Analysis of ASCAT Ocean Backscatter Measurement Noise

The Advanced Scatterometer (ASCAT) level 1b products provide spatially averaged calibrated backscatter measurements along with their Kp values which are estimates of the normalized standard deviation of the backscatter values. The Kp values can be regarded as a measure of the error in the mean backscatter caused by speckle noise, instrument characteristics, data processing, and spatial inhomogeneities of the target. This information assists in the retrieval of wind vectors and allows their error characteristics to be determined. This paper describes the algorithm used to calculate Kp. The algorithm considers both the correlations that occur in ASCAT data due to onboard processing and the Hamming weights used in the spatial averaging in the ground processing. ASCAT Kp values over the open ocean are investigated, and we demonstrate that their behavior is as expected and meets requirements. This indicates that the ASCAT instrument is operating as intended and is providing good quality ocean backscatter data. The behavior of Kp over Arctic sea ice is also examined, and we show that it contains geophysical information that may be useful for various applications such as ice-type classification or sea ice tracking.