How satellites will influence the ATC (air traffic control) system of the future is examined from the control system perspective. It is maintained that, in a future environment, where significantly improved safety margins will be required, ADS (automatic dependent surveillance), by itself, will not permit substantial reductions in lateral separation. Likewise, satellite-based navigation systems will not, by themselves, lead to a greatly improved ATC system because if large errors persist at their current rate, improvements in nominal accuracy will not overcome their effect on the overall rate of collisions. It is concluded that only when satellites are integrated into a control system that uses their ability to furnish highly accurate positional data and frequent surveillance updates will they permit anticipated traffic increases while reducing the risk of collision enough to allow separation reductions. >