ADS-B: What is it, and How does it work?
ADS-B: What is it, and How does it work?
What it is, how it works, the transponder modes it builds on, where each is required — and exactly how ADS-B sits alongside TCAS on the flight deck.
ADS-B — Automatic Dependent Surveillance–Broadcast — is the surveillance technology that has fundamentally changed how air traffic control sees you, and how you can see other traffic. Unlike radar, which relies on ground equipment to interrogate your transponder and process the reply, ADS-B turns the model around: your aircraft continuously broadcasts its own position, identity, altitude, and speed to anyone within range who is listening.
No interrogation. No waiting to be swept. Your aircraft transmits roughly twice every second, and every ground station, ATC facility, and suitably equipped aircraft nearby receives that data in near real-time. To understand ADS-B fully, you first need to understand the transponder modes it evolved from — because ADS-B did not replace those modes, it was built on top of them.
The transponder modes: A, C and S
Before ADS-B, SSR — Secondary Surveillance Radar — was the primary way ATC identified and tracked aircraft beyond primary radar. SSR works by sending an interrogation signal to your transponder, which replies with specific information depending on which mode it is operating in. There are three modes every crew should know.
Replies with your 4-digit squawk code only. ATC sees a target on radar and knows which aircraft it is — nothing more. No altitude, no speed, just identity.
Replies with your squawk code plus pressure altitude from your encoding altimeter. ATC now sees both your identity and your altitude on their screen, enabling vertical separation.
Selectively interrogates individual aircraft by their unique ICAO 24-bit address. Replies with identity, altitude, and additional data. Mode S is what TCAS uses — and it is the transponder layer that ADS-B Extended Squitter broadcasts from.
The progression matters operationally. Mode A told ATC who you were. Mode C added where vertically. Mode S added individual addressability and a richer data channel — and it is that data channel which ADS-B Extended Squitter uses to broadcast your full state vector autonomously, without waiting to be asked.
TCAS II interrogates surrounding aircraft using Mode S and Mode C transponders. It does not use ADS-B. This means an aircraft with a functioning Mode S transponder is fully visible to your TCAS regardless of whether its ADS-B Out is working — and an aircraft whose transponder has failed is invisible to TCAS even if it is still broadcasting ADS-B. The two systems use different interrogation paths and must be understood separately.
How ADS-B builds on Mode S
Your aircraft's ADS-B system takes position data from the onboard GNSS receiver — the same GPS source feeding your FMS — and combines it with data from the air data system: pressure altitude, groundspeed, track, and vertical rate. It packages all of this and broadcasts it continuously on 1090 MHz via the Mode S transponder, roughly twice per second, without any ground interrogation required.
Your unique ICAO 24-bit address — already embedded in your Mode S transponder and matched to your registration — is included in every ADS-B transmission. This is how ATC automation and traffic displays associate each broadcast with a specific aircraft and correlate it with a flight plan. The ground station network feeds these broadcasts directly into ATC systems. Where ground coverage does not reach — oceanic tracks, polar routes, remote terrain — space-based receivers on the Iridium satellite constellation pick up the same broadcasts from orbit, extending surveillance to airspace radar has never reached.
"ADS-B doesn't tell ATC where it thinks you are. It tells ATC where you know you are — and that distinction is its fundamental advantage over radar."
ADS-B Out and ADS-B In
ADS-B operates on two distinct and independent links that serve entirely different purposes.
Your aircraft continuously broadcasts its own state — position, altitude, groundspeed, track, vertical rate, and ICAO identity — to the surrounding environment. This is what ATC receives. This is what is mandated in most controlled airspace worldwide. Without it, you are not ADS-B compliant regardless of what you can receive.
Your aircraft receives broadcasts from other ADS-B Out equipped traffic and, where available, uplinked traffic and weather data from the ground. ADS-B In is what populates your cockpit traffic display on the MFD or EFB. It is not mandated in most jurisdictions but significantly enhances situational awareness where installed.
Performance levels: not all ADS-B Out is equal
The quality of an ADS-B Out installation is defined by its performance level — specifically, how tightly the avionics can verify the integrity of the position being broadcast. This is not just a certification detail. It determines what airspace you are approved to operate in and what separation standards ATC can apply to your traffic.
Adequate for general airspace awareness but not accepted for ATC separation in RVSM or high-density controlled airspace. Typically found in older or lighter aircraft installations.
Accepted for en-route separation in many regions. Position containment within 0.3–0.6 NM guaranteed, with high probability of detection if that boundary is exceeded.
Required for terminal operations, RVSM, and oceanic reduced separation. Position containment within 0.1 NM at aviation-grade reliability. Standard on all modern transport-category aircraft.
In plain operational terms: the performance level reflects how confident the avionics are in the position being broadcast — and whether the system would catch and flag a degraded position before transmitting it. That self-monitoring is what makes high-performance ADS-B a qualified source for ATC separation, not just a GPS readout sent over radio.
ADS-B and TCAS: operationally independent
This is the most important distinction for any crew to carry, and confusion between the two remains common.
TCAS II does not use ADS-B data to generate Resolution Advisories. TCAS II has its own independent surveillance system. It actively interrogates Mode C and Mode S transponders of surrounding aircraft and builds its own traffic picture from those replies. The RA logic runs entirely on that picture — independent of ADS-B quality, ground station coverage, or whether the other aircraft has ADS-B Out at all.
An aircraft with failed ADS-B Out will still trigger TCAS RAs as long as its Mode S transponder is functioning. TCAS will see it. ATC may not, but TCAS will.
Your cockpit traffic display runs on ADS-B In. It gives situational awareness. It does not issue RAs and does not replace TCAS. If there is any conflict between what your traffic display shows and what TCAS is commanding — follow the RA. Every time, without exception.
An aircraft that has lost its Mode S transponder is invisible to TCAS even if it is still broadcasting ADS-B. ADS-B In may show it on your display, but TCAS will not protect you from it. This is a scenario where ADS-B In can provide a situational awareness advantage that TCAS alone cannot.
ACAS X, the next-generation collision avoidance system currently in development, is being designed to integrate ADS-B state data into its logic. When it reaches operational service, this independence will change. Under the current TCAS II environment, treat them as two separate and complementary systems — because they are.
What ADS-B does not do
ADS-B has transformed the surveillance environment, but a clear-eyed understanding of its limits is part of operating with it professionally.
It broadcasts what your GPS reports. If your GNSS position is degraded and the avionics do not detect it, that degraded position goes out on the broadcast. High-performance certified installations include integrity monitoring that catches most failures — but the system is only as reliable as the sensors feeding it.
The broadcast is unauthenticated. ADS-B messages carry no cryptographic verification. False targets can be injected into the picture by ground-based transmitters, and GPS spoofing can corrupt the position being broadcast. If your traffic display is showing contacts that do not correlate with TCAS or ATC, treat them with caution and query ATC immediately.
Oceanic coverage depends on the space-based network. Over most North Atlantic tracks and polar routes, the Aireon satellite network provides continuous coverage. In other remote areas, your ADS-B Out may be transmitting without any receiving infrastructure in range — meaning ATC has no picture of you until you re-enter ground station coverage.
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