Mini-review on topside sounding

Ionospheric sounders flown on satellites have returned a wealth of global information of the electron density distribution from the
F
-region electron peak upwards, not obtainable in any other way. This mini-review outlines the techniques used to obtain topside ionograms and reduce them to electron profiles, indicating the most important differences between ground-based and satellite-based ionograms. An attempt has been made here to indicate the most essential features, with the details provided in the cited references.