# Evaluating GNSS Receiver Resilience: A Study on Simulation Environment Repeatability
## 2 GNSS Jamming and Detection Mechanisms
### Receiver Internal Effects and Jammer Detection  
Modern GNSS receivers are designed to anticipate and adapt to interference. Figure 2 depicts a block diagram of a typical multi-frequency, multi-channel GNSS receiver, featuring an analog high-frequency front-end and digital signal processing (DSP) with associated software [41]. While adaptive notch filters within the DSP routine can effectively remove narrowband interference, they cannot prevent its initial impact on the earlier preprocessing stages.  
The most critical weakness resides in the first low-noise amplifier (LNA), often built into the active antenna. This LNA, providing approximately 25 dB of gain, is vital for maximizing the receiver's sensitivity. However, its need for an extremely low noise figure at high gain makes it only conditionally stable, easily disturbed by strong jamming signals. These signals drive the amplifier into unintended operating conditions, typically saturation, producing numerous intermodulation products [42]. These products, when combined with the subsequent mixing stage, generate unwanted responses in the output spectrum, resembling broadband noise--a noise that cannot be removed by de-spreading. Using narrowband antennas or SAW filters offers little relief, as they generally pass the entire GNSS spectrum, including the intentional jamming signals.  
Figure 1: Depiction of a standalone GNSS receiver’s vulnerability to undetected jamming. Jamming signals, below automatic detection thresholds, can corrupt positioning data and disable RTK corrections from reference stations.  
To address this issue, a passive antenna can be used, requiring the fixed LNA gain to be replaced with an automatic gain control (AGC) amplifier, a standard feature in modern GNSS receivers [43]. The AGC amplifier's gain can be dynamically adjusted, enabling adaptive response to changing conditions. However, incorrect adjustments can significantly impair digital sampling before the DSP stage, especially if changes occur during the sampling process of the analog-to-digital converter's sample and hold (S/H) circuit [44]. Furthermore, interference during I/Q demodulation, typically handled digitally, can also create problems. A substantial deviation from quadrature hinders accurate carrier phase synchronization, although this can be mitigated with appropriate countermeasures within DSP routines [45].  
The receiver's software constantly monitors these components and activates the appropriate response to interference. While the core architecture of modern GNSS receivers is largely standardized, significant differences arise in software implementation, particularly in DSP routines, calculation methods, and reactions to unexpected situations. GNSS receiver software is typically secured within the system using various cryptographic algorithms, preventing end-user modification or detailed analysis [41]. Moreover, the software's behavior in response to identical, unforeseen interferences can fluctuate over time, leading to unpredictable adjustments to the AGC amplifier gain, adaptive filter settings, and I/Q quadrature correction. Given the absence of universal standards for interference response, manufacturers employ a wide range of proprietary solutions.  
This research highlights that, even with consistent GNSS signal conditions and measurement durations, a receiver's response to interference remains highly unpredictable due to its software. Therefore, simply observing receiver behavior in interference-rich environments is insufficient. Accurate evaluation demands repeated testing under controlled conditions, followed by thorough statistical analysis of the results.