geodimeter

an instrument designed to measure distances by means of the time required for electromagnetic waves in the optical or infrared regions of the spectrum to cover the distance.

There are both pulse and phase-comparison Geodimeters, classified according to the method used to determine the passage time over the distance to an object and back again. The pulse type measures distance according to the time between the moment a pulse is emitted by the transmitter and the moment when the pulse returns from a reflector erected at the end of the distance being measured. The phase-comparison type makes measurements according to the phase difference between the sinusoidally modulated radiation that is transmitted and the reflected radiation that is received.

Figure 1. Block diagram of a Geodimeter

Phase-comparison Geodimeters are the most common (see Figure 1 for a simplified block diagram). Incandescent lamps (3–30 watts) and gas-discharge lamps (50–100 watts) were formerly used as light sources; they have been superseded by gas and semiconductor lasers. Usually amplitude modulation at frequencies between 10 and 80 megahertz is used so that a phase difference of 1° corresponds to a change in distance of less than 1 cm. The modulator and demodulator are identical in design; their operation may be based on the Kerr effect or the Pockels effect. The alternating voltage that modulates the luminous flux is produced by a scale-frequency generator, so-called because the wavelength corresponding to the scale frequency determines the scale for converting the phase difference to a distance. The modulated light is focused by a lens or mirror-lens optical system into a narrowly directed beam, which is transmitted to the reflector. The reflected light is focused on the demodulator by an optical system similar to that of the transmitting system. The magnitude of the phase difference registered by an indicator at the output of the demodulator depends on the relation between the phases of the received optical signal and the voltage that controls the demodulator; a phase shifter makes it possible to determine the specific relationship and to compute the registered phase difference, from which the distance is calculated. The phase-difference indicator may be the eye of an observer or a photoelectric instrument with a pointer connected to the output.

Geodimeters can measure distances up to 50 km with a mean square error of ±(1 + 0.2D km) cm, where D is the distance measured. The weight of the instrument ranges from 30 to 150 kg, and the power consumption is between 5 and 150 watts.