Aquarius Scatterometer Calibration

In this paper, we discuss the Aquarius scatterometer calibration, starting with the instrument calibration. We examine the stability of Aquarius as quantified using the loop-back power and estimated receiver gain to shown Aquarius has been extremely stable to order $0.1,{rm dB}$ since mission start. We show the temperatures of scatterometer components not contained in the loop-back path have been controlled precisely to 0.5°C to minimize any temperature-dependent losses. Combined, these results show Aquarius produces accurate $sigma_0$ over the mission lifetime. In the next section, we discuss the stability as quantified using external models and again show stability to order $0.1,{rm dB}$ in very good agreement with instrument-only methods. Then, we discuss the methods used to absolutely calibrate Aquarius $sigma_0$ with respect to previous L-band radar systems. We show that Aquarius is relatively calibrated to order $0.1,{rm dB}$ for copolarization channels and better than $0.2,{rm dB}$ for cross-polarization channels. Finally, we discuss the calibration of the Aquarius wind speed product. We compare the Aquarius wind speed with radiometer wind speed products, other radar scatterometers, and numerical weather products. We show that the Aquarius wind speed product is on par with previous scatterometers in data quality.