Imaging Mechanism And Algorithms Of Spaceborne-airborne Bistatic SAR

Compared to mono-static SAR, bistatic SAR propose many advantages, for example, improving object scattering factor, increasing system survival, and enhancing anti-stealth etc. It can be widely applied to terrain mapping, battlefield surveillance, and detecting moving targets. Transmitter/receiver of BiSAR can be carried by different platforms such as satellite, aircraft and even the stationary on the high tower. SA-BiSAR is a subclass of bistatic SAR, which consists of a spaceborne transmitter and an airborne receiver.According to its application fields, satellite can be divided into communicate, weather, scout, navigation mapping, earth resource and multi-useful satellites etc. Compared to navigation satellite, spaceborne SAR possesses the potential ability of forming high-resolution images, and its signal processing is relatively easy. Although its application is limited by the satellite illuminating time and region, high resolution imaging can be implemented with low cost, high reliability and stability using native or foreign satellite resources at present.Due to the great discrepancy of satellite and aircraft speed, the configuration SA-BiSAR implies an essential asymmetry, which has different range history and obvious two-dimension spatial variant properties. Hence, some aspects of SA-BiSAR are deeply studied such as imaging mechanism, image formation algorithm and motion error and compensation. The contents in this paper are as follows:1. Imaging mechanism and echo simulation are investigated. General bistatic geometry model are presented by the projection of transmitter/receiver trajectory. The variation of technology parameters with bistatic geometry is exhibited by formulae and simulations including baseline range, bistatic angle and range/ azimuth resolution. Considering with earth rotation, we solved the range problem between near earth and out space by coordinate transform method, implemented the more accurate echo simulation at SA-BiSAR arbitrary geometry model. The direct path signal can also be simulated.2. Azimuth space-variant and Doppler properties are studied. Aiming at the parallel mode, the formulae of azimuth space-variant are derived with Taylor second order expansion. Theory analysis is coincided with simulations. At the same time, we put forward to compute Doppler centric frequency and Doppler frequency rate by vector method. From footprint trajectory of transmitter and receiver, we derived the formulae of integrated time and azimuth resolution. Simulations are given and results are analyzed in this chapter.3. Imaging formation algorithms of BiSAR are investigated. Under the non-parallel mode, we evaluated the focus performance of BP algorithm. Under the parallel mode, we presented the improved RD algorithm which is adapted to the SA-BiSAR system; on the estimation of polynomial phase signal, we formulate the maximum likelihood method which is to choose the initial value by HAF method and search the loacal minimum by NM simplex method. Under the lower SNR, it improved the estimated properties.4. Motion error and compensation of SA-BiSAR are demonstrated. We formulated the geometry model including velocity and attitude errors, derived range error equation, induced the three kind errors, and analyzed these properties. With simulating echo data, the results are coincided with theoretical analysis after imaging formation algorithms.5. Oscillator phase noise impacts on SA-BiSAR image quality. We studied phase noise change when airborne SAR is at static and vibration conditions. Based on independent and non-identical distribution oscillators, the formula of integration side lobe ratio of SA-BiSAR is derived when airborne SAR is at static conditions or perturbing by random vibration. Simulations are given and results are analyzed in this chapter.