Principle of FMCW SAR and Signal Model
Principle of FMCW SAR
The range resolution of SAR is determined by the bandwidth of the transmitted signal. FMCW is an applicable technique to improve the range resolution of SAR without a high frequency modulation rate. In FMCW SAR systems, the radar transmits chirp signals during the entire PRI. In pulse SAR systems, the pulse duration is on the order of a few microseconds, whereas the pulse duration is on the order of milliseconds in FMCW SAR systems. Therefore, with the long pulse duration, the bandwidth of the transmitted signal in FMCW SAR systems is increased, and the range resolution is improved, as shown in Fig. 5.1.
Fig. 5.1 Comparison of signals in pulse SAR and FMCW SAR. a Transmitted signal in pulse SAR. b Transmitted signal in FMCW SAR
In pulse SAR systems, the stop-and-go approximation is commonly used, and the instantaneous slant range from the antenna to the target is assumed constant during the pulse duration. In the case of FMCW SAR, however, this approximation is inaccurate due to the long pulse duration.
Moreover, given that the transmitted bandwidth is large in FMCW SAR systems, the deramping operation is used to reduce the range sampling rate. The deramping operation is shown in Fig. 5.2. In Fig. 5.2, the transmitted signal is represented by the black line, and the received signals from near range and far range are represented by red and blue lines, respectively. The reference signal is represented by the green line. The RVP is induced by the deramping operation, which needs to be corrected during the imaging process .
It can also be noted from Fig. 5.2 that after the deramping operation, the der- amped signals are monochromatic signals. Set the near range signal as an example, the deramped frequency /near can be expressed as
Fig. 5.2 Principle of the deramping operation in FMCW SAR where Rnear is the near range, and Br is the bandwidth of the transmitted signal. The second-order phase error of the beat signal is compensated, and the range compression of FMCW SAR can be accomplished by range FFT .