## 4 Discussion of the Outcomes  
To produce coverage data, an appropriate software tool, which accounts for spacecraft propagated orbits, yaw-steering maneuvers of both spacecraft, sensors pointing geometry, Earth rotation and estimates the targeted area, has been developed. At each time point of the simulation, whose overall \begin{table}
\begin{tabular}{|l|r|} \hline zenith incidence angle \(\theta_{\rm i}\) [deg] & 35 \\ zenith scattering angle \(\theta_{\rm s}\) [deg] & 1 \\ azimuth scattering angle \(\varphi_{\rm s}\) [deg] & 180 \\ Semi-major-axis [km] & 7,159.48 \\ Eccentricity & 0.00115 \\ Inclination [deg] & 98.5 \\ Perigee argument [deg] & 90 \\ Ascending nodes difference (\(\Delta\Omega\)) [deg] & 4.20 \\ Mean anomaly difference (\(\Delta M\)) [deg] & 0.91 \\ \hline \end{tabular}
\end{table} 
Table 2: Selected bistatic configuration (in terms of zenith incidence angle, and zenith and azimuth scattering angles), and orbital parameters at the initial epoch (static design). Figure 4: Bistatic observation geometry at the initial time in terms of zenith incidence (\(\theta\)) and scattering (\(\theta_{\rm s}\)) angles, receiver (Rx) and transmitter (Tx) near range (NR) and far range (FR) points, and heights (\(H\)) of the satellites. period is 35 days, _i.e._, the orbit repeat cycle of Envisat, the software evaluates the area of the Earth surface observed simultaneously by the two antennas on the basis of their aperture angles and of the satellites' positions. The portion of this area which is actually observed according to the observation geometry selected in section 2, _i.e._, \(\theta\)\(<\) 35\({}^{\circ}\), \(\theta\)\(\in\) [0\({}^{\circ}\)-8\({}^{\circ}\)] and \(\varphi_{\rm s}\)\(\in\) [90\({}^{\circ}\)-270\({}^{\circ}\)], is the area target. To perform the simulation, we have considered the transmitter able to illuminate according to the real access capabilities of the reference mission (_i.e._, Envisat, see Table 1). As for the receiving antenna, we have assumed a main lobe azimuth aperture of 6\({}^{\circ}\) (see [17]) which is capable to point toward the area illuminated by the transmitter by means of attitude maneuvers or electronic steering. The latter does not ensure the zero Doppler condition of bistatic measurements, but its purpose is the superposition of the antenna footprints. Table 3 reports the results. It can be noted that WSM presents the best results in terms of duty cycle (~40%) thanks to the greater access capability. Higher resolution modes achieve duty cycles not less than 12% (actually 11.9%) and that can exceed 21%, which is a still acceptable performance, considering that we are in the hypothesis of a fully non-cooperative system, _i.e.,_ the maximum duty cycle is 50% for a fixed looking side of the transmitting antenna. It is worth noting that we have decided to discard the swaths smaller than 10 km in the computation of the fraction of orbital period during which the bistatic data are acquired and this choice implies an overall decrease of the resulting duty cycle. ISM1 and WSM reach the highest latitudes (almost 72\({}^{\circ}\) in both the geographic hemispheres). In addition, the far range capability of the considered transmitter operating mode limits the latitude range of bistatic acquisitions. As stated when discussing Figure 4, since the passive satellite orbit is defined according to the maximum incidence angle identified for the application (_i.e._, 35\({}^{\circ}\)) during static design, the operational modes ISM1, ISM2 and ISM3 do not perform bistatic acquisitions at low latitudes but they focus on mid-latitude belts. Figure 5 shows the Earth maps with the acquired targeted areas (red dots) for WSM (upper panel), ISM4 (central panel) and ISM2 (lower panel). The ground tracks of the active and passive satellites are plotted in green and blue, respectively. Although our simulation considers the entire Envisat orbit repeat cycle (35 days), Figure 5 regards a time-window of one week, for the sake of clarity.