Active stations are GPS receivers which constantly write Rinex data out to the Ordnance Survey OS Net website. The files are publically downloadable, and allow measurements taken in the field to be post-processed for greater accuracy.
Early active stations had conical antennae and were made by ???. They are often seen in small arrays. A major upgrade to the active network was started in December 2008 and most, if not all, active stations are now made by Leica and feature a dome shaped antenna.
A commercial offering called smartnet is available, which uses OS data to provide customers with RTK accuracy of a few centimeters.