The orbital sensor net
An orbital sensor net was deployed around a planet controlled by the Mokra Order in the Delta Quadrant to detect approaching starships. Detection of a ship by the net might be followed by a scan, or being fired upon by surface-based phased ion cannon.
Surrounding the entire planet, Harry Kim upon studying it in 2372 declared it to be the most sophisticated system he had ever seen, declaring it could not be disabled and saw everything the USS Voyager did. However, ships could approach and hide behind the planet's moon and inside a magnetic storm at the northern magnetic pole of the planet without triggering the sensor net. (VOY: "Resistance")