The transponder that answers a controller's radar has quietly become one of the most capable radios on the aircraft. Mode S and ADS-B are why a modern airframe can be tracked precisely, selectively, and even where there is no radar at all. Here is how they build on the basic transponder.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
From Mode A and C to Mode S
A classic transponder replies to Secondary Surveillance Radar with the four-digit identity code in Mode A and, with Mode C, its pressure altitude. The weakness is that the ground station has to interrogate everyone in the beam and sort out the overlapping replies, which becomes congested in busy airspace.
Mode S (the S is for Select) fixes that with three additions, as SKYbrary and the ICAO technical provisions set out:
- A permanent 24-bit aircraft address unique to the airframe, separate from the squawk code the pilot sets.
- Selective interrogation: the radar can address one specific aircraft by its address, rather than triggering every transponder in range, which cuts radio congestion.
- A two-way data link, which lets the aircraft downlink extra information and underpins ADS-B.
Elementary and enhanced surveillance
Mode S surveillance comes in two service levels:
- Elementary surveillance (ELS) adds the flight identification (the callsign you filed) and basic status, so the controller's label reads the flight, not just a code.
- Enhanced surveillance (EHS) goes further and downlinks aircraft parameters, such as the selected altitude set in the autopilot, the magnetic heading, indicated airspeed or Mach, and the vertical rate. A controller seeing the selected altitude can spot a level bust being set up before it happens.
What ADS-B actually is
ADS-B stands for Automatic Dependent Surveillance, Broadcast, and each word matters:
- Automatic: it transmits on its own, with no interrogation and no pilot action.
- Dependent: it depends on the aircraft's own navigation, typically GPS, for the position it sends.
- Surveillance: it serves the same purpose as radar, letting others know where the aircraft is.
- Broadcast: it is sent out for any suitably equipped receiver, on the ground or in the air.
ADS-B Out is the aircraft broadcasting its position, velocity, altitude and identity. ADS-B In is the aircraft receiving those broadcasts, which can drive a traffic display and, on some systems, weather and flight information. Because the position comes from GPS rather than a radar return, ADS-B can be more accurate and update faster than older radar, and it works where ground radar is impractical, which is why it extends surveillance over oceans and remote terrain. The FAA AIM describes the system in detail.
Two data links: 1090ES and UAT
ADS-B is carried on two different links:
- 1090 MHz Extended Squitter (1090ES) is broadcast by Mode S transponders and is used worldwide, including by airline aircraft.
- The 978 MHz Universal Access Transceiver (UAT) is used in the United States for lower airspace, and it carries FIS-B (flight information, such as weather and notices), which is available only on UAT. TIS-B (a ground rebroadcast of traffic) is uplinked on both UAT and 1090ES.
Equipping ADS-B Out is now mandatory in much of the world's controlled airspace. In the United States it has been required in the airspace listed in 14 CFR 91.225 since 1 January 2020, and other regions, including Europe, have their own mandates, so check the rule for where you fly.
Common pitfalls
- The 24-bit address is not the squawk. The address is fixed to the airframe; the four-digit code is still set by the pilot and can change on every flight.
- ADS-B is only as good as its position source. Because it is dependent on on-board navigation, a degraded GPS input degrades what it broadcasts.
- FIS-B weather is a US, UAT feature. Do not expect the weather uplink on a 1090ES-only installation, though TIS-B traffic is carried on both links. Treat any uplinked weather as situational, not a substitute for the official briefing.
In Pilot EFB
Pilot EFB keeps a quick squawk-code reference for the transponder codes you set by hand, and understanding what Mode S and ADS-B add explains why that four-digit code is only part of what your aircraft is telling the world. It is a personal reference held on your device; Pilot EFB is offline-first and is not a certified Electronic Flight Bag, so your transponder and avionics manuals and ATC remain the authority on what your equipment transmits.