## 1 Introduction  
Recently, the demand for safety-critical applications (e.g., civil aviation, aircraft landing) using Global Navigation Satellite Systems (GNSSs) has gained extensive and increased interest. With the development of the new GNSSs, such as the European Union's Galileo system [1], the U.S.'s modernized GPS [2] and China's Beidou/Compass [3], more efforts have been devoted to the design of high performance GNSS receivers, which are expected to increase the accuracy, availability, integrity and continuity of service, especially in the field of safety of life (SOL) applications (e.g., the accuracy needed during the landing of an aircraft). High precision positioning and reliable SOL services represent the main challenges for the upcoming satellite navigation systems. The vulnerability of GNSS receivers to interference has become an increasingly important issue. The direct sequence spread spectrum (DSSS), used in most of the satellite navigation systems, spreads the received GNSS signal power over a wider bandwidth; this ensures a dispreading gain in the GNSS receiver, which can reduce impairments caused by the undesired disturbing signals. Although GNSS has a certain capability to be immune from interference, since DSSS is utilized, due to the reception of the very low GNSS signal power, the presence of intentional or unintentional disturbing signals, such as spurious, harmonics and electromagnetic interferences, will result in serious performance degradation of GNSS receivers, including the inability to determine the correct position. Among all of the different error sources that can potentially corrupt GNSS signals, radio frequency interference (RFI) is particularly harmful, since, in some cases, it cannot be mitigated by a simple correlation process [4]. The jamming environment is threatening for satellite navigation systems. Many systems rely on the transmission of radio frequency (RF) energy in the L-band. For example, the European Galileo E5a and E5b radio bands, located within 1164-1214 MHz, occupy frequencies already allocated for aeronautical radio navigation services (ARNS), such as tactical air navigation (TACAN), distance measuring equipment (DME) and secondary surveillance radar (SSR) [5]. In addition, illegal portable jamming devices are becoming popular to protect traffic drivers from being tracked by GNSS receivers in their vehicles. These so-called personal privacy devices radiate disruptive interfering signals in the GNSS frequency bands, which can make a conventional GNSS receiver inoperable. The interference impairments can heavily degrade the reception of useful GNSS signals and deny the GNSS-based services in a geographical area with a radius of several kilometers [6]. It emerges that interfering signals are expected to become a very serious threat for satellite navigation operations, and countermeasures have to be taken to prevent interference from blocking the GNSS receivers, especially for safety-critical applications. In the literature, several interference detection and mitigation techniques have been proposed and investigated, and each of them differs according to the operation domain (time, frequency or space). These techniques can be classified according to the specific processing domain. In the time domain techniques, temporal filtering can be usually adopted to suppress temporally-correlated (narrowband) interference, since wideband interference cannot be easily discriminated from thermal noise. Frequency domain techniques are generally based on spectral estimation of the incoming signal, which is obtained by applying signal processing techniques, such as discrete Fourier transform (DFT). These frequency domain techniques are typically performed by comparing the spectrum of the received signal with a theoretical threshold, which is usually determined according to a statistical model representing the received signal [7]. In addition, spatial filtering is often used in the antenna array receivers for interference suppression [8].