Global Positioning System
Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). Utilizing constellation of at least 24 medium Earth orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine location, speed and direction, and time.
Developed by the U.S. Department of Defense has officially designated NavStar GPS (Contrary to popular belief, NavStar not an acronym, but simply by the name of John Walsh, a key decision maker when it came budget GPS program [1]). Satellites are managed by the U.S. Air Force 50 Space Wing. System maintenance cost is approximately U.S. $ 750 million per year, [2], including the replacement of aging satellites, and research and development. Although these costs, GPS is free for civilian use than the public interest.
GPS has become a widely used aid to navigation worldwide, and a useful tool for map making, land surveying, commercial and scientific uses. GPS also provides precise time reference used in many applications including scientific study of earthquakes and synchronization of telecommunications networks.
Simplified approach
GPS receiver calculates its position by measuring the distance between itself and three or more GPS satellites. By measuring the delay, sending and receiving any GPS microwave signal gives the distance to each satellite, because the signal passes through a known speed – the speed of light. These signals also carry information about the location of satellites and overall health (the so-called Ephemeris and almanac data). Determining the location and distance of at least three satellites, the receiver can compute its position using trilateration. [3] Receivers typically do not completely accurate clocks and therefore track one or more satellites, atomic clocks used to correct the receiver’s own clock error.
[Edit] Technical description
Unlaunched GPS satellite in the present San Diego Aerospace Museum
Unlaunched GPS satellite in the present San Diego Aerospace Museum
[Edit] System segmentation
The current GPS consists of three main sections. These are the space (SS), control segment (CS), and the user segment (U.S.). [4]
[Edit] space
Space segment (SS) consists of GPS-orbit satellites or spacecraft (SV) GPS parlance. GPS design calls for 24 SVS divided equally between six circular orbits. [5] focus on orbits the Earth, not rotating the distant stars. [6] Six of these were about 55 ° inclination (tilt compared to the Earth equator) and separated by 60 ° right ascension ascending node (angle along the equator from the reference point of the orbit of the intersection). [2]
Orbiting at an altitude of about 20200 km (12600 km, or 10,900 nautical miles orbital radius is 26600 km (16,500 mi or 14,400 NM)), which makes two full SV rotate every sidereal day, so it runs in the same place on Earth once each day. Orbit is arranged so that a minimum of six satellites are always within sight almost everywhere on Earth. [7]
As of September 2007, has 31 active yleisradiointisatelliittien GPS constellation. More GPS satellites to improve the accuracy of calculations by providing redundant measurements. Increased the number of satellites constellation changed nonuniform arrangement. Such an arrangement has been shown to improve the reliability and usability of the system in relation to a single system, when multiple satellites are not. [8]
[Edit] Control segment
Flight paths of satellites monitoring the U.S. Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado, as well as monitor stations operate the National Geospatial-Intelligence Agency (NGA). [9] The tracking information is sent to Air Force Space Command, the master station at Schriever Air Force Base in Colorado Springs, acting for the 2d Space Operations Squadron (2 SOP) U.S. Air Force (USAF). SOP 2 contacts each GPS satellite navigation upgrade on a regular basis (using the ground antennas is Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs). These updates synchronize the atomic clocks on board satellites of one micro-second and adjusts the Ephemeris for each satellite within the orbit model. Updates have been created Kalman filter which uses inputs from monitoring stations, space weather, and many other inputs. [10]
GPS receivers come in different forms, devices integrated with cars, telephones, and clocks, is to use the equipment as shown in the manufacturers Trimble, Garmin and Leica (left to right).
GPS receivers come in different forms, devices integrated with cars, telephones, and clocks, is to use the equipment as shown in the manufacturers Trimble, Garmin and Leica (left to right).
[Edit] User segment
The user’s GPS receiver is the user segment (U.S.) GPS system. Usually consists of the GPS antenna that is tuned to the frequencies transmitted by satellite, receiver-processors, and highly stable clock (often a crystal oscillator). They may also display provides position and velocity information to the user. Receiver is often described as the channels: this means, how many satellites can monitor simultaneously. Originally, only four or five, this has gradually increased in recent years, so that since 2006, receivers typically have between twelve and twenty channels.
A typical OEM GPS module, which is based on the SiRF Star III chipset, measuring 15 x 17 mm, and used in many products.
A typical OEM GPS module, which is based on the SiRF Star III chipset, measuring 15 x 17 mm, and used in many products.
GPS receivers may include materials differential corrections using the RTCM SC-104 format. This is usually in the form of RS-232 port on the 4800 bit / s speed. The data are actually sent to a much slower rate, thereby limiting the accuracy of the signal through RTCM. Collectors internal DGPS receivers can outperform those using external RTCM data. Since 2006, the low-cost units commonly include Wide Area Augmentation System (WAAS) receivers.
Many GPS receivers can relay position data to a computer or another device with NMEA 0183 protocol. NMEA 2000 [11] is a newer and less-approved protocol. Both are proprietary and oversees the U.S. National Marine Electronics Association. References to the NMEA protocols have been gathered from public registers, whose open-source tools such as product safety directive to read the protocol without violating intellectual property rights. Other personal records also exist, such as the SiRF and MTK protocols. Receivers can interface to other devices such as the methods of serial connection, USB or Bluetooth.
[Edit] navigation signals
Main article: GPS signals
GPS signal
GPS signal
Each satellite continuously sends GPS Navigation message, 50 bit / s to give time-of-day, week number and GPS satellite health information (all to provide a first part of the message) Ephemeris (supplied with the second part of the message) and calendar (later part of the message). Ephemeris data to provide the satellite’s exact path and the production of more than 18 seconds, repeated every 30 seconds. Ephemeris updated 2 hours and is usually valid for 4 hours and 6 hours out provisions. The time needed to acquire Ephemeris is becoming a major part of the delay of the first positioning because, as the hardware may change, time to lock into the satellite signals shrinks, but the Ephemeris data, 30 seconds (worst case) before it has a low data transfer rate. The calendar contains coarse orbit and status information for each satellite in the constellation, and lasts for 12 seconds for each satellite at the time about the new satellite is transferred every 30 seconds (15 to 5 minutes 31 satellites). The purpose of the information is to help start-up by the acquisition of the satellite receiver to produce a list of visible satellites based on stored position and time when the Ephemeris for each satellite is needed to compute position fixes using that satellite. Parents lack of hardware almanac is a new receiver would cause long delays, which have created a position, because the search for each satellite was a slow process. Advances in hardware have made the acquisition process much faster, so it is not the calendar is no longer a problem. The most important thing to note about navigation data is that each satellite sends Ephemeris only on their own, but provides an almanac for all satellites.
Each of the satellite navigation message is transmitted at least two different spread spectrum codes: coarse / acquisition (C / A) code, which is freely available to the public, and precision (P) code, which is usually protected and reserved for military applications. C / A code is a 1023-chip pseudo-random (PRN) code is 1 023 million chips / sec so that it repeats every millisecond. Each satellite has its own C / A code, so that it can be identified and received separately from other satellites to send the same frequency. P-code is 10 23 mega chip / s PRN code that repeats only once a week. When the “anti-spoofing” is on, because it is the normal operation of the P-code encrypted Y-code to produce P (Y) code, which can be decrypted with the services a valid encryption key. Both C and P (Y) codes to convey the precise time-of-day user. Includes frequencies used by the GPS
* L1 (1575. 42 MHz): Mix of Navigation Message, coarse-acquisition (C / A) code and encrypted precision P (Y) code, and a new L1C on future Block III satellites.
L2 * (1227 60 MHz): P (Y)-code, L2C code and a new Block IIR-M and later satellites.
* L3 (1381. 05 MHz): Used in Nuclear Detonation (NUDET) Detection System Payload (NDS) is the signal detection of nuclear detonations and other high-energy infrared events. Used to monitor nuclear test ban treaties.
* L4 (1379. 913 MHz): more research into ionospheric correction.
* L5 (1176 45 MHz): Proposed for use in the civilian safety-of-life (SoL) signal (see GPS modernization). This frequency is an internationally protected various aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite, which offers this message to be launched in 2008.
[Edit] Calculate the positions of
[Edit] Using the C-code
To start off, the receiver chooses which C / PRN codes to listen to the latest figures, based on alma bugging information has previously acquired. Since the identification of each satellite’s signal can be identified for a separate C / A-code model, then measure the delay for each satellite. To do this, the receiver produces the same C-series using the same seed number as the satellite. Both at the bottom of the stairs, the receiver can be measured directly and to calculate the distance from the satellite, called the pseudorange [12].
Overlapping pseudoranges, represented as curves, be amended so as to obtain a probable location
Overlapping pseudoranges, represented as curves, be amended so as to obtain a probable location
Next, the orbital position data, or from the Ephemeris Navigation Message is then downloaded to calculate the exact location of the satellite. More sensitive receiver will potentially acquire Ephemeris data faster than a less sensitive receiver, especially in a noisy environment. [13] Knowing the location and distance from the satellite receiver is located somewhere on the surface of an imaginary ball in the middle, satellite and whose radius is the distance from it. Receivers can be replaced by a height of one satellite, which the GPS receiver to translate the pseudorange measured from the center.
Places to not be counted as three-dimensional space, but the four-dimensional spacetime, that is precisely measured by time-of-day is very important. Measured pseudoranges from four satellites have already been defined receiver’s internal clock, and thus an unknown quantity at the error. (The clock error or a real time does not matter to the original pseudorange calculation, because it is based on how much time has elapsed between the reception of each signal. [Clarify] [edit]) four-dimensional point equidistant from the pseudoranges calculated guess as the receiver location, and the factor used to adjust pseudoranges are cut at the four-dimensional point to give a guess that the receiver clock offset. For each guess, the geometric dilution of precision (GDOP) vector is calculated based on the relative sky positions of the satellites used. The more satellites are picked, pseudoranges from more combinations of four satellites can be processed more guesses the location and time offset. The receiver then determines which combinations to use and how to calculate the estimated position of the weighted average of these positions and clock offsets. Once the final location and time are calculated, the location is expressed in a special coordinate system, eg latitude / longitude using a WGS 84 geodetic datum or a local characteristic of the country.
[Edit] Using the P (Y) code
Calculation of the position of P (Y) signal is generally similar in concept, if it can extract. Encryption is essentially a safety mechanism: if the signal can be successfully decrypted, it is reasonable to assume that it is a real signal is sent to the GPS satellite. [Edit] By comparison, civil receivers are highly vulnerable to spoofing since correctly formatted C / signals can be produced using readily available signal generators. RAIM features do not protect against spoofing, since RAIM only checks the signals maritime perspective.
[Edit] Accuracy and error sources
GPS position is calculated at the time of calls, satellite status and the measured delay of the received signal. Position accuracy is primarily dependent on satellites, and signal delay.
Measure the delay, the receiver compares a bit sequence received satellite internally created version. By comparing the rise and after the edges of the bit transitions, modern electronics can measure signal to reduce the accuracy of about 1% of a bit of time, or approximately 10 nano seconds, and C-code. Since GPS signals are moving close to light speed, this represents an error of about 3 meters. This is the smallest error possible to use only the GPS C / signal.
Position accuracy can be improved by using a higher chip rate P (Y) signal. Assuming the same 1% bit time accuracy, high-frequency P (Y) signal leads to accuracy of about 30 cm.
Electronics errors are one of the many adverse effects of precision in the following table. When one autonomous civilian GPS horizontal position fixes are typically accurate about 15 meters (50 feet). These effects also reduce the more precise P (Y) code’s accuracy.
Sources Responsible User Range Errors (UERE) Source Effect
Ionospheric effects ± 5 m
Ephemeris errors ± 2 5 m
Satellite clock errors ± 2 m
Multipath distortion ± 1 meter
Tropospheric effects ± 0 5 m
Numerical errors ± 1 meter
[Edit] Atmospheric effects
Inconsistencies in the weather conditions affect the speed of GPS signals as they pass through Earth’s atmosphere and ionosphere. Correcting these errors is a big challenge to improve GPS accuracy. These effects are smallest when the satellite is directly overhead and grow closer to the satellites on the horizon since the signal seems longer. When the receiver’s location is roughly known, the mathematical model can be used to assess and compensate these errors.
Because ionospheric delay affects the speed of microwave signals are often based on property known as dispersion and frequency bands can reduce this error. Some of the military and expensive survey-grade civilian receivers compare the different delays in the L1 and L2 frequencies to measure atmospheric spread, and apply more accurate correction. This can be achieved civilian receivers without encryption P (Y) signal carried in L2, followed by a modulated carrier wave instead of the code. To facilitate this, a low-cost receivers, the new civilian victims of the code signal L2, called L2C was added to the Block IIR-M satellites, which was first launched in 2005. It enables a direct comparison between L1 and L2 signals coded signal instead of the carrier wave.
Effects of the ionosphere generally change slowly, and may be averaged over time. Impact of any particular geographical area can easily be calculated by comparing the GPS-measured position to a known collection location. This adjustment also applies to other receivers in the same general location. Many systems send information over radio or other links, so that L1 only receivers to make ionospheric corrections. Ionospheric data transmitted by satellite Satellite based augmentation systems such as WAAS, which supplies the GPS frequency of specific pseudo-random number (PRN), so only one antenna and receiver is needed.
Humidity also causes a variable delay, when the errors are responsible for ionospheric delay, but in the troposphere. This effect is both more local and changes more quickly than ionospheric effects, and not frequency dependent. These features make a precise measurement and compensation for humidity errors more difficult than ionospheric effects.
Changes in altitude also change the amount due to delays in the signal passes through less atmosphere of greater gain. Since the GPS receiver calculates its approximate altitude, this error is relatively easy to fix.
[Edit] Multipath effects
GPS signals can also affect the multipath issues, where the radio signals reflect off surrounding terrain, buildings, canyon walls, hard ground, etc. These delayed signals can cause inaccuracy. Different techniques, particularly narrow correlator spacing is developed to mitigate multipath errors. Long delay multipath, the receiver itself can recognize the stubborn sign and dispose of it. To address shorter delay multipath signal is taken out of the country, which is specialized antennas to reduce the signal power at the antenna. Short delay reflections are harder to filter out, because they interfere with the actual signal, causing effects almost indistinguishable from routine fluctuations without delay.
Multipath effects are much less stringent than moving vehicles. When the GPS antenna is moving, false solutions using reflected signals quickly each other, not only the direct signal and lead to stable solutions.
[Edit] Ephemeris and clock errors
Satellite navigation message is sent only once every 30 seconds. In reality, the information contained in the messages usually have “outdated” by an even larger sum. Think about where the GPS satellite is boosted back to the proper orbit, some time after the maneuver, the calculation of the satellite receiver’s position wrong until it receives a second Ephemeris update. Onboard clocks are extremely accurate, but they suffer from some clock drift. This problem is usually very small, but it can add up to 2 meters (6 feet) is a blur.
This error class is more “stable” than ionospheric problems and tends to shift the days or weeks instead of minutes. This makes correction fairly simple by sending an accurate calendar in a separate channel.
[Edit] Selective Availability
GPS includes a feature called Selective Availability (SA) that introduces intentional, slowly changing random errors of up to one hundred meters (328 ft) becomes publicly available navigation signals, such as unexpectedly long-range guided missiles at precise targets. Additional information is available for precision signal, but in an encrypted form, which was only available in the U.S. army, its allies and a few others, mostly government users.
SA typically added signal errors of up to about 10 meters (32 ft) horizontally and 30 meters (98 ft) vertically. Inaccuracy of the civilian population, the signal is deliberately encoded so as not to change very quickly, for example, the entire eastern U.S. area might read 30 m off, but 30 m off everywhere and in the same direction. To improve the usefulness of GPS for civilian use navigation, Differential GPS was used in many civilian GPS greatly improve accuracy.
During the Gulf War, a shortage of military GPS and the wide availability of civilian personnel, and knowledge of staff led to the decision to remove Selective Availability. This is ironic, as SA had been introduced specifically in those situations where friendly troops to use precise navigation signal, but at the same time deny its enemy. But since SA was also prohibit the same accuracy of thousands of friendly forces, or deactivate it by making the mistake of zero meters (effectively the same thing) presented a clear benefit.
During the 1990s, FAA began to put pressure on the military shut down permanently in SA. This would save the FAA millions of dollars each year servicing its own radio-navigation systems. The military objected to most of the 1990s, and it eventually led to the leadership of SA from the GPS signal. Number of errors were added to “zero” [14] at midnight on 1 May 2000, when the declaration, U.S. President Bill Clinton, whose error-L1 users. Fri directive error caused by SA was amended to add any error to the public signals (C / A code). Selective Availability is still a system capability of GPS, and error could theoretically re-enabled at any time. In practice, taking into account the risks and costs of this cause the United States and foreign shipping, it is not likely to again, and the various agencies, such as the FAA [15] have argued that it is not intended to be re-established.
The United States Army has developed the ability to prohibit the GPS locally (and other navigation services) to hostile forces in a given area the crisis affecting the rest of the world or its own military systems. [14]
One of the interesting side effect of Selective Availability hardware is the ability to correct the GPS frequency of cesium and rubidium atomic clocks accuracy of about 2 × 10-13 (fifth trillion euros). This represents a significant improvement in the raw precision clocks. [Edit]
19. September 2007 U.S. Department of Defense announced that they will not buy any more satellites capable of implementing SA. [16]
[Edit] theory of relativity
According to the theory of relativity, because they are constantly in motion, and height in relation to Earth-centered Inertial frame, clocks satellites by their speed (special relativity) as well as their gravitational potential (general relativity). So that the GPS satellites, general relativity predicts that atomic clocks are GPS orbital altitudes will happen more quickly, about 45,900 nanoseconds (ns) per day, because they are in a weaker gravitational field than atomic clocks of Earth’s surface. Special theory of relativity predicts that atomic clocks moving in orbit GPS speed tick slower than stationary clocks in the country about the so-called 7200 a day. Combined with the difference of 38 micro-seconds per day, the difference between 4 465 parts of 1010 [17]. To ensure that this account of the frequency standard onboard each satellite is due to replace before the launch, which will run slightly slower than the desired frequency on Earth, particularly at 10 22999999543 MHz instead of 10 23 MHz. [18]
GPS observation processing must also be replaced with another relativistic effect, Sagnac effect. GPS schedule is defined inertial system but observations are processed, the central Earth, Earth-fixed (co-rotating) system, a system which at the same time is not explicitly defined. Lorentz transformation between the two systems to change the signal transit time, a correction having opposite algebraic signs of satellites in Eastern and Western Hemisphere sky. Ignores this effect is produced by the East-West error are hundreds of nano seconds, or tens of meters in place. [19]
Atomic clocks on board GPS satellites are precisely tuned, and the system design practical application of scientific theory of relativity is a real-world environment.
[Edit] GPS interference and jamming
Since the GPS signal from the terrestrial receivers are relatively fragile, it is easy for other electromagnetic radiation, desensitize the receiver, and acquiring and tracking satellite signals difficult or impossible.
Solar flares are one such naturally occurring emissions with degrade GPS reception, and their impact can affect reception of more than half of the world like the sun. GPS signals can also be addressed by naturally occurring geomagnetic storms, predominantly found near the poles Earth’s magnetic field. [20] Another reason for the problems of a metal embedded in some car windscreens to prevent ice, degrading reception just inside the car.
Man-made interference can also disrupt, or jam, GPS signals. Together with the well-documented case, the entire port could not receive GPS signals, because the incidental harassment caused by malfunctioning TV antenna preamplifier. [21] Intentional jamming is also possible. Generally, stronger signals can interfere with GPS receivers, if they are in radio range, or visual contact. In 2002, a detailed description of how to build a short range GPS L1 C / jammer was published in online magazine Phrack. [22]
The U.S. government believes that such jammers were used occasionally in 2001, the war in Afghanistan and the U.S. military claimed to destroy the GPS jammer GPS-guided bomb during the Iraq war. [23] Such a jammer is relatively easy to detect and locate, making it an attractive anti-radiation missiles. British Ministry of Defense tested jamming system of the United Kingdom West Country 7 and 8 June 2007. [24]
Some countries are allowed to use GPS repeaters, so that reception of GPS signals indoors and in obscured locations, but the EU and UK laws, use of these is prohibited in the signals can cause interference to other GPS receivers That may obtain information and the GPS satellites and the repeater.
Since the potential for both natural and man-made noise several techniques will be further developed to address problems. The first is to rely on GPS as the only source. According to John Ruley “IFR pilots should have a fallback plan if a GPS malfunction”. [25], receiver autonomous integrity monitoring (RAIM) is a feature now includes some receivers, which is designed to give warning to the user if jamming or another problem is detected. U.S. Army has also used their Selective Availability / Anti-Spoofing Module (SAASM) and the Department of Defense Advanced GPS Receiver (DAGR). In demonstration videos, DAGR is able to detect jamming and maintain its lock encrypted signals during interference which causes civilian receivers to lose lock. [26]
[Edit] Techniques to improve accuracy
[Edit] Augmentation
Main article: GNSS Augmentation
Augmentation ways to improve the accuracy of external information must be included to perform the calculations. There are many such systems, and they are generally named or described based on how the GPS sensor receives information. Some systems provide additional sources of error (such as clock drift, Ephemeris, or ionospheric delay), others provide direct measurements of how much the signal is out of the past, while a third group of additional navigational or vehicle information to be integrated calculation of the process.
Examples of augmentation systems including Wide Area Augmentation System, Differential GPS, Inertial Navigation Systems and Assisted GPS.
[Edit] Precise monitoring
Accuracy of calculation can also improve the accurate monitoring and measurement of the current GPS signals in additional or alternative ways.
After SA, which is disabled, the maximum error of GPS is usually unpredictable delays in the ionosphere. Probe sent to the ionospheric model parameters, but the errors are. This is one reason the GPS spacecraft transmit at least two frequencies, L1 and L2. Ionospheric delay is a well-defined function of frequency and total electron content (TEC) on the road, so the measurement time of arrival difference between the amount of spectrum in TEC and thus the precise ionospheric delay for each frequency.
Receivers encryption keys can be interpreted as P (Y) code, supplied both L1 and L2. However, these keys are reserved for the military and “authorized” agencies and not available to the public. Without the keys, it is still possible to use less code technology to compare the P (Y) codes on L1 and L2 to get a lot of the same information about the error. However, this technique is slow, so it is currently limited to specialized surveying equipment. In the future, the new civil codes are expected to provide the L2 and L5 frequencies (see GPS modernization, below). Then all users are able to make the two-frequency measurements and directly calculate the ionospheric delay errors.
The exact form of monitoring is called Carrier-Phase Enhancement (CPGPS). Error, which here will be corrected, because the pulse and PRN is not instantaneous, and thus the correlation (satellite-receiver series matching) operation is imperfect. CPGPS approach to take advantage of L1 carrier wave, which is the time to 1000 times smaller than the C / little time to act in addition to a clock signal and resolve the uncertainty. Phase difference error in the normal GPS amounts between 2 and 3 meters (6-10 ft) of ambiguity. CPGPS works for more than 1% of the full transition to reduce this error to 3 cm (1 inch) of ambiguity.
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