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System Design Scheme of the Airborne Mechanic Scanning SARAn important issue of the mechanic scanning mode is to maintaining N constant. Equation 4.2.5 is the expression of N. In an airborne SAR system, V_{a}, Ав and X are constant values. If pMF is also constant, it can be noted that the sharpening ratio changes with the value of в. And it is not acceptable since the resolution of one image differs at different scanning angles. There are mainly two methods to solve this problem. First, set the pulse integration amount M constant, and let the PRF changes with the scanning angle [5]; second, set the PRF constant, and M changes with the scanning angle [6]. The first method increases the complexity of the radar system, while the second method increases the signal processing burden by using DFT. Algorithms are also proposed to adjust the sharpening ratio with both PRF and M constant, such as prefiltering algorithm [7] and frequencydomain interpolation algorithm [8]. However, both algorithms increase the complexity of the DBS algorithm. The mechanic scanning mode is originally applied on the firecontrol radar, of which the typical antenna scanning range is 30°75°. Under this range, the value of cos в varies from 0.259 to 0.866. In this condition, the sharpening ratio is significantly changed. In the application of the mechanic scanning mode into airborne SAR, we wish to maintain the sharpening rate constant without increasing the complexity of both the system structure and the signal processing by the optimizing the system design. According to article [2, 9], a new system design scheme is presented. In this scheme, the scanning range is 18° to 18°. In this condition, the value of cos в varies from 0.951 to 1. Since that DBS is not a high resolution imaging algorithm, the sharpening ratio under this scheme can be viewed as constant. The system parameters are illustrated in Table 4.1. According to the parameters in Table 4.1, the radar works at Kuband, and the central range is 15,455 m. As to a target in the observation area, the echo pulse amount is N_{t} = Ав PRF = 388.8 « 389. Consider the impact of coherent sparkle noise, the target is not integrated during the full synthetic aperture in general cases. In the coherent pulse amount is small, a target can be indicated in multiple coherent Table 4.1 System parameters of airborne SAR mechanic scanning mode
pulse interval (CPI), which not only increases the indication possibility but also eliminates the coherent sparkle noise. Since the coherent integration processing is operated by FFT, set N_{t} = 384, then can be valued as 128. As to different scanning angle, the Doppler bandwidth is 632.4664.95 Hz. Substitute the values of Af_{d} and M into Eq. 4.2.5, the value range of the sharpening ratio is 40.4742.56 and the azimuth resolution varies from 22.183 to 23.382 m. Therefore, it is proved that the sharpening ratio is almost constant. 
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