"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)?

It's pretty simple. The radiated energy from the transmitted pulse takes
12.36 microseconds to travel one mile and back. The radar uses that timing
to determine range. No corrections for altitude are made. The system that
receives transponder signals (which is an independent system which is
correlated later) works exactly the same way.