## 2 System Description  
The radar system exploits the pulsed radar technology and can operate at different carrier frequencies as Sounder and Synthetic Aperture Radar (SAR). More specifically, the Sounder operates at 165 MHz, whereas the SAR may operate either at 450 MHz (SAR-Low mode) or at 860 MHz (SAR-High mode). Summing up, three operational modes are enabled, namely, Sounder, SAR-Low and SAR-High, each working at a different frequency. The bandwidth of the transmitted (chirp) pulses is 40 MHz, leading to a (slant) range resolution of 3.75 m (in air). Moreover, in the SAR-High mode, through the stepped chirp technology [60], an overall bandwidth of 80 MHz can be reached, that is, a (slant) range resolution of 1.87 m. The SAR system is full polarimetric thanks to two separate receiving channels. The overall system basically consists of a radar module along with three different antennas, one for each operational mode. The radar module consists of three main blocks, namely, the Radar Digital Unit (RDU), the Radio Frequency Unit (RFU) and the Power Supply Unit (PSU). In particular, the RDU is fully programmable and allows the parameters setting, the timing generation and the data handling. It also includes the Analog to Digital Converter (ADC) and the data storage unit. The RFU embeds the frequency generation unit (which generates all the synchronization and radio frequency signals) and the chirp generator unit (which generates the low frequency modulated chirp signal by means of the digital direct synthesis technology). RFU also includes the high power amplification unit, which is based on the solid state technology, and an antenna front-end that allows the correct switching among the transmitted and received signals and among the different polarizations of the SAR mode. The PSU provides the power supply to whole system by an external 28 V DC voltage. The system is completely stand-alone: the power supply connector is the only electrical interface. Most radar modules are shared by the three different operational modes of the system. In particular, base band signal generation, base band data sampling and data handling are common to the Sounder and the SAR modes. As noted above, different antennas are used for the three different operational modes of the system. In particular, for the Sounder mode, a low-gain log-periodic antenna is deployed; the antenna points to the nadir and radiates the main beam with Half Power Beam Width (HPBW) [61] of 68\({}^{\circ}\) and 50\({}^{\circ}\) in the range and azimuth directions, respectively, and with a (maximum) gain [61] of 7 dBi[1]. Exploitation of the overall azimuth aperture thus can lead to an azimuth resolution of about 1m [62, 63]. Two different side-looking arrays of micro-strip antennas are instead employed for the two different SAR modes [64]. In particular, these two SAR antennas have the same shape and size, to allow an easier swap when they cannot be installed simultaneously on the airplane/helicopter. More specifically, the array used for the SAR-Low mode consists of 4 patches deployed along the azimuth direction: its (maximum) gain is 11 dBi and its HPBWs are 75\({}^{\circ}\) and 20\({}^{\circ}\) in the range and azimuth directions, respectively. Exploitation of the overall azimuth aperture thus leads to an azimuth resolution of about 1m [62, 63]. For the SAR-High mode (which operates at a carrier wavelength scaled about of a factor two respect to the SAR-Low mode), a planar array antenna of \(8\times 2\) patches is deployed, with the same overall dimensions of the array used for the SAR-Low mode. The corresponding HPBWs are thus reduced of a factor two (in both directions) with respect to the SAR-Low mode. This allows increasing the (maximum) antenna gain of 6 dBi, retaining practically the same achievable azimuth resolution obtainable with the SAR-Low mode. Both SAR antennas are dual-polarized, to enable the full polarimetric capability of the SAR modes. Currently, during the flight the user can select manually one of the three operational modes of the system.