Carl Zeiss of Oberkochen, Germany, has taken an important step in the completion of a mammoth ring laser gyroscope with the delivery and installation of the 10-ton, 4.25-m Zerodur disc that is the centerpiece of the instrument. The gyroscope, which was commissioned by the German Federal Cartography and Geodesy Office of Frankfurt am Main, is slated to begin operation at an underground laboratory in Wettzell in 2001. The instrument will monitor and detect perturbations in the Earth's rotation. Geophysicists hope the precision of the device will enable them to better understand the interior of the planet by observing such changes in its rotational velocity. Satellites will use the gyro to improve the accuracy of navigation technologies such as the Global Positioning System.