a wide angle field of view 215 imager 205 acquires an image and provides the image in the form of image data 220 to a comparator 225 comparator 225 also receives image orientation data 230 from an orientation detector 235 or other source as discussed below imaging apparatus 200 is preferably sited near the antenna 240 of a gnss receiver 245 so that the aquired image represents surroundings of antenna 240 gnss receiver 245 receives ephemeris data from the gnss satellites or from other sources as is known in the art the ephemeris data describes the orbits of the gnss satellites so that at any given time the respective position of each gnss satellite can be determined whether from gnss receiver 245 or from another source satellite position data 250 is supplied to comparator 225 comparator 225 thus is provided with the image represented by image data 220 with image orientation data 230 so that azimuthal orientation of the image can be determined and with satellite position data so that an expected position of each satellite can be determined these information sources enable the expected position of each satellite to be compared with a corresponding region of the image usability of the signals of each satellite is classified accordingly for example if the expected satellite location corresponds to a region of the image representing unobstructed sky the satellite 39 s signals are.