Flying in class-D today (with permission of course) the control tower at one point said they (thier radar I assume) showed me 3/4th of a mile inside the perimeter while iFly had shown me to have just exited their airspace.
(It was an amiable communication, but a bit awkward and embarrassing to be reporting I`d just exited and be told I was still 3/4th mile inside. And I suppose under other circumstances could have been a serious miscommunication issue.)
On at least one other occassion IIRR control tower and my iFly 720 had dissagreement of about this much distance, and IIRR one time the control tower had me at 3500 feet when iFly had me at about 3800.
Now as far as I`ve been able to tell I`ve never seen iFly horizontally off by more than, say, 50 feet, and usually less.
For example, I turned it on in my driveway (iFly, not iFly Streets) and on screen it showed me right were expected.
And when parked on a ramp at a known airport altitude iFly has have been off by more than 50 feet in altitude.
So althought I`m surprised to tentitively conclude this: It appears to me ATC`s radar is nowhere near as accurate as the GPS even only 5 miles from the tower. Makes me understand the motive to go to ADS-B.
FWIW: Note my plane has no transponder, and is a ragwing.... both of which make it a poor radar target. Don`t know if that affects accuracy of location.
Also, as far as I can know the radar that the tower uses is on a mountain top at 7000 feet about 15 miles from the actual airspace.
So my two question here are:
Has anyone else had occassion to notice ATC has dissagreed with where your GPS showed you were?
Anyone have any info on how accurate ATC radar is in reading distances?
Anyone have any reason to belive a normally operating iFly 720 could be off horizontally by 3/4th of a mile?
Alex
NOTE ADDED June 11, 2014 6:04 PDT:
I now believe that 99% of this disparity can be explained as follows:
Apparently the iFly Sectional diagram shows the radius of the MFR airspace as 5 statute miles.
As far as I know both the iFly and the sectional (from which iFly gets the data) are officially correct in doing that.
I just called the MFR tower and spoke to a controller there and he said they use 5 NAUTICAL MILES (which is 5.75 statute miles) as their radius.
That would perfectly explain why they are seeing me as about 3/4th of a mile inside their airspace when my moving map ifly is showing I am just at their perimeter.
If this resolves it (and I think it does) the remaining questions are
1) should they be using 5nm radius (I am not sure I want to go down that path of discussion with them).
2) are other control towers doing this?
Thanks to Jim`s and Walter`s comments that led me realize that possible explaination and thus to pick up the phone and ask them if they used 5nm for their radius.
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NOTE EDITED IN HERE ON June 26, 2014 5:54pm PDT
To see how this finally resolved see complete post further down the thread by me, computerdoc, a few minutes ago.