A ground-based source of a jamming signal capable of disrupting a GNSS satellite-based navigation system, for example a GPS jammer, is located with a detector carried by a suitable platform, for example an airborne UAV or missile. The detector, when the platform is at a first location, measures a characteristic of the jamming signal. The platform and its detector are then moved to a chosen second location, from which a further measurement of the jamming signal is made. The measurements made by the detector are then used to determine the location of the source of the jamming signal.