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Atmospheric effects[ edit ] Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they pass through the Earth's atmosphereespecially the 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 path through the atmosphere is longer see airmass. Once the receiver's approximate location is known, a mathematical model can be used to estimate and compensate for these errors.
Ionospheric delay of a microwave signal depends on its frequency.
It arises from ionized atmosphere see Total electron content. This phenomenon is known as dispersion and can be calculated from measurements of delays for two or more frequency bands, allowing delays at other frequencies to be estimated.
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 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 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. 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.
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. Its effect varies with local temperature and atmospheric pressure in quite a predictable manner using the laws of the ideal gases. These delayed signals cause measurement errors that are different for each type of GPS signal due to its dependency on the wavelength.
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.
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. Ephemeris and clock errors[ edit ] While the ephemeris data is transmitted every 30 seconds, the information itself may be up to two hours old.
Variability in solar radiation pressure  has an indirect effect on GPS accuracy due to its effect on ephemeris errors. If a fast time to first fix TTFF is needed, it is possible to upload a valid ephemeris to a receiver, and in addition to setting the time, a position fix can be obtained in under ten seconds.
It is feasible to put such ephemeris data on the web so it can be loaded into mobile GPS devices. 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.
In the s when receivers were quite expensive, some methods of quasi-differential GPS were developed, using only one receiver but reoccupation of measuring points.LAWS OF EXPONENTS - To multiply powers of the same base, add their exponents. Thus, 2 2 times 2 3 = 2 5 = 32 PROOF: 2 2 = 4; 2 3 = 8; 2 5 Therefore; 4 x 8 = 32 To divide powers of the same base, subtract the exponent of the divisor from the exponent of the dividend.
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