Free tool

Mode S message builder

Enter a position, a callsign or a raw payload and get a valid 112-bit frame back with a correct CRC-24.

What to build

ICAO address

Latitude

Longitude

Altitude (ft)

Enter an address, a position and an altitude to build the even and odd pair.

What a valid frame requires: A Mode S extended squitter is 112 bits. The first 5 are the downlink format, the next 3 the capability, then 24 bits of ICAO address, 56 bits of payload (the ME field), and a 24-bit CRC. The CRC is computed over the first 88 bits and appended at the end. A frame with a wrong CRC is rejected by any receiver or decoder that checks it.
A position comes as a pair, never one frame: One position message does not contain a position. It holds a CPR index that repeats every few degrees, and it takes an even frame and an odd frame together to resolve a coordinate. A single position frame is ambiguous by design.

Paste any frame and see every field with the bit range it came from.

See the CRC-24 residual, what it means for each downlink format, and which single bit to flip when a broadcast fails.