## 4 Discussion
No presuming and no decimation filtering was applied. To process the SAR data, a common reference radar output grid, characterized by a spatial sampling of 75 cm in range and 4 cm in azimuth, was assumed for all the images relevant to the same flight track. Concerning the processing of the navigation data, the integration of data recorded by the system mounted on board the aircraft and the data derived from the Crustal Dynamics Data Information System (CDDIS) and two Italian networks, namely the Rete Integrata Nazionale GNSS (RING) of the Istituto Nazionale di Geofisica e Vulcanologia (INGV) and the Permanent Stations Network of the Campania Region, was carried out by means of the APPLANIX POSPac MMS(r) tool [33]. For the case at hand, this procedure took about 2 h.  
The external DEM was first downloaded onto the IREA-CNR IT platform. Then, the area of interest was cut out and converted onto the reference radar output grid. For this procedure, it is necessary to reserve a dedicated time of about a couple of hours. It is noted that in the case of repeated SAR acquisitions in the same area, this procedure is performed only once. Conversely, in the case of unscheduled acquisitions, as in the case of natural disaster monitoring in areas not previously illuminated by radar, this procedure must be executed before the processing of the acquired SAR dataset. As observed above (see Table 2), the 16 SLC images (two single-pass InSAR SLCs for each straight acquisition track in Figure 10) were focused with a geometric resolution of 75 cm in range and 27 cm in azimuth following the procedure described in Section 2.2.4. Since the overall acquired X-band dataset turned out to be unaffected by RFI, the procedure aimed at detecting and removing these interferences was not applied. Following the generation of the SLCs, the InSAR processing procedure described in Section 2.2.4 was applied, along with a complex multi-look procedure bringing the interferometric products to a resolution of 1.5 m in range and 2 m in azimuth (see again Table 2). The processing time required by the PhU procedure, the STopBE phase offset estimation and the phase-to-height conversion procedure (see Section 2.2.4) is much shorter than that related to the focusing procedure, which represents the most time-consuming processing step. In the considered test case, 12 h of computing time were required to execute the overall data processing chain, allowing us to generate, from the SAR raw data, the final mosaicked DEM shown in Figure 15. It is stressed that achieving such a relatively limited processing time was possible due to the parallel programming strategies adopted for all the data processing steps, particularly the AC procedure performed in the time-domain (see Section 2.2.4), with the aim of taking advantage of the multi-core capability of the IREA-CNR IT platform. In particular, we used up to 448 cores of the platform. Note that with only one core and the same computing time (12 h), just a portion of only one SLC characterized by 2000 range and 23,000 azimuth lines can be focused. Summing up, to evaluate the current response time of the presented IREA-CNR airborne SAR infrastructure in a possible emergency scenario, it must be, first of all, considered that the activation time of the MIPS sensor is on the order of 3 h, without accounting for the time needed for the operators responsible for instrument installation to be operational at the airport. Moreover, according to the flight autonomy of the specific airplane used in the Stromboli campaign case study, it must be also considered that the duration of one flight mission is on the order of 4 h. Generally, depending on the distance between the airport and the area of interest, as well as on the extension and the characteristics of the latter, one acquisition campaign typically requires at least a couple of missions for each operational mode of the system. Therefore, once the MIPS system is activated, at least 10 h are necessary to conclude the overall campaign.