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The ionosphere can cause delays in signals traveling through it due to variations in electrons. This delay is frequency-dependent, with longer wavelengths experiencing more severe impacts. Correction methods include using an ionospheric model to calculate VTEC and applying it to equations to determine ionospheric phase delay. Another method is the big-range spectrum method, which splits the signal spectrum into two sub-bands and removes the ionospheric phase delay. More information can be found in their paper. When sub-signals travel through the ionosphere, the variation of electrons can delay the signals. The ionospheric phase delay is modeled by the formula. As you can see, it is frequency-dependent. So, longer wavelengths experience more severe impacts of ionospheric error, such as L and P bands. And it is not usually corrected for short-wavelength signals, such as C and X bands. There are several correction methods. Ionospheric model provides the VTEC. So, you can apply the VTEC to the above equations to form ionospheric phase delay. Big-range spectrum method has grabbed attention recently. This method splits the spectrum of sub-signals into two sub-bands, and then forms the ionospheric free equations to remove the ionospheric phase delay. You can read their paper for details.