Separation (aeronautics) uses radar
Radar separation
Radar separation is applied by a controller observing that the radar returns from the two aircraft are a certain minimum horizontal distance away from each other, as observed on a suitably calibrated radar system. The actual distance used varies: 5NM (9.26km) is common in en route airspace, 3 NM is common in terminal airspace at lower levels. On occasion, 10 NM may be used, especially at long range or in regions of less reliable radar coverage.
As a practical example of the rules, US FAA rules are published in the FAA Order 7110.65; the rules are complicated, and they allow different separation distances (from 3NM to 10NM) depending on the aircraft, distance from radar antenna, type of radar used, flight level, etc.