Study about ionospheric effects induced by irregularities on MEOSAR range imaging quality

Medium-Earth-Orbit Synthetic Aperture Radar (MEOSAR) is one of the most promising next-generation space borne synthetic aperture radar (SAR) concept for its excellent capabilities, such as images with a wide swath and high time-resolution, high precision reproducible orbit interferometric measurements, ability to track a moving target, rapid response to emergencies and so on. However, spaceborne SAR will inevitably be affected by the ionosphere. Many researchers have studied the ionospheric effects on the image quality of SAR, but almost all the studies are based on LEOSAR. Because of the short synthetic aperture time, narrow swath and lower orbit height for LEOSAR, the ionospheric effects induced by irregularities are usually small. Many studies have been concerned about the ionospheric effects on SAR, however, there are very few papers published discussing ionospheric effects on SAR range imaging induced by ionospheric irregularities, especially for MEOSAR. It has been shown that ionospheric irregularity will result in distortion and extra group delay for radio pulses propagating through ionospheric random media. Usually, the signal of SAR is a pulsed Linearly-Frequency-Modulated (LFM) signal, and therefore extra delay time when propagating through ionospheric irregularities and distortions will inevitably arise. An analysis model for ionospheric effects induced by irregularities on MEOSAR is established based on the system characteristics of MEOSAR and ionospheric irregularity parameters. Extra time delay and pulse width for pulses propagating through the ionospheric irregularity round-trip are studied. The degradation of range image quality, including deterioration of resolution and displacement due to ionospheric irregularities, and the relation between the deterioration and the parameters of irregularities and SAR orbit height are analysed for the first time. The results of the analysis show that the effects induced by ionospheric irregularities on range imaging are usually much smaller than the effects induced by the background ionosphere, but the effects will be very serious when the inner and outer scale of the ionospheric irregularities are small and the fluctuation is strong. Furthermore, the degradation of resolution and the displacement of the image will become more and more serious with the increase of SAR orbit height. Therefore, ionospheric effects on range imaging due to irregularities not commonly considered for Low-Earth-Orbit SAR (LEOSAR) should be studied for MEOSAR.