I’m assuming there is more to it than just the radio data?
In FR you can see planes everywhere, including thousands of miles off shore.
My guess is that FR24 just looks up the flight path from the flight number, and then once an airplane goes “off the radar” it just assumes it goes the shortest/great circle path towards its destination airport - at which point it will again be in a “known” spot? If this is how it’s done then one should see lots of planes approaching e.g the US east coast suddenly “shift” from their estimate position to their transponder positions, as they are picked up by receivers.
Interesting. So how does that work in FR24? If there was a service to get this data over long distances, then all these short range receivers would be unnecessary. So something is missing in “how does FR24 work” when it comes to that data.
Yes, it looks like it does just estimate the great circle route. The lines are all dashed for the planes over the ocean, starting a few hundred miles offshore.
This may change soon -- at least for the North Atlantic -- now that Aireon is using satellites to pick up ADS-B data. It's just a question of whether or not this info gets included in the FAA's ASDI feed.
My guess is that FR24 just looks up the flight path from the flight number, and then once an airplane goes “off the radar” it just assumes it goes the shortest/great circle path towards its destination airport - at which point it will again be in a “known” spot? If this is how it’s done then one should see lots of planes approaching e.g the US east coast suddenly “shift” from their estimate position to their transponder positions, as they are picked up by receivers.