The Federal Aviation Administration is modernizing the Air Traffic Control system to improve flight efficiency, to increase airspace capacity, to reduce flight delays, and to control operating costs as the demand for air travel continues to grow. Promising new surveillance technologies such as Automatic Dependent Surveillance Broadcast and multisensor track fusion offer the potential to augment the ground-based surveillance and controller-display systems by providing more timely and complete information about aircraft. The resulting improvement in surveillance accuracy may potentially allow the expanded use of the minimum safe-separation distance between aircraft. However, these new technologies cannot be introduced with today's radar-separation standards, because they assume surveillance will be provided only through radar technology. In this article, we review the background of aircraft surveillance and the establishment of radar separation standards. The required surveillance accuracy to safely support aircraft separation with National Airspace System technologies is then derived from currently widely used surveillance systems. We end with flight test validation of the derived results, which can be used to evaluate new technologies.