Well, this reveals this if you click on the most recent flight (where the arrow points)
Don’t fall over laughing 
My passenger was taking photos of various things.
I was on a local Shoreham squawk, 3673 or something like that. The GS is totally wrong on FR24, usually.
So FR24 does show Mode S and yes it uses triangulation (multilateration).
I will set up an alert for a really common type (PA28) and advise… on a quick and dirty look I suspect PA28s and Cessnas are filtered out.
For me every flight of more than 1 hour is spent at above 7000ft. So much enjoying flying 150KTAS at 23L/hour in my Europa monowheel :-)
I can just imagine your flight plan for that (if you had needed one)
EGKA dct LOST dct EGKA 
on a quick and dirty look I suspect PA28s and Cessnas are filtered out
But how would they do that? They’ve no idea what a squwak is? Ah! I see. Mode S has a hex number so they can know the aircraft. Well…there probably lies the clue. I would place a bet on the fact that the only aircraft that you will see on it, are ones with mode S who have flown at least 1 recorded IFR flight.
From the IFR flight, they get details from EuroControl, that allows them to put together the hex code, registration and aircraft type. They store that in a database. Then they only show aircraft with known hex codes.
If you never have flown on a EuroControl flight plan, then your hex number is unknown (or if you only have mode C/A, then you don’t have a hex code), and you don’t get displayed on FR24.
That would explain why there are so few GA aircraft there, and no Cessna’s and Pipers (why would you filter them out intentionally, and leave in TB20?).
And it would also explain why you don’t see many home builds on there either!
The FR24 (and other) community(ies) has databases of everything. The 24-bit code for example is on G-INFO. For N-regs and a few others it can be algorithmically derived (both ways) – we have had threads here on that.
One can see the homebuilts but at lower levels.
I think they simply filter out the most common types unless some criteria are met, and it can’t be IFR because FR24 doesn’t know that.
Gliders sometimes set up a “ground base” flarm that they hook up to FR24. Mode S, isn’t that only required above FL20 or something?
There is nothing like one data point which disproves a popular theory!
I am looking now at a PA28-235, squawking 7000! Flying at 6600ft near Lausanne. So FR24 doesn’t simply drop 7000 squawks. Probably the reason people think it filters out 7000 is because it filters out by some minimum altitude IF it is one of the popular types.
But I’ve seen zero PA28 alerts in all of Europe since my post above.
A guy that runs a FR24 MLAT receiver at home showed me his data and the thing is that for most VFR traffic there simply isn’t enough data to calculate the position.
So the reason you see so few VFR aircraft on FR24 is because they’re not flying high enough to get a fix.
Position is calculated using multilateration, and the signal need to be received by at least 4 ground stations.
So the reason you see so few VFR aircraft on FR24 is because they’re not flying high enough to get a fix.
That would make sense. But also I would expect the density of detected traffic of popular types to be strongly dependent on
But this argument just feeds the argument that a lot of GA flies almost entirely at low levels. And if FR24 suppresses 7000-squawk aircraft, it does it only if some other criteria are met.
vic wrote:
Mode S in Germany is required above 5000 ft msl or 3500ft agl. Vic
Checked the AIP. Mode S is not required anywhere here, and the CAA has no plans to introduce any requirement either.
