1. A method of processing sensor data, the method comprising: (i) using a sensor, measuring parameter values of a parameter over a region of interest to produce an image of the region of interest, the image having 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 a number of said pixels that have a gradient orientation value within that range; (iv) determining a number of groups of predetermined ranges, wherein each group of predetermined ranges satisfies a criteria that: for each predetermined range in that group, a number of said pixels that have a gradient orientation value within that range is above a threshold value; and if that group contains more than one predetermined range, each predetermined range in that group is contiguous with at least one other predetermined range in that group; and (v) using the determined the number of groups, discriminating between the presence of a man-made object and clutter in the image.