X-band UAV-SAR System Design Using Wideband Chirp Signal Generator
Synthetic aperture radar (SAR) is an active sensor that is operated using the moving platforms such as satellite, aircraft, or unmanned aerial vehicle (UAV). As a kind of imaging radar, it uses microwave to detect the target in remote area. Due to the characteristics of microwave, the SAR can be operated regardless of the weather conditions. Also, it offers highresolution images of targets by utilizing the wide-bandwidth transmit signal.
Due to the features of SAR, it is widely used in several area such as surveillance, land monitoring, urban managing, disaster monitoring etc. To conduct the continuous monitoring for some specific areas, the SAR payloads are loaded on UAV and operated recently. Several UAV-SARs exist already in research fields and practical uses (e.g. AirMOSS of NASA). These platforms usually perform its missions on 1-2 km above the ground. The specifications of the recent UAV-SARs and the conceptual design of the proposed UAV-SAR are listed in Table 1 .
To achieve the high-resolution, the SAR system uses a linear frequency modulated (LFM) signal called chirp. As the bandwidth of transmit signal is inverse proportional to the resolution, the SAR system requires wide-bandwidth signal to offer the better performance. There are several types of chirp signal generators: memory-map based type, signal generators using frequency multipliers, direct digital frequency synthesizer
(DDFS) type, and parallel DDFS type.
The memory-map based chirp signal generator stores the predefined chirp waveform in the PROM or other memory devices and loads the signal using the counter. It has the advantage of high-precision output. However, it requires the large sized ROM to store the wide-bandwidth signal and hard to update the predefined signal especially the SAR is operated in space mission.
Compared to the previous one, the DDFS generates only the phase of the signal using registers and full-adders. Next, it combines the phase signal and the amplitude signal that is stored look-up table (LUT). When DDFS generates the desired signal, it controls the frequency control word (FCW). The fast switching speed, less memory dependency, simple structure, easy
configurability, and etc. are the features of DDFS. However, as this system truncates the I/O bits between the phase signal generator and LUT, the signal characteristics are degraded due to the spurs.
The signal generator using frequency multiplier is proposed to realize the wide-bandwidth signals. First, it generates the relatively narrow band signal. Next, using the frequency multiplier and band-pass filter (BPF), it expands the bandwidth of transmit signal. Using the frequency multiplier, SAR system can achieve the wide-bandwidth with simple structure. However, as the bandwidth of signal is multiplied with the hardware, also the residual noise of the signal gets larger.
In this paper, the parallelized DDFS type chirp signal generator is presented. The signal generators are operated based on a digital circuit. Therefore, the baseband output bandwidth of the presented signal generation methods is limited due to the clock frequency of the devices. However, by using the multiple memory and the high-speed multiplexer, the parallelized DDFS can generate the wide-bandwidth chirp signal with relatively low clock frequency.
As the recent SAR system requires the high-resolution images, the SAR system equipped with wideband signal generator is also needed. In this paper, the conceptual design for X-band UAV SAR and the parallelized DDFS chirp signal generator for the wideband chirp pulse (up to 800 MHz) have been proposed. Using the circular polarization, the proposed SAR system can be used to full-polarimetric SAR. Also, due to the wideband chirp pulse, the SAR system can offer the images of sub-meter resolution.
E. Chapin, A. Chau, J. Chen, B. Heavey, S. Hensley, Y. Lou, R. Machuzak, and M. Moghaddam, “AirMOSS: an airborne P-band SAR to measure root-zone soil moisture,” Radar Conference (RADAR), 2012 IEEE, pp. 693-698.
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