range finder

Instrument used to measure the distance from the instrument to a selected point or object. The optical range finder, used chiefly in cameras, consists of an arrangement of lenses and prisms set at each end of a tube. The object's range is determined by measuring the angles formed by a line of sight at each end of the tube; the smaller the angles, the greater the distance, and vice versa. Since the mid-1940s, radar has replaced optical range finders for most military targeting, and the laser range finder, developed in 1965, has largely replaced optical range finders for surveying and radar in certain military applications.

Range Finder

an instrument for measuring distances. It is used extensively in engineering geodesy (in the construction of transportation and communications lines, hydraulic-engineering structures, and electric power transmission lines), topographic surveying, the military (principally for determining distances to targets), navigation, astronomical research, and photography.

A distinction is made between range finders of the geometric and physical types on the basis of their principle of operation. The measurement of distances with range finders of the geometric type is based on the determination of the height h of the isosceles triangle ABC(Figure 1)—for example, from the known side AB = l (the base) and the opposite acute angle β(the so-called angle of parallax). For small β(expressed in radians), h = l/β. One of the quantities l or β is usually constant; the other is variable (the one being measured). On this basis a distinction is made between range finders having a constant angle and range finders having a constant base.

Figure 1. Diagram illustrating the principle of operation of a geometric range finder: AB is the base, β is the angle of parallax, and h is the distance to be measured

A crosshairs range finder with a constant angle is a terrestrial telescope with two parallel hairs in the field of view. A telemeter rod with equidistant divisions serves as a base of the range finder. The distance measured by the range finder to the base is proportional to the number of rod divisions visible through the telescope between the hairs. Many geodetic instruments (such as theodolites and leveling instruments) have a crosshairs range finder. The relative error of a crosshairs range finder is ˜0.3-l .0 percent.

More intricate optical range finders of the geometric type have their own fixed base. They are divided into two groups, monocular and binocular (stereoscopic).

A monocular range finder (Figure 2) is constructed in such a way that the image of the object (target) can be seen in the eyepiece E as formed by two halves divided by a horizontal line; the halves of the image are constructed by rays that pass through the various optical systems of the range finder (0₁ and O₂). In the case of a very remote object, when the rays A₁and A₂ incident upon the range finder are virtually parallel, both halves of the image are located at the same point on the horizontal dividing line and form the whole image. As the object approaches the range finder, the parallelism of rays A, and A₂ is disrupted, and the halves of the image diverge along the dividing line. To measure the distance to the object it is necessary to bring together the displaced halves of the image

Figure 2. Construction of a monocular range finder: B₁ and B₂ are reflectors placed at the ends of the base, O₁ and O₂ are optical systems that construct the images, C is a special reflector (prism) that brings together both images in a common focal plane F₁and Eis the eyepiece. The image visible through the eyepiece before focusing (a) and after focusing (b) is shown in the circles.

by using an optical compensator housed in one of the optical systems. The result of the measurement is read from a special scale. The error of monocular coincidence range finders is ˜0.1 percent at distances up to 1 km.

Monocular range finders with a base of 3–10 cm are widely used as photographic range finders. Photographic range finders are usually combined with the viewfinder of a still or movie camera into one optical system. The light rays from the object being photographed pass into the range finder (Figure 3) through two different optical systems (main and auxiliary). The images constructed by these systems are seen separately in the eyepiece. To focus the lens and produce a clear photograph, the two images are brought together by moving the optical compensator, which is linked to the focusing mechanism of the camera lens.

Figure 3. Photographic range finder: C₁ and C₂ are prisms, Bis the camera lens, and kis the lever. Before focusing, the eye sees two images (a) In the viewfinder; after focusing —by rotating the lens and moving the lever attached to the prism —just one image (b) remains

A stereoscopic range finder with a fixed base (Figure 4) is a double telescope with two eyepieces. The operation of the range finder is based on the stereoscopic effect: images viewed separately by each eye are fused into one composite image in which the difference in the location of the objects in depth can be perceived. To determine the distance to the object (target) the image of the object is combined with the image of a special mark (label) located in the focal plane of the range finder. The object and the mark should appear to be located at the same distance from the observer. The displacement of the optical compensator necessary to bring the mark and the target together is proportional to the distance being determined. The accuracy of stereoscopic range finders, particularly those having a base of several meters, is an order higher than the accuracy of monocular range finders.

The principle of operation of physical range finders-light, radio, and acoustic-consists in the measurement of the time required for the signal sent by the range finder to traverse the distance to the object and back. The speed of propagation of the signal (the speed of light c or the speed of sound v) is considered to be known.

Light range finders, or electrooptical range finders, are divided into pulse and phase types. Range finders of the former type measure directly the time interval τ in which the light pulse covers twice the distance to the object 2L, so that L = CT/2 + k, where k is the range finder constant.

In phase range finders a continuous light beam with artificially generated high-frequency modulation of its intensity is used. When the change in the frequency of modulation is smooth, the difference in the modulation phases changes in the sent and reflected light beams. In consequence, maximums and minimums of light intensity are observed in the range finder, and the time τ and then L are determined from the number of maximums and minimums. According to size and accuracy, light range finders are divided into large, medium, and small (topographic), which allow the measurement of distances of 20–25 km with an accuracy of 1:400,000, 5–15 km with an accuracy of 1:300,000, and 5–6 km with an accuracy of 1:10,000 to 1:100,000. A reflector for a laser

Figure 4. External view (a) and schematic diagram (b) of a stereoscopic range finder: A₁ and A₂ are the apertures, β, and B₂ are reflectors (prisms), O, and O₂ are optical systems that construct the images, C is the compensator for bringing the mark together with the image, C, and C, are prisms, and E is the eyepiece; (c) field of view, with marks

range finder designed to measure the distance to the moon (about 385,000 km) with an accuracy of several meters was installed on Lunokhod I.

In range-only radar (radio range finders), electromagnetic waves of the centimeter and millimeter bands are usually used. Distinction is made between pulse range-only radars and range-only radars with continuous radiation.

Because of the strong absorption and dispersion of light and radio waves by condensed mediums (liquids and solids), light range finders and range-only radars are used only under atmospheric conditions and in space. Acoustic range finders (sonars) are used to determine distances in the depths of the oceans and seas, since the absorption of ultrasonic vibrations is insignificant.

In theory the radius of operation of physical range finders is determined by the power of the transmitted signals and by the sensitivity of the range finder receiver that picks up the reflected signal. The capabilities of range finders are illustrated by the following example: during the flight of the Venera 7 interplanetary station in the distance between the earth and Venus (more than 60 million km) was measured to accuracy of 1 km.

REFERENCES

Kratkii topografo-geodezicheskii slovar’-spravochnik. Moscow. 1968.
Kondrashkov. A. V . Elektroopticheskie dal’nomery. Moscow. 1959.
Provorov. K. L. Radiogeodeziia. Moscow, 1965.
Borodulin. G. 1. “Obzor sovremennoi svetodal’nomernoi apparatury.” Geodeziia i karlografiia, 1970. no. 7.

IU. N. DROZHIN-LABINSKII

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