Improving the Intercalibration of sigma0 Values for the Jason-1 and Jason-2 Altimeters

The normalized backscatter from a radar altimeter sigma 0 is a measure of the surface roughness at scales of a few radar wavelengths; over the ocean, this is used to infer wind speed. Long-term studies of wind speed rely on consistent measurements within an altimetric mission and good intercalibration between missions. For the Jason-1 and Jason-2 altimeters, the derivation of sigma 0 from the full waveform data is known to be sensitive to the recovered value for psi 2 , a term encompassing both mispointing and inhomogeneities within the altimetric footprint. The six months of data from the Jason-1/2 tandem mission reveal that different sigma 0 corrections are needed for these two causes of nonzero psi 2 values. With these corrections implemented, the rms difference of K u -band sigma 0 values for Jason-1 and Jason-2 drops from 0.15 to 0.05 dB, with the bias between the two showing a clear trend with wind speed; Jason-1 being 0.04 dB greater in high winds but 0.19 dB greater in low winds. No clear change in offset is noted during the six months of overlapping data. Implementation of this correction will improve consistency of Jason-1 sigma 0 values and may impact on orbit-fitting procedures.