A precision ranger system for measuring the large machining pieces or structures is described. The system uses the principle of beat-wave measurement. The light source of this system is a frequency stabilized laser with a frequency stability of 1×10-7 (open air), which does not depend on any reference frequency. It operates in two longitudinal modes; i.e., the laser emits two beams, which generate in the common resonator of the laser. The beat frequency is about 790 MHz and the wavelength is about 380 mm. The two beams are naturally coaxial. The node of the beat-wave is used as sampling flag and the distance between the node and the close point is measured with double frequency interferometer, which is combined in the same optical system and its resolution is 0.08. As a precise ranger, it does not need a long guide or any heavy machinery. The system described here has the measuring range of 0~20 meters and the measuring uncertainty of 30 μm / 10 m.
