## 1 Introduction  
Bistatic radar is today a research field of great and renewed interest. Recent studies have revealed the potential of spaceborne bistatic radar for large-scale Earth observation by using transmitting and receiving antennae flying in formation for integrated operation [1-5]. Moreover, nowadays a great demand exists for Digital Elevation Models (DEM) on wide areas for an ample range of commercial and scientific applications [6]. The most mature method for wide area DEM generation by means of microwave space remote sensing is SAR Interferometry (InSAR) [7]. InSAR coherently combines the signals from two SAR antennae to calculate the interferometric phase difference in each image point. The phase difference directly depends on the local relief. However, the phase interferogram must be unwrapped to resolve the modulo 2 ambiguity before estimating topography. Different approaches exist for phase unwrapping [7], usually a time-consuming and difficult task that cannot be avoided. In forested or mountain slope areas phase coherence can be very low, and an uncorrected solution of the phase unwrapping problem might be derived, often leading to diverging errors in interferometric DEM generation [8].  
InSAR is not the only technique for DEM generation from SAR data [9]: a previously existing technique is Stereo-Radargrammetry [10]. Both InSAR and Stereo-Radargrammetry are based on the notion of detecting relative displacements between two images, except that InSAR uses phase domain data and, operating on a wavelength scale, is intrinsically more accurate, but also less robust if phase decorrelation is strong [11, 12], whereas Stereo-Radargrammetry exploits only range/Doppler and amplitude measurements. Stereo-Radargrammetry is able to provide good results only for large stereo intersection angles [13], for this reason it was applied only on repeat-track spaceborne SAR data [8, 13-16]. However, according to the present trend in bistatic spaceborne SAR missions [1, 2], the possibility of forming a very large (from tens to hundreds of kilometers) controlled separation between the antennae (baseline) by means of two satellites operating simultaneously and flying in formation is concrete.  
In a previous work [17] the authors analyzed stereo-radargrammetric methods applied to spaceborne monostatic-bistatic SAR with reference to an observation geometry based on parallel tracks, i.e. monostatic and bistatic antennae flying in formation along parallel trajectories at the same height, with identical velocities and with no squint angles. These hypotheses represent a simplification of actual spaceborne scenarios, since it is not possible to achieve perfectly parallel orbits, so differences in antenna altitudes, velocities and pointing angles between monostatic and bistatic sensor exist. The main goal of this paper is to investigate performance of a spaceborne monostatic-bistatic SAR mission accounting for actual and time-varying orbital configuration. To this end, first of all key orbital design issues for spaceborne monostatic-bistatic acquisitions able to assure single-pass stereo-radargrammetric coverage are pointed out. After introducing models adequate to derive terrain height measurements from stereo radargrammetric pairs, a performance evaluation is carried out considering the Italian COSMO-SkyMed mission [18, 19] as reference for the monostatic satellite and a bistatic receiving-only sensor flying in formation with it [2, 20]. Finally, simulation results are presented, showing quantitatively the height measurement accuracy that can be achieved and its time history along the orbit.