High-Precision ISAR Method for Nonlinear Rotating Targets: Imaging for Drones

In this letter, an Inverse synthetic aperture radar (ISAR) imaging method uses second harmonic signals to extract motion parameters of multi-rotor autonomous aerial vehicles in an environment containing other natural objects for high-precision ISAR imaging. The core of the drones, which include powerful radio frequency (RF) circuits, is proven to have strong nonlinear effects. Translational motion parameters are estimated from prominent points in second harmonic signals, and the rotational motion of the drone is corrected by instantaneous imaging. An integrated RF system commonly used in drones has been tested, and its nonlinear phase characteristics have been obtained. The real phase error due to the nonlinearity was carried over into the subsequent simulation and compensated, which did not lead to defocusing in the final image. The feasibility of using second harmonic signals to compensate for fundamental frequency motion errors is verified by simulations. The micro-Doppler effect, generated by the high-speed rotation of the blades, has been demonstrated in the simulation images. System integration with a harmonic signal-based motion correction (H-MC) approach for motion compensation is shown to demonstrate a solution for accurate imaging for drones.