Experimental observation and statistical analysis of the vertical TEC mapping function

The TEC data obtained from GPS is actually the slant TEC, i.e., the integration of electronic density along the GPS signal path. In many applications the slant TEC should be converted into vertical TEC. This is done by the so called mapping function which is usually determined by certain models of electron density profile. In this paper we proposed a new method to obtain mapping functions from TEC observation of GPS network. We first discuss the dependence of the mapping function on the 'ionospheric height', which is the layer height for Single Layer Model (SLM), or the peak electron density height for the Chapman Model, and found that the value of the mapping function decreases obviously with the increase of the ionospheric height. We then estimated the 'experimental mapping function' from the observation of vertical TEC (from JPL GIMs) and slant TEC (from IGS GPS data) during the whole year of 2006. After the comparison between both results, we find the values of the mapping functions from experimental data are much larger than that from the model with reasonable ionospheric height (e. g., 400 km SLM height), when the zenith angle is large enough. We attribute this to the effect of the plasmasphere which exists above about 1000 km altitude hence may increase the effective 'ionospheric height'. It is concluded that the present experimental results may be used to model or improve the vertical TEC mapping function. This might be helpful for retrieving TEC from GPS network.