Monitoring and Cross-Calibration of Altimeter σ0 through Dual-Frequency Backscatter Measurements

Abstract

The normalized backscatter, σ0, observed by an altimeter at its Ku band is used to infer wind speed above the ocean surface, and for the dual-frequency TOPEX altimeter, algorithms exist to calculate wind stress and rainfall from simultaneous observations at its two frequencies (Ku band and C band). However, the creation and application of such algorithms rely on the long-term stability of an altimeter, or even coreferencing the values obtained by one altimeter to those of another. This paper proposes a method of monitoring an altimeter?s backscatter values using the constancy of the correlation between the values at its two different frequencies. Although the amplitude of the scattering by the ocean surface may vary by more than an order of magnitude, the coherency of its behavior at different spatial scales enables it to be used as a constant reference surface for dual-frequency altimetry. Using the observed close correlation between σ0C and σ0Ku for each TOPEX cycle, the drift in σ0 calibration may be assessed through the shifts in the mean relationship. Application to the first 150 cycles of data from TOPEX side A shows good general agreement with the already applied corrections (derived from cycle averages of σ0); however, these corrections have overcompensated slightly leaving a remnant drift of 0.03 dB yr?1. In a particular instance, the response of the altimeter can be seen to take a day to recover from an instrument shutdown. Initial investigations for TOPEX side B (which apart from the antenna is an identically constructed but separate instrument) confirm that dual-frequency methods can be used to cross-calibrate nonsimultaneous sensors.