FR24 Question

[philbky]

I'm currently in Cheadle Hulme a couple of miles from the 23 threshold at MAN. I'm using FR24 on an android tablet. All FlyBe flights and many biz jets appear off track on approach and are behind in time. Everything else on approach appears where it should be, when it should be. I am also seeing biz jets at altitude, many of which are shown as unknown Citations which show long and meandering tracks and then disappear.

At home in Kerry I haven't noticed either the off track/behind time returns or any spurious biz jets. Any ideas?

[philbky]

Could the mod who moved this please explain why. It is evident from the question that this is likely to be best answered by people with computer knowledge rather than spotters.

[DaveReidUK]

The aircraft you are seeing apparently behaving erratically are ones that don't send their position via ADS-B, and so FR24 uses a technique called multilateration (broadly analogous to triangulation) to calculate their locations.

Multilateration (MLat) requires extremely accurate time synchronisation between each of the receivers involved, otherwise positional calculations are prone to significant errors.

I would expect that there are many more FR24 MLat receivers in the Manchester area than in Kerry, so I guess it's possible that synchronising them all presents more of a challenge.

That's my theory, anyway. :O

[HEATHROW DIRECTOR]

FR24 does produce interesting viewing and I have often watched Heathrow traffic "landing" in Richmond Park or Windsor Great Park! Traffic landing at Farnborough apparently taxies off the end of the runway for some distance. Aircraft showing to pass directly over my house are not sen nor heard.

A few nights ago I watched the regular AN12 crossing the south coast heading for East Midlands (or somewhere that way). Roughly west abeam Heathrow it made a sharp left turn and headed for the Gloucester area where it made a sharp turn back to a position a few miles north of where it had turned west... then turned north and went merrily on its way!

Don't believe all you see.

[philbky]

David, as far as I am aware FlyBe Embraers do use ADS-B yet are ahead of their plots on FR24. I'm aware of multilateration and its limitations having been a base station for Planeplotter since the very early days of Kinetic receivers and the off track antics of non ADS-B equipped aircraft are well known so the theory regarding the number of contributing receivers is well made. I have used the tablet on FR24 in many parts of the UK, Hong Kong, Australia, New Zealand and the USA during the last 12 months but the deviations here are far greater than I've seen elsewhere.

The really interesting point is that the delay on approach for non ADS-B aircraft is totally consistent and seemingly not dependent on the aircraft speed or, presumably, the number of receivers contributing. The "offending" aircraft are always showing over the railway viaduct when in fact they are passing my location probably just under a mile further on.

The wandering Citations are odd. They always show up as unidentified Citation Excels and will either follow fairly standard routings for tens or even over a hundred miles, in seemingly level, but low altitudes and then disappear or they will just wander all over the place at varying altitudes before vanishing.

I put this query in the computer section in case anyone with an android tablet had come across these oddities. The type is an Acer A210, the service provider is BT and the download speed is a reasonably constant 20mbps.

[Morrihell]

FlyBe Embraers most definitely do not use ADS-B...

The position shown seems to be "where it was" when it was last plotted, as you say, they're over your head when "passing the viaduct" there's a delay involved, they're not ADS-B so it's not (reasonably) recent position information.

Keep using your viaduct as the overhead for the non ADS-B and you'll likely see them then!

I think it's an FR24 thing rather than a specific computer/laptop/tablet thing, I get varying results with this computer compared to an ACER chromebook. You get used to it after a while!

[DaveReidUK]

I put this query in the computer section in case anyone with an android tablet had come across these odditiWhat happens when you look at a non-ADS-B flight simultaneously on your tablet and on your laptop/PC ?

[KelvinD]

It may not be down to ADS-B or FR24.
A couple of years ago I was stunned to see an American Airlines 777-200 and a Kuwait 777-200 flying between Ockham and Biggin at 10,000ft and a speed of 0kts! I am led to believe this is due to the aircraft having INS rather than a more modern GPS based system and it is this that is feeding inaccurate info to the ADS.
I frequently see the same effect at Heathrow when Delta 757 & 767 aircraft show up in Twickenham and then appear milliseconds later taking off right in front of me!
I have seen FR24 show a BA 747 land in Cranford, then taxi up the Bath Road.
Also, at Manchester, I have seen aircraft landing in Styal and on the M56.

[DaveReidUK]

A couple of years ago I was stunned to see an American Airlines 777-200 and a Kuwait 777-200 flying between Ockham and Biggin at 10,000ft and a speed of 0kts! I am led to believe this is due to the aircraft having INS rather than a more modern GPS based system and it is this that is feeding inaccurate info to the ADS.Not quite.

For older B772s, while it's true they don't have GPS-driven ADS-B, the reason for seeing 0kts displayed by most receivers and networks is that the aircraft is actually sending airspeed (usually TAS) and heading, rather than velocity over the ground, and those values aren't being decoded.

[philbky]

Thanks to all for the info on FlyBe Embraers and the other observations. Very helpful and informative. DaveReidUK I,m currently house sitting/ hospital visiting for my in laws both of whom are in hospital so I have no other equipment with which to compare the tablet's output. When I do at home, I suppose your previous comment about lack of input in my area is the reason for the lack of discrepancy between my Kinetic output to Planeplotter and the FR24 display to which I don't contribute.

The "ghost" Citations are still a mystery!

[ifonly]

The "ghost" Citations are still a mystery!

Could it be that they are being received by multilateration (where I think FR24 requires 4 or more receivers to be in range), but as they go out of range below the required number they start to 'wander' then disappear altogether?

[PAXfips]

FR24 (and others) "projects" the flightpath with the parameters from the last known data. When the next data packet comes in, it will be recalculated/repositioned. Also there might be a delay with exactly that if your download-speed at the current location is slow you might even miss updates.

[STATSMAN]

I live in Cheadle Hulme near the station and use an I Pad and have aircraft landing in Styal. Now on 5L so watch out Wythenshawe!!

[Double Hydco]

http://i1297.photobucket.com/albums/ag21/belfast97/IMG_3411_zpscch3uvqn.png

[AeroSpark]

I have often watched Heathrow traffic "landing" in Richmond Park or Windsor Great Park!

Thats actually happens, just THEY don't want you to know about it. Apparently thats where they fill up with chemtrail chemicals;)

[HEATHROW DIRECTOR]

Ahaaa.. many thanks for that. Naturally I won't mention it to a soul.

[Captivep]

I remember reading, on some nutty chemtrail conspiracy site, that they had proof that a secret island existed, off the coast of Africa, where the "chem" was loaded onto aircraft.


Their proof? A screenshot of a FR24 page showing an airliner leaving Brazil (I think) to a destination on Europe but making a detour and then a sharp turn at the intersection of the equator and the prime meridian. Fairly obviously a glitch in FR24 to most people, but definitive proof of a Thunderbirds style island to the tinfoil hat brigade!