The Effects of Ionospheric Scintillations on GPS

Mark Knight and Anthony Finn

Abstract:	Ionospheric scintillations are rapid variations in the amplitude and phase of radio signals resulting from density irregularities in the ionosphere. At GPS frequencies and in the mid-latitudes, scintillation effects are generally negligible. However, at low latitudes and in particular in the hours immediately after sunset, scintillation activity can become a problem for GPS. This is particularly true during periods of high solar activity when the ionosphere is also at its most active, As the next maximum of the 11 year solar cycle is expected to occur around the year 2000, it is anticipated that the levels of scintillation activity will gradually increase over the coming years. Scintillations have the potential to affect all GPS systems, including both single and dual frequency SPS and PPS receivers and both code and carrier phase DGPS, The effects of scintillations include an increase in errors on the code and carrier phase range measurements, and an increase in the probability of losing lock. Depending on the geometry of the satellite constellation and the locations of the irregularity patches within the ionosphere, these effects can translate into poorer navigational accuracy. The objectives of this work are two-fold: (i) to quantify the effects of scintillations on GPS performance, and (ii) to provide a capability for predicting these effects. The fwst objective has been met by combining a widely accepted stochastic model of scintillation activity with a generic delay locked loop/Costas phase locked loop model of a GPS receiver channel. The results of this work suggest that scintillation activity must be very strong in order for the tracking loops to lose lock, although lower levels of scintillation activity can introduce errors into carrier phase DGPS systems. The level of scintillation activity required to cause loss-of-lock and the likelihood of its occurrence are discussed in relation to the receiver’s bandwidth, discriminator type, and loop order. The second objective has been addressed by combining the Wide Band ionospheric scintillation MODel (WBMOD) with the analytical tracking loop models outlined above, WBMOD is a combined climatological and propagation model, which provides statistical information about scintillations including their intensity and probability of occurrence. By including information about the satellite and receiver locations, the effects of scintillations on the navigational accuracy of a receiver can be predicted at a given time and location.
Published in:	Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998) September 15 - 18, 1998 Nashville, TN
Pages:	673 - 685
Cite this article:	Knight, Mark, Finn, Anthony, "The Effects of Ionospheric Scintillations on GPS," Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998), Nashville, TN, September 1998, pp. 673-685.
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