Passive and active. Passive relies on triangulation of, say, visual detection of reflected sunlight. When only have one probe sighting then you don't know the distance, speed or size of the target, only the direction. With two you can triangulate the position and track it to calculate speed and estimate size
Passive and active. Passive relies on triangulation of, say, visual detection of reflected sunlight. When only have one probe sighting then you don't know the distance, speed or size of the target, only the direction. With two you can triangulate the position and track it to calculate speed and estimate size