Flightradar 24 / FR24 - how exactly does it work?

I played with the geometry of multilateration this morning. To get that kind of accuracy, FR24 must have a lot of receivers contributing at widely varying distances, along with some pretty good hardware.

I was thinking about some kind of airborne multilateration, where a FLARM-like device in each aircraft could receive and (using GPS time) stamp the mode-S messages, sharing them over the same open bands as FLARM for everyone to do their own local calculation. Sadly, I don’t think its practical: the allowable power in the unlicensed bands limits you to a 10-15km range and you need at four least receivers with reasonable spacing to get a good position fix. Perhaps it could be done with mesh networking, but coverage would be very spotty.

The answer is of course TIS-B.

I wonder what the motivation was for providing such a facility? It is possible to break it using brute force, I think.

AOPA pushed for this anonymous mode as a privacy issue. With mode S, in the US, the 24 bit address is algorithmically related to the N number, so even VFR aircraft not talking to any ATC are identified. With a mode A/C transponder, there is no identifier other than the squawk code, so when the VFR squawk code of 1200 is used and the pilot is not talking to ATC, their radar reply can’t be used to identify the aircraft. With UAT ADS-B Out supporting the anonymous mode (self assigned identifier), use of the feature by pilots does not disclose their identity, thus preserving their privacy.

A brute force determination of the identity would have to select all possible locations that the mode might have been selected and try this with possible times that the mode might have been selected and then check for a valid identifier. Unless you had a target in mind and knowing their identity had a great economic value, the work would not be worth the effort. Even so, you would likely come up with multiple possible valid choices.With multiple choices being possible, it would not hold up as identity in a court or enforcement action, such as an airspace bust.

Using it to bust airspace is not defensible, but an accidental bust is understandable. We do not have a mode S mandate, so a mode A/C transponder squawking 1200 is anonymous. VFR flight in the US is practical and may be conducted at altitudes below 18000 MSL in class E or G throughout the country without talking to ATC. Class B areas require a clearance, so no anonymity is possible, but one can fly below or above class B areas without a clearance. Class C has communication and transponder requirements which makes the aircraft not anonymous. Towered airports require communications which makes the aircraft not anonymous, but not all towers have radar available. I can take off at my airport near the east coast and fly to an airport on the west coast and never have a need to talk to ATC or identify myself to anyone, even though as a good citizen, I will keep my transponder on squawking 1200 and altitude. Even if I file a flightplan for all of my legs, this does not involve talking to ATC to activate it and close it. My flight is my business. If I ask for ATC flight following or any other ATC service, I have voluntarily identified myself.

Government installing infrastructure to routinely track the movement of citizens is not an easily defensible moral position. ADS-B Out being mandatory in some areas would never have passed scrutiny in the US if there wasn’t a way to turn off identity reporting when outside of those areas. What annoys me is reading the regulation to become effective in 2020, which requires the system start automatically with ID on and require pilot action to shut it off.

These are all the same rehashed issues that stopped Mode S in any significant sense for private aircraft in the US. The difference now is that the new law allows a tiny sliver of untracked movement, the very least I think the bureaucracy could get away with, and requiring pilot action to turn it off, not on.

There are 50 quite autonomous states in the US with free, uncoordinated, unreported movement between them. It’s that way by constitutional design, and for about 2/3 of the countries history (pre FBI) there were no national police agencies, also by design. I’ve mentioned before that my planes have been flown all around US with no radio contact, one with only a hand held radio and no transponder. It’s no problem at all, and I do think its important that groups like AOPA help Congress understand the issues and keep it that way.

The argument for Mode-S in Europe was not being able to identify aircraft but to increase capacity in areas with a lot of traffic. Mode-S is an international standard in which the US participate, it’s not the invention of Orwellian Eurocrats.

I’m sure the US government lacking the technical wherewithal to use cell phone location data to routinely track its citizens whereabouts is impatiently waiting for the implementation of ADSB out for that purpose…

“It’s that way by design, and for about 2/3 of the countries history (pre FBI) there was no national police force, also by design.”

Except for the US Marshal service founded in 1789 – which had (and has) federal law enforcement jurisdiction if I’m not mistaken…

I’m not sure about the Marshall Service, will have to look it up. The FBI is generally considered the first US national police force, started in the 30’s. Obviously you can turn off a cell phone or not carry one at all but there are in fact other more flagrant examples that can’t be turned off, e.g. widespread automated car license plate readers that are legal only because they are within x miles of an international border. I think the agencies will always push as far as they can, until pushed back, and that’s equally true for any country anywhere.

Silvaire, I can see where we are going with this thread (lots of deja vu) and suggest we get back to FR24 rather than the politics of privacy!