Error analysis for the Global Positioning System: Difference between revisions

Revision as of 00:20, 21 June 2021

Revision as of 20:38, 29 June 2021

Line 72:

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>[https://web.archive.org/web/20140522193825/http://www.navipedia.net/index.php/Earth_Sciences#Troposphere_Monitoring Navipedia: Troposphere Monitoring]{{deadlink|date=June 2021}}</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>[https://web.archive.org/web/20120430015157/http://www.navipedia.net/index.php/Tropospheric_Delay Navipedia: Tropospheric Delay]{{deadlink|date=June 2021}}</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>[https://web.archive.org/web/20120430025036/http://www.navipedia.net/index.php/Multipath Navipedia: Multipath]{{deadlink|date=June 2021}}</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.

Line 257:

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&nbsp;m (164&nbsp;ft) horizontally and about 100&nbsp;m (328&nbsp;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>{{Cite web |title=President Clinton Orders the Cessation of GPS Selective Availability |url=https://clintonwhitehouse4.archives.gov/WH/EOP/OSTP/html/0053.html |archive-date=2000-05-01}}</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.

Line 349:

Some countries allow the use of GPS repeaters to allow the reception of GPS signals indoors and in obscured locations; while in other countries these are prohibited as the retransmitted signals can cause multi-path interference to other GPS receivers that receive data from both GPS satellites and the repeater. In the UK Ofcom now permits the use of GPS/GNSS Repeaters<ref>[http://stakeholders.ofcom.org.uk/consultations/gnss-repeaters/statement/] Ofcom Statement on Authorisation regime for GNSS repeaters</ref> under a 'light licensing' regime.

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, "[[Instrument flight rules|IFR]] pilots should have a fallback plan in case of a GPS malfunction".<ref>Ruley, John. AVweb. [https://www.avweb.com/avionics/gps-jamming/ GPS jamming]. February 12, 2003.</ref> [[Receiver Autonomous Integrity Monitoring]] (RAIM) is a feature included in some receivers, designed to provide a warning to the user if jamming or another problem is detected. The U.S. military has also deployed since 2004 their [[SAASM|Selective Availability / Anti-Spoofing Module]] (SAASM) in the [[Defense Advanced GPS Receiver]] (DAGR).<ref>[https://rdit.army.mil/GPS/CustomContent/gps/ue/dagr.html US Army DAGR page] {{webarchive|url=https://archive.istoday/20120805212524/https://rdit.army.mil/GPS/CustomContent/gps/ue/dagr.html |date=2012-08-05}}</ref> In demonstration videos the DAGR was shown to detect jamming and maintain its lock on the encrypted GPS signals during interference which caused civilian receivers to lose lock.

== See also ==