Is radar location corrected for reported altitude?

Discussion in 'Change to my Frequency...' started by arnoha, May 7, 2017.

  1. arnoha

    arnoha Line Up and Wait

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    So, curious about ATC radar equipment: When there is a Mode C report, does the screen adjust the blip position to reflect elevation error? Alternately, is there an average correction factored in or none at all?

    I can definitely see this mattering, especially close to the radar. If you're 9,500' inside a surface Bravo, you're two miles up and probably less than five miles from the radar. That gives you about a half mile of position error.
     
  2. bflynn

    bflynn En-Route

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    Elevation error? Mode C transponders in Alt mode send your pressure altitude-your equipment. The only error might come if your unit is out of calibration or if there is an unmeasured pressure difference where you are vs what they think the pressure is. It could happen with a weird meteorological situation, but that would be really uncommon.

    Something else that I am not thinking of at 5am?
     
  3. GeorgeC

    GeorgeC Pattern Altitude PoA Supporter

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    I think he's asking if the reported altitude is used with slant range to display the correct ground range on the scope.
     
  4. Velocity173

    Velocity173 Final Approach PoA Supporter

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    ATC radar doesn't use a height calculator like military tactical radar.

    The primary radar provides only lateral position (azimuth & range) while altitude is completely by secondary radar's mode C component. That is a digitized pressure altitude that is automatically converted on the scope through a baro altimeter feed, or manually through controller input.

    At extremely close to the radar site or "main bang" area, the target will drop completely or go into "coast" but that's only temporary.

    Edit: oh, position errors due to altitude and distance from the site. No, no compensation on a single source display that I know of. FUSION radar fixes that error though.
     
    Last edited: May 7, 2017
  5. luvflyin

    luvflyin En-Route PoA Supporter

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    It doesn't adjust. It is slant range just like primary radar "echos." It doesn't "move the blip" by doing the math. There may be some newer radar systems out there that do, but I haven't heard of any.
     
    Last edited: May 7, 2017
  6. MAKG1

    MAKG1 Touchdown! Greaser!

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    I see few hundred foot altimeter setting changes routinely in the summer. Not rare. Crossing a mountain range into a coastal microclimate will do it. There are several examples under Class B shelves.
     
  7. luvflyin

    luvflyin En-Route PoA Supporter

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    The question is not about Mode C error. It's about where the target is located on the Radar scope in relation to where the airplane is. Radar measures how far the airplane is from the radar antenna. If you have 2 airplanes directly over and under each other, over the exact same geographical point, the higher one will appear on the scope as farther from the antenna. Just like DME, radar is slant range. The difference is exagerated when the airplanes are close in. This happens with both primary(echo, skin paint) and secondary(transponders) radar. The OP's question is does the radar use the Mode C altitude, do a little trigonometry, and "move" the target on the scope to put it exactly where it is in relation to the ground. It doesn't.
     
    Last edited: May 7, 2017
  8. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    You got that backwards LUV. The higher one is farther away (slant distance) and will appear farther away on the radar.
     
  9. luvflyin

    luvflyin En-Route PoA Supporter

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    Oops. Now I know why I flunked math. Or was that English. Being corrected
     
  10. arnoha

    arnoha Line Up and Wait

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    Guess you guys answered my question. No correction.

    Also, something I should have realized, but didn't, is that the areas where the correction would be large enough to matter (high, but close), the radar also has no visibility. It doesn't illuminate anything above it.

    With FUSION, I wonder if the internal coordinate system is still on some sort of azimuth/range system like a radar would be, or if it's converted to lat/long...
     
  11. luvflyin

    luvflyin En-Route PoA Supporter

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    Yes, there is a "cone of silence" for Radar just like VOR. FUSION Radar just might make some adjustments in location based on altitude. I don't know but it sure seems it would be possible. If that's true, then dealing with non mode C aircraft would complicate things. Position from the radar antenna would be the starting point and that would be in azimuth/range. That probably gets converted to something like Lat/long so the positions from all the different radars are talking the same language. The details are probably out there on the web. I think there are some currently working controllers here who may have that info