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Flightradar 24 / FR24 - how exactly does it work?

For my flight from Prague to Shoreham, yesterday, I see this

The part of the route which is shown is correct – as far as I can see from the Eurocontrol track

and I am sure my Oziexplorer track log would confirm it.

What is interesting is the rest of it.

When I changed to Munich (not sure where but they gave me a DCT RAPET GASKA) I got a new code of 4145 (previously 1412). This probably explains the missing (dashed) piece.

There is no track departing from LKPR, yet there was continuous radar coverage and thus continuous Mode S from the aircraft, and the same applies to the arrival area where the track disappears in the descent through around FL100. I thought this data comes from owners of the Mode S / ADS-B receivers who are internet connected and feed time-stamped data into a central server. And these people are everywhere. There must be 100 of them just along the SE UK coast.

Also the EGKA-LKPR flight is not shown. Am I using the site correctly?

The Eurocontrol tracking facility (to which Flightradar should not have access – it comes with various confidentiality conditions, to B2B customers only) tracks the flight better – but still also drops it sometimes, especially at squawk changes. But a squawk change does not necessarily cause tracking to drop. It just tends to.

Last Edited by Peter at 28 Jul 20:24
Shoreham EGKA, United Kingdom

Google is your friend, and all shall be revealed here.

I was aware that is how it works in general terms but they don’t give the detail.

I guess my tracking was done using MLAT but it doesn’t explain the dropouts. It also doesn’t explain why they lost it on (apparently) the squawk change and then picked it up again. How would they know it’s the same one? There was a long delay before they picked it up again. Too long to work on “it must be him – he has hardly moved and there is nobody else around” principle.

Shoreham EGKA, United Kingdom

Could it be because you were out of range of one of their feeds?

EHLE / Lelystad, Netherlands, Netherlands

As it’s an amateur network (outside the US), I guess there must be gaps in the coverage. The way I understand ADS-B is that it transmits the position (and other data) no matter the squawk code. Therefore knowing ‘hey, that’s Peter!’ isn’t difficult. Could it be that the squawk change is just coincidence? In my experience flying in that part of the world, you would have spoken to Munich a lot earlier than the dashed line.

OK… but I don’t have ADS-B (and neither has some 99.9% of light GA).

They can track me in only one of two ways

  • my Mode S emissions, which require SSR interrogation, and which return only my reg # and the 24-bit ID code, and this would require triangulation because I don’t emit position data
  • my Mode C emissions, which require SSR interrogation, and this would require triangulation and an initial fix where the reg # is tied to the target, and I can’t see how a temporarily lost target could be re-acquired in a fairly busy traffic environment

The squawk change could be a coincidence, sure…

Shoreham EGKA, United Kingdom

If you look at your trace on FR24, you will see that the source radar is T-MLAT, meaning multilateration. Your position is being triangulated from multiple Mode-S receivers, rather than your GPS position transmitted as part of the Mode-S data block. The missing parts of your track suggest that coverage from multiple receivers was not sufficient to fix your position at those times.

FR24 almost certainly does not use your Mode-A squawk for any purpose other than informational display. Remember some codes are used by multiple aircraft, such as 7000, 7010, 1177 (London Information) or the various listening squawks.

To identify and track you, FR24 will use the Mode-S 24-bit address assigned to your aircraft by the FAA, which your transponder includes in every the data block. Your Mode-A squawk is included in some Mode-S data blocks (known as DF5).

Despite the proliferation of Mode-S receivers, I suspect a great many of them are compromised in some way – perhaps poorly sited, blocked by terrain/buildings/foliage and so coverage may be limited or directional, and/or they may be relying on the tiny whip antenna that tends to be supplied with these units. These are fine for picking up high-alt airline traffic, but very poor for picking up anything at lower levels. Now combine those factors with the need to have data for the same target from multiple receivers in order to triangulate the position.

That said, the coverage is amazing. I find it fascinating to watch and I would have loved something like this in the spotting-days of my youth when I struggled to identify the airliners flying high over my head. I was happy if I could identify the airline.

Last Edited by Finners at 28 Jul 21:30
EGTT, The London FIR

It seems to me that FR24 does display GA IFR traffic, while VFR traffic is not shown.

Is that because VFR traffic is generally at a lower altitude? Or is there some matching being done between the Mode S 24-bit address and the flightplan?

Many thanks for the explanation.

Or is there some matching being done between the Mode S 24-bit address and the flightplan?

They “can’t” have my flight plan. That info is not public. Well, one can do AFTN queries with tools like Afpex, but one isn’t going to be doing that on a large scale like FR24.

It’s an interesting point about VFR. Most of it will be at a very low level but I have done some VFR test flights to FL195 recently, around the middle of June. Is there an archive which goes that far back?

Last Edited by Peter at 28 Jul 22:10
Shoreham EGKA, United Kingdom

Or is there some matching being done between the Mode S 24-bit address and the flightplan?

You cannot easily do that. The 24bit Mode-S address is not part of the flight plan. While it’s easy to compute the 24 bit address for N reg (or HB reg…) from the registration, that doesn’t work for G reg or D reg where the address and registration don’t have an algorithmic relationship, and in the case of D reg the database is not public

LSZK, Switzerland
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