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Radar accuracy

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  • Radar accuracy

    We occasionally hear/read someone complaining that they were told off, either on air or afterwards, for flying just the wrong side of an airspace boundary, yet the alleged offender insists that their GPS showed them clear of the airspace. There's a thread running elsewhere on this topic at the moment.

    Question: does anybody on here understand the technicalities enough (NATS Jonathan, you there?) to explain how accurate an ATC primary radar is at the boundaries of CAS? Does it, for instance, show the slant range from the radar head or can it resolve height to give a true horizontal position?

    No, it wasn't me... just curious

  • #2
    Radar accuracy


    I always fly with two GPSs, an Aware unit and a Garmin 296, and a transponder operating with Mode C. Occasionally I get a twitchy Farnborough North ATC guy ask me to change course away from the Stansted TMZ/CTR boundary, when I know I'm in the right place, both from visual info and from the GPS units. And of course with Mode C there cannot be a slant range issue, if the software is doing its sums properly.

    To answer your question about primary radar, without any information on height only slant range can be derived. When I last played with primary radar plot extractors and primary radar tracking, the radar resolution was about 16th to 32nd nautical mile, but this is resolution in slant range not ground truth. With the introduction of multihead tracking, with positional information coming from several radar heads, the slant range problem is a bit less of an issue provided you can get detections at long range and low angles. I suspect this doesn't apply to skin and bone microlights though when detections are flaky to say the least. So in a primary only scenario your on board GPS is likely to be much more geographically correct than your primary radar picture. Bear in mind though for a radar based within the circle of interest, for example at or very close to the airport in question, the derived slant range is such that it is likely to place you further away from the radar head on the radar display than is actually the case.



    • #3
      Radar accuracy

      One reason to explain a touchy controller, even if you (we/I) are on the correct side of the line:

      The controller is likely to have the predictable track feature selected.
      Here the aircraft's current track is extended ahead of the aircraft by a period of time... 1 minute, 5 minutes 10 minutes......
      So every time you (we/I) drift away from the line we are following (and away from controlled airspace), and then turn back towards that controlled airspace (totally aware that in a few seconds, the intention is to turn to be on that line again), the controller is being presented with a display indicating the aircraft's current extended track is going far into his controlled airspace.



      • #4
        Radar accuracy

        With the big proviso that anyone that close to the boundary is in the wrong place for all sorts of reasons if you are looking at accurate positioning it gets very interesting.
        As I understand it we are looking at radar, visual and GPS for confirmation of position and while not being very good at knowing where I am at the best of times or being an expert in this it would be nice to be educated so this is how I see it.
        Visual is the primary means of position which is represented on the map- you lean out the plane and look down with maybe a bit of practice to recognise the bits on the ground and how they are represented on the map. The map will have errors but as all are using it as the reference its no problem. Am I correct so far??
        I know nothing about radar and am to low or light to figure much in this and want to keep it that way.
        I presume the map on the GPS is a copy of the paper map so there should be no errors there except the original map errors?
        If I remember and its a long time ago the boundary is generated from and on the radar/gps as an electronic circle from a position which is the center of the longest runway or coordinates? Now completely ignoring the fact that I have never seen a radar head in the middle of the runway I presume that unless you visit the source and confirm that position by GPS positioning the rest can be out by map error.
        The GPS knows where it is but you are using a bit of paper as the referee.
        I am old enough to remember time pieces which were mechanical and if you could keep within 2 minutes a week it was a good one, nobody was concerned about the seconds and time has moved on with seconds now being considered as maps are now a problem in so much as Google earth is more representative because its electronic from actual photos though I would be interested in knowing the errors?
        Mick Broom
        Member 909
        Shadow G-MWTN


        • #5
          Radar accuracy

          As I think have mentioned before elsewhere, my "wee strip" is (well may.. or may not be) right under the edge of our local Airport's CTA.
          In fact using Ordnance Survey map, measuring from the Airport's official datum, the centre of my runways is just within the , but measuring from a local known O/s "spot height". it is just "on the line" (which in itself covers a significant width) and using the average of 3 different GPS units it is just about outside

          Using Memory Map, Garmin and Sky Demon gave different results as to the "edge of the CTA" by as much as the length of the strip (OK only 203M),

          Some time ago, I carried out an "agreed unofficial" series of checks with our ATC (during a quiet holiday period) at various points right on what we had agreed was the edge of their CTA and also over the mentioned centerpoint of my strips, in order to establish "just where they thought I was" at any time using only primary returns.
          Their info was mostly different to mine.... occasionally I would be shown as inside... sometimes outside etc.and different controllers gave different results from the same place. often being different to my "Mark 1/2 eyeball" (I am getting old after all!):-(
          (please bear in mind we were trying to get an idea of "actual" accuracy etc)

          Interestingly, some controllers were not 100% certain just where "their boundaries" lay............................. on the line... the inside or outside edge etc, bearing in mind that the delineating line itself on an airchart covers a defined area of ground

          So, as in everything in this life, please remember all measurements etc DO have tolerances, NOTHING can ever, ever be 100% accurate.

          Please always remember also that sometimes multiple tolerances, add.. sometimes they subtract.

          There are "always" multiple errors involved in positioning, be they mapping radar head surveying.. gps and display accuracy... eyesight ... both yours and the controllers.......etc etc....

          As Mick says...why be there in the first place and risk potential problems?


          • #6
            Radar accuracy

            If you were carrying out trials with primary only radar then the odds are that if they showed you were just on the dividing line, you were actually inside their airspace, because of the range stretching that goes hand in hand with uncorrected slant range. It would be interesting to invite a mate in with a secondary radar transponder with mode C and do the trials again. As the secondary radar displays should show real geographically correct position based on height corrected slant range ( provided their superimposed geographic map is accurate) you will know whether you are inside or outside. However there is a simpler way, just using a good map, agreed airfield centre point, and the published geographic controlled airspace extent. Alternatively, to give you some leeway for circuits, go-arounds etc, approach the airfield in question to set up your local agreement.



            • #7
              Radar accuracy

              I think you actually reinforcing my point that if I was showing as "on the line", I may physically have been just in... or out of their space due to multiple tolerances... their radar... the air chart.. my eyes etc.

              The point I was trying to make is that EVERY measurement, by the very nature of measurement itself, has some inbuilt accuracy tolerances and so why "tempt fate by not keeping well away from the fine line"