With worldwide increased Global Navigation Satellite System (GNSS) receivers,it is possible to obtain the ionospheric total electron content (TEC) and hence monitor the ionosphere with GNSS.Using a thin layer assumption of the ionosphere and dual-frequency GPS observations from 16 geomagnetically quiet days in four seasons of 2006,this paper adopts the spherical harmonic model to fit TEC and investigates the effects of two network constitutions on global TEC derivation,one with 275 GPS receivers and the other with 125 GPS receivers.The results show that the data can be well fitted for both network constitutions.The derived TECs are consistent with each other for four seasons.This is especially true for TECs at low- and mid-latitude.The derived satellite and receiver biases are stable during the year.The standard deviation of the satellite and the receiver biases are less than 0.5 ns and 3 ns,respectively.