i) Those in which a beam of light is the carrier and which is reflected back from a mirror located at the other end. Such instruments are less expansive because one active instrument and a battery are only needed at one end and instrument at the other end is simply a reflecting mirror centered over the ground station mark. Such instruments are used for shorter distances only. When the distance between the stations is comparative large, a telescope is used for aiming the beam at the mirror and laser is used as carrier, because this has a very small angle of spread.
ii) Those which transmit short radio waves (micro-waves) as carrier. In these type of instruments, an active instrument and operator are acceded at each end of the line. The signal is re-transmitted on another carrier wave back to the originating instrument. The wavelengths of the carrier of such instrument varies from 8 to 10 mm. These instruments do not require precise directional setting because the spread angle of carrier waves is between 5 degree to 10 degree. Instruments using microwaves are generally used for the long distances required in primary triangulation and precise traversing.