Abstract: Electromagnetic waves, when propagate through the ionosphere, would get delays due to the ionosphere refraction. According to certain experimental data, the difference of the group delay is up to 10⁵ ns. It is close to the time of the earth surface echoes when the signal generator is about 15 kilometers above ground in the air, and they are difficult to be distinguished (30 km/(3×10⁸ m·s^-1) = 10⁵ ns). So the time when the transient electromagnetic waves reach satellite receiver cannot be utilized directly in doing Time Difference of Arrival (TDOA). In this paper, the ionospheric Total Electron Content (TEC) is derived by Dual-Frequency Method, and on this basis, the time received by satellite receiver is modified. The result shows that it is inaccurate with the error being about 100 ns. The potential origins of the error are provided.