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 enables a GPS receiver’s position, speed and direction determine and time. Developed by the United States 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 decision-makers when it comes to the budget for the GPS Program [1]). The satellite constellation is determined by the United States Air Force 50th Space Wing manages. The cost of maintenance of the system is approximately $ 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 widely used aid to navigation around the world and a useful tool for mapping, land surveying, commercial and scientific purposes. GPS provides a 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. Measuring the time delay between sending and receiving of any GPS microwave signal gives the distance to each satellite, since the signal at a known speed – the speed of light. These signals lead information on the location of the satellites and the general health system (such as almanac and ephemeris data “). The determination of the 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 atomic clocks to the receiver’s own clock to correct errors. [Edit] Technical description Unlaunched GPS satellite on display at the San Diego Aerospace Museum Unlaunched GPS satellite on display at the San Diego Aerospace Museum [Edit] System segmentation The current GPS consists of three segments. These are the space segment (SS), a control segment (CS), and a user segment (U.S.). [4] [Edit] Space segment The space segment (SS) is the GPS satellites in orbit, or Space Vehicles (SV) GPS usage together. The GPS will design calls for 24-SVS evenly distributed among six circular orbits. [5] The orbits 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 of the earth and have a 60 ° right ascension of the ascending node (angle along the equator from a reference point until it is isolated, to 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)) makes every two complete orbits each sidereal day SV, so it goes on the same spot on Earth once every day. The orbits are arranged so that at least six satellites always in line of sight from almost everywhere on the 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 growing number of satellites, the constellation was changed to an irregular arrangement. Such an arrangement has been shown that reliability and availability of the system, based on a single system, if not improve several satellites. [8] [Edit] Control segment The flight paths of the satellites are monitored by the U.S. Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island Iceland, Diego Garcia, and Colorado Springs, Colo., will be operated in conjunction with monitor 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 SOPS) of the United States Air Force (USAF) sent. 2 SOPS contacts of the GPS satellites regularly with a navigational update (using the ground antennas at Ascension Iceland, Diego Garcia, Kwajalein and Colorado Springs). These updates synchronize the atomic clocks aboard the satellites within a microsecond and adjust the ephemeris of each satellite’s internal orbital model. The updates are used by a Kalman filter which inputs from the ground stations, space weather information and various other inputs created. [10] GPS receivers come in a variety of formats, from devices in cars that integrates mobile phones and watches, dedicated devices such 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 such as illustrated here by the manufacturers Trimble, Garmin and Leica (from left to right). [Edit] User segment The user GPS receiver is the user segment (U.S.) of the GPS system. In general, the GPS receiver are together from one antenna to the frequencies of the satellite-receiver-processors designed to transfer, and a highly stable clock () is often a crystal oscillator. You can also use a display for providing position and velocity information for the user. A receiver is often described by the number of channels: that is, how many satellites it can monitor simultaneously. Originally limited to four or five, this has gradually increased over the years, so that from 2006, receivers typically have twelve to twenty channels. A typical OEM GPS receiver module, based on the use SiRF Star III chipset, dimensions 15 × 17 mm, and in many products. A typical OEM GPS receiver module, based on the use SiRF Star III chipset, dimensions 15 × 17 mm, and in many products. GPS receivers can also be an input for differential corrections, with the RTCM SC-104 format. This is usually in the form of an RS-232 interface at 4800 bps speed. The data are actually much more at a lower price, which limits 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, even low-cost units are often the Wide Area Augmentation System (WAAS) receiver. Many GPS receivers relay position data to a PC or other device using the NMEA 0183 protocol. NMEA 2000 [11] is a newer and less widely adopted protocol. Both are copyright protected 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 the minutes without reading any 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 with GPS signal with Each GPS satellite continuously broadcasts a navigation message at 50 bps is the time-of-day (GPS week number and satellite health information all) transmitted in the first part of the message, an ephemeris (transmitted in the second part of the message) and an almanac (later) part of the message. The ephemeris data indicates the satellite’s own precise orbit and spent about 18 seconds and repeat every 30 seconds. The ephemeris is every 2 hours and is usually valid for 4 hours, 6 hours with the provisions for time-outs. The time needed to obtain the ephemeris, is always an important element of the delay to resolve the first position, because, as the hardware is better able to block, at the time of the satellite signals shrinks, but the ephemeris Data required 30 seconds (worst case) before it is received, due to the low transmission rate. The calendar consists of coarse orbit and status information for each satellite in the constellation, and takes approximately 12 seconds for each satellite so far transferred, with information for a new satellite every 30 seconds is (15. 5 minutes for 31 satellites). The purpose of the data is in the acquisition of satellites at power, by giving support to create the receiver based on a list of satellites visible to stored position and time, while an ephemeris from each satellite is needed to compute position fixes with this satellite. With older hardware would be, the lack of an almanac in a new receiver in long delays, which result in a valid position, because the search for each satellite was a slow process. Advances in hardware will have made the acquisition process much faster, so not with an almanac is no longer an issue. An important thing is to establish data on the menu that each satellite transmits only its own ephemeris, but transmits a calendar for all satellites. Each satellite transmits its navigation message with at least two different spread spectrum codes: the Coarse / Acquisition (C / A code), which is freely accessible to the public, and the precise (P) code, which encrypts a rule and reserved for military applications. The C / A code is a 1023-chip pseudo-random number (PRN) Code on 1 023 million chips per second so that it repeats every millisecond. Each satellite has its own C / A code so that it can be clearly identified and separated from the other satellites, received at the same frequency. The P-code is a 10th 23 Mega chips per second PRN code that repeats only every week. If the “anti-spoofing” mode as it is in normal operation, the P-code encrypted by the Y-code in order to produce the P (Y) code, the only units with a valid decryption key 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 (1575th 42 MHz): Mix of Navigation Message, coarse-acquisition (C / A) code and encrypted precision P (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): Will the Nuclear Detonation (NUDET) Detection System Payload (NDS) for the detection of nuclear explosions and other high-energy infrared events signal. Used to enforce nuclear test ban treaties. * L4 (1379. 913 MHz): As an additional 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 into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that would give this signal, will be launched in 2008 to life. [Edit] Calculation of positions [Edit] Using the C / A code At the beginning of the recipient takes the C / A codes to listen to PRN number, based on the almanac information it previously acquired. As it detects each satellite signal, he calls it by its distinct C / A code pattern, then measures the time delay for each satellite. To do this, the receiver produces an identical C / A sequence with the same number as the seed satellite. By lining up the two sequences, the receiver can measure the delay and calculate the distance to the satellite, called the pseudorange [12]. Overlapping pseudo-ranges, is presented as curves are modified to yield the probable position Overlapping pseudo-ranges, is presented as curves are modified to yield the probable position Subsequently, the orbital position data, or ephemeris, from the Navigation Message is then downloaded to the exact position of the satellite to 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 the 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 for an altitude satellite, the GPS receiver translates to a pseudorange measured from the center of the earth. Locations are not calculated in three dimensional space, but in four-dimensional spacetime, that 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 thus an unknown amount of clock error. (The clock error or actual time does not matter in the first pseudo-range calculation, because that is based on how much time is passed between the receipt of individual signals. [Clarify] [edit]) The four-dimensional point of equal distance removed from the pseudo ranges than a presumption, as calculated on the location of the recipient, and the factor by which these intersect with pseudo-ranges to adapt to that four-dimensional point is a presumption with regard to the clock of the receiver balanced. With each guess will be charged a geometric dilution of precision (GDOP) vector, based on the relative positions of sky using satellites. The more satellites are picked up, pseudo-ranges can be processed by more combinations of four satellites to more conjecture about the location and clock offset add. The receiver then determines which combinations to use, and how the estimated position by determining the weighted average of these positions and clock offsets to be calculated. Are calculated after the last time and place is the location specified in a coordinate system, e. g. expressed as latitude / longitude, with the WGS 84 geodetic or a 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 it. The encryption is essentially a safety mechanism: if a signal is successfully decoded, it is reasonable to accept it, is a real signal sent from a GPS satellite. [Edit] In comparison, civil receivers are highly vulnerable to spoofing since generated correctly formatted C / A signals with readily available signal generators. RAIM is not working against spoofing protection, since RAIM only the signals from a navigation perspective controls. [Edit] Accuracy and error The position of a GPS receiver calculates the actual time required, the position of the satellite and the measured delay of the received signal. The position accuracy is primarily dependent on the delay position and satellite signal. 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 the bit transitions, modern electronics can measure signal offset in approximately 1% of a bit of time, or about 10 nanoseconds for the C / A code. Since GPS signals propagate nearly at the speed of light, this is an error of about 3 meters. This is the smallest mistake can only with the GPS C / A signal. Position accuracy can be improved through the use of higher chip rate P (Y). Assuming the same 1% accuracy is a bit of time, the high frequency P (Y) to an accuracy of about 30 centimeters. Electronics errors are listed a number of precision-depleting effects in the table below. When taken together, are an autonomous civilian GPS horizontal position fixes usually about 15 meters (50 ft). These effects also reduce the more precise P (Y)-code accuracy. Sources of User Equivalent Range Errors (vere) Source Effect Ionospheric effects ± 5 meter Ephemeris errors ± 2 5 meters Satellite clock errors ± 2 meters Multipath distortion ± 1 meter Troposphere ± 0 5 meters Numerical errors ± 1 meter [Edit] Atmospheric effects Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they pass through Earth’s atmosphere and ionosphere. Correcting these errors is a major challenge for improving the accuracy of GPS position. These effects are smallest when the satellite is directly over the time will be larger and closer on the horizon for satellite because the signal is affected for a long time. Once the receiver is known, the approximate location, a mathematical model can be used to estimate and compensate for these errors. Because ionospheric delay of the speed of microwave signals a different effect on the frequency of a characteristic known as dispersion is based on both frequency bands can be used to reduce this error. Some military and expensive survey-grade civilian receivers compare the different 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 modulated code. To facilitate this known at a lower cost receivers, a new civilian code signal on L2, L2C, was added to the Block IIR-M satellite, 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 can be averaged over time. The implications for a given geographic area can be easily surveyed by comparing the GPS calculated position measured at a known location. This correction also applies to other recipients in the same general location. Several systems send this information via radio or other links to allow L1 only receivers to make the ionosphere corrections. The ionospheric data are transmitted by satellite in Satellite Based Augmentation Systems such as WAAS, who will pass them on to the GPS frequency using a special pseudo-random number generator (PRN), so that only one antenna and receiver is required to 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 more quickly than ionospheric effects, and is not frequency dependent. These properties make precise measurement and compensation of humidity errors more difficult than ionospheric effects. Changes in the amount and the amount of delay through the signal by less of the atmosphere at higher altitudes. Since the GPS receiver calculates the approximate altitude, this error is relatively easy to correct. [Edit] multipath effects GPS signals can also be due to multipath issues, where the radio signals reflect to be affected from the surrounding terrain, buildings, canyon walls, hard ground, etc. These delayed signals can cause inaccuracies. A variety of techniques have been especially narrow correlator spacing, designed to reduce multipath errors. For the long delay of the multipath receiver itself can recognize the wayward signal and discard it. To address shorter delay multipath from the signal reflecting the earth, special antennas can be used to reduce the 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 atmospheric delay. Multipath effects are much less strict in moving vehicles. If the GPS antenna is in motion, 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 satellite only once every 30 seconds. In reality, the data contained in these messages is usually “out of date” with an even larger amount. Consider the case when a GPS satellite will be raised again in the right path, and for some time after the maneuver, the receiver to calculate the position of the satellite be wrong until it receives another ephemeris update. The onboard clocks are very accurate, but they suffer at some clock drift. This problem tends to be very small, but up to 2 meters (6 feet) of inaccuracy. This class of errors is “stable” than ionospheric problems and tends to change over days or weeks instead of minutes. This correction is fairly simple, by providing a more accurate almanac on a separate channel. [Edit] Selective Availability The GPS includes a feature called Selective Availability (SA), which leads intentional, slowly changing random errors of up to one hundred meters (328 ft) in the publicly available navigation signals to confuse, for example, guiding long range missiles to precise targets. Additional accuracy was the signal is available, but in an encrypted form that only the U.S. forces, their allies and a few others who had most users government. SA has usually signal errors of up to about 10 m (32 ft) horizontally and 30 meters (98 ft) in vertical direction. The inaccuracy of the civilian signal was deliberately not change in coded form as quickly as the entire 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 high accuracy. During the Gulf War, the shortage of military GPS units and the wide availability of the civilian staff, in which a decision to disable Selective Availability conducted. This was ironic, as SA had been specially imported for these situations to deny the use of a friendly troops to the signal for accurate navigation, while at the same time to the enemy. But since SA was also denying the same accuracy for thousands of friendly troops, or turn it off him to an error of zero meters (effectively the same thing) presents a clear advantage. In the 1990s, the FAA began pressure on the military to turn SA permanently. This would save the FAA millions of dollars each year for the maintenance of their own radio-navigation systems. The military resistance to most of the 1990s, and it has 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 after an announcement by U.S. President Bill Clinton, with the user access to the L1-error signal. By directive, the induced error of SA was changed to no fault of the public record signals (C / A code). Selective Availability is still a system of GPS capability and errors can, in theory, be reintroduced at any time. Would in practice with regard to the risks and costs for the U.S. and encourage foreign, marine, it 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 locally deny 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 the Selective Availability hardware is the possibility of the frequency of the GPS cesium and rubidium atomic clocks correctly) with an accuracy of about 2 × 10-13 (one of five trillion. This was a significant improvement over the raw accuracy of the clocks. [Edit] On 19 September 2007 announced the United States Department of Defense that it would give no more satellites for the implementation of SA. [16] [Edit] Relativity According to the theory of relativity, which due to their relatively constant movement and height to the earth-centered inertial frame, the clocks on the satellites by their speed (special relativity) and affected by the gravitational potential (general relativity). For the GPS satellites, general relativity predicts that atomic clocks in GPS orbital altitudes bit faster, to approximately 45,900 nanoseconds (ns) per day, because they are in a weaker gravitational field than atomic clocks on Earth’s surface. Special relativity predicts that atomic clocks moving at GPS orbital speeds slowly than stationary ground clocks tick at some 7200 ns per day. In this combination, the gap 38 microseconds per day, a difference of 4 is 465 parts in 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, particularly at 10 22999999543 MHz instead of 10th 23 MHz. [18] GPS observation processing 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-) rotating system in which a system of simultaneity is not clearly defined. The Lorentz transformation between the two systems modifies the signal run time, a correction with opposite signs for satellites in the eastern and western celestial hemispheres. Ignore this purpose will produce an east-west error in the order of hundreds of nanoseconds, or tens of meters in position. [19] The atomic clocks aboard the GPS satellites are precisely tuned, making the system a practical engineering application of the scientific theory of relativity in a real environment. [Edit] GPS interference and jamming Since GPS signals at terrestrial receivers tend to be relatively weak, it is easy for other sources of electromagnetic radiation to desensitize the receiver, which the acquisition, and tracking the satellite signals difficult or impossible. Solar flares are one such naturally occurring emissions with the potential to interfere with GPS reception, and their impact can affect reception over the half of the earth from the sun. GPS signals can also be affected by naturally occurring geomagnetic storms, mainly 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 formation of ice, degrading reception just inside the car. Man-made interference can also disrupt, or jam, GPS signals. In a well-documented case, an entire harbor was unable to receive GPS signals due to unintentional interference by a malfunctioning TV antenna preamplifier causes. [21] Intentional Interference is also possible. Generally, stronger signals can interfere with GPS receiver, if within radio range or line of sight. In 2002, a detailed description of how the construction of a published short range GPS L1 C / A-jammers in the online magazine Phrack. [22] The U.S. government believes that such jammers on occasion during the war in Afghanistan in 2001 and the U.S. military said was used to destroy a GPS jammers with a GPS-guided bomb during the Iraq war. [23] Such a jammer is relatively easy to detect and locate, making it an attractive target for anti-radar missiles. The British Ministry of Defense tested jamming system in the West Country of the United Kingdom on 7 and 8 June 2007. [24] Some countries allow the use of GPS repeaters for the reception of GPS signals in confined spaces and places, however, hides under EU and UK law is the use of these signals as interference to other GPS receivers can cause data may be prohibited from receiving both GPS satellites and the repeater. Due to the potential for both natural and man-made noise, numerous techniques continue to be developed to treat the disorder. The first is to not rely on GPS as the only source. [26] [31] 1. pp. 2. 5. 6. 11. html 14. 17. 20. 21. 31.