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Surveillance at Sixty Thousand Feet By Eric Forrest Viewing the earth from 60,000 feet requires something a bit stronger than the naked eye. To give a boost to long-distance observation, the Global Hawk unmanned aircraft system (UAS) will soon demonstrate a new surveillance capability suggested by MITRE staff members eight years ago. The new capability is an outgrowth of the Multi-Platform Radar Technology Insertion Program (MP-RTIP). MITRE has been involved in the conceptual feasibility, requirements development, design, and testing of the MP-RTIP for nearly a decade. The system was originally conceived for large-scale airborne platforms. Our staff, however, saw the usefulness of extending the radar's capability to the Global Hawk. Design Innovation Overcomes Skeptics MITRE staff first developed designs for a UAS high-altitude ground surveillance capability when the Air Force's Joint Surveillance Target Attack Radar System (Joint STARS) debuted. At first, many in the defense community expressed skepticism. For one thing, an airborne radar system like Joint STARS must radiate enough power to support ground target detection at long stand-off ranges. Other MITRE engineers also noted that a UAS did not have adequate prime power generation nor the endurance to support a relatively large (24 foot) antenna, such as the one used on Joint STARS. Members of MITRE's sensor center offered a different design strategy for the ground surveillance mission. We proposed using antenna designs from the fighter radar community to more efficiently convert prime power to radiated power. This design calls for an active electronically scanned antenna (AESA), which has several hundred monolithic microwave integrated circuits, each with its own transmit, receive, and phase control circuitry. Each component can be synchronized to rapidly direct the radar energy in most directions without physically moving the antenna. We also conducted extensive computer simulations to affirm that ground clutter—radar echoes from fixed objects on the ground—could be reduced with smaller antennas and advanced digital signal processing. Thus, the AESA antenna enables returning radar energy to be collected and processed by many receivers located at different positions on the antenna. The antenna's beam shape can be adapted to mitigate the effects of ground clutter return using a technique called space-time adaptive processing (STAP). Introducing STAP, however, also increases the required computational throughput. At the time, estimates suggested that weight, volume, and power constraints for the unmanned system were so demanding that implementing the signal processor on an unmanned platform posed a significant risk of failure. Fortunately, by delaying the acquisition and allowing the necessary improvements in circuitry and miniaturization to occur, the risk could be mitigated. Two Systems Working Together As a result of our feasibility studies, the Air Force changed its acquisition plans to upgrade the Joint STARS design and include the development of both large (for manned platforms) and small (for UAS) radar systems—the design that ultimately became the basis for the MP-RTIP. Both designs use microelectronic technology from fighter radars to support intelligence, surveillance, and reconnaissance functions. Although these functions are similar, they are not identical. The Air Force also expects new synergies to develop by having both systems perform ground surveillance. Specifically, one system can continue to track a ground target, while the other system conducts a maneuver to reposition itself. In addition, both have the potential to collaborate and provide a better estimate of a target's absolute location than either system operating alone. We look forward to successful flight testing to corroborate the feasibility claims proposed by the MITRE team eight years ago. |
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| For more information, please contact Eric Forrest using the employee directory. Page last updated: August 22, 2007 | Top of page |
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