## 2 Materials and Methods
### The Ground Segment
#### 2.2.4 SAR Data Processing Chain
However, to achieve the parallel RC of the raw data relevant to different flight tracks, different jobs are simultaneously executed on different WNs and cores available on the platform. Radio Frequency Interference (RFI) detection and removal  
This operation is aimed at filtering out interfering signals that could possibly corrupt the useful radar data. It is noted that RFI is typically relevant in the lower bands of the microwave spectrum, (e.g., L- and P-bands) [45; 46]. The mitigation strategy that we adopt is based on a two-step RFI identification approach that works on RC SAR data [47; 48], followed by a proper RFI suppression procedure. The first step of RFI identification acts only on the data samples relevant to the pre-nadir region and provides a rough estimate of the overall fraction of RFI-corrupted data. Based on this information, in the second step, RFI detection is carried out on the overall RC data by means of statistical tests applied azimuth by azimuth. Finally, the achieved detection results are used to drive a simple RFI suppression procedure easily integrated in the SAR focusing processor. Specifically, the latter treats the detected RFI as missing data. As an example, in Figure 7, we report the results achieved with an L-band MIPS dataset acquired over the Campi Flegrei area of the Campania region, southern Italy, in 2021. In particular, the top panel reports the Multi-Look Complex (MLC) image achieved without applying any RFI filtering strategy, whereas the bottom panel reports the MLC achieved after applying the RFI filtering strategy proposed in [47]. Figure 7: RFI removal from L-band MIPS data: MLC images (amplitude) achieved without applying any RFI filtering strategy (**top**) and after applying the RFI filtering strategy presented in [47] to the RC SAR data (**bottom**). Range pixel spacing: 2.38 m (SLC) and 14.3 m (MLC). Range resolution: 2.38 m (SLC) and 14.3 m (MLC). Azimuth pixel spacing: 1.84 m (SLC) and 14.75 m (MLC). Azimuth resolution: 2 m (SLC) and 14.75 m (MLC). Range extension: 8 km. Azimuth extension: 22 km. As for the implementation of this processing procedure on the IREA-CNR IT platform, a single job is responsible for the processing of the data relevant to one flight track. To achieve parallel RFI detection and the removal of data relevant to different flight tracks, different jobs are simultaneously executed on different WNs and cores available on the platform. Azimuth Compression (AC) of the radar data  
AC is a fundamental processing step applied to RC SAR data with the aim of generating SLC SAR products. The algorithms that implement this procedure are divided into time-domain and frequency-domain algorithms. Frequency-domain algorithms [1], which are very computationally efficient, are well tailored to SAR data acquired from straight tracks. In the case of airborne SAR acquisitions, where the flight tracks are characterized by deviations from an ideal, straight track, original frequency-domain algorithms must be properly adapted through the implementation of so-called MOtion COmpensation (MOCO) techniques [43; 49; 50; 51], which involve approximations [52] to preserve the computational efficiency typical of the original algorithms. As a matter of fact, the more significant the track deviations, the more unsuitable the MOCO approximations and the greater the impact on the quality of the achieved SLCs [44]. On the other hand, time-domain algorithms [53; 54; 55; 56; 57], often referred to as back projection algorithms (BPAs), do not use approximations and are very accurate, regardless of the characteristics of the acquisition track. Indeed, they operate on a pixel-by-pixel basis, thus being suited to handling arbitrary and nonlinear flight paths. Moreover, they allow for the processing of data in coordinates other than the native slant range azimuth, e.g., directly in cartographic coordinates. These advantages come at the cost of computational efficiency, which is generally worse than that of frequency-domain algorithms.