A flight tracker shows aircraft, but the rules that shape their paths come from air-traffic control (ATC). Controllers keep aircraft safely separated by distance and altitude, sequence them into and out of airports, and pass each flight from one sector to the next. The orderly streams, holding patterns and stacked arrivals you see on the map are the visible result of that work, even though the controllers’ instructions are not in the data.
Airspace is layered by altitude and divided into regions. Near busy airports, controlled airspace is tightly managed; high above, en-route airspace is carved into sectors, each handled by a controller team. Aircraft generally cruise along published routes and airways — think of them as highways in the sky — which is why long-haul traffic often follows the same arcs rather than straight lines.
Those arcs are also shaped by the planet. Long flights follow great-circle routes — the shortest path over a sphere — which on a flat map look curved, bending toward the poles. Wind matters too: eastbound flights ride the jet stream to save time and fuel, so a route and its return can differ noticeably.
Altitude tells a story as well. Aircraft climbing out of an airport, cruising at a steady flight level, or descending on approach are doing recognisably different things, and the vertical rate in the state vector distinguishes them. Reading altitude and vertical rate together turns a dot into a phase of flight — departure, cruise or arrival.