Is ADS-B UAT required below 18,000 ft for ADS-B ES equipped aircraft?
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.