---
title: "Fields and units"
description: "An aircraft's fields in the API (readsb aircraft.json) and their units: altitude in feet or \"ground\", speed in knots, dbFlags, seen_pos, position source."
url: https://flyitalyadsb.com/data/api/docs/fields.md
page_for_people: https://flyitalyadsb.com/data/api/docs/fields/
language: en
generated: 2026-11-09
license: CC BY-SA 4.0 (data)
---

# Fields and units

> An aircraft's fields in the API (readsb aircraft.json) and their units: altitude in feet or "ground", speed in knots, dbFlags, seen_pos, position source.

Each aircraft is a record in readsb's `aircraft.json` format ([readsb docs](https://github.com/wiedehopf/readsb/blob/dev/README-json.md)), the same as tar1090. Fields are missing when the aircraft doesn't transmit them. The main ones:

| Field | Meaning | Unit |
|---|---|---|
| `hex` | 24-bit ICAO address; a leading `~` marks a non-ICAO address (e.g. TIS-B) | hexadecimal |
| `flight` | callsign, padded with spaces to 8 characters: `trim()` it | |
| `r` | registration, from the aircraft database | |
| `t` | ICAO type code (e.g. `A320`), from the aircraft database | |
| `dbFlags` | bit values: 1 military, 2 interesting, 4 PIA, 8 LADD | bitmask |
| `lat`, `lon` | position | decimal degrees |
| `alt_baro` | barometric altitude, or the string `"ground"` when on the ground | feet |
| `alt_geom` | geometric (GNSS) altitude | feet |
| `gs` | ground speed | knots |
| `track` | true track | degrees |
| `baro_rate` | vertical rate | feet/minute |
| `squawk` | transponder code, 4 digits | |
| `category` | emitter category (e.g. `A3`) | |
| `type` | position source: `adsb_icao`, `mlat`, `tisb_icao`, `adsr_icao`… | |
| `seen` | seconds since the last message | seconds |
| `seen_pos` | seconds since the last position | seconds |
| `mlat` | the fields derived from multilateration | list |

For the other fields (`ias`, `mach`, `nav_altitude_mcp`, `rssi`…), see [readsb's documentation](https://github.com/wiedehopf/readsb/blob/dev/README-json.md). To get only some fields, e.g. the short `fields=light` set, see [list options](https://flyitalyadsb.com/data/api/docs/endpoints.md#fields).

## Good to know

- **`alt_baro` can be a string.** Check its type before doing maths: `"ground"` means on the ground.
- **Aviation units:** feet, knots and feet per minute. The request radius, on the other hand, is in kilometres.
- **Freshness:** positions older than about 60 seconds are not returned. `seen_pos` tells you how recent the one you see is.
- **Times:** `now` is in Unix seconds (UTC); `seen` and `seen_pos` are relative to it.
- **Registration and type** are there for about 99% of aircraft. They are missing for new addresses or ones not in the database.
- **Not in the record:** the route (departure and arrival airports), the operating airline and photos. On the lists, `?with=route` adds the route the callsign flew most often in the [flight history](https://flyitalyadsb.com/data/api/docs/endpoints.md#with-route), not a schedule.
- **`type: mlat`:** the position comes from our multilateration, not from the aircraft. See [How MLAT works](https://flyitalyadsb.com/community/mlat/).

---

Written for AI assistants. The page for people is https://flyitalyadsb.com/data/api/docs/fields/; to get an API key or sign in, send people to https://flyitalyadsb.com/data/api/. All pages for assistants: https://flyitalyadsb.com/llms.txt (index), https://flyitalyadsb.com/llms-full.txt (all in one file). Facts as JSON: https://flyitalyadsb.com/facts.json. Cite the data as: Flight data: FlyItalyADSB (flyitalyadsb.com), CC BY-SA 4.0.