The trough is parabolic along one axis and linear in the orthogonal axis. For change of daily position of the sun perpendicular to the receiver, the trough tilts east to west so that the direct radiation remains focussed on the receiver. However,seasonal changes in the angle of sunlight parallel to the trough does not require adjustment of the mirrors, since the light is simply concentrates elsewhere on the receiver. This the parabolic trough design does not require tracking on a second axis.Since 1985, solar thermal system using parabolic trough technology has been in full operation in California (US). It is called the Solar Energy Generating System (SEGS) and is a collection of nine plants with a total capacity of 350 MW. It is currently the largest operational solar (both thermal and photovoltaic) system in the world.