TIS-B and ADS-R: Relayed ADS-B Traffic Explained

TIS-B (Traffic Information Service-Broadcast) is radar traffic that FAA ground stations rebroadcast to ADS-B equipped aircraft, so you can see airplanes that have a transponder but no ADS-B Out. ADS-R (ADS-B Rebroadcast) relays ADS-B Out traffic from one frequency to the other, 1090 MHz to 978 MHz and back.

Both are client-based. The ground system builds them for aircraft with qualifying ADS-B Out. A receiver without ADS-B Out, like a portable Stratux in an airplane with no Out, only sees what's built for other aircraft nearby. That's why the traffic on your screen can look patchy.

The big-picture version is in our pilot's guide to how ADS-B works. This page goes deeper on the relayed traffic.

TIS-B: radar targets rebroadcast to the cockpit

The AIM defines TIS-B as "the broadcast of ATC derived traffic information to ADS-B equipped (1090ES or UAT) aircraft from ground radio stations" (AIM 4-5-8). The FAA builds it from radar, wide area multilateration and surface systems like ASDE-X (FAA ADS-B In pilot page).

What goes into it:

Transponder-equipped aircraft. The AIM says only Mode A/C or Mode S targets are sent. An aircraft with no transponder won't show as TIS-B traffic. The FAA's equipment FAQ notes one exception: on airport surfaces, TIS-B also uplinks vehicles with no transponder.
ADS-B aircraft that don't meet the performance numbers. The FAA pilot page calls these TIS-B targets too.

What it needs, per the AIM:

A ground station set up for TIS-B uplinks. Not every station does it, because some have no radar feed.
The target inside coverage of, and detected by, at least one radar serving that station. No radar means no TIS-B. Radar siting can limit coverage at low altitude near some airports.
Altitude at or below FL240 (FAA pilot page).

TIS-B is slower than direct ADS-B. The AIM puts TIS-B updates at about every 3 to 13 seconds, depending on the radar. ADS-B updates about once a second. A TIS-B target can lag a little behind where the airplane really is.

ADS-R: the bridge between 978 and 1090

The US runs ADS-B on two frequencies. Airliners and anything flying in Class A use 1090 MHz. Many GA aircraft below 18,000 ft use 978 MHz UAT. A 978 MHz receiver can't hear a 1090 MHz broadcast, and a 1090 receiver can't hear 978.

ADS-R fixes that on the ground. The AIM calls it "a datalink translation function" that takes ADS-B messages on one frequency and rebroadcasts them on the other (AIM 4-5-10). Aircraft on the same frequency talk directly and don't need it.

Two details from the FAA:

Dual-band aircraft don't get ADS-R. If the ground system sees an aircraft with ADS-B In on both links, it only provides TIS-B. That aircraft already hears both frequencies air to air (FAA Ins and Outs).
Same-link rebroadcast on the surface. In surface service volumes, ADS-R also rebroadcasts on the same link. The FAA says this helps when hangars and buildings block direct air-to-air messages on the airport (FAA pilot page).

The client hockey puck

The ground system doesn't send every target to everyone. It builds traffic for each client aircraft inside a cylinder around it, 15 nm in radius (30 nm across) and ±3,500 ft. Pilots call it the hockey puck.

To be a client, the FAA says an aircraft must (Ins and Outs):

Be where TIS-B or ADS-R is offered.
Broadcast ADS-B Out with a valid position in the last 30 seconds.
Have ADS-B In on only one link.
Since January 4, 2016, meet these performance values: NACp > 4, NACv > 0, SDA > 0, SIL > 0 and NIC > 4.

An aircraft with ADS-B In on both links still gets TIS-B, just no ADS-R (see above). Any other-link aircraft inside the puck goes to the client by ADS-R. Any transponder aircraft without ADS-B Out inside the puck goes by TIS-B.

Why ADS-B In without Out sees partial traffic

This catches most portable-receiver owners. If your airplane has no ADS-B Out, or its Out doesn't meet the numbers above, the ground system doesn't know you're there. No puck gets built around you.

You still pick up relayed traffic when you're near a client, because it's a broadcast and anyone in range can hear it. The FAA calls this piggybacking. It says it "will result in an incomplete representation of traffic around the aircraft" (FAA pilot page).

Here's what that means in the air:

Relayed traffic needs a client nearby. Fly alone in a quiet area and you only see aircraft broadcasting ADS-B Out directly.
The puck is centered on someone else. Relayed targets are within 15 nm and ±3,500 ft of that client, not of you. Some shown targets can be far away, and a radar target close to you can be missing.
It changes as the client moves. Targets can pop in and out as clients come and go.

The FAA says the most reliable traffic an In-only receiver gets is direct air to air (Ins and Outs). A dual-band receiver hears both 978 and 1090 directly, so it gets every ADS-B Out aircraft in radio range without waiting on the ground to relay it. Treat a quiet screen as "no traffic shown," never "no traffic."

TIS-B quirks you'll see

The AIM lists a few limits worth knowing:

Your own airplane as a target. You may get an intermittent TIS-B target of yourself, usually in climbing turns, because radar tracks slower than ADS-B. Our ghost traffic guide shows how to filter it out in Stratux.
Duplicates. When the ground system can't match an ADS-B report to the radar return for the same airplane, you can see two symbols. The AIM calls them TIS-B shadows.
No warning when you leave coverage. Nothing tells you when you're inside or outside the TIS-B service volume.

TIS-B and ADS-R are advisory only. The AIM says no avoidance maneuvers are authorized just because a target shows on your display. Keep looking outside.

What Stratux shows

Stratux decodes relayed traffic from both radios. On 1090 MHz, it reads ADS-R and TIS-B from the extended squitter control field. On 978 MHz, it uses the address type in each UAT report. Source: main/traffic.go in the Stratux code.

On the Traffic page of the web interface, each target's color shows which radio heard it and whether it came direct or relayed. Every target uses the same airplane symbol.

Source	Color on the Traffic page	Callsign prefix
1090 MHz ADS-B, air to air	cornflower blue	ea
1090 MHz ADS-R	sky blue	er
1090 MHz TIS-B	sky blue	et
978 MHz ADS-B, air to air	dark orange	ua
978 MHz ADS-R	orange	ur
978 MHz TIS-B	orange	ut or ur

Colors are from web/js/main.js in Stratux v2.0. On 978 MHz, a TIS-B target that carries an ICAO address gets the same "ur" prefix as ADS-R. Only TIS-B targets without one get "ut".

Turn on Show Traffic Source in Callsign and Stratux adds that prefix to each callsign. The switch is on the Settings page in developer mode (see web interface). The code comment calls it a way "to show the source of the traffic on moving maps," so you see it in your EFB too. A TIS-B target with no ICAO address shows an FAA track file ID in the Code column, marked TFID.

Seeing targets in the web interface but not in your app? See traffic in the web UI but not the EFB.

Frequently asked questions

What is TIS-B?

TIS-B (Traffic Information Service-Broadcast) is radar and multilateration traffic that FAA ground stations rebroadcast to ADS-B equipped aircraft. It shows transponder-equipped aircraft that don't broadcast ADS-B Out.

What is ADS-R?

ADS-R (ADS-B Rebroadcast) is a ground-station function that relays ADS-B Out reports from one frequency to the other. It lets aircraft on 1090 MHz and 978 MHz see each other.

Why does my portable receiver show only some traffic?

TIS-B and ADS-R are built for client aircraft with qualifying ADS-B Out. Without your own ADS-B Out, you only see relayed traffic built for nearby clients. The FAA calls that picture incomplete.

Does Stratux need ADS-R?

Less than a single-band receiver does. A dual-band Stratux hears 978 and 1090 MHz ADS-B Out directly. It still decodes and shows any ADS-R traffic that ground stations send for nearby clients.

Relayed traffic depends on where the ground stations are. Read ground stations and coverage next, or the weather side in FIS-B explained.