Assessing jamming and spoofing impacts on GNSS receivers: Automatic gain control (AGC)
Emile Ghizzo, El-Mehdi Djelloul, Julien Lesouple, Carl Milner, Christophe Macabiau
Abstract
In modern GNSS receivers, the Automatic Gain Control (AGC) monitors the received signal level to optimize quantization and mitigate interference. This paper characterizes the jamming and spoofing impact on AGC and received signal. It first expresses the AGC gain as a function of the received signal level. Under nominal conditions, the AGC leverages the ergodic properties of the received signal to estimate its level over time. Two physical quantities, namely time-based power and signal distribution, are typically considered. However, in the presence of interference, these ergodic properties are no longer guaranteed, posing challenges in modeling the behavior of these quantities. This paper proposes a probabilistic framework for interpreting temporal estimation and computing time-based power and distribution in order to characterize AGC gain under jamming and spoofing. First, this study models the spoofing impact for both unique and multiple emitted spoofing signals as a function of the re-radiated noise power and the spoofing signals’ characteristics (e.g., number of emitted signals, amplitudes, modulation). Furthermore, it reveals the non-uniformity of the jamming chirp phase, which introduces distortions in power and signal distribution, consequently affecting AGC gain and demonstrates the convergence of the jamming signal toward a continuous wave signal at high frequencies. • Expression of the AGC dynamic behavior as a function of the received signal level. • Definition of a new probabilistic framework to interpret time-based estimation. • Approximation of multiple spoofing signals into a Gaussian model. • Highlighting the non-uniformity and non-ergodicity of chirp phase. • Approximation of the chirp signal as a continuous wave at intermediate frequency.