BBC News, 8/29/12
Known as Unattended Ground Sensors (UGS), the technology has been around since the 1970s. But advances mean the sensors are now tiny, solar powered and capable of operating for decades.
They are backed up by watchtowers fitted with infra-red radars and optical sensors purchased under SBInet before it was cancelled…
The sensors are supported by drone aircraft known locally as “desert phantoms”. The unmanned planes are capable of detecting people and vehicles from a height of 6,000 metres (19,685ft).
The Predator comes equipped with radar, seven video cameras, an infra-red sensor and a powerful zoom. Each one costs $20m and, according to the Department of Homeland Security, nine of them patrol the skies over Arizona, Florida, Texas and North Dakota.