The Effectiveness of Various Methods for Characterising the Ionosphere in the Approach to Solar Maximum

Elizabeth Rooney, David Holmes, Mike Trethewey

Abstract:	The ionosphere can have a large effect on the accuracy of satellite navigation systems. These effects will increase as we approach solar maximum. There are several ways to characterise the ionosphere and remove its effects when computing a navigation solution. The most com-monly used method is the GPS Ionosphere Model using the parameters broadcast in the GPS navigation message. This model has a number of drawbacks and these will be discussed. Dual-frequency receivers are intended to remove effects of the ionosphere. However these results can be affected by biases on the satellites and within the receiver. Al-though the effects for GLONASS are larger because of the different frequencies used we show that there can still be problems with GPS. If these biases are not removed we show that, in times of low ionospheric activity, the error introduced into measurements using dual-frequency corrections can be greater than making no ionospheric corrections at all. Signal Computing has developed a single-frequency io-nospheric algorithm based on a Kalman filter and, to demonstrate its accuracy, we compare it to dual-frequency GPS and GLONASS measurements. We show that it is possible to derive consistent bias estimates which, when applied to dual frequency measurements, produce more consistent ionospheric delay values. Over long periods the Single-Frequency Algorithm gen-erates large amounts of data. We demonstrate a more ef-ficient method of approximating and storing these data to allow long term analysis of ionosphere activity. These data are then compared with GPS model parameters and various measurements of solar activity. We show that single-frequency measurements more closely follow solar activity and that the GPS Ionosphere Model, as it is im-plemented in its current form, cannot react to short term effects. Having therefore established confidence in the algorithm we also look at long and short term trends in ionospheric activity as we approach solar maximum.
Published in:	Proceedings of the 55th Annual Meeting of The Institute of Navigation (1999) June 27 - 30, 1999 Royal Sonesta Hotel Cambridge, MA
Pages:	387 - 394
Cite this article:	Rooney, Elizabeth, Holmes, David, Trethewey, Mike, "The Effectiveness of Various Methods for Characterising the Ionosphere in the Approach to Solar Maximum," Proceedings of the 55th Annual Meeting of The Institute of Navigation (1999), Cambridge, MA, June 1999, pp. 387-394.
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