O.O. Bezvesilniy, D.M. Vavriv, V.V. Vynogradov, V.A. Volkov, R.V. Kozhyn

Over the last years, theoretical researches and practical developments are being carried out at the Institute of Radio Astronomy of the National Academy of Sciences of Ukraine on creation of synthetic aperture radar (SAR) systems intended for deploying onboard small aircrafts. The application of small aircrafts as SAR platforms has several advantages such as lowering of exploitation costs and an ability to perform quick surveillance of ground scenes. However, the development of SAR systems for small aircrafts is still a rather difficult problem. The main difficulty is related to significant trajectory and orientation instabilities of small aircrafts. Motion error compensation procedures are usually applied in order to compensate such instabilities based on accurate measurements of the aircraft trajectory and orientation by an expensive navigation system. However, even with such motion compensation it is not always possible to obtain high-quality SAR images in the case of significant motion errors. In this paper, we describe scientific and technical solutions implemented in two SAR systems, RIAN-SAR-Ku (2-cm wavelength) and RIAN-SAR-X (3-cm wavelength), developed and produced at the Institute of Radio Astronomy. These SAR systems are designed to be operated from small aircrafts producing SAR images in real time. We have proposed an effective method for estimation of the antenna beam orientation angles from Doppler frequencies of backscattered radar signals in real time. Due to this method we are able to simplify the SAR navigation system considerably since now there is no need in measurements of the aircraft orientation angles. The proposed solution allows accurate tracking of variations of the antenna beam orientation and enables a reliable automatic adjustment of SAR filters for matching them with the backscattered radar signals. In the RIAN-SAR-Ku system, the aperture synthesis is performed by a well-known method based on time-domain convolution with range migration correction by interpolation. This method forms each pixel of the SAR image by using separate reference function and migration curve. Therefore, the algorithm is well suitable for SAR processing under unstable flight conditions. In particular, the above-mentioned SAR filter adjustment is applied in this SAR system. We have found that the aircraft trajectory can be calculated accurately by integration of the aircraft velocity measured by a GPS receiver. The accuracy of such approach is sufficient for the application of the obtained trajectory for the motion error compensation. This solution has been implemented in the RIAN-SAR-X system and allows us to obtain high-quality multi-look SAR images in real time. In this SAR system, the formation of SAR images is performed by using a frame-based range-Doppler algorithm. A processing scheme with half-overlapped frames is applied in order to guarantee the formation of a ground strip image without gaps despite of motion instabilities. Several algorithms for post-processing of the recorded SAR data have also been developed, among which we may highlight a SAR processing algorithm with built-in geometric correction, which is able to produce high-quality multi-look SAR images in the case of significant flight instabilities. During flight test the mentioned SAR systems were installed onboard light-weight aircrafts Antonov AN-2 and Y-12. The obtained results have demonstrated that the developed SAR systems are capable of producing high-quality SAR images under unstable flight conditions. It should be noted that the proposed solutions for SAR systems deployed on small aircrafts could also be useful for SAR systems intended for other platforms.