## 3 Orbit Design  
According to the chosen frequency (_i.e._, C-band), we have made reference to the ASAR instrument onboard Envisat. The real operative Image Swath Modes (ISMs) and Wide Swath Mode (WSM) have been taken into account for the analysis. ISMs guarantee a fairly high resolution, but have a smaller near and far range capability that implies worse monostatic (and thus bistatic) spatial coverage. WSM presents opposite characteristics. Table 1 reports some parameters of the ASAR illuminator in Image and Wide Swath modes. Note that, among the seven Envisat/ASAR Image Swath Modes, only four were selected, because the other ones observe the Earth with incidence angles larger than 35\({}^{\circ}\), which are not compatible with our requirements for SMC retrieval. We have firstly designed the orbit of the receiver by choosing its observation geometry at the equator based on the requirements for SMC estimation (_i.e._, the static design) and then the orbits of both platforms have been propagated to evaluate the antenna footprint superimposition, _i.e._, the spatial coverage of the bistatic acquisitions. Note that the time when the passive satellite is over the equator has been assumed as the initial one [17]. We have made a number of assumptions. Firstly, that the bistatic system is non-cooperative, so that orbit, attitude (including yaw-steering) and antenna pointing of the active system are not subordinated to the bistatic acquisition requirements. This is of course a worst case assumption, which aims to use existing radar as source of opportunity. Secondly, the transmitting antenna is right-looking. Moreover, the satellite carrying the receiver (hereafter denoted also as the passive satellite) flies in formation with that carrying the illuminator (the active one), on a parallel orbit, thus establishing a \"parallel orbit pendulum\" configuration, in which the platforms move along orbits with the same inclination, but different ascending nodes. In principle, also the \"leader-follower\" configuration, in which the spacecraft fly on the same orbit, but with different crossing times of ascending node, would be possible. However, it does not allow any of the bistatic observation geometries previously selected if a non-cooperative bistatic system and a side-looking illuminator are assumed. Another hypothesis we have made for the static design of the orbits is that the receiver performs bistatic acquisitions only along the ascending pass, since the crossing of the orbits near the poles makes the transmitter to illuminate out of the footprint of the receiver. Note that an eventual left/right looking capability of the receiver would dramatically change the bistatic angles with respect to the \begin{table}
\begin{tabular}{|c|c|c|c|c|} \hline
**Mode** & **Swath [km]** & **Near Range [deg]** & **Far Range [deg]** & **Range Resolution [m]** \\ \hline ISM1 & 105 & 15.0 & 22.9 & 30 \\ ISM2 & 105 & 19.2 & 26.7 & 30 \\ ISM3 & 82 & 26.0 & 31.4 & 30 \\ ISM4 & 88 & 31.0 & 36.3 & 30 \\ WSM & 405 & 15 & 37 & 100 \\ \hline \end{tabular}
\end{table} 
Table 1: Some parameters of the ASAR illuminator in Image Swath and Wide Swath Modes. Note that both the swath and the ground range resolution refer to the backscattering acquisition; regarding the latter, its value is related to a nominal incidence angle (e.g., 23\({}^{\circ}\) for ISM2). required ones, whereas only a cooperative transmitter with left/right looking capability would enable valuable data acquisition in the descending pass. The orbit of a spacecraft is described by means of the well-known six Keplerian parameters: semi-major-axis, eccentricity, inclination, right ascension of the ascending node (\(\Omega\)), perigee argument, and mean anomaly (\(M\)). We have considered Envisat as the active satellite, so that its orbital parameters are fixed. The passive satellite semi-major-axis, eccentricity, and inclination have been chosen coincident with those of the active one, in order to reduce orbit maintenance operations.