Beamforming Sensitivity of Airborne Distributed Arrays to Flight Tracking and Vehicle Dynamics

In this paper, we identify the beamforming sensitivity of 2D synthesized arrays for Unmanned Aircraft Systems (UAS) based radars generated from closely spaced flight tracks to in-flight disturbances. Computer- based simulations are used to assess the sensitivity of array beamwidth, and sidelobe level sensitivity to the number of flight-lines (antennas) and the array perturbation in flight path. We assess expected flight path deviations and vehicle attitude using flight data gathered from a small fixed-wing UAS recently deployed to Greenland for polar ice sheet surveys. For circularly distributed flight path offsets, it was found that flight path deviations must be kept within similar to 2% of the operating wavelength to achieve 30 dB Sidelobe Level (SLL) and 20-degree half-power beamwidth (BW). For array sizes of seven elements and greater the maximum average SLL was typically within 1 dB regardless of the magnitude of the offset. SLL are more sensitive to vertical deviations than horizontal, though the difference is small. Analysis of the UAS flight-track perturbations reveals that the radiation parameters of 30 dB SLL and 20 degrees BW are expected to be achieved less than 10% of the flight time (when assuming a 9-element array and an operating frequency of 14 MHz). A simulated nine-element array demonstrated slightly improved performance, including over arrays with more elements. To meet the beamforming parameters for 95% of the flight time would require an operating frequency of 2.76 MHz or lower; however, it is impractical to integrate the large antenna required at 2.76 MHz into a small UAS due to its size. In addition, changing the operating frequency to meet beamforming requirements may not be practical for the sensing application. Thus, phase compensation methods will be investigated in future studies.