Also known as: Mode S, Mode-S, mode select
Mode S (mode select) is the selective-addressing aviation transponder protocol on 1090 MHz that carries each aircraft’s unique 24-bit ICAO address and forms the foundation of ADS-B. It is the modern layer of secondary surveillance radar (SSR): a ground interrogator can address one specific aircraft instead of prompting every transponder in the beam, cutting the mutual interference (“garbling”) that plagues the older Mode A/C scheme. Messages are 56- or 112-bit frames protected by a CRC-24.1
Overview
A Mode S transaction is defined by the interrogator on 1030 MHz and the reply on 1090 MHz. The downlink reply begins with a fixed four-pulse preamble and then a data block whose first five bits are the downlink format (DF) that names the message type. Short 56-bit replies cover altitude (DF4/DF20), identity (DF5/DF21), and all-call/acquisition (DF11); long 112-bit replies carry the Comm-B and Extended Squitter content. The waveform is the same pulse-position modulation at 1 Mbps used by ADS-B — each 1 µs bit places its pulse in the first or second half-microsecond. Every frame ends with a 24-bit CRC whose parity is overlaid with the aircraft address on addressed replies, so a receiver both checks integrity and recovers the address in one step; on Extended Squitter the address is sent in the clear and the CRC stands alone.2
Squitters are unsolicited transmissions a transponder emits without being interrogated. The short acquisition squitter (DF11) announces the 24-bit address roughly once a second; the Extended Squitter (DF17/DF18) carries the ADS-B position, velocity, and identification payloads. This is the key point for SDR users: ADS-B is not a separate radio system but a specific set of Mode S message types.
Technical characteristics
| Property | Value |
|---|---|
| Uplink | 1030 MHz, DPSK interrogation |
| Downlink | 1090 MHz, PPM reply, 1 Mbps |
| Frames | 56-bit (short) / 112-bit (long) |
| Address | 24-bit ICAO aircraft address |
| Format field | 5-bit downlink format (DF) |
| Integrity | CRC-24, address-overlaid on addressed replies |
| Key formats | DF4/5 (altitude/ID), DF11 (all-call), DF17/18 (ADS-B) |
History
Mode S was developed from the 1970s–80s to overcome the capacity and interference limits of Mode A/C, in which every transponder in an interrogator’s beam replies at once. By assigning each airframe a permanent 24-bit address, Mode S lets a ground station poll aircraft individually and exchange data-link messages (Comm-A/Comm-B). It was standardised in ICAO Annex 10 Volume IV, with RTCA and EUROCAE MOPS elaborating the transponder and Extended Squitter behaviour. Mode S also underpins airborne collision avoidance: TCAS II coordinates escape manoeuvres between aircraft over the Mode S link. The Extended Squitter layer was then repurposed to carry ADS-B, making Mode S the substrate for modern broadcast surveillance.1
Deployment
Mode S transponders are mandatory for most powered aircraft in controlled airspace worldwide, and the elementary/enhanced surveillance (ELS/EHS) Comm-B registers report extra parameters such as selected altitude and heading to air-traffic control. Ground-based multilateration systems time the arrival of the same Mode S reply at several receivers to locate aircraft that emit only Mode S and not ADS-B position, and the same technique underlies volunteer “MLAT” networks.
Decoding it with GopherTrunk
GopherTrunk decodes Mode S as part of its 1090 MHz ADS-B path: it correlates the four-pulse preamble, slices the PPM bits, reads the downlink format, verifies (or address-recovers from) the CRC-24, and hands DF17/DF18 payloads to the ADS-B state machine. Short frames (DF4/5/11) are recognised for address and altitude even when they carry no position. GopherTrunk can also accept Mode S frames from an upstream feeder in the BEAST binary format. Interactive interrogation, TCAS coordination, and the 1030 MHz uplink are outside its scope — it is a passive receiver. See the ADS-B decoder page.
Sources
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Secondary surveillance radar — Mode S — Wikipedia, for the Mode S selective-addressing transponder protocol, 1030/1090 MHz link, 24-bit ICAO address, downlink formats, frame sizes, and CRC. ↩ ↩2
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The 1090 MHz Riddle — mode-s.org — Junzi Sun, TU Delft, an open technical reference for Mode S downlink formats, CRC-24 with address overlay, and Extended Squitter decoding, corroborating the frame details here. ↩