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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|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>[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>[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.
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=== Computation of geometric dilution of precision ===
{{Duplication|section=yes|dupe=Dilution of precision (navigation)#Computation|discuss=Talk:Dilution of precision (navigation)#Duplication|date=June 2021}}
The concept of geometric dilution of precision was introduced in the section, ''error sources and analysis''. Computations were provided to show how PDOP was used and how it affected the receiver position error standard deviation.
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