Abstract: This paper introduces a regional ionospheric model derived from dual-frequency GPS pseudo-range measurements,and deduces the formula for VTEC from the GPS observation equation. The ionosphere creates a delay in received GPS signal,which can range from several meters to more than one hundred meters.This will seriously corrupt the positioning and time transfer results unless these effects are taken into account.To mitigate the ionosphere effects,a number of ionosphere modeling methods have been developed including single frequency Klobuchar model,dual-frequency correction,differential method,and network modeling,each of which is appropriate in certain situations.Klobuchar model is typically accurate to a few meters along the vertical direction at mid-latitude.The dual-frequency method is based on the dispersive property of ionosphere and requires the use of a dual-frequency GPS receiver.Differential GPS is another method that can be used to remove or reduce the ionosphere effects.DGPS however works effectively only in local areas within,for instance,50 kilometers.Therefore,wide area differential GPS(WADGPS) and regional area differential GPS(RADGPS)methods become popular in recent year which can overcome the drawbacks associated with the conventional DGPS method. A WADGPS network includes a number of widely distributed reference stations with baselines typically in the range of several thousand kilometers,each of which is equipped with a GPS receiver(usually dual-frequency) and communication equipment.In this case the ionosphere is monitored using the GPS observation from all the reference stations.The ionosphere corrections are then produced over the covered area by some appropriate models such as grid-model,spherical harmonics,polynomial fitting.In this paper polynomial fitting model is adopted,and the VTEC is expressed as the function of the single layer puncture point location(longitude,latitude). Preliminary test shows that accurate to sub-meter along the vertical direction can be achieved by this model,and the predicted accuracy can reach sub-meter level in half an hour.Ionosphere model based on carrier phase should be studied further in the future in order to obtain the centimeter level accuracy.