All of the tests to date have used either a GPS receiver or a "C-band beacon" attached to the warhead to give the defense detailed information on the location of the mock warhead early in flight. The reason is that defense needs trajectory data on the warhead relatively early in flight to generate a "weapons task plan," which specifies where and when to launch the interceptor. It is assumed that a radar would provide this information in an operational situation, but there is currently no radar that can provide this information early enough. The Missile Defense Agency therefore argues that using the GPS receiver or C-ban beacon as sources of track data is a necessary artificiality at this point in the testing program.

In tests IFT-5 through IFT-8, the C-band beacon was the defense system's primary source of track data on the warhead early in its flight. The C-band beacon is tracked by the FPQ-14 C-band radar located at Kaena Point, Oahu, Hawaii, and is apparently needed since the C-band radar does not have sufficient power to detect and track an object with the radar cross-section of the warhead effectively on its own. In these tests, the track information from the C-band radar was used to generate the weapons task plan and to calculate an approximate intercept point in space, and then to launch the interceptor toward the estimated intercept point. It was also used to cue the X-band radar on Kwajalein-that is, to give the X-band radar an approximate location in space where it should search to detect the mock warhead.

However, a key artificiality is that the beacon has apparently provided the defense with very high quality tracking information on the warhead. Moreover, since the beacon is attached to the mock warhead, it provides the location of the warhead itself rather than just the location of a basket containing the target cluster.

In particular, the distance between the actual intercept point in IFT-6 and the estimated intercept point the defense calculated using the beacon data and used to launch the interceptor, was apparently only 400 meters.

The fact that the beacon data allowed the battle management system to predict the intercept point (which is the future location of the warhead) to within 400 meters means the defense knew the location of the warhead with an uncertainty that is considerably smaller than the separation distance between the warhead and the other objects in the target cluster. This allowed the defense to launch the interceptor on a trajectory that was aimed essentially directly at the warhead, rather than toward a larger basket that included the full target cluster.

The test thus assumed that the defense knew the location of the warhead within the target cluster before it launched the interceptor. This situation therefore assumes that the X-band radar has discriminated the warhead from decoys before the kill vehicle is launched. While the Missile Defense Agency clearly hopes to discriminate the warhead as early as possible, this very likely will not be the case in an operational situation.

Using beacon data to launch the interceptor may be appropriate at this stage in the testing program, but it has implications for what can be learned about the endgame from these tests.

For example, the test conditions that have been used so far represent the least stressing case for the defense. Since the kill vehicle was apparently launched on a trajectory that headed it essentially straight toward the target, the kill vehicle needed only minor, if any, course updates from the ground-based radar, and the total maneuvering required of the kill vehicle to hit the target was small compared to what might be expected in an operational scenario. In particular, in cases in which defense had not discriminated the warhead from the target cluster by the time the interceptor was launched, the kill vehicle would be launched toward a basket containing the full target cluster, which could easily be tens of kilometers in size, and homing would require a considerably greater level of maneuvering than is demonstrated in the current tests.

Even if the test assumes that the X-band radar successfully discriminates the warhead, it still matters how accurate the beacon data is and when that information is made available to the kill vehicle. It is not publicly known how the quality of the tracking data supplied by the beacon compares to tracking data that would be expected from the X-band radar in an operational situation, nor when in the engagement the tracking data would be expected to be available to the kill vehicle. If highly accurate data was supplied at interceptor launch-as it was in IFT-6-rather than at the final in-flight update, the kill vehicle would be required to do much less maneuvering.

The beacon will continue to be the primary source of early track data until the battle-management software is upgraded to allow the defense to use track data from the early warning radar at Beale, CA, possibly in 2003.

This artificiality may be justified at this early stage in the development process if the tests are intended to test the homing process under conditions that do not stress the system. However, such tests should not be construed as demonstrating the capability of the system under operational conditions, which could be considerably more demanding.