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. For the first time, the TOPEX altimeter allows for a direct measurement of this effect, rather than relying on numerical models as in previous satellite-altimeter missions. This is done through the use of a dual-frequency altimeter, one frequency in the C band (5.3 GHz) and the other in the traditional Ku band (13.6 GHz). Because the ionospheric delay is, to a good approximation, inversely proportional to frequency squared, the two altimeter height measurements yield a direct first-order correction for the time delay. Further discussions of the TOPEX ionosphere measurements can be found in: D. A. Imel, Journal of Geophysical Research, 99, 24895, 1994.
The raw ionosphere correction in TOPEX measurements is slightly noisy and requires smoothing before it is applied to the altimeter range measurements. This is done by a local Gaussian convolution filter, with a width of 20 one-second altimeter measurements. The figure adjoining shows a long section of altimetry (beginning approximately at MJD 49062.8) with the raw ionospheric corrections and their smoothed counterpart in red.
The ionosphere's characteristic bimodal enhancement about the equatorial electrojet, its dependence on local time, and its dependence on solar declination are all apparent in the following plots, which display the mean ionosphere correction during the first two months of 1994 at four different local times. As anticipated, the maximum correction occurs at mid-day. Also, since the sun is in the southern hemisphere during January and February, the ionosphere correction is decidedly enhanced in that hemisphere.
| 00:00 - 02:00 | 06:00 - 08:00 |
|---|---|
 |  |
| 12:00 - 14:00 | 18:00 - 20:00 |
 |  |
Units are millimeters. Mean local time of measurements given above each plot.
