The ionosphere is the part of the upper atmosphere where we see the aurora. It's important because it affects radio signals in ways that can either enhance or degrade our ability to communicate and navigate.

For example, the BBC world service is broadcast using high-frequency (HF) radio signals. It's only possible to receive it in remote places because it is refracted back to the Earth’s surface by the ionosphere. In a sense, the upper atmosphere acts like a mirror.

Your sat nav system is affected by the ionosphere in a different way. Sometimes the signals are delayed more than other times and may even be lost completely as they pass through the ionosphere.