The synthetic aperture radar has become a popular targeting sensor in recent years, thanks in part to its ability to "see" through clouds and operate in daytime or night. In a typical airborne application, the system transmits microwave radiation and forms an image based on the relative range and range-rate of the reflected energy.
Synthetic aperture radar and the principles of relative targeting. In a typical airborne application, the system transmits microwave radiation and forms an image based on the relative range and range-rate of the reflected energy. When the operator selects a pixel in the image, it is a measure of the relative range and range rate to the target. |
If this process is used to identify targets prior to a military strike, measures must be taken to prevent an unacceptable increase in target location error. If the targeting vehicle's navigation system has a position bias, that same bias will affect the estimate of the target coordinates because the range and range-rate measurements are relative to the estimated position of the radar's antenna at imaging time. Velocity errors will also produce targeting errors. The key to managing navigation errors for targeting is to keep the position error as constant as possible and the velocity error as close to zero as possible during imaging. This will produce an error in the target position estimate that is very close to the position error of the targeting vehicle at imaging time.
Such control of position and velocity errors is possible with GPS, assuming proper satellite selection before and during the imaging period. If the satellite selection is frozen to the same four satellites during imaging, the range error would stay roughly the same because the satellite geometry would not appreciably change during that time. Because the position error is nearly constant, the velocity errors approach zero. Proper filtering of the GPS measurements and the use of a high-quality INS achieves extremely stable position errors and velocity errors almost too small to measure.
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Since the synthetic aperture radar measures the relative range and range rate to the target, it will have a bias in the target coordinate estimate. The weapon can be forced to incur the same bias by suitable satellite selection. |
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One of the benefits of using synthetic aperture radar in conjunction with GPS/INS guidance is that the weapon can be made to approach the target from a preferred direction to minimize any residual targeting errors. In fact, one can predict the three-dimensional targeting error ellipsoid by knowing the location and velocity vector of the targeting vehicle at each synthetic aperture radar imaging time. One direction usually has a larger targeting error uncertainty due to the projection of synthetic aperture radar errors into the targeting space. If the weapon is controlled to approach the target along this largest direction of uncertainty, the resulting impact error can be minimized. This error projection property can be very useful in mission planning if the target requires a preferred approach angle—such as a tunnel entrance.