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SESK: Stuff Everyone Should Know · sesk.info/pilot-adsb-1090es-class-a

Which type of ADS-B Out equipment is required in Class A airspace, beginning at 18,000 feet MSL?

1090ES, the extended squitter on 1090 MHz.

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  1. UAT on 978 MHz
  2. Either type, pilot's choice
  3. 1090ES on 1090 MHz ✔

Why: The US uses two ADS-B links. Below 18,000 feet you may use either UAT on 978 MHz or 1090ES. In Class A airspace, 1090ES is required.

Huh, didn't know that: Weather and flight information broadcast to cockpits (FIS-B) goes out only on the 978 MHz UAT link.

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Why did the ADS-B unit talk so much? It was always broadcasting its position.
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References

Answer: 1090ES on 1090 MHz. The US uses two ADS-B links. Below 18,000 feet you may use either UAT on 978 MHz or 1090ES. In Class A airspace, 1090ES is required.

  1. AIM Chapter 4 Section 5, Surveillance Systems www.faa.gov/air_traffic/publications/atpubs/aim_html/chap4_section_5.html
    In general, operators flying at 18,000 feet and above (Class A airspace) are required to have 1090ES equipment.
    Checked 2026-10-09.

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