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.

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.

ADS-B Message Decoder

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

on the roadmapHardware, Radio & Telecomfree · no sign-up

This bench is being wired up

The adsb-decoder decoder is on the roadmap. The heavier binary and cryptographic parsers ship after the core text and token decoders — all of which are already live and working.

Browse the live decoders

Input is processed on your device. Nothing you paste is sent anywhere.

What is the ADS-B Message Decoder?

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.

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.