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HomeHomeDiscussionsDiscussionsiFly General Di...iFly General Di...ATC and iFly disparity in location?ATC and iFly disparity in location?
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6/4/2014 3:33 PM
 

The place to start with your questions is not here. It is with a phone call to the tower. Ask for the manager or a duty supervisor. Guaranteed you will learn a lot. One thing you may find out is that the tower does not have radar. Many do not. Others near bigger airports may have a D-Brite screen which is a sort of remote display of what the many-miles-away TRACON radar is seeing. The ATCT guy or gal will be happy to explain to you what they have, how a "primary" target (non-transponder) shows up, and to noodle with you about why you saw the discrepancy that you did. If the situation was at all recent, they might even pull the "tapes" and replay the radio action.

I probably call an ATC facility at least a couple of times a year with a question. I have never been blown off, cut off, or treated rudely. 100% of the time it has been both a pleasant and an informative phone call.

If you call in for information, even in connection with some minor sin, nobody is going to hassle you or write you up. In addition to being 99% nice people, controllers hate forms and paperwork as much as anyone. So don't worry; make the call. (If you have just forced an Airbus 380 to make a go-around, though , that might not be the best time to call!)

 
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6/5/2014 10:26 AM
 
Tinker wrote:

The place to start with your questions is not here. It is with a phone call to the tower. ......



Actually that was my first impulse. I was a bit nervous that an inquiry could be interpreted as me being adversarial, which I was not. Would not want to get on the wrong side of these controllers as I fly you in the near their space. Turns out as you say no reason to avoid contacting.

Call yesterday evening. One controller on duty but not busy.
Happy to chat.

In brief:

Not surprisingly, he said this disparities such as i reported are not uncommon. Many times someone using their GPS reports as 6 miles out when they see them at 7 or whatever. He agreed it's highly unlikely my GPS was off three quarters of a mile, and that he's never figured out the source of the disparity and would be very curious to understand it himself. Best. We could come up with in our blind leading the blind discussion was there could be a disagreement between the sectional and their radar about what constitutes the centerpoint of the airport/airspace.
He said he'd be real interested in having a pilot flying the pilots's GPS indicated line of the airspace boundary some time and he track it on the radar.

So we have an agreement that I'm going to go out there and fly there sometime when they're not too busy and do my best to exactly fly the GPS indicated as airspace boundary ... but for safety do so about 1500 feet above their airspace.
He he went so far as to give the other controllers my N-number and explain our proposed experiment, do there would be no confusion or static when I was doing this.

Should be very interesting. Will report the results here. My best guess is we may find two circles that are slightly offset from each other, perhaps as much as a mile.

I appreciate all the caveats everyone else here posted (along the lines of "never trust the position and altitude anyone,including ATC , reports as precisely accurate, let alone precisely sink with Sarah GPS altimeter readings and cut sufficient slack for potential large disparities")........ seem worth remembering.

Alex

 
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6/5/2014 10:42 AM
 

<blockquote> <p>&lt;p&gt;&amp;lt;div class="Quote"&amp;gt;&amp;lt;em&amp;gt;ComputerDoc wrote:&amp;lt;/em&amp;gt;Call yesterday evening. One controller on duty but not busy. Happy to chat.&amp;lt;br /&amp;gt; &amp;amp;#160;&amp;lt;/div&amp;gt;</p> </blockquote>

Good job. You found exactly what I have found repeatedly. They are good guys, happy to help, and they don't bite. Be sure to report back here after you've done your test run.

 
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6/6/2014 10:04 AM
 
Walter Boyd wrote IN PART

Interesting thread.

......... Since transpoders all report pressure altitude, this error really doesn't matter...we're all working off the same error.

  • GPSes are very good at triangulating horizontally. The satellites you're tracking typically have a large variety of different horizontal distances from your current location, so it's pretty easy to measure and conclude where you are horizontally. But the relative vertical distance between you and the satellites is tiny in comparison. So unless you have a couple of satellites directly overhead, there is very little information to calculate your position vertically (altitude).
  • I typically assume my GPS altitude will be different than my pressure altitude by 10% AGL. This variation also factors into our traffic alerting system. ADSB traffic will be sometimes be reported as pressure altitude, and sometimes GPS altitude. And we won't know which.

    Cheers,
    Walter



    GPS as you say isn't as good at altitude as horizontal..... But when comparing what iFly reports to known surveyed altitudes I am at (on runway, etc) I have never seen it off by more than 50 feet. By contrast even when recently having been reset to nearest officially reported pressure I've seen my altimeter read , IIRR, as much as 150 feet off a known reference altitude .

    As for "Since transpoders all report pressure altitude, this error really doesn't matter...we're all working off the same error.":
    Unfortunately you can't count on that Walter. Remember, a few of us are flying without transponders. I could be reporting my altitude off my GPS , the tower could be telling you they see me at 4500 however their radar reads altitude (since they get no transponder signal from me), or o could be reading it of my altimeter which, unless by coincidence is set it standard pressure, is as you know giving a different number.

    Disappointing, although useful, to hear that we won't know if ADSB traffic altitude is reported as GPS or pressure altitude. Guess the prudent practice will remain ... As one guy here said... If within 500 feet assume traffic is AT my altitude.

    Alex
     
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    6/6/2014 11:34 AM
     
    Tom Murrell wrote IN PART:

    ........Alex I'm just curious but how do you get by without a transponder in controlled airspace? I thought is was required.

    Tom Murrell



    Hi Tom,
    One other guy here asked me the same question earlier in the thread.
    As I said to him ... Guess you've forgotten some of the details and fine print we all had to study for our FAA knowledge tests. ;-)

    No transponder required in class-D airspace.
    Nice to have, but perfectly legal and not that unusual to happen.
    As a practical matter I do fine in class-D without transponder.... at least the few I've been in.
    Did all my early training in class D MFR with no transponder.
    Traffic generally relatively light. I talk and listen on radio, and tower has me on radar.

    Can even fly into class-C without transponder if you ask for and get prior clearance.
    I believe aircraft with no electrical system may even be legal in class-C with no transponder, but don't quote on that.)
    Can fly under the class-C without transponder.
    (Not a great idea, though, IMO to get anywhere near class C or higher withour transponder).
    Someone else here reminded me that you must have a transponder anywhere within 30 miles of class-B.

    Alex
     
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