Ionospheric Effects on GPS

T HE ionosphere is an important source of range and range-rate errors for users of the global positioning system (GPS) satellites who require highaccuracy measurements. At times, the range errors of the troposphere and the ionosphere can be comparable, but the variability of the Earth's ionosphere is much larger than that of the troposphere, and it is more difficult to model. The ionospheric range error can vary from only a few meters, to many tens of meters at the zenith, whereas the tropospheric range error at the zenith is generally between two to three meters. Fortunately, the ionosphere is a dispersive medium; that is, the refractive index is a function of the operating frequency, and twofrequency GPS users can take advantage of this property of the ionosphere to measure and correct for the first-order ionospheric range and range-rate effects directly. Unlike the troposphere, the ionosphere can change rapidly in absolute value. Although the range error of the troposphere generally does not change by more than ±10%, even over long periods of time, the ionosphere frequently changes by at least one order of magnitude during the course of each day. The major effects the ionosphere can have on GPS are the following: 1) group delay of the signal modulation, or absolute range error; 2) carrier phase advance, or relative range error; 3) Doppler shift, or range-rate errors; 4) Faraday rotation of linearly polarized signals; 5) refraction or bending of the radio wave; 6) distortion of pulse waveforms; 7) signal amplitude fading or amplitude scintillation; and 8) phase scintillations. In order to understand the reasons for these potential effects on GPS performance, a brief description of the major characteristics of the ionosphere is necessary.