Objectives:
The study examines intentional interference with GNSS signals, with emphasis on jamming, as an element of hostile activity in asymmetric and hybrid operations. It aims to characterise technical features and types of intentional GNSS interference, assess its effects on navigation and timing, present measurements from the Gulf of Gdańsk, and discuss related legal issues.

Methods:
The research combines analysis of scientific and technical sources, expert interviews and empirical field measurements. GNSS interference was recorded in the Gulf of Gdańsk using a low-cost SDR receiver and dedicated software for spectrum observation. The legal dimension was examined through analysis of international and national regulations on harmful interference and state responsibility.

Results:
Field measurements in the Gulf of Gdańsk L-band (1525–1600 MHz) revealed anomalies in the L1/B1 bands and a narrowband continuous-wave interference event near the GPS/Galileo L1 frequency. Its stable, narrow spectrum and level above the noise floor indicate an anthropogenic emitter and confirm the feasibility of low-cost SDR-based GNSS interference monitoring.

Conclusions:
Intentional GNSS jamming is already an operational tool of asymmetric and hybrid activities, posing concrete risks for navigation, timing and critical infrastructure. The findings highlight the need for resilient GNSS-dependent systems, low-cost monitoring networks and more effective international and national legal safeguards against harmful interference.