Radar screen and a flight path

Range Circle and Bearing

A radar sits at one point. It cannot see coordinates; it can only read how long an echo took to come back (a distance) and where the antenna was pointing when it did (an angle). Everything it knows about the sky is that pair of numbers. Now an aircraft crosses on a straight route. Whether it is ever seen depends on neither its current position, nor its speed, nor its departure time — only on a single number fixed the moment the route was drawn.