Guide
ADS-B vs MLAT: two ways to find an aircraft
ADS-B is when an aircraft broadcasts its own GPS position. MLAT (multilateration) is when receivers on the ground work out an aircraft's position from the tiny differences in when its signal arrives. ADS-B is precise and needs only one receiver. MLAT works for aircraft without ADS-B but needs at least four receivers to hear the same signal.
ADS-B: the aircraft tells you where it is
An ADS-B Out system takes the aircraft's position from a certified GPS receiver and broadcasts it, together with identity, altitude, velocity and status, about twice a second. A single receiver in range can decode it. Accuracy is typically within a few metres to tens of metres, and the aircraft also reports how accurate its position is (the NIC and NACp integrity values).
The US mandate
Since 1 January 2020, 14 CFR 91.225 has required ADS-B Out in most controlled airspace in the US, including:
- Class A airspace (18,000 ft MSL up to FL600),
- Class B and Class C airspace around major and medium airports,
- within 30 nautical miles of the primary Class B airports (the "Mode C veil"), and
- Class E airspace at and above 10,000 ft MSL, excluding airspace at and below 2,500 ft above the ground.
The equipment must meet performance standards in 14 CFR 91.227. Aircraft that fly only outside this airspace, such as some light aircraft, gliders and balloons, are not required to equip, although many have done so voluntarily.
1090ES and 978 UAT
The US uses two ADS-B frequencies:
- 1090 MHz Extended Squitter (1090ES) is the international standard, used by airliners, business jets and most aircraft worldwide. It is required for flight in Class A airspace, at and above FL180.
- 978 MHz Universal Access Transceiver (UAT) is a US-only option allowed below 18,000 ft. It is popular in general aviation because it also lets the aircraft receive free weather (FIS-B) and traffic data.
A receiver for one frequency cannot hear the other, which is why many community feeders run two radios.
TIS-B and ADS-R
FAA ground stations help aircraft see each other across the two systems. ADS-R (rebroadcast) relays ADS-B targets from one frequency to the other. TIS-B (Traffic Information Service–Broadcast) rebroadcasts radar targets of aircraft without ADS-B. Trackers sometimes display TIS-B targets, which are less precise and updated less often than real ADS-B.
MLAT: the receivers work it out
Many aircraft still carry only a Mode S transponder, which replies to radar interrogations and sends identity and altitude but not position. When four or more receivers hear the same reply, a central server compares precise arrival times. Each pair of receivers defines a curved surface on which the aircraft must lie; with enough receivers, those surfaces intersect at one point. This technique is called time difference of arrival, and it is the same principle that air traffic control's own wide-area multilateration systems use.
MLAT accuracy and limits
- Needs geometry: receivers must surround the aircraft at good angles. Coverage is best over dense receiver networks such as the Northeast, Florida, Texas and California.
- Less precise: positions can be off by tens to hundreds of metres and may jump between updates.
- Altitude from the transponder: MLAT uses the barometric altitude in the Mode S reply.
- Slower: positions are calculated a few times per minute to every few seconds, not twice a second.
Which one am I looking at?
Most of what you see on the heywhatsflying map is ADS-B. MLAT matters most for military aircraft, older business jets and some helicopters that have not equipped with ADS-B Out. On the airplanes.live globe, the aircraft details panel shows the source.
| ADS-B | MLAT | |
|---|---|---|
| Position source | Aircraft's own GPS | Signal timing at ground receivers |
| Receivers needed | 1 | 4 or more |
| Typical accuracy | Metres | Tens to hundreds of metres |
| Update rate | About 2 per second | Seconds |
| Works for | ADS-B Out equipped aircraft | Mode S transponders, including many military |
Related reading
Questions
Is ADS-B required in the US?
Yes. Since 1 January 2020 the FAA has required ADS-B Out in most controlled airspace, including Class A, B and C airspace, the 30 nm Mode C veil, and Class E airspace at and above 10,000 ft MSL, excluding airspace at and below 2,500 ft above the ground.
How many receivers does MLAT need?
At least four receivers must hear the same transponder reply. More receivers at good angles around the aircraft give a more accurate position.