What the Data Fields on ADS-B Exchange Mean (Hex ID, MLAT, Squawk, Altitude)

Updated

ADS-B Exchange's aircraft detail panel shows live and historical flight data, but many fields are abbreviated or use aviation/technical jargon. Here's what each one means, based on the site's own labels and tooltips.

Hex ID (ICAO 24-bit address)

The hex ID is the aircraft's unique identifier — normally a 24-bit ICAO address assigned to the airframe. It's the primary key ADS-B Exchange uses to match a contact to a registration, type, and operator.

Not every contact has one. The site distinguishes several cases:

Label Meaning
Non-ICAO hex The hex value isn't a valid ICAO 24-bit address
No ICAO 24 bit address No ICAO address is present at all
Anonymized MLAT result The position came from MLAT with the identity obscured
ATC contact broadcast The contact was broadcast via ATC rather than a standard ADS-B message
Dynamic hex ID The ID changes and can't be tied to a type or registration

When the hex is dynamic or non-ICAO, the Reg. (registration), Type, and Airline fields typically show n/a — there's nothing stable to look up.

MLAT (multilateration)

MLAT estimates an aircraft's position by timing how long its signal takes to reach multiple ground receivers. Per the site's own note, MLAT requires the contact to have a ModeS transponder and reception by at least 4 receivers. Fewer than 4 and no MLAT position is produced.

This matters because MLAT is how ADS-B Exchange shows aircraft that aren't broadcasting full ADS-B position data — but the resulting position is an estimate, not a direct broadcast, and it may appear as an "anonymized MLAT result" with no identifiable hex.

Common flight data fields

Field What it shows
Altitude Barometric and/or WGS84 (geometric) altitude, depending on what the aircraft broadcasts
Speed Ground speed, plus true/indicated airspeed and Mach where available
Squawk The 4-digit transponder code (Mode A)
RSSI Received signal strength — a rough indicator of how well the receiver is hearing the aircraft
Msg. Rate / Messages How many messages per second, and total, the aircraft is transmitting
Pos. / Distance Position and distance from the receiver or reference point
Signal Source Whether the data came from ADS-B, MLAT, or another source
Last Seen / Last Pos. Timestamps for the most recent contact and position

The panel also groups fields into Flight Activity (history with playback), Spatial (position, altitude, track, vertical rate), FMS SEL (selected altitude/heading, wind, temperature), and Accuracy (NAC P, SIL, NAC V, NIC BARO — ADS-B quality metrics).

Where to find the explanations

The site states you can hover over each data label to see a definition of that Mode S data type. That's the fastest way to decode a field you don't recognize, since the tooltip is tied directly to the live value.

Practical notes

  • n/a is common and expected. Fields populate only when the aircraft broadcasts them or when MLAT can resolve them. A missing registration usually means a non-ICAO or dynamic hex.
  • UAV data shown is flagged as Part 107 (US commercial drone rules), and the site invites UAV operators to register.
  • Ground vehicles and Non-ICAO targets can be toggled on separately — these are radar tracks or airframes with unknown identity, so treat their data as lower confidence.
  • History is per-aircraft; if the panel says "No history available for this aircraft," there's no stored trace to play back.

If a field still looks wrong, check the Signal Source first — an MLAT-derived position behaves differently from a direct ADS-B broadcast, and that distinction explains most of the odd values.