What is ADS-B, and why is your neighbour listening?

The aircraft does the talking

An airliner is an unlikely correspondent. It is busy carrying people, burning fuel and making a noise that persuades dogs to look up. Yet an equipped aircraft also broadcasts brief radio messages about itself. A receiver on the ground can listen, and software can turn those messages into aircraft on a map.

ADS-B stands for Automatic Dependent Surveillance-Broadcast. Automatic means the broadcasts do not need a pilot to press a button each time. Dependent means the information depends on the aircraft's own systems, including its position source. Surveillance describes the tracking job. Broadcast means the message is available to compatible receivers in range, rather than addressed to your particular garden. The FAA distinguishes the transmitting function, ADS-B Out, from receiving equipment, ADS-B In. [1]

You do not make an aircraft transmit by visiting a tracking website. You also do not need to transmit anything back to receive it. This guide concerns a receive-only hobby station, not installing equipment in an aircraft.

From a pulse to a moving dot

Your antenna collects radio energy. A software-defined radio, usually shortened to SDR, converts a slice of that signal into samples. A decoder recognises messages. A local map displays the results. An optional feeder sends selected observations to a tracking network. These are different jobs, even when an image or container bundles them together. [2] [3] [4]

This distinction saves an extraordinary amount of reinstalling. A blank map can mean that the decoder has no data, or that the map cannot reach the decoder. Replacing both at once makes it harder to discover which one was innocent.

1090 and 978 are not rival subscription plans

For an Ireland-first receiver, start by investigating 1090 MHz equipment and 1090 Extended Squitter, usually written 1090ES. In the United States there is also 978 MHz UAT. The FAA's equipment rules distinguish their uses, including 1090ES for Class A airspace. Do not turn a US shopping guide into a worldwide shopping list. A 978 MHz receiver is not a substitute for a 1090 MHz receiver. [5] [6]

Before buying, write the frequency on your parts list. Then check the antenna, filters, receiver and software against it. The fact that two plugs fit together is not a certificate of suitability.

A dot is several kinds of information wearing one icon

A flight tracker may combine received messages with registration databases, schedules and other receivers. The aircraft's destination, a photograph and the airline's logo need not come from the aircraft's broadcast. Plane Finder describes these separate sources in its explanation of how its service works. [7]

Mode S is the wider transponder-message family you will encounter. Receiving Mode S messages does not mean every aircraft supplied a usable ADS-B position. Some positions come from multilateration, a calculation using several receivers. Others can be rebroadcast traffic information. Provenance matters when you compare stations or forward data. [8] [9] [10]

A smooth moving icon can also include interpolation between observations. Treat the presentation as a presentation. A missing contact does not establish that the sky is empty, and a convincing animation does not establish a fresh measurement.

Your first useful experiment

Open a local receiver map, when you have one, and choose an aircraft. Notice which fields change, which are absent and how old the position is. Then compare the same aircraft on a network map. Write down the differences rather than deciding that one display must be wrong.

You have just separated reception, decoding, enrichment and display. Those four ideas will be more useful than memorising fifty acronyms. For the deeper protocol, Junzi Sun's The 1090 Megahertz Riddle is the next book to open. [8]