slide 20

Range compression is used to increase range resolution, and now we will learn how to increase azimuth resolution using the same concept. The Doppler effect naturally generates a chirped signal, which changes the frequency of a signal when the observer and the source move closer or farther apart. By considering multiple footprints as observers and sources, we can observe the Doppler shift and generate chirped signals to increase azimuth resolution. This method is called Doppler beam sharpening or synthetic aperture radar. The SAR receiver is more sensitive to the geometric phase change than the Doppler shift. Next, we will discuss how to convert the geometric phase change to the Doppler shift. After understanding how to use range compression to increase the range resolution, I'm going to introduce you how to increase azimuth resolution. Actually, it uses the similar concept to the range compression. The difference is that the chirped signal is naturally generated by the Doppler effect. Let us review the basic principle of the Doppler effect. If an observer and source of a signal approach, the observed frequency of the signal becomes higher. If the observer and the source of a signal move away, the observed frequency of the signal becomes lower. The change of the frequency due to the Doppler effect is called the Doppler shift. Let us imagine that an object is detected by several footprints in the azimuth direction because the footprint is large. We take the center of the several footprints as the origin. In other words, the origin is like an observer. Then, the received signals from other footprints are like the source of the signal. From the perspective of the origin, the received signals from other footprints approach or move away from the origin. So, the frequencies of the observed signals in different footprints also have the Doppler shift. Then, the chirped signals generated from the Doppler shift can be used to increase the azimuth resolution, just like what range compression does. This method is called Doppler beam sharpening. It is well known for its alternative name, synthetic aperture radar. In reality, the SAR receiver is not sensitive enough to observe the Doppler shift. It is more sensitive to the geometric phase change related to the origin. Next slide, I will talk about the deviation to convert from geometric phase change to Doppler shift.