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The effects of the ionosphere generally change slowly, and can be averaged over time. Those 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 System]]s (SBAS) such as [[Wide Area Augmentation System]] (WAAS) (available in North America and Hawaii), [[EGNOS]] (Europe and Asia), [[Multi-functional Satellite Augmentation System]] (MSAS) (Japan), and [[GPS Aided Geo Augmented Navigation]] (GAGAN) (India) which transmits it on the GPS frequency using a special pseudo-random noise sequence (PRN), so only one receiver and antenna are required.
[[Humidity]] also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the [[troposphere]]. This effect is more localized than ionospheric effects, changes more quickly and is not frequency dependent. These traits make precise measurement and compensation of humidity errors more difficult than ionospheric effects.<ref>[http://www.navipedia.net/index.php/Earth_Sciences#Troposphere_Monitoring Navipedia: Troposphere Monitoring]{{deadlink}}</ref>
The [[Atmospheric pressure]] can also change the signals reception delay, due to the dry gases present at the troposphere (78% N2, 21% O2, 0.9% Ar...). Its effect varies with local temperature and atmospheric pressure in quite a predictable manner using the laws of the ideal gases.<ref>[http://www.navipedia.net/index.php/Tropospheric_Delay Navipedia: Tropospheric Delay]{{deadlink}}</ref>
== Multipath effects ==
GPS signals can also be affected by [[multipath interference|multipath]] issues, where the radio signals reflect off surrounding terrain; buildings, canyon walls, hard ground, etc. These delayed signals cause measurement errors that are different for each type of GPS signal due to its dependency on the wavelength.<ref>[http://www.navipedia.net/index.php/Multipath Navipedia: Multipath]]{{deadlink}}</ref>
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 (e.g., a [[choke ring antenna]]) 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.
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== Ephemeris and clock errors ==
While the [[ephemeris]] data is transmitted every 30 seconds, the information itself may be up to two hours old. Variability in solar radiation pressure<ref>{{Cite web |title=''IPN Progress Report'' 42-159 (2004) |url=http://ipnpr.jpl.nasa.gov/progress_report/42-159/159I.pdf
The satellites' atomic clocks experience noise and [[clock drift]] errors. The navigation message contains corrections for these errors and estimates of the accuracy of the atomic clock. However, they are based on observations and may not indicate the clock's current state.
These problems tend to be very small, but may add up to a few meters (tens of feet) of inaccuracy.<ref>{{
For very precise positioning (e.g., in [[geodesy]]), these effects can be eliminated by [[differential GPS]]: the simultaneous use of two or more receivers at several [[Benchmark (surveying)|survey points]]. In the 1990s when receivers were quite expensive, some methods of ''quasi-differential'' GPS were developed, using only ''one'' receiver but reoccupation of measuring points. At the TU Vienna the method was named ''qGPS'' and post processing software was developed.{{Citation needed|date=September 2011}}
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When visible GPS satellites are close together in the sky (i.e., small angular separation), the DOP values are high; when far apart, the DOP values are low. Conceptually, satellites that are close together cannot provide as much information as satellites that are widely separated. Low DOP values represent a better GPS positional accuracy due to the wider angular separation between the satellites used to calculate GPS receiver position. HDOP, VDOP, PDOP and TDOP are respectively Horizontal, Vertical, Position (3-D) and Time Dilution of Precision.
Figure 3.1 Dilution of Precision of Navstar GPS data from the U.S. Coast Guard provide a graphical indication of how geometry affect accuracy.<ref>{{
We now take on the task of how to compute the dilution of precision terms. As a first step in computing DOP, consider the unit vector from the receiver to satellite i with components <math>\frac{(x_i- x)}{R_i}</math>, <math>\frac {(y_i-y)}{R_i}</math>, and <math>\frac {(z_i-z)}{R_i}</math> where the distance from receiver to the satellite, <math>\ R_i </math>, is given by:
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SA errors are actually pseudorandom, generated by a cryptographic algorithm from a classified ''seed'' [[key (cryptography)|key]] available only to authorized users (the U.S. military, its allies and a few other users, mostly government) with a special military GPS receiver. Mere possession of the receiver is insufficient; it still needs the tightly controlled daily key.
Before it was turned off on May 2, 2000, typical SA errors were about 50 m (164 ft) horizontally and about 100 m (328 ft) vertically.<ref>Grewal (2001), p. 103.</ref> Because SA affects every GPS receiver in a given area almost equally, a fixed station with an accurately known position can measure the SA error values and transmit them to the local GPS receivers so they may correct their position fixes. This is called Differential GPS or ''DGPS''. [[Differential GPS|DGPS]] also corrects for several other important sources of GPS errors, particularly ionospheric delay, so it continues to be widely used even though SA has been turned off. The ineffectiveness of SA in the face of widely available DGPS was a common argument for turning off SA, and this was finally done by order of President [[Bill Clinton|Clinton]] in 2000.<ref>{{
DGPS services are widely available from both commercial and government sources. The latter include WAAS and the [[US Coast Guard|U.S. Coast Guard's]] network of [[Low frequency|LF]] marine navigation beacons. The accuracy of the corrections depends on the distance between the user and the DGPS receiver. As the distance increases, the errors at the two sites will not correlate as well, resulting in less precise differential corrections.
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During the 1990–91 [[Gulf War]], the shortage of military GPS units caused many troops and their families to buy readily available civilian units. Selective Availability significantly impeded the U.S. military's own battlefield use of these GPS, so the military made the decision to turn it off for the duration of the war.
In the 1990s, the [[Federal Aviation Administration|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 amount of error added was "set to zero"<ref name="FAA">{{
On 19 September 2007, the [[United States Department of Defense]] announced that future [[GPS modernization|GPS III]] satellites will not be capable of implementing SA,<ref>{{
== Anti-spoofing ==
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The effect of gravitational frequency shift on the GPS due to [[general relativity]] is that a clock closer to a massive object will be slower than a clock farther away. Applied to the GPS, the receivers are much closer to Earth than the satellites, causing the GPS clocks to be faster by a factor of 5×10<sup>−10</sup>, or about 45.9 μs/day. This gravitational frequency shift is noticeable.
When combining the time dilation and gravitational frequency shift, the discrepancy is about 38 microseconds per day, a difference of 4.465 parts in 10<sup>10</sup>.<ref>Rizos, Chris. [[University of New South Wales]]. [http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap3/312.htm GPS Satellite Signals] {{Webarchive|url=https://web.archive.org/web/20100612004027/http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap3/312.htm |date=2010-06-12}}. 1999.</ref> Without correction, errors of roughly 11.4 km/day would accumulate in the position.<ref>{{
To compensate for the discrepancy, the frequency standard on board 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.<ref name="Nelson">[http://www.aticourses.com/global_positioning_system.htm The Global Positioning System by Robert A. Nelson Via Satellite], November 1999</ref> Since the atomic clocks on board the GPS satellites are precisely tuned, it makes the system a practical engineering application of the scientific theory of relativity in a real-world environment.<ref>Pogge, Richard W.; [http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html "Real-World Relativity: The GPS Navigation System"]. Retrieved 25 January 2008.</ref> Placing atomic clocks on artificial satellites to test Einstein's general theory was proposed by [[Friedwardt Winterberg]] in 1955.<ref>{{
=== Calculation of time dilation ===
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That is, the satellites' clocks lose 7,214 nanoseconds a day due to [[special relativity]] effects.
: Note that this speed of {{val|3874|u=m/s}} is measured relative to Earth's center rather than its surface where the GPS receivers (and users) are. This is because Earth's equipotential makes net time dilation equal across its geodesic surface.<ref>{{
The amount of dilation due to gravity will be determined using the [[gravitational time dilation]] equation:
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Since GPS signals at terrestrial receivers tend to be relatively weak, natural radio signals or scattering of the GPS signals can [[Desensitization (telecommunications)|desensitize]] the receiver, making acquiring and tracking the satellite signals difficult or impossible.
[[Space weather]] degrades GPS operation in two ways, direct interference by solar radio burst noise in the same frequency band<ref>Cerruti, A., P. M. Kintner, D. E. Gary, A. J. Mannucci, R. F. Meyer, P. H. Doherty, and A. J. Coster (2008), Effect of intense December 2006 solar radio bursts on GPS receivers, Space Weather, {{doi|10.1029/2007SW000375}}, October 19, 2008</ref> or by scattering of the GPS radio signal in ionospheric irregularities referred to as scintillation.<ref>{{
== Artificial sources of interference ==
In automotive GPS receivers, metallic features in windshields,<ref>{{
Man-made [[electromagnetic interference|EMI]] (electromagnetic interference) can also disrupt or [[radio jamming|jam]] GPS signals. In one well-documented case it was impossible to receive GPS signals in the entire harbor of [[Moss Landing, California]] due to unintentional jamming caused by malfunctioning TV antenna preamplifiers.<ref>{{
The [[Federal government of the United States|U.S. government]] reported that such jammers were used occasionally during the [[War in Afghanistan (2001–present)|War in Afghanistan]], and the U.S. military destroyed six GPS jammers during the [[Iraq War]], including one that was destroyed with a GPS-guided bomb, noting the ineffectiveness of the jammers used in that situation.<ref>American Forces Press Service. [http://www.defenselink.mil/news/newsarticle.aspx?id=29230 Centcom charts progress]. March 25, 2003. {{webarchive |url=https://web.archive.org/web/20091203004107/http://www.defenselink.mil/news/newsarticle.aspx?id=29230 |date=December 3, 2009}}</ref> A GPS jammer is relatively easy to detect and locate, making it an attractive target for [[anti-radiation missile]]s. The UK Ministry of Defence tested a jamming system in the UK's West Country on 7 and 8 June 2007.{{cn|date=September 2020}}
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