ATC Radar Question

"Jackie" wrote in message
...
Brian Whatcott wrote:
Jackie wrote:
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)?


I am late into the thread.
But you are presumably talking about an area surveillance radar.
Its fan beam does not typically stick 45 degrees up into the sky. Too
wasteful of energy.

That's interesting. So if what you say is correct, an airport
surveillance radar has very little coverage of the area, say at the top of
a class B airspace because to cover that high an angle is "wasteful." For
example, a VFR plane flying legally just above the B ceiling could very
well be out of coverage of the radar that is supposed to be also
monitoring another high performance aircraft poking through that ceiling
at a high rate of speed. I'm not sure I agree with such an energy saving
measure.

The antenna design of surveilance radars allows for a pretty wide coverage
area in the vertical plane due to the beam pattern. If you look at the
antenna, you'll see that it's a modified parabolic for this very reason.
http://upload.wikimedia.org/wikipedi...ar_Antenna.jpg
Notice how the antenna is more focused horizontally than vertically.
Imagine the spray pattern of a pressure washer with a fan tip and you get
the idea.

There is a "cone of silence" above all surveilance radars, but it is not
that big. Many airports locate their radar off the airport a mile or so for
this reason, and because there is also a receiver recovery time of
approximately 1/4 mile.