## 2 Materials and Methods
### The Ground Segment
#### 2.2.4 SAR Data Processing Chain
As remarked in Section 2.1, the MIPS system of the IREA-CNR infrastructure may operate at two different carrier frequencies. In particular, it is equipped with a single-pass InSAR configuration in the X-band. Accordingly, the IREA-CNR SAR data processing chain is, first of all, tailored to the generation of single-pass InSAR DEMs. It is remarked that this is a key activity in the framework of the agreement with the Department of Civil Protection of the Italian Presidency of the Council of Ministers for the monitoring of effects related to eruptive activities. By referring to the standard InSAR processing [1; 2; 4], the IREA-CNR chain overcomes the co-registration procedure [1], since the BP algorithm described in Section 2.2.4 is capable of generating SLCs in arbitrary output grids (that is, geometric coordinate systems). Therefore, all SAR SLCs of the considered InSAR datasets are generated with respect to the same output grid. Following the interferometric beating, the phase unwrapping procedure [67; 68; 69; 70; 71; 72; 73; 74] is implemented through the generalized approach for sparse data [73; 74]. Then, the constant offset of each unwrapped interferogram is estimated using the STopBE [75] or IPBE [76] method, which exploit CRs properly deployed in the observed area or an available external DEM when CRs are not present in the monitored zone. Finally, a pixel-by-pixel and target dependent phase-to-height conversion procedure is applied [1]. In particular, we apply a strategy that exploits the actual trajectory of the aircraft and takes into account the changing pointing direction of the antenna. First single-pass InSAR experiments with data acquired by the X-band MIPS system were conducted over the volcanic area of Stromboli, Italy [31; 32]. A more detailed discussion of such experiments is provided in the next section. Turning to the processing of airborne repeat-pass InSAR data, this procedure is generally more complicated than the processing of single-pass InSAR data because the motion errors and the attitude variations affecting two generic interferometric acquisitions are independent of each other. Therefore, they do not tend to compensate for each other as in the case of single-pass InSAR acquisitions. Generally speaking, airborne repeat-pass InSAR is more critical at higher frequencies [44] for different reasons. First, the higher the frequency, the more sensitive the radar system is with respect to the errors induced by the inaccuracy of the navigation system mounted on board the airplane [44]. Secondly, for a fixed azimuth resolution, at higher frequencies the same squint angle difference between the two generic repeat-pass InSAR acquisitions produces a higher relative Doppler shift between the two corresponding images [1]. Thirdly, the lower the frequency, the higher the temporal coherence [1] of repeat-pass interferograms. For this reason, among the two operative frequencies of the MIPS system, the L-band is more appropriate for repeat-pass InSAR applications. The IREA-CNR InSAR data processing chain is currently capable of dealing with interferometric acquisitions characterized by different squint angles, as is the case for single-pass cross-eyed InSAR acquisitions [42], and by different motion errors. The performance assessment of this processing procedure with repeat-pass MIPS data acquired in the L-band is a matter of ongoing work.