# Introduction to Radar Scattering Application in Remote Sensing and Diagnostics: Review
## 2 Introduction: The Radar System  
Radar can be equipped either with an active sensor which illuminates the objects, and in this case, it is defined as active, or with a passive sensor, which relies on external sources (e.g., natural fonts of energy such as Sun and stars), and in this case, the radar system is defined as passive. In the literature, active radars are usually addressed simply with the term radar. Active radar is known as bistatic if the receiving antenna differs from the transmitting one; otherwise, it is defined as monostatic. In this paper, we present applications of both active and passive radar systems. The (active) radar operates either by transmitting a continuous wave or trains of electromagnetic energy pulses: in the first case, we have the so-called Continuous Wave (CW) radars, for instance, Stepped Frequency Continuous Wave(SFCW) radar, while in the second case radars are defined pulse radars. An alternative form of radar is represented by pulse-Doppler radar, which combines the features of the two radar systems previously mentioned. Independently of the particular waveform employed, the radar application can be briefly described as follows: the radar system transmits electromagnetic energy through the transmitting antenna, either in a preferred direction or in any direction. These radiated waves are reflected back, typically in all directions. A certain portion of the reflected radiated signal is received through the radar receiving antenna and processed through digital signal processing tools, which constitute the so-called reception chain: in Figure 1, a block diagram illustrating the basic working principle of a monostatic pulse radar is shown [2]. The transmitting chain is constituted by a pulse modulator and a power amplifier; those generate a signal that is transmitted by an antenna having the appropriate radiation pattern. The duplexer allows a single antenna to be used for both the signal transmission and reception; this element is therefore absent in bistatic radars. The first block in the receiving chain, following the receiving antenna (and eventually the duplexer), is represented either by a low-noise amplifier or by a mixer. The local oscillator and mixer convert the received Radio Frequency (RF) signals into Intermediate Frequency (IF) signals: in typical radar applications, such as air-surveillance, the IF amplifier has a center frequency of 30 or 60 MHz with 1 MHz bandwidth. The IF amplifier is designed as a matched filter (i.e., the Signal-to-Noise Ratio (SNR) at the output is maximized) for signal processing. Its objective is to separate the information (wanted signal) from noise (unwanted signal). Then, the radiated pulse modulated signal is captured by a second detector and amplified by a video amplifier, so that the signal can be eventually displayed on a screen [3]. The monostatic pulse radar uses the same antenna for both transmission and reception of signals. Radar that operates with pulse signal for detecting stationary targets is called pulse radar. Radar that operates with CW signal for detecting non-stationary targets is called CW radar (see Figure 2). It has two antennas: one antenna is used for transmitting the signal and the other antenna is used for receiving the signal. It produces an analog signal having a frequency of \(f_{o}\). Local oscillator produces a signal having a frequency of \(f_{l}\). The output of local oscillator is connected to the mixer. The output of CW transmitter is connected to both transmitting antenna and mixer. Mixer can produce both sum and difference of the frequencies that are applied to it. The signals having frequencies of \(f_{0}\) and \(f_{l}\) are applied to mixer. Thus, the mixer will produce the output having frequencies \(f_{o}+f_{l}\) or \(f_{o}-f_{l}\). As the name suggests, sideband filter allows a particular sideband frequencies either upper sideband frequencies or lower sideband frequencies. The IF amplifier detects the signal, which has Doppler frequency \(f_{d}\).