Because it's going to come several times before this all is resolved, a word about flight tracking websites such as
Flightradar24 and
ADS-B Exchange:
1. These are not all radar tracks. They are primarily crowd-sourced receiver data, mostly from:
- Independent ADS-B (Automatic Dependent Surveillance-Broadcast) receiver stations, both ground and space-based. ADS-B transponders
actively emit ADS-B data (this will be important later).
- MLAT (Multilateration) Mode S (interrogation-based transponder data). This method uses >= 4 ground stations to derive a transponder's location since Mode S does not transmit GPS data.
- North American radar data with coverage of parts of the Atlantic and Pacific
- Australian radar data with some coverage of Indian and Pacific Oceans
2.
ADS-B is an expansion of the data provided by Mode S transponder on 1090MHz (required in Class A airspace (above Flight Level 180 [basically 18,000ft MSL]), in Class B airspace, and internationally), and 978MHz (below FL180), but Mode S requires a ground station to interrogate the transponder.
ADS-B uses the existing Mode S transponder data fields:
- Aircraft ID (squawk code)
- Altitude
- Registration
- Aircraft type
And expands it to include the transponder's GPS location.
3. Some factors to keep in mind:
- Higher-altitude aircraft in remote areas are more likely to appear than lower-altitude aircraft. That means an aircraft disappearing in a relatively remote location like the ocean doesn't mean they have turned off transponders. Flightradar24 may be more likely to be able to track remote aircraft due to their satellite integration.
- In the case of Iran, data feeds getting cut off inside of the country via Internet blackout means there may be aircraft transmitting ADS-B or Mode S that are not appearing because the ground stations are offline.
- F-35s in my area generally appear only by a reduced set of TIS-B MLAT and may appear to change Hex ID multiple times because they pass in and out of areas with enough receivers to provide MLAT.
As an example of this, you can check the Los Angeles area where the LAPD air unit often takes advantage of the ADS-B exception for law enforcement and military flights.
- Remember the part about ADS-B GPS location being broadcast by the transponder? This means that it is simple for GPS locations to be spoofed by a transponder with that capability (which we certainly have).
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