The Impact of Ionospheric Scintillations on GPS Performance

Mark Knight and Anthony Finn

Abstract:	Ionospheric scintillations are rapid variations in the amplitude and phase of radio signals resulting from irregularities in the ionosphere. At GPS frequencies and in the mid-latitudes the effects of these phenomena are generally negligible. However, at low latitudes and in particular in the hours immediately after sunset, scintillation activity can become a significant problem. This is particularly true during periods of high solar activity when the ionosphere is at its most active (the next maximum of the 11 year solar cycle is expected around the year 2000). Scintillations have the potential to effect all GPS systems, including both single and dual frequency SPS and PPS receivers and code and carrier phase DGPS. Amplitude scintillations can lead to periods of reduced signal levels at the GPS antenna which results in an increase in the measurement noise within the code and carrier tracking loops. Phase scintillations increase the dynamic stress on the carrier tracking loops which results in additional phase measurement jitter. Both effects result in an increase in pseudo-range measurement errors and under extreme conditions can lead to complete loss of signal lock. This paper discusses the use of a simple deterministic model which relates the velocity, density, and dimensions of typical ionospheric irregularities to the scintillation effects they produce. The irregularities, which are assumed to be located at ionospheric F2 layer heights (300-600 km), are modeled as an assembly of Gaussian variations on the background Total Electron Content (TEC). The patterns of amplitude and phase variations produced by these irregularities are derived using Fresnel- Kirchoff diffraction theory. The resulting variations are then applied to numerical models of the GPS receiver’s code and carrier tracking loops to obtain estimates of the pseudo-range measurement errors and tracking loop performance. These models demonstrate the conflicting requirements of a large tracking loop bandwidth for a high tolerance to phase scintillations, and a narrow tracking loop bandwidth for a high tolerance to amplitude scintillations. The impact of the loop filter order and velocity aiding on the tolerance to scintillations is also discussed.
Published in:	Proceedings of the 9th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1996) September 17 - 20, 1996 Kansas City, MO
Pages:	555 - 564
Cite this article:	Knight, Mark, Finn, Anthony, "The Impact of Ionospheric Scintillations on GPS Performance," Proceedings of the 9th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1996), Kansas City, MO, September 1996, pp. 555-564.
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