The ionosphere correction to the altimeter data compensates for the propagation delay in the travel time caused by the presence of free electrons in the atmosphere, primarily the ionosphere. When lacking direct altimeter measurements of the ionospheric delay such as provided by the TOPEX altimeter, we must rely on other data or on models of the ionosphere. For ERS-1, ERS-2, and Geosat altimetry, we use the IRI95 model (International Reference Ionosphere '95, specifically Version 13, dated January 1996) in combination with a special ionospheric-effective, solar index. This index is obtained from ionosonde measurements, which during the Geosat time period comprised about 70 globally distributed stations.
Although the IRI95 model represents a marked improvement over earlier models used, for example, on the original Geosat GDRs, the model is by no means perfect. By comparing it to actual ionosphere observations made by the TOPEX altimeter, it is easy to find periods when the IRI95 model yields an inaccurate correction for altimetry. This plot is an example. Near the equatorial electrojet, the IRI95 underestimates the required correction by order 10 cm. (Also shown for comparion is the ionosphere correction from the DORIS system, as used for POSEIDON altimetry.) We emphasize that this plot has been purposely selected as an example of inferior performance by the IRI95 model; it is not typical.
