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How ADS-B Works: A Pilot's Guide to Weather and Traffic

ADS-B puts free weather and traffic on your iPad. What you actually see depends on which frequency you receive, whether your airplane transmits, and how close you are to a ground station.

~650FAA ground stations across the US
1×/secposition updates, vs. 5–12 s for radar
978 + 1090two frequencies, two jobs (MHz)
15 nmradius of the TIS-B "hockey puck"

1What ADS-B is

ADS-B stands for Automatic Dependent Surveillance–Broadcast, and each word tells you something about how it works:

AAutomaticTransmits on its own. No pilot input.
DDependentPosition comes from the aircraft's own GPS, not from radar.
SSurveillanceReports 3-D position and identification.
BBroadcastAnyone with a receiver can hear it, including you.

An ADS-B Out transmitter sends the aircraft's GPS position, altitude, ground speed and other data about once per second. Radar updates every 5 to 12 seconds. ADS-B ground stations are also far smaller than radar sites, so the FAA can put them where radar can't reach.

2ADS-B Out vs. ADS-B In

ADS-B Out is the transmitter. Since January 1, 2020, the FAA rule (14 CFR 91.225) has required it in most controlled airspace:

FAA diagram of airspace requiring ADS-B Out: Class A, Class B, Class C, Class E at and above 10,000 ft MSL, the Mode C veil, and over the Gulf within 12 nm of the coast
Where ADS-B Out is required: Class A (1090ES only), Class B and C, the 30 nm Mode C veil, Class E at and above 10,000 ft MSL (except at and below 2,500 ft AGL), and over the Gulf of America at and above 3,000 ft MSL within 12 nm of the coast. Image: FAA

ADS-B In is the receiver, and it's optional. It's what puts traffic and weather on your display, either built into a panel or as a portable receiver that talks to your tablet. In the FAA's words, free traffic is available on all ADS-B In receivers, and free weather and flight information on receivers that can receive UAT.

3978 MHz vs. 1090 MHz: two links, two jobs

The US system runs on two frequencies, and they don't carry the same things:

1090 MHz"1090ES" · Extended Squitter
  • Airliners, jets and most turbine aircraft, plus many GA aircraft
  • Required at and above FL180
  • Works worldwide
  • No FIS-B weather
978 MHzUAT · Universal Access Transceiver
  • Many GA aircraft flying below 18,000 ft
  • Allowed below 18,000 ft in the US
  • US, and Canada via the CIFIB network
  • The only link that carries FIS-B weather

A dual-band receiver hears both, so you get weather and direct air-to-air traffic from every ADS-B Out aircraft, without depending on the ground to relay it.

4FIS-B, TIS-B and ADS-R: what the ground sends

FIS-B Flight Information Service–Broadcast (weather)

Broadcast continuously on 978 MHz to anyone in range, with no transmitter required. It's not sent on 1090, so ADS-B weather needs a 978 MHz receiver.

NEXRADRegional & CONUS radar
METAR & TAFObservations and forecasts
AIRMET / SIGMETIncl. convective SIGMETs
Winds & tempsAloft forecasts
PIREPsPilot reports
NOTAMs & TFRsIncl. temporary flight restrictions
SUA statusSpecial use airspace
Always onNo ADS-B Out needed
FAA illustration of FIS-B: weather from FIS-B providers goes through an FAA control station to a ground-based transceiver, which uplinks it to UAT-equipped aircraft
FIS-B: weather flows from providers through the FAA's control station to a ground-based transceiver (GBT), which uplinks it on 978 MHz. Only UAT receivers get it. Image: FAA

TIS-B Traffic Information Service–Broadcast (radar traffic)

Many aircraft still don't have ADS-B Out. TIS-B fills that gap: ground stations take radar and multilateration targets (aircraft with a working transponder) and rebroadcast them on both frequencies. It's available at or below FL240, and only where there's both ground-station and radar coverage.

FAA illustration of TIS-B: radar detects a non-ADS-B aircraft and the ground-based transceiver uplinks its position to UAT and 1090ES aircraft
TIS-B: radar sees an aircraft without ADS-B Out, and the ground station uplinks its position to ADS-B In aircraft on both links. Image: FAA

TIS-B is client-based. The ground station builds traffic for each ADS-B Out–equipped "client" aircraft, inside a cylinder around it 15 nm in radius and ±3,500 ft, nicknamed the hockey puck.

FAA diagram of the TIS-B and ADS-R hockey puck: a cylinder 30 nm in diameter and plus or minus 3,500 ft around the client aircraft
The client "hockey puck": 30 nm across and ±3,500 ft around an aircraft with qualifying ADS-B Out. Image: FAA

ADS-R Automatic Dependent Surveillance–Rebroadcast

ADS-R bridges the two links. When a 1090 aircraft and a UAT aircraft are near each other, the ground station relays each one's broadcast on the other frequency so they can see each other. Like TIS-B, it's client-based and available at or below FL240.

FAA illustration of ADS-R: a ground-based transceiver cross-links a UAT aircraft and a 1090ES aircraft
ADS-R: the ground station cross-links 978 and 1090 traffic so each side sees the other. Image: FAA

5What a portable receiver sees without ADS-B Out

This part surprises most pilots. If your airplane has no ADS-B Out (or it doesn't meet the performance requirements), the ground stations don't know you're there, so they don't build a hockey puck around you. You only see ground-relayed traffic when you happen to be inside someone else's puck:

Traffic picture with and without your own ADS-B Out Left: with ADS-B Out, the TIS-B and ADS-R cylinder is centered on you and you see all relayed traffic around you. Right: without ADS-B Out you only see relayed traffic inside a nearby client aircraft's cylinder, so traffic close to you outside that cylinder is missing. With your own ADS-B Out The puck is built around you You Without ADS-B Out You piggyback on another aircraft's puck Client aircraft You not shown
You Relayed traffic (TIS-B / ADS-R) Direct air-to-air ADS-B (always visible) Nearby traffic you won't see

Without ADS-B Out you still get all FIS-B weather and direct air-to-air traffic from every aircraft with ADS-B Out on the frequencies you receive, which is the most reliable traffic you'll get. The FAA notes that piggybacked TIS-B and ADS-R give an incomplete picture, so treat a quiet display as "no traffic shown," never "no traffic." With your own qualifying ADS-B Out, a portable receiver gets the full picture built around you.

6Ground stations and range

The FAA network has roughly 650 ground stations, all on our ADS-B ground station map. Each UAT station is assigned one of four altitude tiers (high, medium, low and surface), which sets the area it covers and the weather products it uplinks.

ADS-B is line of sight: terrain, buildings and the curve of the earth block it. That's why altitude matters so much. Drag the slider to see how far a receiver could hear a station in theory:

1,000 ft AGL
51 nm theoretical line-of-sight range
Typical in practice: 40–60 nm in the air.
Ground station 51 nm 050 nm100 nm150 nm200 nm

Radio-horizon estimate: d ≈ 1.23 × (√haircraft + √hantenna) nm, heights in feet, assuming a 100 ft station antenna and flat terrain. Real range is shorter: terrain, buildings, antenna placement and transmit power all cut it down. Not to scale vertically.

In the real world, receivers typically hear a station 40–60 nm away in the air but often under 1 nm on the ramp, where hangars, terrain and a low antenna block the signal. Most pilots start seeing weather around 1,000 ft AGL near a station, so no weather while you taxi is normal.

In Canada, the volunteer-run CIFIB network runs 978 MHz UAT stations in Ontario, Alberta, Saskatchewan and Nova Scotia. They broadcast the same way the FAA network does, so any US UAT receiver works with them. They're on the map in red.

Map of ADS-B ground stations across the United States and Canada, colored by tier
Every FAA and CIFIB station, colored by tier. Open the interactive map →

7Using ADS-B safely

FAA figure of a multifunction display in traffic map mode showing a traffic advisory 500 ft above and a target 300 ft below
Traffic on a multifunction display: relative altitude in hundreds of feet (+05 is 500 ft above you) and a trend arrow (climbing or descending). Image: FAA Instrument Flying Handbook (FAA-H-8083-15B), fig. 11-18
Traffic is advisory

TIS-B isn't a collision avoidance system and doesn't replace see-and-avoid. Use it to spot traffic, not to maneuver around a target you can't see.

Not everyone shows up

Aircraft without a transponder, or outside radar coverage, won't appear as TIS-B targets.

Check weather timestamps

Uplinked radar can be several minutes older than it looks. Use it for strategic decisions, not to pick a path through a line of storms.

Mind the altitude filter

Many EFBs only show traffic within a band around you, commonly ±3,500 ft.

8What you need in the cockpit

You need two things: an ADS-B In receiver and a display, usually an EFB app on a tablet or phone. A portable receiver sits on the glareshield, creates a Wi-Fi network, and streams weather, traffic and GPS position to your EFB.

Stratux dual-band ADS-B receiver in a white case with two antennas

Stratux is an open-source, community-built ADS-B In receiver. It's dual-band (978 + 1090), so you get FIS-B weather and direct traffic on both links. It has GPS, and an optional AHRS gives you a backup attitude display (a valuable backup, not certified for IFR). It works with most popular EFB apps, including apps compatible with ForeFlight®, and because it's modular, every part can be repaired or replaced.

Ready-to-fly units Build your own EFB compatibility

Frequently asked questions

Do I need ADS-B In?

No. The rule (14 CFR 91.225) requires ADS-B Out in most controlled airspace. ADS-B In is optional, but it gives you free weather and traffic in the cockpit.

Can a portable receiver show traffic if my airplane has no ADS-B Out?

Yes, partly. You'll see aircraft that broadcast ADS-B Out directly, plus TIS-B and ADS-R traffic generated for nearby aircraft that have it. That picture is incomplete; full service requires your own qualified ADS-B Out.

Why is there no ADS-B weather on the ground?

FIS-B comes from ground stations by line of sight, and on the ramp you're usually out of range. Weather typically appears after takeoff, often by about 1,000 ft AGL near a station.

Does ADS-B In work outside the United States?

1090 MHz traffic from other aircraft can be received worldwide. 978 MHz UAT and FIS-B weather are available in the United States and, through CIFIB, in parts of Canada.

Sources and image credits