This article is meant as an introduction to online aircraft tracking. It explains the basics. There are many resources online for those interested in more detailed information or specs of the ADS-B signal.
There are several good web sites that allow world-wide tracking of aircraft. These web sites include ADSBExchange, FlightAware and Flightradar24. These sites receive their data from a network of receivers all over the world that pick up the ADS-B signal broadcast by most aircraft. With that data aircraft can be tracked in real time on an online map.
Per FAA regulations all aircraft in the United States operating above 18,000ft must be equipped with an ADS-B transponder. This is a fairly new regulation and not all aircraft have their transponder turned on or set up properly. But as of this writing in 2021 almost all civilian aircraft in the United States and many military "heavies" have their transponder on. Most smaller military aircraft currently have their ADS-B transponder turned off, meaning they will not show up on aircraft tracking sites. Aircraft operating in military controlled airspace such as the NTTR are also not bound by the FAA requirements and often turn off their transponder when they enter that air space. You can see this for Janet aircraft to Area 51. They generally turn off their transponder where they enter the NTTR air space near Mercury. In that case the track just ends near Mercury, seemingly in mid-air.
As mentioned above the ADS-B data is fed to the tracking web sites from a network of receivers that are maintained by aviation enthusiasts all over the world. These receivers have a limited coverage that decreases with lower aircraft altitude. Our receiver in Rachel for instance picks up aircraft up to 200 NM (Nautical Miles) away at altitudes greater than 30,000ft, but covers only about 80 NM for aircraft below 20,000ft (see plot below). Mountain ranges also impact the coverage as you can see in our coverage plot. Our Rachel receiver feeds data to all three tracking web sites mentioned above.
Due to the limited receiver range there are gaps in the coverage. This can result in tracks ending in mid-air or in partial tracks, connected by straight dotted lines. These lines are not neccessarily the flight path of the aircraft but simply connect two known locations with missing coverage in between.
In some cases aircraft transponders are configured to only report their ID but no position data. In order to show these aircraft on the map the tracking web sites use a process called Multi Lateration (MLAT). MLAT is basically triangulation of the aircraft position based on data from several receivers and timing analysis. Due to inherent errors this often results in jagged tracks that look like the aircraft is jumping all over the map. But most of the time these MLAT tracks still give a pretty good idea about the general track. The more receivers are in a given area the smoother the MLAT tracks are.
All three tracking web sites mentioned above show detail information for a selected aircraft in one way or another. That data includes the registration, type and owner of that aircraft. But you will also see aircraft without that information, in particular military aircraft. That is because the ADS-B signal only contains an ID of the aircraft, the so-called Mode-S code. With that code the tracking sites get the aircraft information from a database. If the aircraft is not yet included in the database the detail information cannot be displayed.
Some aircraft are blocked from tracking upon request from their owners or for other reasons. These aircraft will not show up in Flightaware or in FlightRadar24. ADSBExchange is the only tracking site that shows unfiltered data. All tracking sites also have more or less useful functions to look at historic flight data for a given aircraft. Unfortunately none of the sites provide a way to play back historic tracking for ALL aircraft in a given area.