The extremely precise measurements are realised by a laser interferometer that measures the distance between two free-floating heavy masses with reflective surfaces. In the case of LISA Pathfinder, the accuracy of the interferometer measurements is tested by the Optical Metrology Subsystem (OMS; managed, designed, and tested by Astrium GmbH) which comprises several units: a reference laser unit (developed by Tesat, Germany), a laser control unit (by Kayser Threde, Germany) with a laser modulator (developed by RUAG, Switzerland), an optical bench interferometer (built by the University of Glasgow, UK), a phase meter (University of Birmingham, UK), and a data management unit (ICE and NTE, Spain).