What is the ionospheric delay, and how is it mitigated in GNSS?

• A. A non-dispersive delay caused by tropospheric water vapor, mitigated by weather models.
• B. A dispersive delay caused by free electrons in the ionosphere; mitigated by dual-frequency measurements or ionospheric models.
• C. A delay due to satellite clock errors, mitigated by precise clock products.
• D. A delay caused by multipath reflections, mitigated by antenna shielding.

Answer: (B)

Ionospheric delay is a dispersive delay caused by the presence of free electrons in the ionosphere. Because the ionosphere behaves like a plasma, the speed of GNSS radio waves depends on frequency, so lower frequencies experience more delay than higher ones. The amount of delay also depends on the total electron content along the signal path, so it changes with time of day, solar activity, and satellite geometry.

This delay is best mitigated by using dual-frequency measurements. With two frequencies, the receiver can compare the delays at the two different colors of light and separate the ionospheric component from the geometric range. The ionospheric delay scales roughly with 1/f^2, so the difference between the two frequencies isolates the ionospheric impact, allowing either an ionospheric-free combination to be formed or a direct estimate of the total electron content along the path that can be removed.

An alternative approach uses ionospheric models, which predict the delay based on parameters like time, location, and solar activity, and are especially useful for single-frequency receivers. In practice, GNSS users often combine both methods: dual-frequency measurements for robust correction, with model-based corrections supporting single-frequency setups.

For context, tropospheric delay, in contrast, is non-dispersive and is treated with weather/climate models; satellite clock errors are handled with precise clock products; multipath is addressed through antenna design and shielding.