Improving the intercalibration of σ0 values for the Jason-1 and Jason-2 altimeters

The normalized backscatter from a radar altimeter, σ0, 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 σ0 from the full waveform data is known to be sensitive to the recovered value for ψ2, 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 σ0 corrections are needed for these two causes of non-zero ψ2 values. With these corrections implemented, the r.m.s. difference of Ku-band σ0 values for Jason-1 and Jason-2 drops from 0.15 dB 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 6 months of overlapping data. Implementation of this correction will improve consistency of Jason-1 σ0 values and may impact on orbit-fitting procedures.