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This paper investigates the characteristic of the ionospheric delay and forecasting using modified statistical Holt-Winter method over Malaysia. This was carried out by using dual frequency GPS Ionospheric Scintillation and TEC Monitor (GISTM) receivers installed at the National University of Malaysia (UKM) (geographic coordinates 2.55°N, 101.46°E and geomagnetic coordinates 7.10°S, 174.05°E), and Langkawi national observatory (Langkawi) (geographical coordinates 6.19°N, 99.51°E and geomagnetic coordinates 3.39°S, 172.42°E), during 2011. In this work, the Holt-Winter forecasting method is modified to provide a better forecasting result over Malaysia. The diurnal, monthly, and seasonal variations of actual ionospheric delay have been analyzed and compared with forecasted ionospheric delay. Based on the results, the diurnal variability of ionospheric delay was maximal mostly between 12:00– 17:00 LT, and minimal nearly at 05:00 LT. The maximum monthly ionospheric delays occurred in October, which is attributed to the geomagnetic storm disturbance that caused increase in ionospheric delay. The lowest seasonal ionospheric delay occurred in the summer and the highest was during the equinox. The maximum ionospheric delay on a quiet day was around 41.9 TECU, while it reached up to 86.6 TECU during the geomagnetic storm. The Holt-Winter method exhibited the highest difference between the measurements and the forecasts in March, with an error of 6%. Hence, the modified Holt-Winter method forecast ionospheric delay effectively during quiet and disturbed periods over Malaysia.
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