With the escalating prevalence of radio-frequency interference, the vulnerability of Global Navigation Satellite System (GNSS) receivers to potential jamming or spoofing threats has become a critical concern. The proliferation of GNSS repeaters, commonly known as meaconers (electronic devices that intercept GNSS signals, amplify them, and subsequently rebroadcast them to GNSS receivers in sight) contributes to this threat landscape, by compromising the operating performance of the nearby GNSS receivers. This work investigates the impact of onboard meaconers on aircraft GNSS receivers, emphasizing their detrimental effects on the accuracy, availability, and integrity of the GNSS estimated positions. Through mathematical modeling and highly realistic simulations, the influence of meaconing on the GNSS observables (code, phase, smoothed pseudoranges, carrier-to-noise density ratio C/N0 estimations), and on the main processing blocks of a standardized aircraft GNSS receiver (C/N0 threshold, measurement quality monitoring, step detector, fault detection procedure and protection level checks) has been deeply characterized. The findings indicate that onboard meaconers can induce substantial degradations in the GNSS signal tracking, resulting in significant positioning errors and availability drops that compromise both the flight operations and safety. For specific meaconer characteristics, the meaconer could completely jeopardize the aircraft’s receiver ability to compute a position, induce position errors up to 40 meters, or provoke continuous misleading position information and integrity hazards. The study highlights the importance for aviation bodies to consider the onboard meaconing threats. Additionally, the findings present valuable guidance for pilots and manufacturers in identifying and interpreting onboard meaconing interference, thereby strengthening the reliability of GNSS-based navigation systems in the aviation sector.