## 3 GNSS Anti-Jamming Techniques  
Modern GNSS use direct sequence spread spectrum communication (DSSS), which has an inherent level of protection against signal interference, but these signals have low signal power levels, which make them conversely susceptible to signal jamming. Hence, anti-jamming techniques are required under jamming conditions to guarantee continuous navigation/positioning availability. Various anti-jamming methods have been documented in the literature, which could be employed at any of the different GNSS signal processing stages, i.e., from signal reception incident at the receiver antenna through the navigation solution processor [15]. Accordingly, they are classified as such. For example, [16] categorized anti-jamming techniques such as adaptive time-domain filtering, time-frequency domain processing, subspace processing, adaptive antennas, and amplitude-domain processing techniques. The selection of an anti-jamming technique depends on various factors, including cost, size, and power consumption [17]. Nonetheless, we can categorize anti-jamming techniques into two main classes: signal processing-based and antenna-based methods. A nonstationary anti-jamming technique was proposed in [18] based on the assumption that the received GPS signal is a composite of the jammer signal, additive noise, and the desired GPS signals for GPS receivers. The method uses what is called the time-frequency distributions to characterize the jammer time-frequency characteristics. These are then used to build a mask that is applied to the received signal samples to block the jammer energy using orthogonal projection allowing only the GPS signals. The paper used signal simulations to demonstrate the performance of the proposed method with more than the 3 dB gain improvement in the signal-to-noise ratio (SNR) for a one-antenna versus two-antenna case. Unlike other anti-jamming techniques, research in [19] presented an interference suppression method that exploits spatial signal processing but is based on the known temporal self-coherence structure of the GPS C/A signals. C/A-code patterns are repeated 20 times per every navigation message bit; thus, GPS signals inherent solid between-chip rate samples self-coherence. This fact is utilized to eliminate interference signals, which typically have temporal structures that are very different from those of the GPS signals. Results show that the proposed method could successfully cancel directional jammer signals at as high as 50 dB jamming to signal ratio (JSR). Further advantages of the algorithm are that it does not require synchronization between the satellite and the receiver nor the knowledge of the visible satellite positions. Performance evaluation of several anti-jamming techniques such as adaptive temporal filtering (ATF), space-time adaptive processing (STAP), and overlapped fast Fourier transform (OFFT) was done in [15]. The paper also investigated the advantage of combining two of these algorithms in one method. Results indicate that individual algorithms show moderate jamming resistance at JSR levels around 40 dB, whereas algorithm combinations show better jamming resistance at JSR as high as 60 dB. A new approach of jamming mitigation is introduced in [20], which combines a time-domain notch filtering and a spatial filtering technique. This is to take advantage of the complementary characteristics of both techniques as the time-domain filtering is efficient in suppressing the continuous wave and narrowband interference since it addresses the signal in the time domain. Spatial filtering, on the other hand, is more applicable in mitigating the narrowband and wideband interference based on the direction of arrival of both signals and interferences. Thus, the suggested combination can mitigate the narrowband and wideband interference at once. This combination selection was also selected to avoid higher hardware requirements often demanded by frequency domain and space domain filtering combinations.