## 2 Materials and Methods
### The Ground Segment
#### 2.2.4 SAR Data Processing Chain  
Once the acquired data are transferred to the IREA-CNR laboratory, the SAR data processing chain can be applied. The implementation of such data processing procedures on the IT platform described in Section 2.2.1 involves the use of:  
* a single Computing Element (CE) acting as a master node in terms of job scheduling policy control;
* several Worker Nodes (WNs) ensuring the required computation capability, particularly to execute data processing steps based on a parallel programming strategy;
* several Storage Elements (SEs) that are installed to store the data. The data processing procedures are aimed, first of all, at generating Single Look Complex (SLC) images from the acquired raw airborne SAR data [1]. Following this step, named SAR focusing, other processing procedures are implemented to achieve higher-level SAR products, such as radiometrically calibrated SAR images, interferometric DEMs, soil moisture maps and so on. SAR focusing is a 2-dimensional image processing procedure [1; 29; 30] that, in the airborne case, also involves compensation for the so-called motion errors, which affect the acquired radar data. Motion errors are caused by platform attitude instabilities and deviations of the trajectory from an ideal rectilinear planned track [42; 43; 44]. For this aim, knowledge of the phase center position and pointing direction of the radar antennas during the whole acquisition, as well as the availability of an external DEM of the illuminated scene [42; 43; 44], is needed. The overall procedure consists of the cascade of the following main operations:  
* processing of the navigation data;
* Range Compression (RC) of the radar data;
* Radio Frequency Interference (RFI) detection and removal at the range-compressed data level;
* Azimuth Compression (AC) of the radar data. Note that RFI detection and removal is an optional procedure, developed at IREA-CNR, which we apply only if RFI signals corrupt the acquired radar data. The processing of the navigation data is performed by using commercially available software, namely the APPLANIX POSPac MMS(r) tool. The remaining procedures implement, mainly through the Interactive Data Language (IDL), algorithms jointly developed at IREA-CNR and University Parthenope. More details on the four processing procedures listed above are provided in the following paragraphs (a)-(d). * Processing of the navigation data  
The processing of the navigation data is a delicate and fundamental procedure, especially when using radar data acquired at high operating frequencies (e.g., X, Ka and Ku bands). To achieve the needed centimeter level position measurement, it is necessary to correct systematic errors (related to the receiver, the satellites and propagation delays) by augmenting the basic GNSS system mounted on board the airplane. Specifically, we use the APPLANIX POSPac MMS(r) tool [33], which imports and processes data recorded by the Applanix INS/GNSS navigation system and by a network of reference stations. * Range Compression (RC) of the radar data  
RC is an image processing operation whose specific implementation depends on the characteristics of the transmitted radar signal and, more generally, of the used radar system. In particular, when pulsed radar is used, RC is achieved through matched filtering [1; 29; 30]. Instead, in FMCW systems, RC is achieved by means of a signal beating procedure (typically carried out on board by the radar receiver) followed by a Fourier transform [28]. The implementation of the RC procedure on the IREA-CNR IT platform described in Section 2.2.1 exploits, to some extent, the multi-core capability of the platform. In particular, a parallel programming strategy is not adopted to carry out the RC of the data relevant to one flight track. Indeed, since this kind of procedure is not particularly time-consuming, a single job is responsible for it.