# Interference Mitigation for GNSS Receivers Using FFT Excision Filtering Implemented on an FPGA
###### Abstract
Spoofing can interfere with the security system, allowing access to the shipment. Other applications such as autopilots use GNSS data for navigation and by spoofing the receiver and can steer the vehicle in the wrong direction. This is especially dangerous since military units such as drones rely on GNSS for navigation. Figure 2 shows a spoofing attack on the GNSS receiver. Figure 1: Illustration of a jamming source attack on a warship. The robustness of a GNSS receiver that has the capability to be protected from unintentional and intentional (jamming) interference is necessary to ensure reliability in GNSS services. Current GPS receivers have begun including anti-jamming algorithms that provide defence against jamming and unwanted interferences [13]. Current GNSS receivers have RF sections with automatic gain control (AGC) and multi-bit analogue-to-digital converters (ADCs) to provide protection against signal fluctuations and pulsed interference [14]. Moreover, the direct sequence spread spectrum (DSSS) is used in GNSS receivers to spread the received signal power over a wider bandwidth than the one used for information data. As a result, this spreading of the gain of the receiver reduces the effect caused by the narrowband interference [15]. However, these techniques are insufficient for applications such as military, aviation, and emergency services that require a high level of reliability. Over the past years, many researchers have detailed the interference effect and mitigations at different stages of the GNSS system, including antennas, low noise amplifier (LNA), AGC, intermediate frequency (IF), signal processing, and time-frequency domain techniques [16]. Interference increases the effect, which can be observed firstly in the receiver's RF section, a decrease in AGC, and change in the distribution across ADC. When the interference power is too high, the available carrier-to-noise density ratio (\(C/No\)) is reduced, degrading the performance of the receiver acquisition and tracking loops [14, 17]. As the interference exceeds the GNSS systems' anti-jamming ability, it will degrade the performance and ultimately result in the failure of the system. There are different types of interferences to be considered, broadly categorised into narrowband and wideband interference. This work addresses narrowband interference sources as wideband interference is generally addressed by multi-antenna beamforming techniques such as Novatel GAJT [18]. Within narrowband interference, continuous-wave interference (CWI), pulsed CWI, and sweeping interference are the most harmful interference impacting the quality of the received GNSS signals [17, 19, 20]. Therefore, CWI interference and sweeping interferences will be used in this research to illustrate the effect of the mitigation technique and compare it against other related techniques in the literature. There is a huge array of algorithms that can improve the GNSS signal. Until recently, such devices were only discussed as theoretical solutions due to their computational costs. In this paper, the proposed interference excision filters were performed using fast Fourier transforms (FFTs), which were used as a powerful interference mitigation method. Furthermore, a novel hardware implementation for this mitigation technique was designed and tested using field programmable gate arrays (FPGAs), preserving the desired performance of GNSS receiver operations such as tracking, acquisition, and navigation against the required hardware resources. Moreover, a comparison between the results of  
Figure 2: Illustration of a GPS spoofing attack on a vehicle. the GNSS software receiver and the GNSS hardware implementation was conducted to demonstrate the proof of concept. This interference mitigation algorithm was developed with a GPS L1 C/A signal generated by the front-end NSL Stereo at the intermediate frequency of 6.5 MHz, a 3 dB bandwidth of around 2.75 MHz, and frequency sampling at 26 MHz with a 2-bits resolution. This paper is organized as follows.