Research On The ISAR Imaging Of Ship Targets Under Circular Scanning Mode For Airship-borne Radars In Near Space

With the rapid development of the airship technology in the near space,it is significant to develop the marine surveillance radars mounted on airships.Because the airship is often in a quasi-hover state and its cruising speed is usually slow,it is suitable to acquire the images of ship targets using inverse synthetic aperture radar(ISAR)technology.Accordingly,this paper proposes a new working mode,which is called the circular scanning ISAR mode,for radars mounted on airships.This mode can exploit airships' advantages in large image swath and long observation time over a fixed region,and can obtain ISAR images of multiple targets distributed in a large area in a short time.So far the parameter design and echo simulation for radars mounted on airships are first studied because of the fact that there is no published accessed echo of airship-borne radars at home and abroad.In order to make the simulated echo close to the real case,the simulation method of sea clutter is studied and three dimensional cosinoidal rotation model is adopted as the motion model of ships.On the basis of the above contents,the imaging method of near space airship-borne ISAR is studied.It is found that the traditional RD(Range Doppler)imaging algorithm can still obtain fine imaging results when the antenna rotation speed is slow,but the image quality is poor when the antenna rotation speed is relatively fast.This is mainly due to that the rapid rotation of the antenna makes the imaging time very short,resulting in the low azimuth resolution of the target.For this reason,the ISAR super-resolution imaging method based on compressed sensing is studied.The compressed sensing signal model and the imaging algorithm under the condition of uniform rotation speed are first studied.This model and corresponding algorithm applies to the case that the integration time is very short.Nevertheless,the imaging quality of images generated by the above algorithm is poor when the imaging time length is longer than the maximum time period acceptable in the uniform rotation speed model.Therefore,the compressed sensing signal model and the imaging algorithm under the condition of non-uniform rotation speed are studied.Due to that the integration time in the circular scanning ISAR mode will not be too long,the assumption of uniform acceleration rotation is adopted in the modeling of the compressed sensing signal under the condition of non-uniform rotation speed.The simulation experiments were carried out to solve the problems of parameter designing,echo simulation and target imaging for near space airship-borne radars.The simulation results show that the RD algorithm can be applied to the case when the antenna rotation speed is slow,and also prove that the imaging algorithm based on compressed sensing achieves good imaging results when the antenna rotation speed is fast.When the rotation speed of the antenna is very fast,the compressed sensing signal model and imaging algorithm under the condition of uniform rotation speed is recommend to be used since this algorithm is simple and its processing efficiency is high.When the integration time is longer than the maximum time period acceptable in the uniform rotation speed model,the compressed sensing signal model and imaging algorithm under the condition of non-uniform rotating speed must be employed.