## 2 Selection of a Bistatic Configuration  
### Configurations Suitable for Soil Moisture Retrieval  
Bistatic configurations are defined in terms of frequency, polarization, and transmitter-target-receiver relative geometry. This geometry is shown in the left panel of Figure 1, in which the incident plane is assumed as a reference coordinate plane, and the configuration is identified through the incidence angle (\(\theta_{i}\)) with respect to the vertical, the zenith (\(\theta_{s}\)) and azimuth (\(\varphi_{s}\)) scattering angles, and their acceptable ranges to be compatible with the considered application.  
In [2], we adopted a well-established electromagnetic surface scattering model, such as the Advanced Integral Equation Model (AIEM) [15], to numerically simulate the bistatic measurements, for the purpose of identifying the configurations suitable for soil moisture estimation. We underline that the AIEM model has a large range of validity, so that the study described in [2] was carried out under different conditions. L-, C- and X-bands were considered, both Gaussian and Exponential autocorrelation functions were adopted, and the parameters characterizing the surface roughness, _i.e._, the standard deviation of heights \(s\) and the correlation length \(l\), varied within fairly standard intervals, encompassing most of the typical agricultural fields in bare soil conditions. The results we obtained can be therefore considered fairly general.  
The findings in [2] indicate that, especially at C-band, the quality of soil moisture retrievals improves by complementing bistatic measurements with monostatic ones provided by already-existing SARs. This could be expected because, for bistatic systems, the detected field is determined by a different behavior of the scattering with respect to the monostatic case, and in the specular configuration also by a coherent scattering mechanism as opposed to a diffuse one, thus providing  
Figure 1: Left panel: geometric elements that identify the transmitter-target-receiver (Tx-TG-Rx) bistatic configuration. Right panel: sketch of observing configurations suitable for SMC retrieval.  
independent information on the target parameters. The need to take advantage of various sources of information for tackling the complex and usually ill-posed problem of SMC retrieval was indeed pointed out in numerous literature studies (e.g., [16], where polarimetric data were used).  
The range of valuable scattering directions, for a C-band radar, singled out in [2] is sketched in the right panel of Figure 1. Observations around nadir, at \(\theta_{s}\) below 4\({}^{\circ}\)-10\({}^{\circ}\), form a cone (_i.e._, \(\varphi_{s}\) can assume any value) whose vertical axis corresponds to the \(z\)-axis (in a Cartesian reference system), while the vertex is in the origin. Furthermore, for \(\varphi_{s}\) less than 40\({}^{\circ}\)-50\({}^{\circ}\), the zenith scattering angle can indefinitely be increased by including the specular direction and some grazing observations. The illuminator should observe the Earth with a quite small incidence angle, approximately between 15\({}^{\circ}\) and 35\({}^{\circ}\). In [2] it was demonstrated that, at C-band, the standard deviation of the SMC retrieval error can be reduced up to a factor 3 with respect to that achievable with monostatic observations, by integrating backscattering and bistatic measurements.