What is ADS-B?

ADS-B is a system that tracks and relays aircraft traffic to ground stations and other aircraft.  ADS-B stands for Automatic Dependent Surveillance Broadcast.

ADS-B is:

Automatic – It works on it’s own, the pilot doesn’t need to do anything

Dependent – Unlike radar, ADS-B relies on other aircraft being equipped with the right equipment.

Surveillance –  It tracks other aircraft.

Broadcast – The aircraft also transmit their own velocity and position.

ADS-B Out vs ADS-B In

ADS-B Out is a system that reports the aircraft’s velocity, altitude, and position.  ADS-B Out continually reports – about once a second.  By January 1, 2020 all aircraft that operate in areas that currently require a Mode C transponder will need to become ADS-B Out compliant.  Mode C will still be required in conjunction with ADS-B Out.

ADS-B In is a system that allows aircraft to receive transmissions from other aircraft and ground stations.  ADS- B In also allows pilots to receive traffic and weather.  There is no mandate for ADS-B In.  It is completely optional.

978 MHz – 1090 MHz Frequencies

There are two different datalinks for ADS-B in the US, 978 and 1090 MHz.  There are two different frequencies to help alleviate congestion.

ADS-B Out 978 MHz Universal Access Transceiver (UAT) – This is only allowed below 18,000 ft. and in the United States.  This is a lower cost option designed for general aviation aircraft.  If 978 MHz is being used, the aircraft still needs to be equipped with at least on transponder.  This must be hard-mounted to the aircraft, it cannot be portable.  978 Out is designed for General aviation piston airplanes.

ADS-B Out 1090 MHz Extended Squitter (ES) – This is more or less an upgraded transponder.  This must also be installed in the aircraft.  This will be required above 18,000 ft. and for international ops.  This is designed for turbojets and turboprop aircraft.

ADS-B In 978 MHz Universal Access Transceiver (UAT) – This can detect other aircraft equipped with ADS-B Out 978 transmitters and can receive traffic from ground stations.  978 In can receive weather information.

ADS-B In 1090 MHz Extended Squitter (ES) – This will not receive weather.  ADS-B In 1090 receives traffic from ground stations and can detect aircraft that are equipped with ADS-B Out 1090 MHz transmitters.  This can be a portable or hard-mounted.

How Does ADS-B In it Work?

ADS-B In continuously receives weather.  Think of it as an AM radio, if you are tuned in, you receive it.  However, traffic is only broadcast in response to ADS-B Out.  When ground stations receive ADS-B Out signals, they reply back to your aircraft with traffic.  The ground station sends back all traffic that is within 30nm in diameter (horizontally) of your aircraft and within 3,500 ft. (vertically).  This is known as the “hockey puck”.  If your aircraft only has ADS-B In, it can receive air-to-air signals from another aircraft that is within range and has ADS-B Out, and see what is in that aircraft’s “hockey puck”.


When your aircraft is equipped with ADS-B Out, a WAAS GPS will also be required in the aircraft.  With ADS-B Out, your aircraft is reporting it’s position and ATC uses that position to separate aircraft.  Your position must be reported accurately and that is why WAAS GPS is required.

ADS-B Equipment Requirements

The ADS-B equipment must meet the requirements of 14 CFR 91.225 and 91.227 and TSO-C166b or TSO-C154c.  It is a lot easier to check the FAA website here for a list of ADS-B certified equipment.

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