"Jackie" wrote in message
...
I understand that ATC radar uses an encoding altimeter and a transponder
operating with Mode C to determine an aircraft's altitude. Let's forget about
Mode C for a moment and switch to Mode A only or just a primary return.

If an aircraft is at 18,000 ft (approx 3 nm) and 3 nm away from the radar
antenna, as seen on a map, how does the radar correct for slant distance when
distance is displayed (e.g. using concentric circle distance markers on the
scope or relative to a known distance, such as a marker on the display)?

In other words how does the radar know that the aircraft is actually 3 nm away
laterally and not 4.25 nm (approx slant distance at that altitude)?

Asquared + B squared = C squared, I'd guess.