GPS Guide

Electronics errors are one of several accuracy-degrading effects.  They include ionospheric effects, ephemeris errors, satellite clock errors, multipath distortion, tropospheric effects, and numerical errors.

Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they pass through the Earth’s 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 receiver’s 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 delay in the frequencies to measure atmosphere dispersion and apply a more precise correction.

This can be done in civilian GPS receivers without decrypting the P(Y) signal carried on L2 by tracking the carrier wave instead of the modulated code.  To do this on lower cost receivers, a new civilian code signal on L2 called L2C was added to the satellites.  This new signal 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 corrections.  The date is transmitted via satellite system and transmits it on the GPS frequency using a special pseudo-random number so only one antenna and receiver is required.

Humidity also causes a variable delay resulting in errors similar to ionospheric delay but occuring in the troposphere.  This effect is more localized and changes more quickly than ionospheric effects and is not frequency dependent.  These traits make it much more difficult to make precise measurement and compensation for humidity errors than with the 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.

GPS signals can also be affected by multi-path issues where the radio signals reflect off of surrounding terrain such as buildings, canyon walls, and hard ground.  These delayed signals can cause inaccuracy as a well.