Remote sensing of the ionosphere by the TOPEX dual-frequency radar altimeter

Transionospheric radio pulses suffer group retardation associated with ionospheric refraction effects. The effect is small for operational frequencies in excess of a few GHz, but where accurate timing is required, such as for the GPS navigation system or for radar altimetry, the retardation cannot be neglected. Dual-frequency operation can be used to correct for this effect. This technique is incorporated in the dual-frequency radar altimeter on board the joint NASA/CNES satellite TOPEX/Poseidon. Important by-products of the correction are high resolution measurements of vertically integrated electron content over much of the Earth's surface. Ionospheric data from the TOPEX dual-frequency altimeter are compared with models currently used to provide values for single-frequency altimeter correction, and their performance is assessed with regard to variations with location, and levels of solar and geomagnetic activity.