# ADS-B Message Decoder

> Decode raw Mode-S / ADS-B aircraft telemetry frames.

- Page: https://decoder.tools/adsb-decoder
- Category: Hardware, Radio & Telecom
- Status: live, runs in the browser
- Last reviewed: 2026-10-23

ADS-B is a system in which aircraft continuously broadcast their identity, position, altitude and velocity on 1090 MHz, and an ADS-B decoder parses those 112-bit Mode-S extended squitter frames into readable flight data.

## Example

```text Input
8D4840D6202CC371C32CE0576098
```

```text Output
8D4840D6202CC371C32CE0576098
  DF 17 (ADS-B), ICAO 4840D6, type code 4, CRC OK
  Identification: callsign KLM1023
Messages: 1
```

## What is the ADS-B Message Decoder?

An extended squitter frame is 112 bits: a 5-bit downlink format (17 for ADS-B from a transponder), a 3-bit capability field, the 24-bit ICAO aircraft address, a 56-bit message payload and a 24-bit parity/CRC. The ICAO address is the aircraft's permanent identifier, assigned by registration country — which is how a decoder names an aircraft before it ever transmits a callsign.

The payload's type code says what the message carries: 1–4 is identification (the callsign), 9–18 is airborne position, 19 is airborne velocity, 5–8 is surface position. Positions use compact position reporting (CPR), which splits a coordinate across even and odd frames to save bits — so you need a matched pair of frames, or a known reference position, before a latitude and longitude can be resolved at all.

Everything is unencrypted and unauthenticated by design, which is why hobbyist receivers can feed the global flight-tracking networks, and also why ADS-B spoofing is a recognised aviation security concern.

## How to use the ADS-B Message Decoder

1. Paste the raw frame as hex, typically 28 hex characters for a 112-bit message.
2. Read the downlink format, ICAO address and decoded type-code payload.
3. For positions, supply a matching even/odd frame pair or a reference location.

## When to use it

- Debugging a home ADS-B receiver's decoded output against raw frames.
- Learning the Mode-S message format for an SDR project.
- Analysing captured aviation telemetry offline.

## ADS-B Message Decoder at a glance

| | |
|---|---|
| Frequency | 1090 MHz extended squitter |
| Frame | 112 bits, including a 24-bit CRC |
| ICAO address | 24-bit permanent aircraft identifier |
| Position | CPR — needs an even/odd frame pair to resolve |

## FAQ

### Why can't a single frame give me a position?

Compact position reporting encodes a coordinate across even and odd frames to fit in fewer bits. Without a matched pair — or a known reference position within about 180 nautical miles — the latitude is ambiguous.

### Is ADS-B encrypted?

No. It is broadcast in the clear and unauthenticated, which is what makes public flight tracking possible and spoofing a genuine research concern.

### What is the difference between Mode-S and ADS-B?

Mode-S is the underlying transponder protocol, including interrogated replies. ADS-B is the self-broadcast extended squitter subset in which aircraft periodically transmit position and velocity unprompted.