A method of locating a ground-based source of a jamming signal capable of disrupting a satellite-based navigation system, wherein the method comprises using a detector (20, 22, 24, 120) at different locations to make measurements of one or more characteristics of the jamming signal, the detector being on a vehicle and the method comprising the following steps: - using the detector (20, 22, 24, 120) at a first location to make one or more of the measurements, - choosing a second location from which one or more further of the measurements are to be made by the detector (20, 22, 24, 120), - moving the detector (20, 22, 24, 120) from the first location to the second location, - using the detector (20, 22, 24, 120) at the second location to make said one or more further measurements, - choosing a third location from which one or more yet further of the measurements are to be made by the detector (20, 22, 24, 120), the third location being displaced from the straight line passing through both the first and second locations, and - moving the detector (20, 22, 24, 120) to the third location, - using the detector (20, 22, 24, 120) at the third location to make the one or more yet further measurements, characterised in that the method further comprises: - moving the detector (20, 22, 24, 120) to at least one further location which can be readily reached by the vehicle, the at least one further location being calculated, in dependence on the previous measurements made by the detector (20, 22, 24, 120), to be a location from which one or more further measurements may be made which will improve the accuracy of a determined location of the jamming signal source, as compared to other locations which could also be readily reached by the vehicle, - using the detector (20, 22, 24, 120) at the at least one further location to make the one or more further measurements, and - using measurements made by the detector (20, 22, 24, 120) of the one or more characteristics of the jamming signal to determine a location of the source of the jamming signal. - using the detector (20, 22, 24, 120) at a first location to make one or more of the measurements, - choosing a second location from which one or more further of the measurements are to be made by the detector (20, 22, 24, 120), - moving the detector (20, 22, 24, 120) from the first location to the second location, - using the detector (20, 22, 24, 120) at the second location to make said one or more further measurements, - choosing a third location from which one or more yet further of the measurements are to be made by the detector (20, 22, 24, 120), the third location being displaced from the straight line passing through both the first and second locations, and - moving the detector (20, 22, 24, 120) to the third location, - using the detector (20, 22, 24, 120) at the third location to make the one or more yet further measurements, - moving the detector (20, 22, 24, 120) to at least one further location which can be readily reached by the vehicle, the at least one further location being calculated, in dependence on the previous measurements made by the detector (20, 22, 24, 120), to be a location from which one or more further measurements may be made which will improve the accuracy of a determined location of the jamming signal source, as compared to other locations which could also be readily reached by the vehicle, - using the detector (20, 22, 24, 120) at the at least one further location to make the one or more further measurements, and - using measurements made by the detector (20, 22, 24, 120) of the one or more characteristics of the jamming signal to determine a location of the source of the jamming signal.