Abstract:
The present invention provides a system and method for self-determining a radar signature of a target in which the target has a radar transceiver. The target is positioned at a predetermined distance from a reflective surface, such a flat metal surface, and the transceiver is used to transmit an energy signal toward the reflective surface. The reflective surface is positioned to reflect the energy signal back toward the target. Energy reaching the target reflects from component parts of the target back toward the reflective surface which reflects the energy back toward the target again where the transceiver receives the returned energy and calculates a radar signature indicative of the returned energy signal.

Description:
This invention was made with Government support under Contract Number N00019-97-C-0038 awarded by Naval Air Systems Command. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates generally to radar technology, and, more particularly to a method and system for radar cross section self-test. 
     2. Description of Related Art 
     The survival techniques of aircraft in combat situations has undergone dramatic changes throughout the years. In the earlier days of aviation, survival of a combat aircraft was based upon speed. As avionics progressed, survival techniques progressed from speed to electronic capabilities. During World War I, visual detection in daylight did not exceed 15 miles. Even in the late 1930s, defenders were expected to listen and watch for attacking aircraft. By 1940, however, radar could spot incoming aircraft at a distance of more than 100 miles. Early detection gave defenders much more time to organize their air defenses and to intercept attacking planes, such as radar height-finding assisted anti-aircraft gunners. Primitive airborne radar sets were installed in night fighters in the later years of the war. Now, however, avionics have evolved to the point where one of the big keys to survival of an aircraft is in the stealthyness. 
     Since World War II, the radar game between attackers and defenders has determined who will control the skies. Radar domination allows firepower of air forces to bear against a foe or to deprive an enemy of this most valuable asset. Highly survivable aircraft contribute directly to achieving joint force objectives, and thus the ability to project power with efficient and effective air operations depends on controlling the radar contest. 
     Aircraft survivability depends on a complex mix of design features, performance, mission planning, and tactics. The effort to make aircraft harder to shoot down has consumed a large share of the resources dedicated to military aircraft design in the 20th century. Since the 1970s, the Department of Defense has focused on research, development, testing, and production of stealth aircraft, designed to blunt the power of defenders to detect them and thus defeat and/or destroy them. 
     Stealth technology minimizes aircraft signature in several ways but most notably by greatly reducing its radar signature. Low-Observable (LO) aircraft such as the first operational stealth aircraft, the F-117 and the B-2, demonstrated the feasibility of LO aircraft and their importance to more effective air operations. Like all combat aircraft, they have limitations that must be recognized to ensure proper employment. 
     An aircraft on a mission may become proximate to anti-aircraft fire or fragments which can strike the aircraft. Conventionally, such a fragment typically doesn&#39;t do a great deal of damage however, the fragment is capable of disfiguring the aircraft to actually compromise the aircraft by increasing the radar signature. In order to maintain the stealthyness, the aircraft must be repaired between missions. Unfortunately, the repairs may appear sound upon visual inspection however, the repair work may still be apparent on radar. 
     Currently one method for testing the RCS of an aircraft involves the building of a model of an aircraft and hoisting it up on a big radar pole. The aircraft is then shot with radar and the radar signature or the radar cross section (RCS) is measured. The aircraft signature can also include, for example, infra-red signatures, visual signatures, or acoustic signatures. RCS measurements are customarily made on radar cross section ranges or labs. Such ranges basically consist of a test radar that sends radar signals to a remotely positioned test target and receives and measures any returned radar echo, as may be reflected from the object. Typically the test target is supported upon or suspended from an RCS test mount. 
     When operating LO aircraft, one doesn&#39;t always know if the RCS is as low as that which it was designed. Many actions and events in the aircraft&#39;s life can affect is RCS, e.g., maintenance, battle damage, erosion. Scattering centers may be produced on the LO aircraft by patches of dirt, production defects, exterior damage, or incompletely closed access doors. Such conditions may go unnoticed by maintenance personnel and pilots in the field. Furthermore, repairs and production defects may leave imperfections that may not be detected by visual inspections. As a result, the aircraft may be vulnerable to radar detection. Unless the aircraft is brought to an ISAR test range, these conditions will often remain undetected. 
     SUMMARY OF THE INVENTION 
     The present invention achieves technological advances as a system and method for self-determining a radar signature of a target in which the target has a radar transceiver. The target is positioned at a predetermined distance from a reflective surface, such a flat metal surface, and the transceiver is used to transmit an energy signal toward the reflective surface. The reflective surface is positioned to reflect the energy signal back toward the target. Energy reaching the target reflects from component parts of the target back toward the reflective surface which reflects the energy back toward the target again where the transceiver receives the returned energy and calculates a radar signature indicative of the returned energy signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings, wherein like-numerals refer to like elements, wherein: 
     FIG. 1 illustrates a current system for measuring a radar signature of a target, in this case, an exemplary aircraft; 
     FIG. 2 illustrates a radar self-test system in accordance with an exemplary embodiment of the present invention; 
     FIG. 3 illustrates a top view of the radar self-test system illustrated in FIG. 2 including an exemplary RCS blemish, located on a wing of the target aircraft; 
     FIG. 4 illustrates an exemplary embodiment of a radar self-test method in accordance with the present invention; 
     FIG. 5 illustrates an exemplary embodiment of a radar calibration method in accordance with the present invention; and 
     FIG. 6 illustrates a radar self-test method in accordance with an exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The numerous innovative teachings of the present application will be described with particular reference to the presently preferred exemplary embodiments. However, it should be understood that this class of embodiments provides only a few examples of the many advantageous uses and innovative teachings herein. In general, statements made in the specification of the present application do not necessarily delimit any of the various claimed inventions. Moreover, some statements may apply to some inventive features, but not to others. 
     Referring now to FIG. 1 there is illustrated a current system for measuring a radar signature of a target, in this case, an exemplary aircraft  10 . The aircraft  10  is mounted on a stand  20  or other type support structure which is rigidly affixed to the ground or to a rotatable table, for example. The stand  20  is generally of a specific shape and/or made of a material which exhibits a very low RCS with respect to the target. A source of radio frequency (RF) microwave energy (hereinafter referred to as RF source  30 ) is mounted or affixed to a structure set at a predetermined distance from the targeted aircraft  10 . The RF source  30  transmits the microwave energy toward the aircraft  10  and receives that microwave energy which is reflected back from the aircraft  10 . The received microwave energy is subsequently processed and an RCS or radar signature is determined. Generally, a radar operator specially trained to support and maintain the RF source  30  and other supporting equipment is required for consistent results. The radar signature of the aircraft  10  can be displayed in many different forms specific for a particular application. 
     In another system, the RF source  30  can be a handheld device in which an operator stands at a predetermined distance and directs the microwave energy toward the targeted aircraft  10 . Because of the inherent sensitivities with determining a radar signature, handheld approaches can be very inconsistent and can vary from operator to operator. 
     Referring now to FIG. 2 there is illustrated a radar self-test system in accordance with an exemplary embodiment of the present invention including a target  200 , reflective surface  210 , and radar absorbing shield  240 . The target  200  can be a fighter jet, helicopter, missile or any other vehicle with the capability to transmit and receive RF signals. In the present exemplary embodiment, a fighter jet is illustrated as the target  200 . The fighter jet is equipped with its own radar  260  which is typically enclosed in the nose-cone. 
     The target aircraft  200  is positioned at a predetermined distance directly in front of the reflective surface  210 . Preferably, when the radar is located in the nose of the target aircraft, the nose of the target aircraft  200  is pointed toward the reflective surface  210  on a perpendicular axis with the most center point of the surface  210 . The reflective surface  210  can be any type surface which reflects microwave energy in a predictable manner. For example, the reflective surface  210  can be a large metal surface. The reflective surface  210  is illustrated as a flat surface, however, it can also be parabolic in shape. 
     A low-power, real-beam, map mode with short gate times can be incorporated in the aircraft radar  260  for use in a self-test procedure. The reflector  210  is located directly in front of the aircraft  200 , perhaps 15 feet in front of the radar  260  aperture, for example. The radar  260  aboard the aircraft  200  is activated to determine an RCS of the aircraft  200  itself. Using a real-beam map mode, the radar  260  can scan critical areas of the aircraft in the frontal sector to detect flaws or other problems compromising the aircraft RCS. The reflector can be rotated if needed to obtain data over a small range of azimuth angles. Processing and interpretation of test results can be automated, in a repeatable program, for example, if desired to minimize pilot workload. Post-repair or preflight checks can be made using the aircraft radar  260  and a passive reflector  210 . Go, caution, and no-go limits can be determined from test aircraft and/or lab results, and entered in a “table” or memory associated with radar processing equipment to automate the evaluation process 
     In another embodiment of the present invention the radar absorbing shield  240  is placed in front of the landing gear, which are normally enclosed in the aircraft  200  during flight, to simulate flight configuration. The radar absorbing shield  240  is used to absorb the microwave energy that would otherwise reflect off the landing gear. More than one radar absorbing shield  240  can be used to more effectively cover multiple landing gear from reflecting microwave energy which may otherwise corrupt the radar test. 
     Referring now to FIG. 3 there is shown a top view of the radar self-test system illustrated in FIG. 2 including an exemplary RCS blemish, located on a wing of the target aircraft  200 , to illustrate a use of the self-test system. As illustrated, microwave energy is transmitted from the aircraft radar  260 . The RF signal is directed toward and reflects off of the reflective surface  210 . Incident RF energy reflects off the RCS blemish back toward the reflective surface reflecting back to the radar receiver  260 . The higher reflective energy, relative to an expected energy return, back to the radar receiver indicates that a potential RCS problem may exist. 
     Referring now to FIG. 4 there is illustrated an exemplary embodiment of a radar self-test method in accordance with the present invention. Following initiation of the test procedure  41 , the target aircraft and reflective surface are positioned  43  such that the reflective surface is located directly in front of the aircraft perhaps 15 feet away, for example. Preferably, the nose of the target aircraft is pointed toward the reflective surface on a perpendicular axis with the most center point of the surface. Subsequently, the radar equipment on board the target aircraft is used to transmit RF energy  45  toward the reflective surface. Further, the radar equipment on board the target aircraft is used to receive RF energy reflected back to the radar equipment  45 . Subsequently, a processor associated with the radar converts the raw RF signals received into digital data for imaging and/or comparative analysis  47 . Processing and interpretation of test results can be automated if desired to minimize pilot workload. Lastly, the aircraft can be returned to combat or modified and retested  49 . 
     Referring now to FIG. 5 there is illustrated an exemplary embodiment of a radar calibration method in accordance with an embodiment of the present invention. The calibration  50  includes selecting an aircraft predicted to have an acceptable RCS for a specific application  52  or mission. Subsequently, an RCS is measured or determined for the selected aircraft using the current best techniques available  53 , for example, such as the system illustrated in FIG.  1 . Next, the RCS measurements are analyzed to determine if the RCS was acceptable  55  for the selected mission. If the RCS is not acceptable for the specific application the aircraft is repaired  54  to correct any flaws and/or modified (using know techniques) to anticipate an associated RCS to meet that required for the specific application. If the RCS is acceptable  55 , a self-test procedure is executed  56 . The self-test procedure can be the aforementioned self-test procedure illustrated in FIG. 2,  3  or  4  and can be an extension of the lab type procedure illustrated in FIG.  1 . For example, following the stand mounted RCS procedure illustrated in FIG. 1, a self-test procedure can subsequently be used while the aircraft is still in the same condition. After a RCS has been determined in both the laboratory type environment, as in procedure  53  and in a self-test procedure  56 , the two are compared and a calibration RCS is determined for calibration and comparison purposes with future self-test measurements made in the field. 
     Referring now to FIG. 6 there is illustrated a radar self-test method in accordance with an exemplary embodiment of the present invention. Following an operation  61  or return from mission, a self-test procedure is performed. The self-test procedure can be the aforementioned self-test illustrated in FIG. 2,  3  or  4 . Subsequently, the results of the self-test procedure are compared against a calibration RCS  63 . The calibration RCS can be determined, for example, by the aforementioned calibration method  50  illustrated in FIG.  5 . If the resultant self-test RCS, from procedure  62 , is within a predetermined limit with respect to the calibration RCS the aircraft is determined to be acceptable for further operation  64 . Otherwise, the aircraft operation is aborted  65  and/or repairs or modifications are made to the aircraft and the test is repeated. 
     Although a preferred embodiment of the method and system of the present invention has been illustrated in the accompanied drawings and described in the foregoing detailed description, it is understood that obvious variations, numerous rearrangements, modifications and substitutions can be made without departing from the spirit and the scope of the invention as defined by the appended claims.