"Matt Barrow" wrote:
"Jackie" wrote in message
...
Matt Barrow wrote:
"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.


And how does it separate A and B?

"A" is the radar range, "B" is garnered from the encoding altimiter.

Nit: Actually "C" would be radar range and either "A" or "B" would be from
the altimeter, solving then for "B" or "A".