## 2 Materials and Methods
### The Ground Segment
#### 2.2.3 In-Situ Activities  
The in-situ activities described in Section 2.2.3 are essential to ensure the success of the SAR mission [21; 23].  
Assembly and Disassembly of the SAR Sensor Onboard the Aircraft  
As remarked above, IREA-CNR does not own an aircraft. Therefore, a chartered aircraft is used for the acquisition campaigns, and the MIPS system is assembled on board just before the beginning of the campaign and disassembled just after its end. In this regard, it is worth stressing that the MIPS system is not tailored to a specific aircraft; it can even be mounted on board small/medium size aircrafts such as the Cessna 172 shown in Figure 2.  
Deployment of Corner Reflectors (CRs)  
Recognizable ground reference points such as CRs are of paramount importance to assess the quality of SAR products. Typically, we deploy CRs over a test area where preliminary data analysis procedures, such as that proposed in [40], are carried out before processing the entire dataset acquired during a campaign. When possible, such a test area  
Figure 4: Radiation patterns of the MIPS SAR antennas. Black line: amplitude pattern. Red line: phase pattern. The blue lines mark the edges of the half power beamwidth. From the top: X-band antenna with vertical polarization, L-band antenna with horizontal polarization and L-band antenna with vertical polarization. The left panels refer to the azimuth plane, and the right panels refer to the elevation plane.  
is included in the region of interest for the planned acquisition campaign; otherwise it is chosen near to the departure/landing airport. In any case, it is recommendable that CRs be deployed over the entire range swath of the system--first of all, to assess the quality of the SAR impulse response at different ranges and, more generally, to effectively implement some preliminary analysis procedures, such as those described in [40; 41], which benefit from a wide dispersion, along the range direction, of the exploited ground control points.  
Measurement of CR Positions  
This measurement, which is necessary to correctly locate CRs within SAR data, is carried out using the Differential Global Positioning System (DGPS) technique immediately after the deployment of the CRs, as described above. As an example, in Figure 5, we show the activities related to the measurement of the position of one CR during one MIPS campaign.  
To accurately carry out an SAR campaign with a focus on the external radiometric calibration procedures described in Section 2.2.4, it is necessary to know the orientation and position of each radar antenna with respect to a global reference system during the entire period of radar data acquisition [42]. To do this, the flight information provided by the navigation unit is necessary. Moreover, the antenna orientations and positions must be known with respect to a local reference system centered on the IMU phase center. To this end, the so-called lever arms need to be measured, at least, for 3 different reference points on each radar antenna (see [42] for further details). Such measurements are performed by using a theodolite total station once that the radar antennas and the IMU are mounted on board the aircraft, with the aircraft at rest. As example, in Figure 6, we show the activities related to the measurement of the antenna lever arms during one MIPS campaign.  
Figure 5: Measurement of the position of one CR during a MIPS campaign.  
Figure 6: Measurement of the antenna’s lever arm during one MIPS campaign.  
Data Download and Transfer  
The downloading of data onto hard disks takes place at the landing airport at the end of the mission. Afterwards, the hard disks are physically transferred to the IREA-CNR laboratory in order to download the data onto the IT platform described in Section 2.2.1.