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If an aircraft is flying in the USA with 1090 MHz Extended Squitter (ES) and it is below 18,000 feet, does it then need to also have 978 MHZ Universal Access Transceiver (UAT) so that aircraft flying below 18,000 equipped with 978 MHz (UAT) can detect it?

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    $\begingroup$ From Wikipedia: "UAT users [...] can receive reports from proximate traffic (TIS-B) through a multilink gateway service that provides ADS-B reports for 1090ES-equipped aircraft and non-ADS-B equipped radar traffic. [...] Viable ADS-B UAT networks are being installed as part of the United States' NextGen air traffic system." $\endgroup$
    – mins
    Commented Sep 16, 2017 at 12:08

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No. ADS-B ES equipped aircraft do not have to have UAT to allow ADS-B UAT equipped aircraft to 'see' them. Below FL180, they are tracked by ATC radar and their track is translated and re-transmitted to UAT equipped aircraft via TIS-B service. More information, as well as coverage maps are available at the NextGen ADS-B site.

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Actually, aircraft broadcasting ADS-B Out on one frequency (say 1090Mhz) and equipped with ADS-B In, will receive traffic operating ADS-B Out on the other frequency (in this example, 978Mhz) via the ADS-R (ADS-B Re-broadcast) service.

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