## 2 Materials and Methods
### The Flight Segment  
The flight segment, which ensures data acquisition, is composed of the following two major elements: radar and software tools for flight planning. The aircraft is not owned by IREA-CNR but is provided by external operators. The airborne SAR system is MIPS [27], which is a lightweight, cost-effective FMCW imaging sensor. The radar module has a weight of approximately 35 kg and is accommodated in a compact, easily transportable rack (see Figure 1), whose dimensions are about 50 cm \(\times\) 50 cm \(\times\) 65 cm. MIPS operates at two different carrier frequencies, namely, in the X- and L-bands. The availability of two separate bands of the microwave spectrum allows widening the observation capabilities of the infrastructure. For instance, L-band SAR signals (characterized by a wavelength on the order of 20 cm) exhibit higher penetration capabilities than X-band signals ones (characterized by a wavelength on the order of 3 cm). Therefore, L-band signals carry information on soil properties, even in vegetated areas, whereas X-band signals are stopped by the upper canopy layers. L-band SAR data are, thus, sensitive to the different scattering mechanisms that characterize complex targets and much more robust than X-band signals with respect to temporal decorrelation phenomena occurring in InSAR scenarios [1; 2]. Therefore, the L-band is particularly suitable for the observation of natural areas, where it can be exploited, for instance, for land classification and soil moisture retrieval through the use of polarimetric techniques [3] or for ground deformation monitoring through the use of the Differential InSAR (DInSAR) technique [2]. In this regard, it is worth remarking that the phase unwrapping operation [1] is simplified at L-band, due to the lower fringe rate that characterizes the corresponding interferograms. On the other hand, the short wavelengths of the X-band are able to interact with objects at finer spatial scales compared to those of the L-band. X-band SAR data are, thus, usually collected at high/very high spatial resolutions, which are more suitable for the observation of anthropogenic scenarios, as, for instance, in the monitoring of urban areas based on tomographic approaches [2]. In addition, the short wavelengths of the X-band allow effective single-pass InSAR configurations to be obtained for the generation of highly accurate DEMs [1], even on airplanes of rather small sizes. To take advantage of the peculiarities of these two frequency bands, the MIPS system operates with a single-pass InSAR configuration in the X-band or with a fully polarimetric configuration in the L-band. To achieve these capabilities with the FMCW technology, the system is equipped with three X-band antennas (one transmitting (Tx) and two receiving (Rx)) and four L-band antennas (two Tx and two Rx). The X-band antennas are vertically polarized, while two L-band antennas are vertically polarized and the other two are horizontally polarized. All antennas are off-the-shelf microstrips. Figure 2 shows the two possible antenna layouts relevant to the X-band interferometric configuration (left panel) and the L-band fully polarimetric configuration (right panel) achieved on a Cessna 172 aircraft. More generally, depending on the physical and aerodynamic characteristics of the aircraft, the X-band and L-band antennas can even be simultaneously mounted. In this case, the operator can decide during the flight to switch between the two different configurations described above. More details on the parameters of the MIPS SAR sensor are collected in Table 1.  
The MIPS system includes a modern navigation unit directly connected to the radar module, namely, an Applank POS-AV 610 [33], which includes an embedded Global Navigation Satellite System (GNSS) and an Inertial Measurement Unit (IMU). The use of this navigation unit, coupled with proper postflight processing techniques, guarantees very precise flight parameter measurability [19].