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RAIM features do not protect against spoofing, since RAIM only checks the signals from a navigational perspective [edit] Accuracy and error sources The position calculated by a GPS receiver requires the current time, the position of the satellite and the measured delay of the received signal The position accuracy is primarily dependent on the satellite position and signal delay To measure the delay, the receiver compares the bit sequence received from the satellite with an internally generated version By comparing the rising and trailing edges of the bit transitions, modern electronics can measure signal offset to within about 1% of a bit time, or approximately 10 nanoseconds for the C/A code Since GPS signals propagate nearly at the speed of light, this 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 the higher-chiprate P signal Assuming the same 1% bit time accuracy, the high frequency P signal results in an accuracy of about 30 centimeters Electronics errors are one of several accuracy-degrading effects outlined in the table below When taken together, autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters (50 ft) These effects also reduce the more precise P codes accuracy Sources of User Equivalent Range Errors (UERE) Source Effect Ionospheric effects ± 5 meter Ephemeris errors ± 2 5 meter Satellite clock errors ± 2 meter Multipath distortion ± 1 meter Tropospheric effects ± 0 5 meter Numerical errors ± 1 meter [edit] Atmospheric effects Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they pass through the Earths atmosphere and ionosphere Correcting these errors is a significant challenge to improving GPS position accuracy These effects are smallest 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 receivers approximate location is known, a mathematical model can be used to estimate and compensate for these errors Because ionospheric delay affects the speed of microwave signals differently based on frequency—a characteristic known as dispersion—both frequency bands can be used to help reduce this error Some military and expensive survey-grade 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 signal carried on L2, by tracking the carrier wave instead of the modulated code To facilitate this on lower cost receivers, a new civilian code signal on L2, called L2C, was added to the Block IIR-M satellites, which was first launched in 2005 It allows a direct comparison of the L1 and L2 signals using the coded signal instead of the carrier wave The effects of the ionosphere generally change slowly, and can be averaged over time The effects for any particular geographical area can be easily calculated by comparing the GPS-measured position to a known surveyed location This correction is also valid for other receivers in the same general location Several systems send this information over radio or other links to allow L1 only receivers to make ionospheric corrections The ionospheric data are transmitted via satellite in Satellite Based Augmentation Systems such as WAAS, which transmits it on the GPS frequency using a special pseudo-random number (PRN), so only one antenna and receiver are required Humidity also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the troposphere This effect is both more localized and changes more quickly than ionospheric effects and is not frequency dependent These traits making 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 of the atmosphere at higher elevations Since the GPS receiver computes its approximate altitude, this error is relatively simple to correct [edit] Multipath effects GPS signals can also be affected by multipath issues, where the radio signals reflect off surrounding terrain; buildings, canyon walls, hard ground, etc These delayed signals can cause inaccuracy A variety of techniques, most notably narrow correlator spacing, have been developed to mitigate multipath errors For long delay multipath, the receiver itself can recognize the wayward signal and discard it To address shorter delay multipath from the signal reflecting off the ground, specialized antennas may be used to reduce the signal power as received by the antenna Short delay reflections are harder to filter out because they interfere with the true signal, causing effects almost indistinguishable from routine fluctuations in atmospheric delay Multipath effects are much less severe in moving vehicles When the GPS antenna is moving, the false solutions using reflected signals quickly fail to converge and only the direct signals result in stable solutions [edit] Ephemeris and clock errors The navigation message from a satellite is sent out only every 30 seconds In reality, the data contained in these messages tend to be out of date by an even larger amount Consider the case when a GPS satellite is boosted back into a proper orbit; for some time following the maneuver, the receivers calculation of the satellites position will be incorrect until it receives another ephemeris update The onboard clocks are extremely accurate, but they do suffer from some clock drift This problem tends to be very small, but may add up to 2 meters (6 ft) of inaccuracy This class of error is more stable than ionospheric problems and tends to change over days or weeks rather than minutes This makes correction fairly simple by sending out a more accurate almanac on a separate channel [edit] Selective availability The GPS includes a feature called Selective Availability (SA) that introduces intentional, slowly changing random errors of up to a hundred meters (328 ft) into the publicly available navigation signals to confound, for example, guiding long range missiles to precise targets Additional accuracy was available in the signal, but in an encrypted form that was only available to the United States military, 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 The inaccuracy of the civilian signal was deliberately encoded so as not to change very quickly, for instance 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 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 civilian ones among personnel resulted in a decision to disable Selective Availability This was ironic, as SA had been introduced specifically for these situations, allowing friendly troops to use the signal for accurate navigation, while at the same time denying it to the enemy But since SA was also denying the same accuracy to thousands of friendly troops, turning it off or setting it to an error of zero meters (effectively the same thing) presented a clear benefit In the 1990s, the FAA started pressuring the military to turn off SA permanently This would save the FAA millions of dollars every year in maintenance of their own radio navigation systems The military resisted for most of the 1990s, and it ultimately took an executive order to have SA removed from the GPS signal The amount of error added was set to zero[14] at midnight on May 1, 2000 following an announcement by U S President Bill Clinton, allowing users access to the error-free L1 signal Per the directive, the induced error of SA was changed to add no error to the public signals (C/A code) Selective Availability is still a system capability of GPS, and error could, in theory, be reintroduced at any time In practice, in view of the hazards and costs this would induce for US and foreign shipping, it is unlikely to be reintroduced, and various government agencies, including the FAA,[15] have stated that it is not intended to be reintroduced The US military has developed the ability to locally deny GPS (and other navigation services) to hostile forces in a specific area of crisis without affecting the rest of the world or its own military systems [14] One interesting side effect of the Selective Availability hardware is the capability to correct the frequency of the GPS caesium and rubidium atomic clocks to an accuracy of approximately 2 × 10-13 (one in five trillion) This represented a significant improvement over the raw accuracy of the clocks [citation needed] On 19 September 2007, the United States Department of Defense announced that they would not procure any more satellites capable of implementing SA [16] [edit] Relativity According to the theory of relativity, due to their constant movement and height relative to the Earth-centered inertial reference frame, the clocks on the satellites are affected by their speed (special relativity) as well as their gravitational potential (general relativity) For the GPS satellites, general relativity predicts that the atomic clocks at GPS orbital altitudes will tick more rapidly, by about 45,900 nanoseconds (ns) per day, because they are in a weaker gravitational field than atomic clocks on Earths surface Special relativity predicts that atomic clocks moving at GPS orbital speeds will tick more slowly than stationary ground clocks by about 7,200 ns per day When combined, the discrepancy is 38 microseconds per day; a difference of 4 465 parts in 1010 [17] To account for this, the frequency standard onboard each satellite is given a rate offset prior to launch, making it run slightly slower than the desired frequency on Earth; specifically, at 10 22999999543 MHz instead of 10 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, a system in which simultaneity is not uniquely defined The Lorentz transformation between the two systems modifies the signal run time, a correction having opposite algebraic signs for satellites in the Eastern and Western celestial hemispheres Ignoring this effect will produce an east-west error on the order of hundreds of nanoseconds, or tens of meters in position [19] The atomic clocks on board the GPS satellites are precisely tuned, making the system a practical engineering application of the scientific theory of relativity in a real-world 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, making acquiring and tracking the satellite signals difficult or impossible Solar flares are one such naturally occurring emission with the potential to degrade GPS reception, and their impact can affect reception over the half of the Earth facing the sun GPS signals can also be interfered with by naturally occurring geomagnetic storms, predominantly found near the poles of the Earths magnetic field [20] Another source of problems is the metal embedded in some car windscreens to prevent icing, degrading reception just inside the car Man-made interference can also disrupt, or jam, GPS signals .
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In one well documented case, an entire harbor was unable to receive GPS signals due to unintentional jamming caused umi x3 by a malfunctioning TV antenna preamplifier [21] Intentional jamming is also possible Generally, stronger signals can interfere with GPS receivers when they are within radio range, or line of sight In 2002, a detailed description of how to build a short range 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 in Afghanistan and the U S military claimed to destroy a GPS jammer 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-radiation missiles The UK Ministry of Defence tested a jamming system in the UKs West Country on 7 and 8 June 2007 [24] Some countries allow the use of GPS repeaters to allow for the reception of GPS signals indoors and in obscured locations, however, under EU and UK laws, the use of these is prohibited as the signals can cause interference to other GPS receivers that may receive data from both GPS satellites and the repeater Due to the potential for both natural and man-made noise, numerous techniques continue to be developed to deal with the interference The first is to not rely on GPS as a sole source According to John Ruley, IFR pilots should have a fallback plan in case of a GPS malfunction [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 another problem is detected The U .
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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 its lock on the encrypted GPS signals during interference which causes civilian receivers to lose lock [26][edit] Techniques to improve accuracy [edit] Augmentation Main article: GNSS Augmentation Augmentation methods of improving accuracy rely on external information being integrated into the calculation process There are many such systems in place and they are generally named or described based on how the GPS sensor receives the information Some systems transmit additional information about 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 integrated in the calculation process Examples of augmentation systems include the Wide Area Augmentation System, Differential GPS, Inertial Navigation Systems and Assisted GPS [edit] Precise monitoring The accuracy of a calculation can also be improved through precise monitoring and measuring of the existing GPS signals in additional or alternate ways After SA, which has been turned off, the largest error in GPS is usually the unpredictable delay through the ionosphere The spacecraft broadcast ionospheric model parameters, but errors remain This is one reason the GPS spacecraft transmit on at least two frequencies, L1 and L2 Ionospheric delay is a well-defined function of frequency and the total electron content (TEC) along the path, so measuring the arrival time difference between the frequencies determines TEC and thus the precise ionospheric delay at each frequency Receivers with decryption keys can decode the P-code transmitted on both L1 and L2 However, these keys are reserved for the military and authorized agencies and are not available to the public Without keys, it is still possible to use a codeless technique to compare the P codes on L1 and L2 to gain much of the same error information However, this technique is slow, so it is currently limited to specialized surveying equipment In the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies (see GPS modernization, below) Then all users will be able to perform dual-frequency measurements and directly compute ionospheric delay errors A second form of precise monitoring is called Carrier-Phase Enhancement (CPGPS) The error, which this corrects, arises because the pulse transition of the PRN is not instantaneous, and thus the correlation (satellite-receiver sequence matching) operation is imperfect The CPGPS approach utilizes the 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 phase difference error in the normal GPS amounts to between 2 and 3 meters (6 to 10 ft) of ambiguity CPGPS working to within 1% of 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 to 12 inches) of absolute accuracy Relative Kinematic Positioning (RKP) is another approach for a precise GPS-based positioning system In this approach, determination of range signal can be resolved to an accuracy of less than 10 centimeters (4 in) This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver This can be accomplished by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests—possibly with processing in real-time (real-time kinematic positioning, RTK) [edit] GPS time and date While most clocks are synchronized to Coordinated Universal Time (UTC), the Atomic clocks on 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 contain leap seconds or other corrections which are periodically added to UTC GPS time was set to match Coordinated Universal Time (UTC) in 1980, but has since diverged .
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The lack of corrections means that GPS time remains at a umi x3 black constant offset (19 seconds) with International Atomic Time (TAI) Periodic corrections are performed on the on-board clocks to correct relativistic effects and keep them synchronized with ground clocks The GPS navigation message includes the difference between GPS time and UTC, which as of 2006 is 14 seconds Receivers subtract this offset from GPS time to calculate UTC and specific timezone values New GPS units may not show the correct UTC time until after receiving the UTC offset message The GPS-UTC offset field can accommodate 255 leap seconds (eight bits) which, at the current rate of change of the Earths rotation, is sufficient to last until the year 2330 As opposed to the year, month, and day format of the Julian calendar, the GPS date is expressed as a week number and a day-of-week number The week number is transmitted as a ten-bit field in the C/A and P navigation messages, and so it becomes zero again every 1,024 weeks (19 6 years) GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6, 1980 and the week number became zero again for the first time at 23:59:47 UTC on August 21, 1999 (00:00:19 TAI on August 22, 1999) To determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584 days) to correctly translate the GPS date signal To address this concern the modernized GPS navigation messages use a 13-bit field, which only repeats every 8,192 weeks (157 years), and will not return to zero until near the year 2137 [edit] GPS modernization Main article: GPS modernization Having reached the programs requirements for Full Operational Capability (FOC) on July 17, 1995,[27] the GPS completed its original design goals However, additional advances in technology and new demands on the existing system led to the effort to modernize the GPS system Announcements from the Vice President and the White House in 1998 initiated these changes, and in 2000 the U S Congress authorized the effort, referring to it as GPS III The project aims to improve the accuracy and availability for all users and involves new ground stations, new satellites, and four additional navigation signals New civilian signals are called L2C, L5 and L1C; the new military code is called M-Code Initial Operational Capability (IOC) of the L2C code is expected in 2008 [28] A goal of 2013 has been established for the entire program, with incentives offered to the 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, meaning it has significant applications for both the military and the civilian industry [edit] Military Please help improve this article by expanding this section See talk page for details Please remove this message once the section has been expanded The military use GPS for the following purposes: [edit] Navigation GPS allows soldiers to find objectives in the dark or in unfamiliar territory, and to coordinate the movement of troops and supplies [edit] Target tracking Various military weapons systems use GPS to track potential ground and air targets before they are flagged as hostile These weapons systems pass GPS co-ordinates of targets to precision-guided munitions to allow them to engage the targets accurately Military aircraft, particularly those used in air-to-ground roles use GPS to find targets (for example, gun camera video from AH-1 Cobras in Iraq show GPS co-ordinates that can be looked up in Google Earth) [edit] Missile and projectile guidance GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles and precision-guided munitions Artillery projectiles with embedded GPS receivers able to withstand forces of 12,000G have been developed for use in 155 mm howitzers [29] [edit] Search and Rescue Downed pilots can be located faster if they have a GPS receiver [edit] Reconnaissance and Map Creation The military use GPS extensively to aid mapping and reconnaissance [edit] Other The GPS satellites also carry nuclear detonation detectors, which form a major portion of the United States Nuclear Detonation Detection System [30] [edit] Civilian See also: GPS applications This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing Many civilian applications benefit from GPS signals, using one or more of three basic components of the GPS; absolute location, relative movement, time transfer The ability to determine the receivers absolute location allows GPS receivers to perform as a surveying tool or as an aid to navigation The capacity 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 enables time transfer, which is critical in large communication and observation systems An example is CDMA digital cellular Each base station has a GPS timing receiver to synchronize its spreading codes with other base stations to facilitate inter-cell hand off and support hybrid GPS/CDMA positioning of mobiles for emergency calls and other applications Finally, GPS enables researchers to explore the Earth environment including the atmosphere, ionosphere and gravity field GPS survey equipment has revolutionized tectonics by directly measuring the motion of faults in earthquakes To help prevent civilian GPS guidance from being used in an enemys military or improvised weaponry, the US Government controls the export of civilian receivers A US-based manufacturer cannot generally export a GPS receiver unless the receiver contains limits restricting it from functioning when it is simultaneously (1) at an altitude above 18 kilometers (60,000 ft) and (2) traveling at over 515 m/s (1,000 knots) [31] [edit] History Please help improve this article by expanding this section See talk page for details Please remove this message once the section has been expanded The design of GPS is based partly on the similar ground-based radio navigation systems, such as LORAN and the Decca Navigator developed in the early 1940s, and used during World War II Additional inspiration for the GPS system came when the Soviet Union launched the first Sputnik in 1957 A team of U S scientists led by Dr Richard B Kershner were monitoring Sputniks radio transmissions They discovered that, because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion The first satellite navigation system, Transit, used by the United States Navy, was first successfully tested in 1960 Using a constellation of five satellites, it could provide a navigational fix approximately once per hour In 1967, the U S Navy developed the Timation satellite which proved the ability to place accurate clocks in space, a technology the GPS system relies upon In the 1970s, the ground-based Omega Navigation System, based on signal phase comparison, became the first world-wide radio navigation system The first experimental Block-I GPS satellite was launched in February 1978 [28] The GPS satellites were initially manufactured by Rockwell International and are now manufactured by Lockheed Martin [edit] Timeline * In 1972, the US Air Force Central Inertial Guidance Test Facility (Holloman AFB) conducted developmental fight tests of two prototype GPS receivers over White Sands Missile Range, using ground-based pseudo-satellites