This paper presents the design of an X-band, linear antenna array suitable for UAV-borne radars. The array is synthesized by thirteen reflector-backed, pattern-reconfigurable dipoles arranged in an E-plane configuration. The dipole elements provide three different radiation patterns in the elevation plane (H-plane), where the main beam can be steered towards -40°, 0° and 40° for the three states, respectively. This is achieved through the incorporation of two tunable metasurfaces in the antenna structure. These surfaces are composed of split-ring resonator (SRR) unit cells, which are controlled by diode switches to act either as a pass-band or a stop-band spatial filter for the incident waves. The final array design operates between 9.5 and 10.5 GHz, exhibits a maximum gain of almost 19 dBi and has a scanning range of \pm 65\circ in the azimuth plane for all three states of the dipoles' elevation radiation pattern. By combining the pattern-reconfigurability property of the elements in the elevation plane with electronic scanning in the azimuth plane, the proposed antenna array provides a wide coverage that enables the detection of targets flying at different altitudes and can also be exploited to correct a potential beam misalignment in airborne radar systems. As a result, it is an excellent candidate for low-cost, UAV-borne radar platforms for drone detection, sense-and-avoid, and surveillance applications.