Potential of High Frequencies Radiometer for High Resolution Altimetry
Obligis, Estelle1; Amiot, Thierry2; Picot, Nicolas2

The exploitation of radar altimetry data requires an accurate knowledge of all error sources that influence the measurements. One of them, the atmospheric humidity (mainly concentrated in the troposphere), strongly affects the range measured by the altimeter. Today, no meteorological model can provide this quantity with enough accuracy, so a Microwave Radiometer is added to altimetry missions (Envisat/MWR, Jason/JMR, S3/MWR, AltiKa).
Current radiometers combine channels taken inside and outside the water vapor absorption line centered at 22.235 GHz. Two other channels are required to account for the effect of the sea surface and for the cloud scattering. For these reasons, TMR, JMR and AMR radiometers use three channels, one below the water vapor line (18 /18.7 GHz) where the sensitivity to clouds is low, one in the absorption line (21 / 23.8 GHz) and one at higher frequency (37 / 34 GHz) where the sensitivity to cloud liquid water is higher. In the case of the ERS-1, ERS-2, Envisat, Sentinel-3 and AltiKa microwave radiometers, which do not include the low frequency channel, the surface roughness is taken into account through the altimeter backscattering coefficient. The spatial resolution of these instruments is of the order of 20 km, except about 10 km for the AltiKa instrument.

In the context of future altimetry missions, with new generation of altimeters operating in SAR mode for a better spatial resolution and noise reduction, comes the question of improving in parallel the spatial resolution of the wet tropospheric correction. Algorithmic improvements could be considered but it is also necessary to assess the potential of higher frequencies: indeed, brightness temperatures at higher frequencies present a much better spatial resolution and a lower sensitivity to the surface, allowing also improved performances when approaching the coasts or for hydrology.

The objective of this study is to evaluate the potential of high frequency radiometers for future altimetry missions. This work will be conducted in the context of the Jason-CS mission preparation, for which the presence of 2 different microwave radiometers is considered.