What Is ADS-B? In, Out & the Rule Every Pilot Must Know

Automatic Dependent Surveillance-Broadcast (ADS-B) is a satellite-based surveillance technology in which your aircraft automatically broadcasts its GPS position, altitude, and velocity to air traffic control and to nearby aircraft. It replaces the old “ground radar pings the airplane” model with a system where your airplane tells everyone where it is, several times per second, without you doing a thing.

If you’re learning to fly, ADS-B is one of those topics that sounds like alphabet soup until somebody draws the picture for you. There’s an “Out” half and an “In” half. There’s a rule that says where you must have it. And there’s a real safety payoff sitting in your cockpit display. Let’s get you day-one ready on all of it.

What is ADS-B in simple terms?

ADS-B is a system where your airplane uses GPS to figure out exactly where it is, then automatically broadcasts that position — along with your altitude and speed — to air traffic control and to any nearby aircraft listening. The name breaks down cleanly: Automatic (no pilot action), Dependent (it depends on GPS), Surveillance (it’s how you’re tracked), Broadcast (it goes out to everyone).

The old way of tracking airplanes was ground radar. A spinning antenna sent out a pulse, the pulse hit your airplane, and the return told the controller roughly where you were. Radar works, but it’s limited by where the antennas are and by the slow sweep of the dish.

ADS-B flips that around. Your GPS already knows your position with far more precision than radar ever could. So instead of waiting to be found, your airplane simply announces itself. That’s the whole idea, and it’s why the FAA built its NextGen surveillance backbone on it.

What is the difference between ADS-B Out and ADS-B In?

ADS-B Out broadcasts your aircraft’s position outward to ATC and other traffic; ADS-B In receives data — traffic and weather — back into your cockpit. Out is the transmitting half and the part the FAA requires in regulated airspace. In is the receiving half, and it is optional. Think of Out as your airplane talking and In as your airplane listening.

ADS-B Out is the workhorse of the regulation. When you fly into the airspace covered by 14 CFR § 91.225, your airplane must be transmitting its position. That broadcast is what lets controllers separate traffic and what feeds the safety picture to everyone around you.

ADS-B In is the gift you give yourself. With the right receiver, your tablet or panel display can show nearby traffic and a live weather feed. The FAA does not require ADS-B In, but plenty of pilots install or carry it because the situational awareness is genuinely hard to beat.

One nuance worth locking in early: a properly equipped airplane sends Out automatically. You don’t push a button to broadcast. As long as the system is powered and the transponder is on, it’s working.

Where is ADS-B Out required?

ADS-B Out is required under 14 CFR § 91.225 in most of the same airspace where a Mode C transponder has long been required: Class A, Class B, and Class C airspace; above the ceilings of Class B and Class C up to 10,000 feet MSL; and at and above 10,000 feet MSL in the contiguous 48 states (excluding airspace at and below 2,500 feet AGL). It is also required within 30 nautical miles of a primary Class B airport — the “Mode C veil.”

If that list sounds familiar, it should. The FAA deliberately aligned the ADS-B Out rule with the existing transponder rule in 14 CFR § 91.215, so the mental model you already build for transponders carries straight over. Where you needed Mode C, you now also need ADS-B Out.

Here’s a clean way to hold the high-traffic requirements:

A small but real point for student pilots: there are limited provisions for operating without ADS-B Out in certain airspace, but they require ATC coordination in advance. Don’t plan around the exception — plan around the rule. Know that if your training airport sits under a shelf of Class B or inside a Mode C veil, your airplane almost certainly needs to be ADS-B Out equipped to fly there.

What’s the difference between 1090ES and 978 UAT?

ADS-B Out can be transmitted on one of two data links: 1090ES (1090 MHz Extended Squitter) or 978 UAT (Universal Access Transceiver, on 978 MHz). The 1090ES link is built into a Mode S transponder and is required for flight at and above 18,000 feet MSL. The 978 UAT link is a separate box popular with general-aviation pilots who stay below 18,000 feet.

The reason two links exist comes down to where you fly. Airliners and turbine aircraft operating in the flight levels needed a globally compatible solution, and 1090ES fit that role. The FAA created the 978 UAT link to give light general-aviation aircraft a lower-cost path that also carries a bonus: free weather and traffic uplinks.

Here’s the side-by-side:

For most of the trainers you’ll fly — the Cessna 172s and Piper Cherokees of the world — either link satisfies the rule as long as you stay below 18,000 feet. Which one is installed simply depends on what the owner chose to equip.

What can you actually see with ADS-B In?

With ADS-B In, you can receive two free services: TIS-B (Traffic Information Service-Broadcast) shows nearby aircraft as targets on your display, and FIS-B (Flight Information Service-Broadcast) delivers weather and aeronautical information such as radar imagery, METARs, TAFs, and NOTAMs. Both come at no subscription cost — you only need a receiver.

TIS-B is the traffic picture. The ground network blends radar and ADS-B data and beams it back up so you can see airplanes around you, including some that are not ADS-B equipped. It’s a powerful “see and avoid” aid, though the FAA is clear that it supplements your eyes — it does not replace looking outside.

FIS-B is the weather feed, and it rides only on the 978 UAT link. You can pull up a graphical radar mosaic, current and forecast conditions, and notices to air missions right in the cockpit. One caution drilled into every instrument-bound student applies to you too: FIS-B radar imagery is delayed. The timestamp on the display tells you how old the picture is, and weather can move a long way in those minutes. Use it for strategic decisions, not for threading between cells.

How is ADS-B different from a regular transponder?

A traditional transponder replies to ground radar interrogations with a coded signal — Mode C adds your pressure altitude, Mode S adds a unique aircraft identity. ADS-B Out goes further: it broadcasts a GPS-derived position automatically and continuously, without waiting to be interrogated by radar. On a 1090ES installation, the ADS-B function is actually built right into a Mode S transponder.

So the two aren’t rivals — they’re layers. Your transponder still does its classic job: you set a squawk code, you squawk the assigned number, and you respond to “ident” requests. ADS-B Out adds the precise, GPS-based broadcast on top of that.

That’s also why your old habits stay relevant. You’ll still verify the transponder is on the correct code, still flip to ALT/Mode C, and still treat 7500, 7600, and 7700 as the emergency codes you never set casually. ADS-B doesn’t change cockpit transponder discipline — it amplifies what that little box is already telling the world.

If wiring all of this together — airspace, equipment rules, transponder operations — feels like a lot right now, that’s exactly the gap our Private Pilot Ground School is built to close. We teach the why behind every requirement so it sticks long after the checkride.

A real ADS-B moment over Alaska

I do a lot of flying in Alaska, and if there’s one place that sells you on ADS-B In, it’s a wide river valley up there on a hazy summer afternoon. The terrain funnels traffic. Floatplanes, bush planes, and the occasional 172 like mine all end up using the same low routes between the same gaps, and the visibility isn’t always doing you any favors.

I remember one leg where my tablet lit up with a traffic target converging from my two o’clock, slightly low. I hadn’t picked it up out the window yet — the haze and a darker ridge behind him had him completely camouflaged. The ADS-B In traffic alert told me where to look, I found him, adjusted, and we passed with comfortable room. No drama. That’s the point.

Here’s the honest lesson I give every student, though. The target on the glass made me look, and looking is what kept us apart. ADS-B In is one of the best traffic tools we’ve ever had in light aircraft, but it doesn’t see everything, and it doesn’t fly the airplane. It points your eyes. You still have to do the seeing.

PLT Study Guide

The FAA tags knowledge-test questions with PLT (Pilot Learning Statement) codes. For ADS-B, these three are the ones that actually match the content — study them in plain English:

PLT161 — Recall airspace classes: limits / requirements / restrictions / airspeeds / equipment. This is the big one for ADS-B. Know that ADS-B Out is required equipment under 14 CFR § 91.225 in Class A, B, and C airspace, above the B/C ceilings up to 10,000 ft MSL, at and above 10,000 ft MSL (excluding airspace at and below 2,500 ft AGL), and within the 30 NM Mode C veil. Tie the equipment requirement to the airspace.

PLT497 — Recall transponder: codes / operations / usage. ADS-B Out on the 1090ES link is built into a Mode S transponder, so transponder operation and ADS-B overlap. Know your squawk-code basics, ALT/Mode C reporting, and the emergency codes (7500/7600/7700). The transponder must be on for the ADS-B broadcast to work.

PLT405 — Recall regulations: equipment / instrument / certificate requirements. This covers the regulatory framing of “what must be installed and working.” Connect 14 CFR § 91.225 (ADS-B Out performance) and § 91.215 (transponder) as the equipment rules that govern where you can legally fly your airplane.

(Heads up: an early hint pinned this topic to a “preventive maintenance” code. That’s a mismatch — ADS-B is an airspace-and-equipment subject, not a maintenance one — so we corrected it to the codes above.)

Frequently Asked Questions

ADS-B isn’t hard once somebody shows you the two halves and the one rule. Out broadcasts, In receives, and 14 CFR § 91.225 tells you where Out is required. Learn it that way and the test questions — and more importantly, the real cockpit — stop being intimidating.

FROM CHRIS

Angle of Attack has been in aviation education since 2006, and Chris has been a CFI since 2017. We teach this stuff the way we wish somebody had taught us back when ADS-B was just another acronym on the page — clear, visual, and built to make you a safer, sharper pilot from day one. Get the two halves and the one rule, then go fly.

Throttle On!

Chris Palmer

Founder & Chief CFI, Angle of Attack — Two-Time Master Aviation Educator and Gold Seal CFI