Where is it going
Tap a plane on the map and the card told you its callsign, its type, how high it was and how fast. It never told you the thing most people actually wonder about: where it came from and where it's going.
That gap wasn't laziness. ADS-B doesn't carry a route. An aircraft broadcasts its address, its callsign, a position, an altitude, a speed, a heading and a squawk code, and that's the whole list. No transponder anywhere announces an airport. Every tracker you've used, ours included, gets the route the same way: it takes the callsign and looks it up in a flight database.
What we're reading
Ours is the Virtual Radar Server standing data. It's a corpus of routes that volunteers correct through Andrew Whewell's SDM portal; the credits page for that project names roughly 1500 people who've typed rows into it over the years. The export lives in a GitHub repository under CC0, a dedication to the public domain, and adsb.lol mirrors it hourly as one small file per callsign. CC0 asks for no attribution at all, so the credit in About → Data & licences is courtesy rather than obligation. It seemed like the least we could do for 1500 people's evenings.
There's a second database serving the same airport pairs, and we're not using it. Its terms credit a different lineage and forbid copying, and those names appear nowhere in the VRS route credits, so the two are separate corpora that agree because they describe the same real timetables. Reading the CC0 mirror keeps that restriction off our path entirely.
Where it shows up
The route appears on the hover card, in the phone's bottom sheet, on the radar card after a scan, on the chip under a selected aircraft, and on the plate that unfolds when you fly within nine kilometres of a contact in the simulator. On the map itself there's now a line: the great circle between the two airports, dashed behind the aircraft and solid ahead of it, with both airport codes labelled. Not a straight line on the screen, because that isn't what anything flies. Houston to Frankfurt bows up past Newfoundland, and the drawn arc bows with it.
Three letters aren't much of an answer if you don't already know them, so the route is spelled out as far as each surface has room for. The hover card and the phone sheet show a flag and a city beside every code, and hovering the row gives you the airports' full names. The radar card, which is wide enough, drops the codes and just names the cities. On the arc, each endpoint carries its code with the city underneath it, in the order a departure board uses. Only two places still speak in codes alone: the chip under a selected aircraft, which has one line, and the plate in the simulator, whose eight lines were already spoken for. Both put the full names in the tooltip.
Three honest limits
Most of the sky has no route, and that's normal. Airline flights have schedules. General aviation, business jets, trainers and helicopters mostly fly under their own registration and appear in no timetable, so their cards show no route line at all rather than a permanent "unknown". On our own feed, 13 of 15 callsigns resolved; both misses were light aircraft.
It's the schedule, not the flight. A ferry leg, a diversion or a positioning flight can wear a callsign whose timetable entry says something else. The tooltip says "scheduled route" out loud instead of leaving you to discover it on the day it's wrong.
Private and military aircraft are never asked about. For private traffic that's the entire point: where somebody flew from and to is exactly the kind of fact this project already refuses to publish, which is why we don't draw low-altitude trails for them either. The lookup won't even fire for a callsign shaped like a registration.
One more thing about rotations. Some flights reuse a callsign across several legs, so the database answers with a chain like Dallas → Fort Lauderdale → Dallas. Picking which leg an aircraft is on by distance alone doesn't work there, because an out-and-back rotation flies the same great circle in both directions. So the choice is made by which way the aircraft is actually travelling, and when there's no way to tell, such as an aircraft parked at the airport the rotation turns around in, the card prints the whole chain instead of picking one and sounding certain.