An under vehicle scanning system for detecting a dissimilar object in a vehicle, wherein the system comprises: - a scanner and acquisition module configured to capture a plurality of three dimensional, 3D, images of an undercarriage of the vehicle at multiple instants when the vehicle crosses a structural unit with a slit; - an analysis and processing module configured to process the plurality of 3D images to obtain a plurality of 3D profiles of the undercarriage of the vehicle, wherein the analysis and processing module is further configured to: - normalize variations in orientation of the plurality of 3D profiles of the vehicle's undercarriage; - generate a 3D signature of the vehicle based on the plurality of normalized 3D profiles of the vehicle's undercarriage; - identify a model of the vehicle based on the 3D signature of the vehicle; and - compare the plurality of 3D profiles of the undercarriage of the vehicle with a standard profile of the undercarriage of a same model vehicle using the 3D signature to detect the dissimilar object; and - a display module configured to highlight the dissimilar object when the dissimilar object is detected in the undercarriage of the vehicle, characterized in that the analysis and processing module is further configured to generate the 3D signature of the vehicle by: (a) initializing a point set for a line of the plurality of 3D profiles with a first point; (b) comparing depth of each point along the line with depths of a previous point and a next point to add a current point at a change or discontinuity in the monotonic nature of the depth of the vehicle; (c) continuing step (b) till an end of the line is reached, and adding a last point; (d) calculating a value of, z representing the depth details of the vehicle, based on the line between the first point and the next point for each consecutive pair of points; (e) subtracting the calculated value of z from an actual value of z for all the points in the 3D profile line to obtain a difference value; and (f) adding the difference value to curvature for generating the 3D signature of the vehicle. - a scanner and acquisition module configured to capture a plurality of three dimensional, 3D, images of an undercarriage of the vehicle at multiple instants when the vehicle crosses a structural unit with a slit; - an analysis and processing module configured to process the plurality of 3D images to obtain a plurality of 3D profiles of the undercarriage of the vehicle, wherein the analysis and processing module is further configured to: - normalize variations in orientation of the plurality of 3D profiles of the vehicle's undercarriage; - generate a 3D signature of the vehicle based on the plurality of normalized 3D profiles of the vehicle's undercarriage; - identify a model of the vehicle based on the 3D signature of the vehicle; and - compare the plurality of 3D profiles of the undercarriage of the vehicle with a standard profile of the undercarriage of a same model vehicle using the 3D signature to detect the dissimilar object; and - normalize variations in orientation of the plurality of 3D profiles of the vehicle's undercarriage; - generate a 3D signature of the vehicle based on the plurality of normalized 3D profiles of the vehicle's undercarriage; - identify a model of the vehicle based on the 3D signature of the vehicle; and - compare the plurality of 3D profiles of the undercarriage of the vehicle with a standard profile of the undercarriage of a same model vehicle using the 3D signature to detect the dissimilar object; and - a display module configured to highlight the dissimilar object when the dissimilar object is detected in the undercarriage of the vehicle, (a) initializing a point set for a line of the plurality of 3D profiles with a first point; (b) comparing depth of each point along the line with depths of a previous point and a next point to add a current point at a change or discontinuity in the monotonic nature of the depth of the vehicle; (c) continuing step (b) till an end of the line is reached, and adding a last point; (d) calculating a value of, z representing the depth details of the vehicle, based on the line between the first point and the next point for each consecutive pair of points; (e) subtracting the calculated value of z from an actual value of z for all the points in the 3D profile line to obtain a difference value; and (f) adding the difference value to curvature for generating the 3D signature of the vehicle.