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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. |
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