TOPEX Outreach
Propogation Medium Correction

Included in the category of propogation medium correctionare the effects of: Total sea-state bias of the onboard tracker estimate of mean sea level, and those corrections corresponding to the resultant decrease in the local speed of light due to index of refraction changes as the altimeter's signal travels through earth's atmosphere.

There are two effects which contribute to the discrepancy between the electromagnetic (EM) sea level estimated by the onboard tracking algorithm from averaged return waveforms and that of true mean sea level: Electromagnetic Bias and Skewness Bias

Electromagnetic Bias
This correction arises because of the height difference between mean sea level and the mean scattering surface. The way this occurs is through the differential backscattering of power per unit surface between wave troughs and that of wave crests. This deviation results from the fact that the power backscattered from a small wave facet is proportional to the local radius of curvature of the long wavelength portion of the wave spectrum. In general, ocean troughs have a larger radius of curvature than wave crests; thereby creating a bias in backscattered power toward wave troughs. A greater prominence of small-scale " wavelets" super-imposed on wave crests creates an increased roughness which further scatters the altimeter pulse in directions away from the incident radiation; effectively enhancing the bias. The backscattered power measured by the altimeter is therefore greater from wave troughs than from wave crests, thus inducing an EM sea level bias toward wave troughs. A direct correlation between significant wave height (SWH) growth and EM bias increase allows researchers to estimate the required correction.

Skewness Bias
This error occurs because of the height difference between the mean height of specular scatterers and the median scattering surface that is actually measured by the onboard tracker. This is a direct result of the non-gaussian distribution of the sea surface height which shifts the median from the mean sea level toward wave troughs; which in turn also contributes to the EM bias towards wave troughs.

Corrections corresponding to the resultant decrease in the local speed of light due to index of refraction changes as the altimeter's signal travels through the earth's atmosphere can be separated into:

Ionospheric Correction
This correction takes into account the variation in the number of free electrons present in the sub-satellite ionosphere location. Typically, the electron content varies from day to night (very few free electrons at night), from summer to winter (fewer during the summer), and as a function of the solar cycle (fewer during the solar minimum). The signal delay encountered is inversely proportional to the altimeter monitoring frequency squared.

Tropospheric Correction
This correction is related to water vapor content and other gases present in the subsatellite troposhere location. There is both a wet & dry tropospheric delay which must be accounted for. The dry correction can be modeled via surface pressure measurements. The wet portion is typically adjusted from measurements made by an onboard radiometer. It is significant to note that the dry term includes the weight of the water molecules while the wet term accounts for their additional influence on the index of refraction.

Geoid Modeling Errors


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Last Modified: Sat Jul 17, 1998
CSR/TSGC Team Web