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The authors describe MITRE's approach to overcoming electronic interference. Adaptive space-time processing uses a technique that has a less sophisticated parallel in our everyday lives. Have you ever wondered why the traffic report being broadcast from a helicopter is relatively clear and without the whirlybird's engine sound? The answer to that question has everything to do with adaptive space-time processing. The broadcaster's microphone picks up his voice and the sound of the engine. Knowing something about the sounds of the engine makes the task of filtering it out before you hear it on your car radio relatively simple. The same principle is used in adaptive space-time processing. Adaptive antennae are becoming a critical systems component for combating an environment of increasingly dense intentional and unintentional electronic interference. These antennae, which can be used for communications, navigation and radar systems applications, sample interference obtained from auxiliary antennae or subapertures of the main antenna. The sample is used to filter the "noise", while simultaneously preserving the desired signal, just as in the helicopter example. The simplest adaptive antenna is illustrated at left, and is appropriate for canceling a single jammer. Antenna 2 is an auxiliary and its output voltage V2 is weighted and subtracted from the main antenna voltage V1, with the weight w chosen so as to minimize the output power. This results in a spatial null in the direction of the jammer, blocking the signals from the jammer and allowing the real signal to be received. 
Two-element adaptive antenna. In order to cancel multiple jammers, more antenna elements are required. Furthermore, if the operating bandwidth is large and multipath is strong, it may even be necessary to add adaptive temporal degrees (adaptive time delays) of freedom behind each antenna. To illustrate this, let us consider a seven-element antenna array on an aircraft that is attempting to navigate on the high-precision code using the Global Positioning System (GPS), in the presence of three strong jammers. Simulated values of the output signal-to-jammer plus noise ratio are shown in the Table, and indicate that for the assumed arbitrary scenario without adaptive processing, there is no hope of acquiring a GPS satellite, but that adaptive processing makes acquisition possible. 
Effect of adaptive processing. Additional simulations have illustrated that a seven-element space-time adaptive array can provide protection to a GPS receiver from up to five hostile, strong, broadband jammers plus up to twenty sources of unintentional narrowband interference. To read about MITRE's development of adaptive systems and their applications to communications, navigation and surveillance, see Applications of Adaptive Systems to Communications, Navigation, and Surveillance. For more information, please contact Ronald Fante using the employee directory. |
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