Radar used for tracking commercial aircraft is line-of-sight only. Even for aircraft several miles up, radar can track only for a few hundred kilometers. This means that most air traffic over the Atlantic or Pacific is out of radar range for most of their flight. Procedural means, such as radio reports of position, speed, direction etc is about all there is to keep track of all the trans-ocean traffic. This is one of the reason the Global Positioning Satellite System was such a big aid to long distance flight as the pilot can determine his location very precisely (since the man on the ground can't).

The military augments their air traffic tracking with airborne planes carrying surveillance radar but that's obviously too expensive and impractical for everyday commercial use. Maybe someday there will be satellite radar to track everything but that's a long ways off.