Re: ATC Radar Question

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.

Another concept to ponder: if its beam WERE able to steer up at 45 degrees or more, what do you think its path would look like on a plan position indicator? (a regular display). You've mentioned its slant range is 4.25 miles at 3 mile distance horizontally. 30 seconds later, it might be overhead: where would it paint in terms of range?
Three miles?? A circular range ring at 3 miles, all round the display?

My distances where small to keep the math simple for discussion purposes, not to suggest what an actual display would include. Geez.
.

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    ... radar antenna, as seen on a map, how does the radar correct for slant distance when distance is displayed? ... You've mentioned its slant range is 4.25 miles at 3 mile distance horizontally. ...
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  • Re: ATC Radar Question
    ... radar antenna, as seen on a map, how does the radar correct for slant ... distance when distance is displayed (e.g. using concentric circle ... a marker on the display)? ... radar system can't extract the horizontal distance from the slant distance. ...
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  • Re: ATC Radar Question
    ... 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)? ... Without altitude information from the pilot or a Mode C transponder, the radar system can't extract the horizontal distance from the slant distance. ...
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