"In 1991, Congress passed the Missile Defense Act of 1991. The act calls for the deployment of defenses, as soon as possible, to protect the United States from attacks by a limited number of long range ballistic missiles, and to protect U.S. troops and allies from attacks by missiles of less than intercontinental range. The act does not, however, specify the size and nature of the defenses that should be deployed. Nor does it specify when a full system of defenses should be in place. The Administration has modified its plans for strategic defenses to respond to the act. But critics have other ideas about what types of defenses are appropriate, when they should be deployed, and how much should be spent on the Strategic Defense Initiative [SDI]. This Congressional Budget Office (CBO) paper analyzes the costs of the Administration's plan for strategic defenses and the costs of alternative approaches. All of the alternatives would provide for eventual deployment of defenses but, at least through 1997, would cost less than the Administration's approach."
Defense Technical Information Center (DTIC): http://www.dtic.mil/dtic/