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3.2.2.3.7. Electromagnetic bias

The Electromagnetic Bias is the correction for bias in measurements introduced by varying reflectivity of wave crests and troughs. It is calculated from models. Bias uncertainty is currently the biggest factor in altimeter error budgets.

Amplitude in metres of the sea state bias correction computed from empirical model for the Ku-band, here for the Jason-1 cycle 223. This map is drawn using the Basic Radar Altimetry Toolbox from the Jason-1 GDR products.

Due to the large footprint radar measurements, the sea surface scattering elements do not contribute equally to the radar return : troughs of waves tend to reflect altimeter pulses better than do crests. Thus the centroid of the mean reflecting surface is shifted away from mean sea level towards the troughs of the waves. The shift causes the altimeter to over-estimate the height of the satellite above the sea surface. The nature of the sea state bias has been investigated using airborne radars and lasers capable of determining for various sea states the strength of the vertically reflected signal as a function of the displacement of the reflecting area from mean sea level. It is given as a function of wind speed and the skewness and kurtosis of the probability distribution of sea surface elevation due to the waves on the sea surface.

EMB is the difference between the mean height of the sea surface specular facets and the mean sea level, it is a purely physical effect linked to the electromagnetic properties of the sea surface: The sea surface radar cross section per unit area varies with displacement from the mean water level. It is smaller towards the crests and larger towards the troughs [Yaplee, 1971]. In other words, the wave troughs are better reflectors than the crests. As a result, the mean height of the sea surface specular facets is below the mean sea level. The difference between the mean height of the specular facets and the mean sea level is the EMB. It is a function of the significant wave height and other sea-state related parameters. It varies with the radar frequency.

References:

• Yaplee, B.S., and al., Nanoseconds radar observations of the ocean surface from a stable platform. IEEE Trans. Geosci.Electron. GE-9 171-174 , 1971.