# X-Band Photonic-Based Pulsed Radar Architecture with a High Range Resolution
## 1 Introduction  
A radar is an electromagnetic device used for the detection and identification of various targets such as aircraft, ships, and ground vehicles. It transmits electromagnetic waves to targets and receives the reflected signals from the target and clutter. The frequency band and waveform of the transmitted signal are determined by the platform and the type of target to be detected [1]. In recent years, as the demand for remote sensing systems has increased, technologies tend to have been expanded to multifunctional, wide-band, and multiband surveillance systems [2; 3; 4]. The expansion of these applications is demanded the very stable radio frequency (RF) sources for obtaining the higher spatial and velocity resolution performance and requires the ability to detect and process broadband ultra-high frequency signals. However, a method based on the DDS (direct digital synthesizer), which is currently widely used, has a limited frequency range of the generated signal to a maximum of several GHz; therefore, in order to up-convert to the desired frequency band, several steps of frequency multiplication are required, and the phase noise of the signal is degraded in this process. To solve this problem, as a new field of optoelectronics, photonic-based microwave technology has been proposed [5; 6; 7]. Microwave photonics offers a wide bandwidth, resistance to electromagnetic interference, lower propagation distortion, lower phase noise, and higher frequency flexibility compare to those of the conventional electronics-based technology. Depending on the type of laser source employed, the radars adopting the microwave photonics technology can be divided into a pulsed radar applying a mode-locked laser (MLL) and a frequency modulated continuous-wave (FMCW) radar using continuous-wave (CW) laser [8; 9; 10; 11; 12]. A pulsed radar using an MLL can detect the lower radar cross-section (RCS) targets with the same power consumptioncompared to an FMCW radar. However, due to the characteristics of the MLL that outputs a signal having a specific frequency using a resonance, it is difficult to generate the wideband signals, so there is a disadvantage that the range resolution cannot be fine. Therefore, when two or more targets are in close proximity to each other, there is a limitation to separate the targets and the quality of the inverse synthetic aperture radar (ISAR) image is deteriorating due to a low resolution when the ISAR image is acquired. In contrast, an FMCW radar has the advantage of being able to easily generate broadband signals without the minimum detectable range limitation as known as a blind region. However, for the detection of low RCS targets with the same range, the power handling capability of the power amplifier used in the transmitter has to be very high compared with a pulsed radar's because of the continuously high energy transmitting [13]. In addition, the large amount of heat, which is generated in the power amplifier, causes the increase in thermal noise and the degradation of the noise figure of the system. Therefore, in order to prevent the generation of thermal noise, an additional cooling system is required, which results in increasing the volume of the system [14; 15; 16; 17]. For these reasons, the previous studied of the photonic-based pulsed radars with an MLL is mainly focused on for long-range target detection, and the acquisition of high-resolution ISAR images has been studied in photonic-based FMCW radars with a CW laser source [8; 9; 18; 19; 20; 21; 22; 23; 24]. According to the results of the previous studies, the photonic-based pulsed radar with MLL can replace the conventional electronic-based shipborne radar or airborne radar, and the photonic-based FMCW radar with CW laser can be applied to automobiles, distance measurement, or missile fuses. In this paper, we propose a photonic-based pulsed radar architecture for the lower RCS target detection with the same average transmission power while maintaining the broadband characteristics of a conventional photonic-based FMCW radar.