The proliferation of ballistic and cruise missiles to Third World countries is becoming a major concern for both the United States and the Russian Federation. These classes of missiles can carry weapons of mass destruction and are difficult to intercept. To date, this concern has been somewhat mitigated because these missiles are relatively inaccurate. But the potential of improving the accuracy of these missiles by using the Global Positioning System (GPS) or the Global Navigation Satellite System (GLONASS) makes them a greater threat. This paper evaluates the error sources that affect missile accuracy and assesses the improvement that could occur by using satellite navigation-aiding of the missile's inertial guidance system. The authors' analysis focuses on the U.S. GPS system; however, the findings would be similar if GLONASS were used. The paper concludes that satellite navigation-aiding can improve the accuracy of current short- and medium-range ballistic missiles by approximately 20-25 percent, and up to 70 percent for advanced ballistic missiles. It can also greatly improve the accuracy of cruise missiles with ranges greater than 50 km. In addition, the U.S. policy of Selective Availability has a marginal effect on controlling missile accuracy in most of the cases the authors examined.