (from left to right).
[Edit] User segment
The user’s GPS receiver is the user segment (U.S.) GPS system. In general, the GPS receiver are set an antenna for frequencies of the satellite-receiver-processors designed to transfer, and a very stable clock () is often a crystal oscillator. You can also use a display to provide position and velocity information for the user. A receiver is often described by the number of channels: it is, 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 use SiRF Star III, size 15 × 17 mm, and in many products.
A typical OEM GPS receiver module, based on the use SiRF Star III, size 15 × 17 mm, and in many products.
GPS receivers can also be an input for differential corrections, with the RTCM SC-104 format. It is usually in the form of a RS-232 speed BPS 4800. The data are actually much more at a lower price, thereby limiting the accuracy of the signal sent sent using RTCM format. Receivers with internal DGPS receivers can outperform the use of external RTCM data. Since 2006, units of low cost yet is often the Wide Area Augmentation System (WAAS) receiver.
Many GPS position data relay to a PC or other device that uses protocol NMEA 0183. NMEA 2000 [11] protocol is a relatively recent and widely adopted. The copyright protected and are 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 the minutes without reading all the laws on intellectual property. Other proprietary protocols 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
GPS Signal
GPS Signal
Each GPS satellite continuously broadcasts a navigation message at 50 basis points is the hour of the day (GPS week number and satellite health information at all) forwarded 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 indicates own precise orbit of the satellite and spent about 18 seconds and repeat every 30 seconds. Ephemerides are all 2 hours and is generally valid for 4 hours, 6 hours with the provisions downtime. The time required to obtain the ephemeris, is always an important element for the delay in resolving the first position, because, as the material is better able to block at the time of the satellite signals shrinks, but the ephemeris data required 30 seconds (worst case) before being received because of the lower bit rate. The schedule consists of large orbit and status information for each satellite in the constellation, and takes about 12 seconds for each satellite to date transferred, with information for a new satellite every 30 seconds (15. 5 minutes for 31 satellites). The purpose of these data is the acquisition of satellites to power, giving support to create the receiver based on a list of satellites visible to the stored position and time, while the ephemeris of each satellite is needed to calculate position corrections to the satellite. With older equipment that would, in the absence of an almanac in a new receiver for long periods, leading to a valid position, because the search for each satellite was a slow process. Advances in hardware have made the acquisition process much faster, not with an almanac is not a problem. One important thing is to compile data on the menu at each satellite transmits its own ephemeris, but transmits a timetable for all satellites.
Each satellite transmits its navigation message with at least two different codes spread spectrum: the Coarse / Acquisition (C / A code), which is freely accessible to the public, and precision (P) code, which encrypts a rule reserved for military applications. The C / A code is a pseudo-1023-chip-random number (PRN) code of the 1 023 million chips per second, so it repeats every millisecond. Each satellite has its own C / A code so that it can be clearly identified and separated from other satellites, received at the same frequency. The P-code is a 10th 23 Mega chips per second NAP code that merely repeat each week. If the “anti-spoofing mode because it is in normal operation, the P-code encrypted by the Y-code to produce the P (Y) code, the only units with a decryption key valid can be decrypted. Both the C / A and P (Y) codes provide accurate time of day for the user. Are frequencies used by GPS
* L1 (1575e 42 MHz): Mix of Navigation Message, coarse acquisition (C / A code) and P accurately quantified (Y) code and the new L1C on future Block III satellites.
L2 * (1227. 60 MHz): P (Y) Code and the new L2C code on the Block IIR-M satellites and new.
L3 * (1381. 05 MHz): Is the nuclear detonation (NUDET) Detection System Payload (NDS) for the detection of nuclear explosions and other events of great energy of the infrared signal. Used to enforce the treaty banning nuclear tests.
* L4 (1379. 913 MHz): As a complementary study of the ionospheric correction.
* L5 (1176. 45 MHz) for use as a civilian safety-of-life proposed (SoL) signal (see GPS) modernization. This frequency falls within a range internationally protected for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that would provide this signal, will be launched in 2008 to life.
[Edit] Calculation of positions
[Edit] Using the C / A code
At the beginning of the beneficiary takes the C / A codes to listen to PRN number, based on the almanac information it previously acquired. As it detects each satellite signal, it is known by its distinct C / A code model, then measures the time delay of each satellite. To do this, the receiver produces a C or an identical sequence with the same number as the seed satellite. By aligning the two sequences, the receiver can measure the delay and calculate the distance to the satellite, called the pseudorange [12].
Nickname overlapping ranges, is presented as curves are modified to give the probable position
Nickname overlapping ranges, is presented as curves are modified to give the probable position
Subsequently, the orbital position data, or ephemeris, from the navigation message is then downloaded to the exact position of the satellite must be calculated. A-sensitive receiver will potentially acquire the ephemeris data faster than a less sensitive receiver, especially in a noisy environment. [13] Knowing the location and distance of a satellite indicates that the receiver centered somewhere on the surface of an imaginary ball on this satellite is located and whose radius is the distance. Receivers can substitute an altitude satellite, the GPS receiver results in a pseudo-distance measured from the center of the earth.
Locations are not calculated in three dimensional space, but space-time into four dimensions, which is a measure of the exact time of day is very important. The measured pseudo-ranges from four satellites are already determined by the internal clock of the receiver and then an unknown amount of clock error. (The error clock or real time is not important in the first pseudo-range calculation, because it’s based on how much time elapsed between the receipt of individual signals. [Clarify] [EDIT ]) The four points equidistant dimensional removed from the pseudo ranges than a presumption, calculated on the location of the recipient, and the factor by which these intersect with the pseudo-ranges to adapt to the four-point scale is a presumption against the clock of the receiver balanced. Each proposal will be charged a geometric dilution of precision (GDOP) vector, based on the relative positions of sky using satellites. Satellites are more compact, pseudo-ranges can be processed by more combinations of four satellites to more speculation about the location and clock offset added. The receiver then determines what combinations to use, and how the estimated position by determining the weighted average of these positions and clock offsets should be calculated. Are calculated after the last time and place is the location specified in a coordinate system, E. G. expressed in latitude / longitude, with the WGS 84 geodetic or local system, especially for a country.
[Edit] Using the P (Y) Code
Calculate the position with the P (Y) signal is provided as a rule, similar in concept, we can decode. The encryption is essentially a safety mechanism: if a signal is correctly decoded, it is reasonable to accept a real signal is sent from a GPS satellite. [Edit] In comparison, civil receivers are highly vulnerable to spoofing since correctly formatted generated C / A signals with signal generators readily available. RAIM does not protect against spoofing, since RAIM only signals from a standpoint of navigation controls.
[Edit] Accuracy and Error
The position of a GPS receiver calculates the actual time needed, the satellite position and measure the delay of the received signal. The positioning accuracy depends essentially on the position and time signal satellite.
To measure the delay, the receiver compares the bit stream from the satellite with an internally generated version received. By comparing the rising and falling edge of bit transitions, modern electronics can measure signal offset to about 1% a short time, about 10 nanoseconds for the C / A code. Since GPS signals propagate nearly at the speed of light, it is an error of about 3 meters. It is the smallest error can only with the GPS C / A signal.
Position accuracy can be improved by the use of higher chip rate P (Y). Assuming the accuracy of 1% is a bit of time, the high frequency P (Y) with an accuracy of about 30 centimeters.
Electronics errors listed a number of precision effects depleting in the table below. When taken together, constitute an autonomous civilian GPS horizontal position statements, typically about 15 meters (50 ft). These effects also reduce the more precise P (Y), the accuracy of the code.
Sources of user range equivalent Errors (Vere) Source Effect
Ionospheric effects ± 5 meter
Ephemeris errors ± 2 5 meters
The satellite clock errors ± 2 m
Multipath distortion ± 1 meter
Troposphere ± 0 5 meters
Numerical errors ± 1 meter
[Edit] Atmospheric effects
Inconsistencies of atmospheric conditions affect the speed of GPS signals as they pass through Earth’s atmosphere and ionosphere. Correcting these errors is a major challenge to improve the accuracy of GPS position. These effects are smallest when the satellite is directly over time will be larger and closer on the horizon for the satellite because the signal is affected for a long period. Once the receiver is known, the approximate location, a mathematical model can be used to estimate and compensate these errors.
Because ionospheric delay the speed of microwave signals a different effect on the frequency of a feature called dispersion is based on both frequency bands can be used to reduce this error. Some investigation and expensive military grade receivers Civil compare various delays in the L1 and L2 frequencies to measure atmospheric propagation, and apply a more precise correction. This can be done in civilian receivers without decrypting the P (Y) signal carried on L2, by tracking the carrier wave instead of the code modulated. To facilitate this known at a lower cost receivers, a new civil code signal on L2, L2C, was added to the IIR-M satellite Block, which was first launched in 2005 to life. It allows a direct comparison of L1 and L2 signals with the coded signal instead of the carrier wave.
The effects of the ionosphere generally change slowly and may stagger. The implications for a given geographic area can be easily studied by comparing the measured GPS position calculated in a known location. This fix also applies to other recipients in the same place. Several systems send the information via radio or other links to allow L1 only receivers to make ionospheric corrections. The ionospheric data are transmitted by satellite in Satellite Based Augmentation Systems such as WAAS, which will go to the GPS frequency using a special pseudo random number generator (PRN), so a single antenna and receiver is necessary for the transfer.
Humidity also causes a variable delay, so that errors such as ionospheric delay, however, occur in the troposphere. This effect is both localized and changes faster than the effects of the ionosphere, and is not dependent on frequency. These properties make precise measurement and compensation of humidity errors more difficult than ionospheric effects.
Changes in the quantity and the amount of delay through the signal within the atmosphere at higher altitudes. Since the GPS receiver calculates the approximate altitude, this error is relatively easy to correct.
[Edit] The effects of multipath
GPS signals can also be due to problems with multipath, which reflects radio signals to be affected by the surrounding terrain, buildings, canyon walls, hard ground, etc. These delayed signals can cause inaccuracies. A variety of techniques have been particularly narrow correlator spacing, designed to reduce the multipath errors. For the delay of a multipath receiver itself can recognize the wayward signal and discard it. To address multipath shorter period from the signal reflecting the earth, special antennas can be used to reduce power as a signal from the antenna. Short delay reflections are harder to filter because they interfere with the true signal, so that effects almost indistinguishable from routine fluctuations in the delay of the atmosphere.
Multipath effects are much less stringent in moving vehicles. If the GPS antenna is moving, fail to converge quickly with the wrong solutions reflected signals and direct signals only lead to stable solutions.
[Edit] ephemeris and clock errors
The navigation message is sent from a single satellite once every 30 seconds. In reality, the data contained in these messages is usually “out of date” with an even greater amount. Consider the case when a GPS satellite will be raised again in the right way, and for some time after the maneuver, the receiver calculates the satellite position is wrong until it receives an updated ephemeris. The clocks on board are very accurate, but they suffer from some clock drift. This problem tends to be very low, but up to 2 meters (6 feet) of inaccuracy.
This error is “stable” than ionospheric problems and tends to change in a few days or weeks instead of minutes. This fix is quite simple, providing a more accurate almanac on a separate channel.
[Edit] Selective Availability
The GPS includes a feature called Selective Availability (SA), which leads intentionally moving slowly random errors of up to one hundred meters (328 ft) in the navigation signal available to the public confused, for example, guided missiles long range to specific targets. Additional precision was the signal is available, but in an encrypted form that only U.S. forces, their allies and others who most government users.
SA generally signal errors to about 10 m (32 ft) horizontally and 30 meters (98 ft) vertically. The inaccuracy of the civilian signal was deliberately not change in coded form as quickly as any region in the eastern United States, 30 meters could be read, but only 30 m from the same everywhere and in the right direction. To improve the usefulness of GPS for civilian navigation, Differential GPS was developed by many civilian GPS receivers used to improve the high accuracy.
During the Gulf War, the shortage of military GPS units and the wide availability of civilian personnel, in which a decision to disable Selective Availability conducted. It was ironic, as SA had been specially imported for these situations to deny the use of friendly troops on the signs to accurate navigation, while at the same time to the enemy. But since SA is also no denying the same accuracy for thousands of friendly troops, or turn it to an error of zero meters (effectively the same thing) has a distinct advantage.
In the 1990s, the FAA started pressuring the military to turn SA permanently. This would save the FAA millions of dollars each year to maintain their own radio-navigation systems. The military resistance to most of the 1990s, and he finally passed a law to have SA in the GPS signal is removed. The amount of error was added to zero [14] at midnight on 1 In May 2000, following the announcement by U. S. President Bill Clinton, the user access to the L1 signal error. By the directive, the error induced by SA was changed to no fault of the public record signals (C / A code). Selective Availability is even a GPS system capacity and errors can, in theory, be reintroduced at any time. Would in practice regarding the risks and costs to the United States and encourage foreign sailor, he is unlikely to be reintroduced, and reported by several government agencies including the FAA, [15 ] have determined that it is not intended to be reintroduced.
The U.S. military has the ability to refuse locally (GPS navigation and other services) to hostile forces in a specific area of crisis without the rest of the developed world or its own military systems. [14]
An interesting side effect of Selective Availability hardware is the ability of the GPS frequency of cesium and rubidium atomic clocks correctly) with an accuracy of about 2 × 10-13 (one of five billion dollars. It was a significant improvement the accuracy of the first clock. [Edit]
On September 19, 2007 announced the U.S. Department of Defense, he would not give more satellites for the implementation of SA. [16]
[Edit] Relativity
According to the theory of relativity, which because of their relatively constant movement and height to Earth-Centered Inertial, satellite clock their speed (special relativity) and affected by the potential gravity (general relativity). For GPS satellites, general relativity predicts that the faster the atomic clocks on GPS orbital altitudes low, approximately 45.900 nanoseconds (ns) per day, because they are in a weaker gravitational field than atomic clocks on the surface of Earth. Special Relativity states that atomic clocks moving at GPS orbital speeds slower than the clocks on the ground to stop to check some 7200 ns per day. In this combination, the difference of 38 microseconds per day, a difference of 4 is 465 coins 1010th [17]. To explain this, the frequency standard onboard each satellite is replaced by the offset to start a sentence before, making it slightly slower than the desired frequency on Earth, in particular 10 22999999543 MHz instead of the 10th 23 MHz. [18]
GPS observation must also compensate for 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-) rotation system in which simultaneity is not clearly defined. The Lorentz transformation between the two systems modifies the run-time signal correction with opposite signs for satellites in the Eastern Hemisphere and the Western sky. Ignoring this effect will produce an error-west in the order of hundreds of nanoseconds, or tens of meters in position. [19]
The atomic clocks on board GPS satellites are precisely tuned, making the system engineering of a practical application of scientific theory of relativity in a real 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 to desensitize the receiver, which acquisition and tracking satellite signals difficult or impossible.
Solar flares are one of those natural emissions have the potential to affect GPS reception, and their impact can affect reception over half the earth’s sun. GPS signals can also be affected by naturally occurring geomagnetic storms, predominantly found near the poles of the magnetic field of the earth. [20] Another source of problems is the metal embedded in some car windows, to prevent ice formation, degrading reception just inside the car.
Man-made interference can also disrupt, or jam, GPS signals. In a well-documented case, a whole port was unable to receive GPS signals due to interference by unintentional causes dysfunction of the TV antenna preamplifier. [21] Intentional Interference is also possible. Generally, stronger signals can interfere with GPS receiver, if radio range or line of sight. [22]
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