Methodology:

During data acquisition, the KuKa radar acquires data on a series of six signal states: the four transmit polarization combinations (VV, HH, HV and VH), a calibration loop signal and a noise signal. Each data block consists of these six signals and are processed separately for each frequency. Data are processed into range profiles of the complex received voltage, through fast fourier transform (FFT). The range profiles for each polarization combination are power-averaged in azimuth for each incidence angle. Both the Ka- and Ku-band scatterometer beam scans at the programmed incidence angle, moving across the azimuth within a prescribed azimuthal angular width. The system then moves up to the next incidence angle, at 5 degree increments, and scans the next elevation line along the same azimuthal angular width. A new file is generated each time the positioner begins a scan. The footprint of the KuKa radar during one complete scan is a function Ku- and Ka-band antenna beamwidth, and the system geometry, with the footprint increasing in area, as incidence angle increases.

The raw Ku- and Ka-band radar data are stored in the KuKa radar system as .dat files, and needs to be manually downloaded from the radar's internal data drive. The radar processing IDL program is used to process the raw files and converts them into readable output data. The IDL code ingests the raw data and is post-processed using the calibration file, to derive the second-order covariance matrix, and fully-polarimetric backscatter (VV, HH, HV and VH; in dB scale). This procedure is done across the entire azimuthal angular width, for every incidence angle. The output data also contains the polarimetric parameters such as the co-polarized phase difference (in degrees) and co-polarized correlation coefficient, derived along with the polarimetric backscatter from the average covariance matrix (derived from the complex scattering matrix), of all azimuthal data blocks, within every incidence angle scan line. Using the polarimetric backscatter, user can derive other parameters such as the co-polarized and cross-polarized ratios. The output file also contains the date and time information (in UTC) and the geographical coordinates (latitude and longitude) at the time of data acquisition. The radar also records the actual incidence angle (accounting for the angle offsets) for every scan and records them in the output file. During the IDL processing, the user will be able to see the power-range and polarization signature plots for every incidence angle step, and can be saved by modifying the IDL script. The user can also modify the configuration .txt files to change the processing routine. The final processed files can be imported into any text reading program such as notepad, wordpad or excel to use the data for analysis.

Data collection:

KuKa radar

Data quality:

An experiment was done to investigate the response of the internal calibration loop in comparison to the KuKa radar's response when a metal plate was placed on the surface. This serves as a vertical height reference for the Ku- and Ka-band radar returns, and demonstrates the response of the system to a flat, highly-scattering surface. The metal plate and calibration loop data are consistent and in good agreement with each other, which indicates that the shape of the return including internal reflections are well characterized in the calibration data. For more details, refer to https://doi.org/10.5194/tc-2020-151.