I was looking through my virtual radar logs one of the days and found this "glitchy" ADS-B behavior. I am almost 100% sure that this is not due to my antenna or setup since two independent different radars confirmed this weird behavior from FlightRadar24. Also A/C before and after this one did not exhibit this behavior.

  1. Does anybody have any thoughts as to what may be happening???
  2. Why is the "skew" at seemingly same angle? Is that anything?
  3. In light of MH370, does this happen often, how reliable is that GPS data?

Tail # N657UA Boeing 767-300 Typical route between EGLL and KORD

Time of occurrence is approximately: 3/16/2014 6:09pm CST

I have also verified FlightAware is ALSO showing the same weird glitch.

See below "yellow" highlighted airplane: enter image description here

Same A/C from FlightRadar24: enter image description here


This seems to be related to THIS aircraft. The explanations given (GPS->INS->GPS switching) still applies in my opinion, but wanted to give another screen shot. Here it is today (3/30/2014) and again displaying this behavior - should their maintenance department be alerted to adjust their GPS antenna?? enter image description here

  • $\begingroup$ Updated with more details. Also verified same glitchy behavior on flightaware. $\endgroup$ – KORD4me Mar 18 '14 at 3:51
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    $\begingroup$ related to near dupe: aviation.stackexchange.com/q/2269/609 $\endgroup$ – ratchet freak Mar 18 '14 at 8:54
  • $\begingroup$ @ratchet freak, while the INS argument is valid I would argue that there's not a constant "drift" to any one direction, which I would expect from miscalibrated INS. The aircraft was on pretty much perfect approach otherwise. Chances of me, flightaware and flightradar24 independently getting that behavior rules out any decoding errors of the signal as well. $\endgroup$ – KORD4me Mar 18 '14 at 12:49
  • $\begingroup$ @KORD4me: The thing is that most positions are from GPS , but some are from INS. The INS ones are offset from the GPS ones, because INS has drifted (because in this particular plane it is not synchronized with GPS in flight; in some it is and in some it is not). $\endgroup$ – Jan Hudec Mar 19 '14 at 5:38

This is a classic example of fallback to INS. Most of the track data is coming from GPS. For some reason the GPS signal is lost momentarily causing a fall back to INS. The INS is offset by about 1km to the Northeast, which appears as a jump. Next position report is from the GPS again, in line with the original track. This happens several times.

In the transmitted data these spikes will be flagged as data of low integrity. This allows for filtering them out.

  • $\begingroup$ I could see that. So it's not really that INS is used the whole time, it's that it intermittently switches to INS. I was always under impression that they had redundant GPS units, but maybe both were failing at the same time? $\endgroup$ – KORD4me Mar 18 '14 at 20:09
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    $\begingroup$ They do have two GPS units but each is connected to a different transponder. Transponders are not switched for a momentary glitch in the GPS. $\endgroup$ – DeltaLima Mar 18 '14 at 20:17
  • $\begingroup$ can you comment on how often does this happen if you have a swag and how may ATC deal with these interruptions? Or do they automatically switch to primary radar? $\endgroup$ – KORD4me Mar 18 '14 at 20:48
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    $\begingroup$ The more important question is why isn't the INS continuously realigned from the GPS. In fact according to some document linked from another question the combined RNP units (especially the RNP0.1 ones) can do that, using each system to filter errors of the other. $\endgroup$ – Jan Hudec Mar 18 '14 at 21:10
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    $\begingroup$ @KORD4me 4 to 5 seconds is the typical rotation period of a TMA (TRACON) radar. And that one is allowed to miss a target every now and than as well, so up to 15 seconds occasionally is acceptable. The ADS-B integrity is in the FTC field, first five bits of the 5th octet IIRC. If FTC is 18, there is no position data integrity. I don't know ADSB#, I use home brewed tools. $\endgroup$ – DeltaLima Mar 19 '14 at 11:46

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