The Federal Aviation Administration (FAA) is considering whether to adopt space-based Automatic Dependent Surveillance-Broadcast (ADS-B) to supplement the existing surveillance capability provided by terrestrial radar and ADS-B networks. Radar and ADS-B provide a realtime, electronic “picture” of aircraft positions, allowing air navigation service providers (ANSPs) to separate airplanes by only 3-5 nautical miles (NM). However, because surveillance technology requires a direct line of sight to the aircraft, radar and ADS-B coverage is limited to the airspace over land. For the airspace over oceans and remote land areas that cover 70 percent of the earth, ANSPs must rely on periodic position reports from the aircraft, and the infrequency of those reports requires that aircraft be separated by 30-120 NM. With space-based ADS-B, low-earthorbit satellites equipped with ADS-B receivers will take the place of ground-based infrastructure, making it possible to track airplanes in real time anywhere above the earth’s surface.