Improving the Intercalibration of $\sigma ^{0}$ Values for the Jason-1 and Jason-2 Altimeters

The normalized backscatter from a radar altimeter sigma⁰ 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 sigma⁰ from the full waveform data is known to be sensitive to the recovered value for psi², 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 sigma⁰ corrections are needed for these two causes of nonzero psi² values. With these corrections implemented, the rms difference of K_u-band sigma⁰ 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 sigma⁰ values and may impact on orbit-fitting procedures.