## 1 Introduction  
A spaceborne bistatic radar system is defined when antennas for reception and transmission are physically separated and located aboard two spacecraft. The system could be either cooperating (transmitter and receiver designed for the specific bistatic application), or non-cooperating (receiver designed independently of the transmitter) [1]. Non-cooperating systems can use already-existing satellite instruments, such as Synthetic Aperture Radars (SARs), as transmitters (or illuminators) [2]. Bistatic applications were investigated in the fields of target detection [3, 4] and planetology [5]. Concerning Earth Observation, the specular reflection from navigation satellites was exploited for scatterometric applications over the oceans [6]. The possibility to envisage bistatic configurations for SAR interferometry was studied in [7, 8]. Recently, experiments involving the X-band have been carried out (e.g., [9, 10, 11]) to assess the additional information contained in the bistatic reflectivity of targets.  
As for land applications, [PERSON] _et al._[12] demonstrated the potential to detect the signal scattered in the specular direction for retrieving bare soil parameters. However, they outlined that measuring the coherent component of the scattered signal by means of bistatic radars is problematic, because the spatial resolution becomes very poor around the specular direction, and ambiguities between surface points before and after the specular point occur [13]. [PERSON] and [PERSON] stated, in a brief report [14], that considering a GPS signal impinging on a rough land surface, soil moisture is related to the ratio between horizontally and vertically polarized scattered waves.  
In a previous work [2], we reported on a theoretical investigation aiming at identifying the best bistatic measurement configuration, in terms of incidence angle, observation direction, polarization, and frequency band, for soil moisture content (SMC) retrieval. In addition, we evaluated the improvement of the estimation accuracy with respect to the conventional backscattering measurements. While in [2] we did not tackle the problem of verifying the identified radar configurations from a technical point of view, this work deals with spatial resolution, spatial coverage and duty cycle (_i.e._, the fraction of the orbit in which suitable bistatic data collection is ensured). It aims at evaluating whether an adequate overlap between the footprints of the transmitting and receiving antennas is ensured for the bistatic observation geometry that guarantees both an improvement of the quality of the SMC retrieval and a quite good spatial resolution.  
We point out that investigations on the design of a spaceborne mission implementing a configuration of bistatic radars devoted to specific environmental applications are generally lacking in the literature. Moreover, the literature generally deals with fixed bistatic configurations (_i.e._, the static design), without considering the orbital dynamics, while the feasibility of maintaining the selected configuration of sensors along the orbit of the spacecraft on which the receiver is installed, ensuring good performances in terms of operational parameters such as spatial coverage, is a crucial aspect.  
In the present study, the nominal bistatic mission is considered, so that problems such as the orbit and attitude controls are not tackled, thus implicitly assuming that the passive system is periodically controlled in order to maintain the designed bistatic formation.  
In Section 2, the findings regarding the bistatic configurations most suitable for SMC retrieval are summarized and the selection of one configuration, based on the evaluation of the spatial resolution, isdescribed. In Section 3, the orbit design approach is depicted, while, in Section 4, the results of the analysis of the spatial coverage are presented and discussed. Section 5 draws the main conclusions.