FlightAware Discussions

Question about ADS-B accuracy

I was watching local traffic this morning and saw this track. Pretty sure it was a successful landing at the airport, but ADS-B would indicate it was off-airport.

I was looking into how ADS-B works and it seems the location is GPS-based. I’ve never seen GPS be this far off. I normally see aircraft right on the runway centerline but the track looks bad and obviously the plane is nowhere near centeline. There was also no rollout. The plane disappeared shortly after the screenshot though the age data value was always less than 1 second.

What could cause this kind of track? Defective ADS-B?

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It looks to be an aircraft being tracked by multilateration - it’s position on your map is being calculated from the timing of received signals at different receivers near to you, not being transmitted by the plane. This is less accurate than when an aircraft broadcasts a gps position directly. How accurate the mlat results you receive are depends on how many receivers are being used to calculate the position, and their positions in relation to the aircraft.

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Thanks @caius! That’s a great explanation! I’ll look for that in the future.

That relatively straight segment before the last turn was probably landing and rollout on 20 then, and the last turn was onto a taxiway.

This may not be the case here, however I have seen planes land at Heathrow before where their track doesn’t line up with the runway. At the time, the model and age of the aircraft suggested that they were using an Inertial Navigtion System (rather than GPS) which has a tendency to drift slightly the longer you fly. That could possibly be an explanation too.

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Thanks, @Coooogz! It all helps understand what I’m seeing. The best information is that there can be good reasons for the offset and noise in the track. I hadn’t seen that before.

Only highly accurate GPS is allowed to be tied into ADS-B.
https://www.ecfr.gov/cgi-bin/text-idx?node=14:2.0.1.3.10#se14.2.91_1227

© ADS-B Out Performance Requirements for NAC P, NACV, NIC, SDA, and SIL—

(1) For aircraft broadcasting ADS-B Out as required under §91.225 (a) and (b)—
(i) The aircraft’s NACP must be less than 0.05 nautical miles;
(ii) The aircraft’s NACV must be less than 10 meters per second;
(iii) The aircraft’s NIC must be less than 0.2 nautical miles;
(iv) The aircraft’s SDA must be 2; and
(v) The aircraft’s SIL must be 3.
(2) Changes in NACP, NACV, SDA, and SIL must be broadcast within 10 seconds.
(3) Changes in NIC must be broadcast within 12 seconds.

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Thanks for the update, I am no expert :slight_smile:

As this aircraft was in the USA and those are USA regulations then that makes sense it is not the case here.

However could still be an issue in other parts of the world.

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That’s not how that works. ADS-B includes information about the precision of the position information being provided in the ADS-B messages themselves. There are compliance requirements around the minimum level of accuracy required in normal operation, but you can certainly transmit degraded position information (be it from INS, or GPS with a poor sky view, or whatever) so long as it’s identified as being degraded. There are also plenty of older ADS-B transponders around that predate the accuracy requirements.

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I understand that might be exceptional, emergency, cases when in flight the accuracy degrades.
But IMO for a compliant installation the FAA requirement says “must”.
Surely the inertial doesn’t get close to those requirements.

Yes, if you’re emitting nothing but NACp 0 or whatever then the FAA’s not going to consider that compliant with the 2020 mandate.

But that does not mean that “only highly accurate GPS is allowed to be tied into ADS-B”, for many reasons. Gracefully degrading is a good plan if you can do it. There are lots of older systems out there which may not be DO-260B compliant, but they’re still emitting ADS-B. The world is larger than just the USA.

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I don’t have time to research but I was convinced that Australian or German or UK requirements for ADS-B are similar if not identical.

Would have helped if the identity of the flight was not deleted in the screenshot. That might have helped with an informed response. Older Boeing 737 and Boeing 757 often arrive “off airport”. I think the INS explanation is valid for those.

So what was the mystery flight?

ADS-B isn’t compulsory in the EU just yet. It’s due to come into force on 7th June this year.

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@idh - sorry about that. I wanted to be sensitive to privacy for the owner so that’s why I colored out the identifying information.

The aircraft is a experimental low cost single engine piston homebuilt monoplane. I’d expect the minimum of radio equipment in it. Not trying to be cagey, but I don’t want to say more as it could identify them. Definitely not an old airliner. :grin:

I fly an airplane with both ADS-B and an IRS. Transport Category jet. You’re right that for a certified installation that a GPS is required primarily. But GPSs can and do fail. Probably more often than the travelling public know. But when that happens many systems will still be providing a position to the ADS-B system. If I want I can even turn off the GPS updating function in the FMC and coast on either just the IRS or switch over to DME/DME/IRS mode where position is computed by the FMC using range from multiple DME stations in your proximity. That mode is even accurate enough that I can fly designated RNAV routes without a GPS at all. In the large jet world, the FMC (Flight Management Computer) derives location from many sensors, of which GPS is just one. And using these multiple sensors computes a location and certainty of your location. In the modern world your location isn’t just your location, you have to be able to prove to a specific probability that you are where you think you are. Look up Performance Based Navigation on wikipedia if you want to learn more about modern navigation.

TL;DR ADS-B will still output data in many airplanes even with a GPS failed if they have other means of navigation or a multi-sensor FMC.

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Could be MLAT.
That could happen if the plane is sending ADS-B via UAT on 978 MHz but still has a normal ModeS transponder transmitting on 1090 MHz.

Your receiver likely is for 1090 MHz and with a couple receivers in the area FA could use MLAT to determine the location.

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I meant that the technical specs for ADS-B in those countries are identical with the US ones, not the legal applicability of ADS-B. Bad phrasing… but in my mind we were discussing about the precision requirements (technical), not the legal ones.

What am I missing here?
Looking at the OP, I see no reason the plane should be on the center-line (of a runway?).
The plane never drops below 6000 ft, so why would its path need to lineup with anything?

The screenshot shows the plane at 3.9 nm, so if it’s a direct received signal, it probably overloaded the receiver.

Field elevation Is 6348.7 ft. so 6125 is actually below field elevation. :grin:

It’s hilly around here but around the airport it’s a little flatter. There may be a 200’ elevation change around there but probably not much more than that and if he was that low, he’d be weed cutting. :grin:

The plane slowed down before stopping and traveled a bit well below stall speed. It also didn’t pop back up which a flying plane would. I can watch aircraft taxi at the airport, fly patterns, and such. There are signal strength variations and some planes that fly low directly overhead swamp the receiver, but I’m pretty sure this wasn’t that. I’ve spent a fair amount of time tuning receiver gain to make sure I see ops at the airport. I continue to be amazed that I can probably tell what hanger planes push out of.

I think the multilateration call is likely the correct one and makes sense for the circumstances.

Cheers!

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It seems that the aircraft is airborne and ground speed is 62 kt when last received.

If that was a landing and taxi it would be most of the way to the highway.

S