In mid-February an SM-3 missile from the cruiser USS Lake Erie (CG-70) hit a large U.S. reconnaissance satellite that was falling out of control and would have re-entered the atmosphere within a few weeks. The missile took three minutes to hit the satellite, which was at an altitude of about 130 miles. That is twice the altitude at which SM-3s are designed to intercept ballistic missiles, but it is far lower than the satellite the Chinese destroyed last year, and far lower than the orbit of reconnaissance satellites. The U.S. satellite had already decayed well out of its intended orbit. Even so, hostile commentators in Russia described the shot as a covert test of an anti-satellite system. The Chinese saw hypocrisy: Americans condemned the Chinese test, but then conducted their own. They carefully avoided marked differences. Quite aside from any strategic or political implications, the Chinese test created debris at an altitude through which many satellites, particularly military ones, pass. The debris from the much loweraltitude U.S. test will almost certainly re-enter the atmosphere and simply burn up. It is unlikely to threaten anyone else's satellites.
The shot was an impressive example of network-centric warfare. Although descriptions of the Aegis system typically emphasize the associated SPY-I phased-array radar, the key to Aegis is the tactical picture the system creates and maintains. The system uses the picture to guide the missile into an interception "basket" within reach of the target. It homes the rest of the way. In the case of SM-3, terminal homing uses infra-red sensors mounted on the kinetic-energy (hit-to-kill) warhead. In the original anti-aircraft version of Aegis, the picture came from the radar on board the ship. That radar cannot possibly suffice for a target as distant as a missile or a satellite. Instead, the ship must exploit a wide range-a network-of other sensors, including satellites and remote long-range radars. What remains from the original anti-aircraft system is the picture-keeping computer directly linked to a fire-control system that commands the missile in flight through its autopilot.
The term "network-centric" is unfortunate. It is certainly true that the cruiser's shot required a network of remote sensors, all working together, with sufficient communication links to bring their data to where it was needed. However, the key was the picture maintained on board the cruiser. Everything had to come together properly in that picture. For example, the cruiser and the sensors all had to know their relative positions, something probably impossible before the advent of the Global Positioning System (GPS). Position includes timing. The precision of the picture also depends on knowing the timing of the data and GPS provides accurate timing signals.
Missile Defense
Even more than a satellite shoot-down, missile defense is obviously a network-centric enterprise. The system is often cued by sensors that look down on the territory from which the missile is fired. In theory, surface over-the-horizon radars could provide the same sort of information, but at a horrific cost and with limited precision. Moreover, a combination of different sensors may well be the best way to filter out decoys and avoid errors.
The satellite shoot-down was, in effect, a test of the combination of sensors the United States deploys. Because the satellite target was out of control and already tumbling at fairly low altitude, its precise path was slightly unpredictable. It really mattered that the SM-3 could home on it. Moreover, the missile only succeeds if it actually hits the target. Many other antisatellite weapons, probably including the one fired by the Chinese, produce fragments over a wide area likely to include the target. In this case the kinetic-kill SM-3 vehicle offered the important advantage of not creating further space debris.
The missiles on the cruiser were modified for the satellite shot, and it took a few days to return them to their usual anti-missile configuration. Presumably that meant a different seeker, using a somewhat different target signature. A missile warhead at an altitude of about 60 miles is already being heated by even the thin atmosphere through which it is passing. That heating makes it much easier to distinguish against the cold of space. The satellite, though far larger than a warhead, was less subject to atmospheric heating, so the key signature was probably reflected light.
Both the Chinese test last year and February's shoot-down dramatized for many the possible vulnerability of satellites, particularly those in low earth orbits such as reconnaissance satellites. This year's official report on Chinese military power, for example, mentions the apparent determination of the Chinese to deny satellite reconnaissance over their country in a time of tension. Since each shoot-down involves a massive missile system (at least for the Chinese), it may be argued that the most effective counter would be to multiply the number of U.S. satellites, and to build a capability to replace them quickly.
This year's defense appropriations bill includes money for satellite defense. That probably means arming a new generation of large reconnaissance satellites with weapons capable of neutralizing approaching anti-satellite weapons. It is not clear just how such weapons would be recognized. Much would depend on how the anti-satellite systems worked. They could be orbited, then maneuvered to intercept, or they could be fired directly from the earth's surface (as in the Lake Erie shot). If orbited, they could be destroyed just like other satellites, assuming that the U.S. system acted promptly enough, directly from the surface.
It is more difficult to imagine a counter to a direct surface shot. It may be that the best defense is simply to multiply the number of satellites by making them individually cheaper and more easily replaceable. It may also be possible to develop countermeasures, ranging from stealth (already reported for satellites) to deception of enemy satellite-tracking radars. A satellite with sufficient maneuvering fuel could also be commanded to maneuver itself out of trouble, if the trouble were recognized early enough.
Ultimately, satellites are vulnerable because they have predictable orbits. Antisatellite measures may thus favor long-endurance UAVs.