An automated method of processing sensor data, the method comprising: (i) using a sensor to measure values of a parameter over a region of interest to produce an image of the region of interest, the image comprising a plurality of pixels; (ii) for each pixel, determining an orientation of a gradient of the parameter, at that pixel, using the measured parameter values; (iii) for each of a plurality of predetermined ranges of gradient orientation values, determining the number of said pixels that have a gradient orientation value within that range; (iv) determining the number of groups of predetermined, ranges, wherein each group of predetermined ranges is determined such that it satisfies the criteria that: for each predetermined range in that group, the number of said pixels that have a gradient orientation value within that range is above a threshold value; if that group comprises more than one predetermined range, each predetermined range in that group is contiguous with at least one other predetermined range in that group; and for each group, the one or more predetermined ranges in that group are not contiguous with any of the predetermined ranges in a different group; and (v) using the determined number of groups to discriminate between the presence of a man-made object and clutter in the image; wherein an object in the region of interest is labelled as man-made if the number of groups is greater than or equal to a further threshold, and is otherwise labelled as clutter. (i) using a sensor to measure values of a parameter over a region of interest to produce an image of the region of interest, the image comprising a plurality of pixels; (ii) for each pixel, determining an orientation of a gradient of the parameter, at that pixel, using the measured parameter values; (iii) for each of a plurality of predetermined ranges of gradient orientation values, determining the number of said pixels that have a gradient orientation value within that range; (iv) determining the number of groups of predetermined, ranges, wherein each group of predetermined ranges is determined such that it satisfies the criteria that: for each predetermined range in that group, the number of said pixels that have a gradient orientation value within that range is above a threshold value; if that group comprises more than one predetermined range, each predetermined range in that group is contiguous with at least one other predetermined range in that group; and for each group, the one or more predetermined ranges in that group are not contiguous with any of the predetermined ranges in a different group; and for each predetermined range in that group, the number of said pixels that have a gradient orientation value within that range is above a threshold value; if that group comprises more than one predetermined range, each predetermined range in that group is contiguous with at least one other predetermined range in that group; and for each group, the one or more predetermined ranges in that group are not contiguous with any of the predetermined ranges in a different group; and (v) using the determined number of groups to discriminate between the presence of a man-made object and clutter in the image; wherein an object in the region of interest is labelled as man-made if the number of groups is greater than or equal to a further threshold, and is otherwise labelled as clutter.