"Matt Barrow" wrote in message
...

"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.



Interesting thread!

But, for the life of me, I can't figure any reason that atc would really
care about the exact map location of an aircraft--especially when it is both
close enough and high enough for the error to be significant.

Given the two most common uses, surveilance approaches to an airport at
nearly the same elevation as the radar and collision avoidance, the map
distance should be a trivial issue.

Peter