## 3 Mission Design  
The orbital design of a spaceborne monostatic-bistatic mission must be developed in order to attain required baselines, to reduce maintenance operations and, thanks also to an adequate pointing strategy, to permit the bistatic sensor to observe the area illuminated by the monostatic one [32]. Moreover, the analysis synthetized in the previous section allows the following requirements to be defined for designing a formation flying missions devoted to radargrammetric DEM generation:  
* parallel orbits, to take advantage of stereo effect;
* bistatic angle larger than 5-10\({}^{\circ}\);
* same-side stereo configuration, to avoid bistatic slant range ambiguities;
* bistatic sensor closer to the target thus limiting too large off-nadir angles for the bistatic receiver and benefiting by stronger echoes.  
In a pendulum formation the minimization of propellant mass is achieved by assuring that the orbital perturbations due to Earth oblateness have the same effects on both satellite orbits [32-34]. To this end monostatic and bistatic satellites must share the same semi-major axis, eccentricity, and inclination. Moreover, if the orbits have the same argument of perigee, velocity differences are minimized too, aiding in reducing too large relative along track displacements. Since pendulum formation establishes the maximum baseline over equatorial area, and, simultaneously, a minimum bistatic angle must be guaranteed for relief reconstruction, the differences in right ascension of ascending node (RAAN) and mean anomaly can be selected in order to maximize the area on which the acquisition with a large bistatic angle can be achieved. On this basis they are established so that the bistatic sensor is in the elevation plane of the monostatic one when passing over the equator; in [32] a mathematical model is presented to evaluate them.  
Assuming the Italian COSMO-SkyMed [18, 19] as a reference for the monostatic mission (see Table 1) the range of monostatic off-nadir angle is from 23.3\({}^{\circ}\) to 43.7\({}^{\circ}\) and the monostatic satellite performs a classic yaw steering manoeuvre to reduce aerodynamic drag and to attain zero monostatic Doppler centroid frequency. Moreover, considering the bistatic sensor working with a constant off-nadir angle, \(\theta\)_\",_ the parameters of the pendulum formation for four different bistatic off-nadir angles are listed in Table 2.  
It is clear that when the satellites move away from the equator the cross track separation decreases and the bistatic satellite leaves the monostatic range elevation plane. Therefore, without envisaging a change of the monostatic/bistatic spacecraft attitude and/or antenna pointing angles, the bistatic coverage is lost. In [35] the following solution is proposed:  
* monostatic SAR elevation steering counteracts cross-track separation;
* bistatic SAR azimuth elevation steering (\(<\)4\({}^{\circ}\)) avoids along-track separations;
* an _ad hoc_ designed yaw-steering manoeuvre for the bistatic satellite can be used to overcome swaths relative rotations.  
This strategy allows bistatic acquisition only during ascending phase within a range of latitudes dependent on the assumed off-nadir angle of the bistatic satellite (see Table 2). It is worth noting that azimuth beam steering can attained in an easier way than elevation steering, since modern SAR antennae are segmented in more tiles in along-track direction, each of them fed separately [3]. In particular, recent studies have pointed out potential, peculiar applications achievable by splitting antenna aperture in along-track direction, such as location of moving targets in high resolution SAR images [36, 37]. Figures 6-8 show baseline components, bistatic angle, and bistatic antenna azimuth and yaw steering angles within the covered latitudes for each of the four selected bistatic off-nadir angles.