Ionospheric Amplitude and Phase Fluctuations at the GPS Frequencies

Jules Aarons, Santimay Basu

Abstract:	The F-region of the ionosphere at times becomes turbulent and develops small scale (<500 m) irregu- larities of electron density. When sufficiently intense, these irregularities scatter radio waves at L-band fre- quencies and generate amplitude and phase scintilla- tion of GPS signals. Amplitude scintillation causes cycle slips and data losses to occur and phase scintil- lation generates fast variations of frequency with which the receiver has to cope. We have utilized the available data sets to establish the constraints imposed by scin- tillation on the operation of GPS receiving systems. This will help GPS users to correctly distinguish re- ceiver problems from ionospheric scintillation. The worst source of scintillation is the equatorial anomaly region. This region corresponds to two belts, each several degrees wide, of enhanced ionization in the F-region at approximately 15” north and 15O south of the magnetic equator. In this region, during the solar maximum periods, amplitude scintillations at 1.5 GHz may exceed 20 dB for several hours after sunset. Ascension Island in the Atlantic, Diego Garcia in the Indian Ocean, Hong Kong and Taiwan in the Pacific are some of the stations that fall directly under the anomaly region. However, areas within a few degrees of the magnetic equator may show only 5-7 dB fades. Occurrence of scintillation in the equatorial region is a function of longitude. For example the region between 0” and 30” West Longitude can have severe fading from September to May while the region from 110” to 130” East has maximum occurrence in the equinoxes with minimum occurrence from November to January (in solar maximum years). The other potentially active regions are at aurora1 and polar cap latitudes. In the central polar cap in years of solar maximum, GPS receivers may suffer > 10 dB fades during the solar maximum period. Relatively rare effects of magnetic storms noted in Japan will be outlined. GPS satellites offer a unique source for measure ments of amplitude and phase scintillation on a global scale. One receiver can record scintillation magnitudes and spectra at multiple propagation paths in the over- head sky. The data can be used to study ionospheric plasma structures, develop weather models of scintil- lation and can be scaled in frequency to support many operational systems. However the interpretation of the fading structure and scintillation spectra for GPS is not straightforward since both the ionospheric motion and the velocity of the satellite control the fading rate of scintillation. With the use of more data the problem of amplitude fading could be organized for the user’s benefit.
Published in:	Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994) September 20 - 23, 1994 Salt Palace Convention Center Salt Lake City, UT
Pages:	1569 - 1578
Cite this article:	Aarons, Jules, Basu, Santimay, "Ionospheric Amplitude and Phase Fluctuations at the GPS Frequencies," Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994), Salt Lake City, UT, September 1994, pp. 1569-1578.
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