Tutorial UAT

Tutorial: UAT (978 MHz in the United States)

What it is

UAT, the Universal Access Transceiver, is the ADS-B link used by general aviation aircraft in the United States below 18,000 ft. On 978 MHz it carries ADS-B position and identity reports, FIS-B weather text products and a relay of other traffic (TIS-B). It is not used in Europe. OnAir shows the aircraft, the weather text and the relayed traffic.

What you need

• A radio that tunes 978 MHz. The mode asks for 2 Msps and the lowest working rate is 1.9 Msps; a typical capture is 1 Msps, so check the status for the rate in use. An RTL-SDR can do it.
• A 978 MHz antenna. A 1090 MHz antenna is close but not tuned; a short quarter-wave whip (about 7.7 cm) is a good start. A band-pass filter at 978 MHz helps near cellular sites.
• Frequency: 978 MHz (the database entry reads "UAT ADS-B", general aviation below FL180 in the US). Outside North America you will normally hear nothing.

Before you start

• Gain as for ADS-B: high enough to reach noise, not so high that strong nearby signals overload the radio.
• Set your position in the ADS-B tuner (the same setting is used) if you want ranges.

Step by step

1. Choose the group Aviation, entry Aircraft, 978 MHz (mode UAT).
2. Press Start. The side panel is headed AIRCRAFT. Your receiver position (latitude and longitude) is set in the tuner.

What you should see

In a busy region, aircraft rows appear with ICAO address, callsign, altitude, speed and range. Weather text appears in the FIS-B tab when a ground station is in range.

Reading the screen

• Aircraft tab: ICAO, Call, Alt ft, Speed, Hdg, Range, Src (where the message came from) and Age. Range needs your receiver position.
• FIS-B tab: the weather and notice text products received from ground stations.
• Log tab: every decoded message, newest first, with its kind.

Troubleshooting

• Nothing at all outside the US: expected; UAT is used in the US only.
• No aircraft in the US: antenna and gain; small aircraft are low, so line of sight matters more than for airliners.
• Rate too low: below 1.9 Msps the mode cannot run as set; check the status.

Tips

Run ADS-B on another receiver slot to see 1090 and 978 MHz traffic together.

Legal and privacy notes

Receiving is lawful in many countries; check yours; see Legal. Do not use the weather text for flight planning: it is not an official source here.

Under the hood

Continuous-phase FSK at 1.041667 Mbit/s with a Reed-Solomon forward-error-correcting code on the data blocks; ADS-B messages and ground uplink frames per RTCA DO-282B.

Related

UAT mode page, Tutorial: ADS-B, Live map.