A Comparison of Predicted and Measured GPS Performance in an Ionospheric Scintillation Environment

Mark Knight, Manuel Cervera and Anthony Finn

Abstract:	Ionospheric scintillations are rapid variations in the am-plitude and phase of radio signals resulting from density irregularities in the ionosphere. At GPS frequencies and in the mid-latitudes, scintillation effects are generally negli-gible. 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 dur-ing periods of high solar activity when the ionosphere is at its most active. As the next maximum of the 11 year solar cycle is expected to occur around the year 2000, it is an-ticipated that the levels of scintillation activity will in-crease significantly over the coming year. Scintillations have the potential to affect all GPS systems, including both single and dual frequency SPS and PPS re-ceivers and both code and carrier phase DGPS. The ef-fects of scintillations include an increase in errors on the code and carrier phase range measurements, and an in-crease 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 for a re-ceiver. It is anticipated that the most significant effects will be observed on the L2 tracking loops of codeless and semi-codeless receivers and for carrier phase DGPS. Previously, models of the code and carrier tracking loops of GPS receivers have been combined with predictions provided by the Wide Band Ionospheric Scintillation Model (WBMOD [1]) in order to attempt to predict the likely impact of scintillations on GPS [2]. The objectives of this paper are as follows: (i) To compare the predictions of GPS receiver perform-ance with measurements made by co-located Novatel Millennium and PAQ12 Ionospheric Scintillation Monitoring (ISM) receivers [3], (ii) To use the ISM data to evaluate the Wide Band Iono-spheric Scintillation Model (WBMOD) for the South East Asian region, and (iii) To suggest improvements to the models described in our previous paper. Although data from the peak of the current solar cycle is not yet available, data from the September 1998 and March 1999 equinoxes show sufficient scintillation activ-ity to begin such a comparison.
Published in:	Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999) September 14 - 17, 1999 Nashville, TN
Pages:	1437 - 1450
Cite this article:	Knight, Mark, Cervera, Manuel, Finn, Anthony, "A Comparison of Predicted and Measured GPS Performance in an Ionospheric Scintillation Environment," Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999), Nashville, TN, September 1999, pp. 1437-1450.
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