Under normal circumstances, ground-based air traffic controllers use radar and other equipment to track all aircraft in their area of reach and direct planes so they are at different altitudes and distances. This enforced separation — vertical and horizontal — prevents collisions. But the planes searching for Flight 370 are operating over a remote patch of ocean that is hundreds of kilometers (miles) from any air traffic controller.
Houston, a former Australian defense chief, called the search effort the most challenging one he has ever seen. The starting point for any search is the last known position of the vehicle or aircraft, he said.