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 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 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 , - moving the detector from the first location to the second location, - using the detector 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 , the third location being displaced from the straight line passing through both the first and second locations, and - moving the detector to the third location, - using the detector at the third location to make the one or more yet further measurements, characterised in that the method further comprises: - moving the detector 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 , 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 at the at least one further location to make the one or more further measurements, and - using measurements made by the detector of the one or more characteristics of the jamming signal to determine a location of the source of the jamming signal. - using the detector 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 , - moving the detector from the first location to the second location, - using the detector 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 , the third location being displaced from the straight line passing through both the first and second locations, and - moving the detector to the third location, - using the detector at the third location to make the one or more yet further measurements, - moving the detector 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 , 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 at the at least one further location to make the one or more further measurements, and - using measurements made by the detector of the one or more characteristics of the jamming signal to determine a location of the source of the jamming signal. <EOS>