Target Suite in Kill Vehicle's Field of View

After its release from the interceptor booster, the kill vehicle uses its on-board sensor to look for the objects released by the attacking missile, and then collect data on them to try to determine which object is the mock warhead. The kill vehicle's sensor can see only a very small area at any time; that is, it has a small "field-of-view." However, it appears that in the tests the target missile has released the mock warhead and balloon decoys in a very special way, so that those objects and the final missile stage (the "bus" that releases the warhead and decoys) appear close enough together that they all appear simultaneously in the kill vehicle's field of view until very late in the homing process, greatly simplifying the job of observing them.

To achieve this special arrangement, the bus appears to be programmed to release the warhead and balloons so that all of the objects are roughly lined up along the direction of sight of kill vehicle's sensors. As a consequence, although the actual spatial separation of the object is several kilometers shortly before intercept, to the kill vehicle they appear to be separated by much less than that. If the bus released the objects in any other direction, as a real attacker would do, this would not be true.

Lining the objects up in this way allows the kill vehicle to view and collect data on all of them simultaneously. Otherwise, it would only be able to see one or two at a time, and would have to maneuver and change its physical orientation to rotate its sensors' field of view from one object to another. Not only is this much more complicated, but it would significantly reduce the amount of time the kill vehicle would have to collect the data on each object needed for discrimination.

Assuming the kill vehicle's infrared sensors have a field-of-view of one degree, then two objects separated by five and a half kilometers could not be seen simultaneously when the kill vehicle was closer than about 300 kilometers. If the objects were separated by one kilometer, they could be seen simultaneously until they were within 60 kilometers of the kill vehicle.

This special arrangement therefore simplifies the job of the kill vehicle and allows it to collect data on the objects for a longer time. This situation could obviously not be expected in general. Indeed, in an actual attack, one might expect that the attacker would consciously try to separate the objects in the target cluster far enough to make observing them more difficult for the kill vehicle. Moreover, this problem becomes more severe at the higher closing speeds appropriate to an operational system, since the kill vehicle would then have less time to collect data on the objects in the target cluster. Since using the operational booster could lead to a closing speed that is twice as large as that in current tests, this could cut in half the time available for the kill vehicle to view the objects.

The figure above shows the same arrangement of objects in the target set in test IFT-6 from two different directions. Object (1) is the balloon decoy, (2) is the mock warhead, and (3) is the "bus" that released them. The kill vehicle is assumed to be approaching the objects in the direction you are looking at the screen. In figure (a), the target set is oriented in such a way that the objects appear so widely spaced that the sensors on the kill vehicle could not see the objects simultaneously as it approached impact. This is illustrated in the figure by the fact that the field-of-view of the kill vehicle's sensors (shown by the black and red squares) cannot contain all of the objects at the same time. In IFT-6, the objects were instead released so that the kill vehicle approached them from the top in figure (a). From this direction, the objects are lined up with one another and appear to the kill vehicle to be much closer together, as shown in figure (b). As a result, they all remain simultaneously in the kill vehicle's field-of-view until very late in the homing process. This figure assumes a one-degree field-of-view for the kill vehicle's sensors, and a closing speed of 7.4 km/s, which was the speed in IFT-6.