In radar altimetry the electromagnetic (EM) bias is originated by the smaller reflectivity of wave crests than troughs, thus the average sea surface height is under-estimated. Bias uncertainty is currently the largest factor in altimetry error budgets. The EM bias in a bistatic forward-scattering configuration at L-band, such as in Global Navigation Satellite SystemsReflectometry (GNSS-R) altimetry, remains one of the major sources of uncertainty in the altimetry error budget. In this work the EM bias is computed using numerical simulations.To do so, a time-dependent synthetic non-Gaussian sea surface is created using the PiersonMoskowitz and Elfouhaily sea surface height spectra and spreading function. The sea surface is then discretized in facets and "illuminated" using a Right Hand Circular Polarization data at C-and Ku-bands. Then, the numerical model is applied at L-band, for bistatic configurations, including different azimuth angles, and different wind speeds. It is found that the EM bias is almost insensitive to the sea surface spectra selected and increases with increasing wind speed and incidence/scattering angle (up to ~20 cm at θi,s = 45° and U10 = 12 m/s), and it also exhibits a non-negligible azimuthal dependence, that must be accounted for in the error budgets of upcoming GNSS-R altimetry missions.

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In previous studies a method was devised to estimate the EM bias in bistatic GNSS-R altimetry (L-band) for a wind-driven sea surface spectrum. In the present study, the synthetic three-dimensional timeevolving wind-driven sea surface is also altered by rain, swell, or sea surface currents. The generated sea surface is illuminated by a Right Hand Circular Polarization (RHCP) L-band electromagnetic wave.Then, the scattered wave is computed from each facet in which the sea surface is discretized using the Physical Optics (PO) method. Finally, the EM bias is computed numerically under the presence of these three natural phenomena listed before. The impact of rain is a moderate decrease (in magnitude) of the EM bias due to the damping of the wind-driven waves, which is more significant as the wind speed increases. The impact of swell is a small increase (in magnitude) of the EM bias due to the increased sea surface roughness. The impact of currents can be either a moderate increase or decrease of the EM bias, depending on the sense of the current with respect to the wind.

This paper presents the computed electromagnetic (EM) bias predicted for GNSS-R altimetry systems. First, a synthetic time-evolving sea surface is generated satisfying an ocean surface height spectrum. Then, a direct GNSS signal recorded by an up-looking antenna is used as the GNSS signal 'illuminating' the synthetic sea surface, and the scattered waves are simulated using the Geometric Optics (GO) method. Finally, using the fundamental definition of the EM bias, it is estimated and compared to the limited existing analytical methods.Peer ReviewedPostprint (author’s final draft