slide 13

The range resolution can be increased by decreasing the pulse width, but this leads to lower signal-to-noise ratio. It is difficult to generate this kind of signal due to technical limitations. Achieving good azimuth resolution for space-bound platforms is impossible. Calculations show that a kilometer-long antenna is needed for a SAR image with 50-meter resolution, which is impossible to make. Scientists have developed techniques to increase resolutions without degrading the signal or using a large antenna. Before discussing this, we need to understand how transmitted signals are known in the range direction. If you look at the formula for the range resolution, you can immediately know that we can try to decrease the pulse width to increase the range resolution. However, it leads to a problem. The shorter the pulse width typically causes the lower the signal-to-noise, SNR, of signals because it doesn't use enough power to transmit signals. Due to the technical limitation, it is difficult to generate this kind of signal. In terms of the azimuth resolution, the good resolution for the space-bound platform is impossible to achieve. Let us do the simple calculation for the C-band space-bound sensor. The central wavelength of the C-band is 0.056 meter. We assume the altitude of the satellite is 700 km. The incident angle of the signal is 40 degree. If we want the SAR image with 50 meter resolution, the required antenna length should be one kilometer. This kind of antenna is impossible to make. Up till now, we know that the resolutions in the raw SAR data are not that high. Some scientists have developed amazing techniques to increase the range and azimuth resolutions without degrading the strength of the signal and making an incredibly large antenna. I will introduce to you how to do this. But before talking about that, I would like to introduce to you how we know the transmitted signals written in the range direction.