Mode S (“Mode Select”) is the secondary surveillance radar mode in which every aircraft transponder has a unique 24-bit address, so a ground station can interrogate one aircraft at a time instead of every transponder in range. It carries identity and altitude, can downlink extra aircraft parameters, and its 1090 MHz extended squitter is the radio link most ADS-B traffic rides on.
§01What is Mode S?
Secondary surveillance radar (SSR) works by interrogation: the ground station interrogates and the transponder replies on a different frequency. Older Mode A transponders reply with an identity code only, and Mode C adds pressure altitude.1Source 1 · SKYbraryTransponderskybrary.aero Mode S operates on the same 1030 MHz (interrogation) and 1090 MHz (reply) frequencies as conventional SSR and stays backward compatible, but gives each aircraft a unique 24-bit address, so the ground can address a single aircraft and exchange data with it.2Source 2 · SKYbraryMode Sskybrary.aero Selective addressing cuts the overlapping replies that plague busy airspace when every transponder answers every interrogation.
| Mode | What the reply carries | Addressing |
|---|---|---|
| Mode A | Four-digit identity (squawk) code | All transponders reply |
| Mode C | Pressure altitude | All transponders reply |
| Mode S | Identity, altitude, plus data link (callsign, aircraft parameters) | Selective, by 24-bit address |
§02What is the difference between Elementary and Enhanced Surveillance?
Elementary Surveillance (ELS) automatically reports the flight’s callsign, altitude in 25 ft steps where the aircraft supports it, and capability flags such as airborne or on-ground status. Enhanced Surveillance (EHS) adds downlinked aircraft parameters, including selected altitude, roll angle, track angle and rate, magnetic heading, indicated airspeed, Mach number and barometric pressure setting.2Source 2 · SKYbraryMode Sskybrary.aero Controllers use selected altitude to spot level busts before they happen.
§03Example: what a receiver sees
A short Mode S surveillance reply is 56 bits. Downlink format 4 carries altitude and format 5 carries identity, the squawk code packed into a 13-bit field.3Source 3 · Junzi Sun, mode-s.orgThe 1090 Megahertz Riddle: Mode S surveillance repliesmode-s.org An ADS-B extended squitter is a longer 112-bit frame (downlink format 17) that the transponder broadcasts unprompted, carrying the same 24-bit address.4Source 4 · Junzi Sun, mode-s.orgThe 1090 Megahertz Riddle: ADS-B basicsmode-s.org A ground receiver that logs both can attach altitude replies and ADS-B positions to the same aircraft without guessing.
§04Common misconceptions
- “Mode S and ADS-B are the same thing.” ADS-B on 1090 MHz uses the Mode S transponder’s extended squitter, but a Mode S transponder can reply to radar without broadcasting ADS-B positions.
- “The hex code identifies the operator.” The 24-bit address identifies the transponder. In principle it does not change over the aircraft’s lifetime, but a transponder can be reprogrammed, and the FAA’s Privacy ICAO Address program issues alternate, temporary addresses to eligible aircraft.4Source 4 · Junzi Sun, mode-s.orgThe 1090 Megahertz Riddle: ADS-B basicsmode-s.org Join on it, but do not over-interpret it.
- “The squawk is the aircraft ID.” Squawks are assigned per flight and reused. The 24-bit address is the stable key.
§05How Kimo uses Mode S
In Kimo Defense Intelligence, icao24 is the primary key of the aircraft dimension and the join key of the tracks model. That lets ADS-B, OpenSky state vectors and multilateration positions merge into one track per aircraft. Mode S altitude and identity replies fill gaps between position reports. Start with the Airspace watch template, or explore live (simulated) traffic in Airspace.
§06Related terms
- ADS-B: position broadcasts carried on Mode S extended squitter.
- Squawk code: the Mode A identity code that Mode S also reports.
- Multilateration (MLAT): locating Mode S transmissions from their arrival times.
Frequently asked questions
What does the “S” in Mode S stand for?
What is a Mode S “hex code”?
Can I receive Mode S with a hobby receiver?
Sources
4 references- Transponder (opens in a new tab)SKYbraryskybrary.aero
Mode A identity, Mode C altitude, Mode S data exchange; emergency codes.
- Mode S (opens in a new tab)SKYbraryskybrary.aero
Selective interrogation by 24-bit address; ELS and EHS parameters; 1030/1090 MHz.
- The 1090 Megahertz Riddle: Mode S surveillance replies (opens in a new tab)Junzi Sun, mode-s.orgmode-s.org
56-bit DF4/DF5 replies; 13-bit identity field; altitude coding.
- The 1090 Megahertz Riddle: ADS-B basics (opens in a new tab)Junzi Sun, mode-s.orgmode-s.org
112-bit extended squitter, DF17; ICAO 24-bit address normally fixed but reprogrammable (e.g. FAA Privacy ICAO Address).
External sources were accessed at the time of writing. Kimo product details, customers and figures in examples are illustrative unless a source is cited.
