5. Ionospheric Experiment Validation and Compensation

The L-band Geosynchronous SAR (GEO SAR) is very susceptible to the temporal-spatial variant ionosphere as its long integration time and large coverage, leading to image drift and degradation. This chapter demonstrates an experimental study of ionospheric influences on GEO SAR, including both the background ionosphere and the ionospheric scintillation. In the experiment, we employ the Global Positioning Satellites (GPS), probe the ionosphere and collect the trans-ionosphere GPS signals. Then the recorded signals are used to create the data basis on which simulations and analysis are based. But GEO SAR has very different orbit trajectories from GPS. Thus in the real operation, the transformation of the temporal-spatial frame between GPS and GEO SAR should be first performed before the focusing and the evaluation are carried out. Then the influences of the background ionosphere and the ionospheric scintillation are analyzed based on the experimental data. Finally, the corresponding compensation or mitigation approaches are presented. For the temporal-spatial variant background ionosphere, the autofocus method can produce a well-focused image. In comparison, the random fluctuations of amplitude and phase induced by ionospheric scintillation are more difficult to deal with. Thus an orbit-optimization strategy is first proposed by utilizing the diurnal and geographical pattern of the ionospheric scintillation occurrence. It can avoid being interfered by ionospheric scintillation by tuning the orbit parameters. Alternatively, once interfered, an iterative algorithm based on entropy minimum is derived to jointly compensate the signal amplitude and phase fluctuations in GEO SAR.