Flight tracking for spotters: using ADS-B, Flightradar and more

Someone at the fence is holding a phone and calls out: “The A380 is on final!” Two minutes later it thunders overhead. For many spotters, live tracking is now as much a part of the gear as binoculars and a telephoto lens. But if you know where the dots on the map come from, you use the services far more purposefully – and understand why an aircraft is sometimes simply missing. This guide explains the technology behind ADS-B, MLAT and flight tracker maps and shows how to use them sensibly at the spot.

The short answer: where does the data come from?

A flight tracker in the browser or in an app is not a radar in the classic sense. Most positions come from signals the aircraft broadcasts itself. A worldwide network of ground receivers – many of them run by volunteers on rooftops and balconies – picks up these signals and sends them to the services. There they are merged with flight plans, fleet data and other sources.

Flightradar24 names sources such as ADS-B, multilateration, satellite reception, radar data from North America, data from the Open Glider Network and estimates. Other services work similarly but weight them differently. For you as a spotter, that means every dot on the map has an origin – and that determines how reliable it is.

ADS-B: the aircraft reports itself

ADS-B stands for Automatic Dependent Surveillance – Broadcast. Behind the unwieldy name is a simple principle: the aircraft determines its position with satellite navigation and broadcasts it regularly without being asked. The data packet includes, among other things:

• the 24-bit address, which belongs permanently to the individual aircraft and usually appears on maps as a six-character hex code,
• the flight identification, that is, the callsign of the current flight,
• the barometric altitude plus status information, such as an emergency code that has been set.

Worldwide, ADS-B runs over the so-called 1090 Extended Squitter of the Mode S transponder. In the US there is also the UAT system on 978 MHz, which is mainly used in general aviation below the high cruising altitudes. For spotters in Europe, 1090 MHz is the relevant frequency.

Who has to transmit ADS-B?

In Europe, ADS-B Out is required for flights under instrument flight rules with aircraft above 5,700 kg maximum takeoff mass or with a maximum cruising speed of more than 250 knots; there are also individual exemptions. In the US, the requirement has applied since January 1, 2020 in defined airspace. In practice, that means airliners such as the Airbus A320neo or the Boeing 737-800 show up reliably. For light aircraft, helicopters, gliders or military aircraft, that is much less certain.

MLAT: when the position is calculated

Not every aircraft sends its position via ADS-B. Many, however, have a Mode S transponder that answers interrogations. This is where multilateration (MLAT) comes in: several ground receivers hear the same signal, each at a slightly different moment. From these time differences, the server calculates where the aircraft must be. The transponder supplies altitude and identification anyway.

This has limits you should know about at the spot:

• Several receivers needed: FlightAware requires at least four receivers that hear the aircraft at the same time for an MLAT position. According to its own statements, Flightradar24 has been able to calculate with as few as three receivers in its network since 2025.
• Altitude counts: Because the receivers need a line of sight, MLAT usually only works above roughly 3,000 to 10,000 feet according to Flightradar24. An aircraft on the apron or on low final therefore often drops out.
• Some delay: FlightAware gives a processing time of usually four to six seconds for the MLAT calculation.

On many maps you can recognize MLAT positions by a separate label in the flight details. If a dot suddenly jumps or disappears shortly before landing, this effect is often the reason.

Satellite, estimates and other sources

There are no ground receivers over oceans and remote areas. Here satellites help, picking up ADS-B signals from orbit. However, this only works for aircraft that transmit ADS-B, and coverage varies with the satellite position. If real data is missing entirely, some services keep estimating the position based on the known route. Spotters should treat such estimates with caution: they show where the aircraft should be, not where it is.

Which source is good for what? The comparison

Source	Where does the position come from?	Strength	Weakness	Use at the spot
ADS-B	From the aircraft itself via satellite navigation	Precise, updated often	Only aircraft with ADS-B Out	Approach order, registration, altitude
MLAT	Calculated from time differences	Also shows aircraft without an ADS-B position	Needs several receivers, patchy near the ground	Military and older aircraft at higher altitude
Satellite	ADS-B reception from orbit	Coverage over oceans	ADS-B only, varying coverage	Follow long-haul flights in advance
Estimate	Extrapolated from route and last position	No gap in the display	Not a real measurement	Rough orientation, never for pressing the shutter
Own receiver	Directly from your antenna	Immediate, unfiltered for your area	Limited range, setup required	Local traffic at your home airport

Why some aircraft are missing

When an aircraft flies loudly over your head and nothing shows up on the map, the question gets exciting. The most common reasons:

• No ADS-B, no MLAT coverage: The aircraft does not send a position and flies too low for a calculation.
• Blocking lists: Owners can have the display restricted, in the US for example through the FAA’s LADD program. Flightradar24 points out that military and government aircraft are sometimes hidden completely and that some restricted flights appear only anonymously with the aircraft type.
• Changing addresses: Through the FAA’s PIA program, operators in the US can use an alternative, temporary 24-bit address so that the aircraft cannot be permanently identified.
• Different filters: Services handle blocked data differently. ADS-B Exchange, part of the aviation data provider JETNET since January 2023, stressed at the acquisition that it intends to stick with unfiltered data – which is also why two maps sometimes do not show the same thing.

How to use live tracking at the spot

The live map shows its value above all in planning and in the minutes before a landing. Here is how to go about it in a structured way:

This is especially helpful at big hubs like Frankfurt Airport or Munich Airport, where traffic is dense and widebody jets like the Airbus A350-900 or the Boeing 777-300ER land at short intervals. At the Munich Airport visitors’ hill or on the Düsseldorf observation deck, the map helps you make good use of the waiting time and not miss the right moment. Why the wind direction determines where you should stand is explained in the guide to the runway direction in use at airports.

Your own ADS-B receiver: is it worth it?

If you want to go deeper, you build your own receiver. As a DIY option, FlightAware describes a Raspberry Pi with a USB ADS-B receiver and the matching software and puts the cost at under US$100 or around €80. You also need an antenna with as clear a view of the sky as possible.

Range depends on line of sight. FlightAware gives a maximum of about 480 kilometers for aircraft at cruising altitude. The lower the aircraft and the lower your antenna, the shorter the distance. The calculator below gives you a rough idea of the theoretical radio horizon.

Many services reward sharing your data. FlightAware offers feeders a free Enterprise account, and Flightradar24 a Business subscription that stays active as long as data keeps flowing. At the same time you improve coverage – for MLAT in particular, every additional station in the region counts.

Which service fits your question?

Instead of arguing about “the best” flight tracker, it is worth choosing tools by the question you have right now. All the big services have their strengths, and many spotters use several in parallel.

Common mistakes with live tracking

• Mistaking the map for a live picture. Every position has an origin and a delay. With MLAT or estimates it can be far off.
• Searching only by flight number. If an aircraft is swapped, you suddenly follow a different aircraft. For collectors, the registration counts.
• Looking only at your phone at the spot. Especially on observation decks, an approach often shows up in the sky earlier than on the map – and the battery lasts longer.
• Treating missing aircraft as an error. Blocking lists, missing coverage and privacy programs are intentional or physics, not a glitch.
• Sacrificing safety to timing. No landing justifies blocking access roads, climbing fences or entering restricted areas. You can read what is allowed under Is plane spotting allowed?

If you are just starting out, you will find the basics in the guide Plane spotting for beginners. The gear checklist shows what belongs in your bag besides your phone, and the article on sun position at the photo spot explains when the light is right for your spot.

Conclusion

Flight tracker maps are an enormous gain for spotters – provided you know what you are looking at. Most dots come from ADS-B signals the aircraft broadcasts itself. MLAT, satellite data and estimates fill gaps, each with its own limits. If you use hex code and registration instead of flight number, can place blocking lists and coverage gaps, and look at the sky rather than the display at the decisive moment, you get more out of every spotting day.