2.1.1. Variables Calculated with DDM

The DDM glistening zone (the area from which scattered signals are observed) depends on the sea state, and the DDM volume has a significant correlation with wind speed [30,31]. Five variables (LES, DDMA, Noise Floor, SNR and NBRCS), extracted from DDM, can better reflect the sea state than the simple DDM volume [10,31]. Meanwhile, these variables are calibrated in the CYGNSS Level 1B product, which is commonly used for the retrieval of wind speed [32]. The LES of the integrated delay waveform, such as that generated with the delay waveforms of five different Doppler shifts, is strongly correlated with wind speed [33]. DDMA is the average of scattered power computed from the center 5 Doppler × 3 delay bin box [34], which is also significantly affected by wind speed. Noise Floor is the average power of DDM pixels which only contain noise. The signal-to-noise ratio (SNR) is defined as 10log(Smax/Noise Floor), where Smax is the maximum value in DDM, which has a strong correlation with sea surface roughness [35]. NBRCS is one of the two observables that were used to produce the global tropical cyclone product of CYGNSS [26], which is effective for wind speed retrieval [32].
