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?
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.