2.1.2. Other Variables

In addition to the five variables derived from DDM data, five other variables were considered, representing the signal status, so that they can be used to enhance the performance of the models [8]. Instrument gain is the black body noise count divided by the sum of the black body power and the instrument noise power, which is an important parameter to calculate the DDM values. Scattering Area is the area of the central part of the DDM; generally, the larger this area, the rougher the reflective surface. Sp\_inc\_angle and sp\_az\_body are the incidence angle and azimuth angle of a given specular point, respectively. By taking sp\_inc\_angle and sp\_az\_body into account, the models can better reflect the situation of the received reflected signal [26]. Additionally, GNSS-R wind retrievals are affected by the ocean state [33]. Ocean swells are waves which travel from a long distance. The significant wave height of a swell (SWH\_swell) will affect the reflection of the GNSS signals, which is a form of interference which can be used as a variable [27]. Table 1 lists all the variables used in this study.


**Table 1.** List of input variables used in wind speed retrieval.
