The height of structures in the United States has been poorly documented. However, the data is a matter of public record, appearing in documents maintained by the Federal Aviation Administration (FAA) and Federal Communications Commission (FCC). Since 1978 the United States has maintained eleven tethered aerostats sites along the southern borders. These balloons rise to 18,000 feet (5,500 m), carrying radar units for drug interdiction purposes. However, since the balloons are aided by buoyancy and are not permanent they are not considered true structures.