Total Electron Content
7
H|0|water|water vapor|67|77|
H|0|process|radar pulse|120|130|
H|0|alt|altimeter|141|149|
G|0|freelect|Free electrons|5|18|
G|0|iono|ionosphere|47|56|
G|0|rangedelay|range delay|422|432|
G|0|accuracy|accuracy|179|186|
    Free electrons are present in the earth's ionosphere. As with water vapor, these electrons delay the return of the radar pulse from the altimeter and thus interfere with the accuracy of sea-level measurements. 
    To correct for this delay the altimeter takes measurements at two radio frequencies. The difference between the two measurements provides both a measure of the electron content and a correction for the range delay. This is the first dual-frequency altimeter to measure the ionospheric range correction directly, providing the first high-resolution view of the ionospheric electron content. 
