# Interference Mitigation for GNSS Receivers Using FFT Excision Filtering Implemented on an FPGA
###### Abstract  
GNSS receivers process signals with very low received power levels (\(<\)\(-\)160 dBW) and, therefore GNSS signals are susceptible to interference. Interference mitigation algorithms have become common in GNSS receiver designs in both professional and mass-market applications to combat both unintentional and intentional (jamming) interference. Interference excision filters using fast Fourier transforms (FFTs) have been proposed in the past as a powerful method of interference mitigation. However, the hardware implementations of this algorithm mostly limited their use to military GNSS receivers where greater power and resources were available. Novel implementation of existing FPGA technology should make interference mitigation feasible with limited hardware resources. This paper details the practicalities of implementing excision filters on currently available FPGAs trading off the achievable performance against the required hardware resources. The hardware implementation of the FFT excision mitigation algorithm is validated with the GNSS software receiver  
devices are used in vehicles in order to hide their location. One such example is an accident that occurred in 2011 at the Newark International Airport. In November 2009, the local-area augmentation system (LAAS) of the airport installed a ground-based augmentation system (GBAS). However, this system was interrupted by the presence of radio-frequency interference. An investigation was launched and discovered the presence of jamming signals whose power exceeded more than 20 dB of the receiver antenna's noise, resulting in tracking errors. After a lengthy investigation that reached two years, it was discovered that a truck driver was using a GPS jammer in order to hide the location of the vehicle. It turns out, however, that the jamming device used in this case was able to disrupt the operation of GNSS receivers within a radius of hundreds of meters. This led to the driver being fined more than 30,000 dollars [6, 7, 8, 9]. Another incident occurred in Hong Kong in 2018, where more than 40 drones fell from the sky during a light show. The Hong Kong Tourism Bond's Executive Director, [PERSON], stated that the reason was a GPS jammer [7, 10]. In 2019, a jammer set up at a nearby pig farm was blamed for the intermittent GPS signal loss experienced by an aircraft landing at Harbin airport in north-eastern China. According to The _South China Morning Post_, the jammer was intended to stop criminal gangs from using drones dropping swine fever-infested packets on the herd, forcing farmers to sell them sick meat at a lower price [11]. In 2020, a French maker of high-precision GNSS technology claimed that GPS and Galileo signals were being frequently disrupted at their factory. After investigating, the French Radio Frequency Agency (ANFR) discovered the cause of the disturbance was a broadband router installed in a citizen's adjacent flat. It was found that the malfunctioning router was producing detrimental interference at a frequency of 1581.15 MHz, which is very close to the GPS L1 and Galileo E1 signals at a frequency of 1575.42 MHz [12]. Figure 1 shows the main concept of the jamming source interfering with an object such as a warship. Spoofing, on the other hand, is an interference technique that records the real GNSS signal, modifies it, and emits it back as if it was the original signal. This in turn causes the GNSS receiver to make false readings. The re-emitted signal is far more powerful than the original and can easily mask it. Simpler spoofing devices simply change the phase of the GNSS signal, whereas more advanced devices can transmit a specific location. This does not directly cause any harm; however, systems that rely heavily on GNSS navigation can be compromised. This includes goods transportation. Some shipments are secured throughout their journey and are unlocked once the final destination is reached. In such examples, the security systems use the GNSS geo-location in order to identify the location of the load.