Difference between revisions of "First-Hand:Sidelobe Cancellers and the Like"
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'''Contributed By''' Dean Chapman, IEEE Senior Life Member
'''Contributed By''' Dean Chapman, IEEE Senior Life Member
Revision as of 22:38, 8 September 2008
Contributed By: Dean Chapman, IEEE Senior Life Member
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For those of us who worked in the area of Radar Systems, it was not uncommon to jump from designing electronic countermeasures (ECM) equipment designed to defeat radars to Electronic Counter-countermeasures (ECCM) equipment intended to defend against such devices. I first worked on ECM and then moved to the ECCM world.
In 1961 after I graduated, I went to work for General Electric’s Light Military Electronics Department (LMED) in Utica, NY. One of the first assignments I had there was working on an airborne externally stored RADAR Jammer nicknamed JAMPAC, officially known as QRC160-1. It was later assigned a production number of ALQ-71. This equipment was mounted in a pod and powered by a ram air turbine. It was designed to attach to the wing of an F-100 or other aircraft in the manor of a weapon or fuel tank. It employed a set of General Electric voltage-tunable magnetrons (VTM’s) and was designed to output broadband noise-modulated signals that would enter the enemy radar through its antenna sidelobes. These VTM’s were developed by General Electric’s Power Tube Department in Schenectady and were a major breakthrough in the state of the art at that time.
The physics of the standoff jammer scenario dictates that while the Radar return signal varies as the 4th power of range to the target, the jamming power varies as the 2nd power (one way versus two way ). Thus the jammer has the advantage for a given radar cross section and jammer power until the range to the target decreases to the so-called “burn-thru” range. That this system was effective was brought home one day when one of the development models of this jammer was mounted on an F-100 for a seemingly innocent test somewhere in Northeastern United States. The system was turned on and was supposed to fly against a designated Radar site. Before the test had been completed, the whole Northeast was scrambled as search radar screens all up and down the coast were masked with unexpected clouds of noise.
In 1968 after working on Electronic Countermeasures for some time, I went to work for Anaren Microwave (more about them later) and then in 1972 for Syracuse University Research Corporation (then called SURC, now SRC). At SURC were two gentlemen who were instrumental in developing a powerful Electronic Counter-Counter Measure (ECCM) technique known as Sidelobe Cancelling. Sidelobe jammers depended upon the fact that typical antennae have sidelobes ranging from 15 to 25 DB below the main lobe peak. Jammers of sufficient strength can stand off the target area and inject noise into the sidelobes of a search or tracking radar at an enemy site. However, if the jammer is located in a sidelobe null of the antenna response pattern, its effect on the Radar can be radically diminished. The sidelobe canceller could sample the jammer input signal and feed it into the radar receiver channel out of phase with the jamming signal in the received path, thus “cancelling” the effects of the jamming noise. Spatially, this results in a steerable null which is automatically aimed at the jamming source.
The invention of the sidelobe canceller is credited to Paul Howells (US Patent 3 202 990). Later mathematical analysis of the device and its extension to adaptive arrays was credited to Sid Applebaum. Both of these gentlemen originally worked for the Heavy Military Electronics Department of General Electric in Syracuse and then later split off to form the Special Projects Lab of SURC. As an aside, I mentioned Anaren Microwave previously. Two other gentlemen, Hugh Hair and Carl Gerst, also started with GE’s HMED, then went to SURC, and in about 1967 split off to form Anaren Microwave, where they specialized in strip line microwave devices. Incidentally, the name Anaren derives from Carl’s wife Anna and Hugh’s wife Rene. To this day, Anaren is a thriving company located in Syracuse. General Electric’s HMED was the spawning ground of several successful spinoffs in the Syracuse area.
The initial design of the sidelobe canceller comprised a single control loop and could steer one null against one jammer. Two jammers spatially separated would render the technique ineffective. Howells and Applebaum then developed a multiple loop design that would handle multiple jammers. Although this technique was to be used in many different Radar applications, the one of most interest to SURC at the time was for Ballistic Missile defense. The problem that the ABM radar had to solve was to reliably pick out the reentering enemy ballistic missile warheads from the chaff, balloon decoys, and other debris in time to launch a missile that would destroy the warhead. The SPRINT missile was typical of this genre of weapons. It featured astronomical acceleration capability. Warhead reentry into the atmosphere would strip away all but the real warheads so that if the anti warhead missile was fast enough, it could still intercept the incoming warhead before impact. This was a very difficult problem and was not unlike trying to hit a moving bullet. Defense designers then became worried about offense payloads that might contain multiple jammers that could be deployed in the vicinity of the armed warheads to attempt to lower the burn through altitude to the point where the anti warhead missile could not be effectively used. SURC proposed a multiple loop sidelobe canceller as a part of ABM radars. They successfully tested it at the HAPDAR (HArd Point Demonstration Array Radar) at White Sands, NM in the 1970’s.
As Sid Applebaum thought more about the multiple loop canceller and the problem as it applied to array antennae, he realized that if each element of the array could be adaptively controlled, the entire radiation pattern of the antenna could be adaptively shaped to reject sidelobe jamming regardless of the number of jammers. This was a major breakthrough and was shown to work through computer simulation. The challenge was to design an adaptive system that could react in real time with sufficient array size to be able to satisfy the other requirements of the radar. Simulations using a simplified model of the antenna yielded very optimistic results. It was then necessary to model the effects of finite bandwidth, channel to channel phase and amplitude mismatch, and element mutual coupling effects to give a more realistic picture of the expected performance. At that time, I did many computer simulations of Adaptive Arrays, experimenting with different configurations and studying the effects of bandwidth, phase mismatch and other effects. My typical day involved making computer runs and plotting results on our trusty “Calcomp” plotter and then sitting in Sid Applebaum’s office trying to make sense of the results while Sid proposed yet another test. One of the last tests SURC ran at HAPDAR was a linear fully adaptive array operating at radar bandwidths.
Naval Research Laboratory became interested in Adaptive Arrays for use in Airborne Early Warning (AEW) applications. It turned out that in addition to the ability to cancel jamming, the Adaptive Array could be used to reduce clutter entering the system through the sidelobes of the AEW array. Sid Applebaum and I were in Washington and driving to a meeting with the NRL staff one day. As we passed the rows of strobe lights at the end of the National Airport runway, Sid’s eyes lit up with an idea inspired by the apparent movement of the strobes as they cycled toward the runway threshold. What resulted was a clever way to simulate the AEW problem statically on the ground on an antenna test range. SURC was awarded a contract to provide the hardware and equipment to run this series of tests. Bill Gabriel of NRL was the project engineer. During this period, Bill organized a special issue of the IEEE Transactions on Antennas and Propagation devoted to Adaptive Arrays (September 1976 Vol. AP-24, Number 5). We at SURC contributed four papers to that issue. Others working on array adaptivity in the areas of Over-the Horizon radar, communications, and in Seismic Arrays also contributed to the issue.
Paul Howells was a very original thinker and the term “thinking outside of the box” applied liberally to him. One day he was in Washington talking with military planners when someone mentioned that the Russian Strella Missile was a particularly dangerous threat to our aircraft at that time. It was a heat-seeking shoulder-launched missile that was being used against our helicopters and low flying planes. On the way home in the airplane, Paul sketched out the design of a CW Radar (not unlike today’s traffic radars) that could be coupled to flare launchers to detect oncoming missiles and decoy them away with flares. His radar cross-section and Doppler analysis calculations were literally scratched on the back of an envelope. When he came in the next day, he handed the envelope over to engineers who immediately started working on a prototype system. It was a relatively simple and economical yet effective answer to a vexing problem. Incidentally, both Paul and Sid worked on early color TV design at General Electric’s E-Lab in Syracuse before becoming interested in radar and ECCM.
It was a unique and thrilling experience for me to be able to work with such original thinkers and pioneers in the field of Radar Systems. Both Sid and Paul have since passed on but their legacy lives on in their contributions to the state of the art in Radar Systems. As I progressed through the first half of my career, I was able to work on both ECM and ECCM systems as well as having the good fortune to work for and with some very talented people. Ironically, the latter half of my career was spent working in the electric utility business.
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