Three systems, one frequency, endless confusion
People talk about "ADS-B receivers" as though there were one thing being received. There are really three, layered on top of one another across seventy years, and they all share 1090 MHz. Knowing which is which explains most of what looks inconsistent on a tracker.
| Mode A/C | Mode S | ADS-B | |
|---|---|---|---|
| Introduced | 1950s–60s | 1990s | 2000s |
| Identity | 4-digit squawk | Unique 24-bit address | Unique 24-bit address + callsign |
| Altitude | Yes (Mode C) | Yes | Yes, barometric and geometric |
| Position | No | No | Yes |
| Speaks when | Interrogated | Interrogated, individually | Continuously, unprompted |
| Trackable by | Not by hobby receivers | MLAT only | Any receiver in range |
Mode A and Mode C: shout and everyone answers
The oldest scheme is brutally simple. A ground radar transmits an interrogation, and every transponder within range replies. Mode A returns the four-digit squawk the crew has set; Mode C returns pressure altitude in 100-foot steps.
The weakness is that the replies carry no addressing. With many aircraft in range, replies overlap and garble each other — an effect known as FRUIT, and the reason a busy sector could saturate the system. There is also nothing in a Mode A/C reply that identifies the aircraft: two aircraft on the same squawk are indistinguishable.
For our purposes the important point is that hobby receivers cannot usefully track Mode A/C at all. The replies only exist in response to an interrogation, they contain no identity, and there is no way to tie one reply to the next.
Mode S: asking one aircraft at a time
Mode S — S for Select — fixed the addressing problem by giving every airframe a permanent 24-bit
address, written as six hexadecimal digits. That address is the hex you see on a tracker,
and it is the closest thing an aircraft has to a serial number in the air.
A Mode S interrogation can be directed at one specific address, so only that aircraft replies. The garbling problem largely disappears and the same airframe can be followed from sweep to sweep with certainty.
The address is allocated by country. UK aircraft sit in a block beginning 40 through
43, Germany around 3C, the United States from A0 upward. That is
why an experienced spotter can often guess an aircraft's country of registration from the hex alone.
The 24-bit address belongs to the airframe, not the flight. It stays with the aircraft when it is repainted, re-registered or sold — though a re-registration does usually mean a new address, since the address is derived from the registration in most countries. This is exactly why aircraft logbooks are kept by tail number: it is the identifier that persists.
ADS-B: the aircraft volunteers everything
The crucial change with ADS-B is not the data, it is the direction. Mode A/C and Mode S are replies — nothing is transmitted unless something on the ground asks first. ADS-B is a broadcast: the aircraft works out its own position from satellite navigation and announces it roughly twice a second whether or not anyone is listening.
The B stands for Broadcast, and that is the whole reason a receiver in a back garden can build a live map. There is no interrogator involved, no permission, and no interaction of any kind. You are simply overhearing something being said continuously in the open.
Technically ADS-B on 1090 MHz rides inside Mode S as an "extended squitter" — a Mode S message the
aircraft sends unprompted rather than in reply. Squitter is an old radar term for a spontaneous
transmission, and it is why decoder software talks about downlink formats: DF17 is the
extended squitter carrying ADS-B, while DF11, DF4 and DF5 are
ordinary Mode S replies.
What this means for what you see
Most of the apparent inconsistency on a tracker follows directly from the layering:
- An aircraft with a hex code but no position is Mode S only. It is being heard perfectly; it simply never says where it is.
- An aircraft with a position but no callsign is transmitting position messages while its identification message has not yet been received, or the crew has not entered the flight number.
- An aircraft with a squawk but nothing else is the oldest layer showing through.
- Position but no geometric altitude usually means the position came from multilateration rather than from the aircraft.
A fourth system, if you are in America
The United States added a second ADS-B link on 978 MHz, called UAT, aimed at general aviation below 18,000 feet. It carries the same position information plus uplinked weather and traffic services. It is not used in Europe, so a European receiver has no reason to cover it — but it explains why American equipment is often described as dual-band and European equipment is not.
Mode A/C is a code and an altitude, shouted back when asked. Mode S adds a permanent address so aircraft can be asked individually. ADS-B is the aircraft telling everyone where it is, continuously, unprompted. Your receiver hears the second and third of those; the map is built almost entirely from the third.