Patent GB-1156548-A (GB)
Title: Improvements in or relating to Distance Measurement
Inventor: FROOME KEITH DAVY
IPC: G01S17/32
Filed: 1966-12-01
Abstract: 1,156,548. Optical range finding. NATIONAL RESEARCH DEVELOPMENT CORP. 29 Nov., 1967 [1 Dec., 1966], No. 53926/66. Heading H4D. The Specification describes a development of the elliptical polarization optical range finding apparatus described in Specification 1,104,021, wherein, by the use of several successive modulation frequencies the range to the distant reflector may be determined in successive orders of magnitude, and the need to determine the number of whole wavelengths contained in said range is obviated. In the apparatus, Figs. 1 and 2, a plane polarized beam of light b1 is elliptically polarized by means of K.D.P. crystals X 1  and X 2 , with the degree of ellipticity varying with the modulation frequency developed in the tuned cavity resonator M, housing the crystals. The elliptically polarized beam is directed to and reflected from a distant reflector. The reflected beam b3 is directed at a variable path length arrangement VP comprising a mirror assembly R4, R5, movable under the control of calibrated tape T. Beam b3 is directed through K.D.P. crystals X 4  and X 3 , where its degree of elliptical polarization is increased or decreased, according to the total go and return path length. The final degree of polarization is determined by an analyser filter P 2 , crossed with respect to polarizer P 1 , and a photomultiplier PM feeding an indicator P.I. The cavity resonator M is tuned (tuning member TM) to the various modulation frequencies by means of a standard cavity resonator SR, pressure and humidity controlled as described in Specification 1,104,021, a frequency multiplier FM, a series of local oscillators LO feeding either the input or the output of the frequency multiplier, and a resonance indicator RI. Tape T, calibrated to give the fractional part of the range with the aid of vernier VR, is supported by a geared drum G1 which meshes with gears G2, G3 and G4, the last two having spindles frictionally carrying pointers B1 and A1 moving respectively over scales B and A indicating hundreds or tens of feet and units of feet respectively. Units of thousands of feet are obtained from scale C indicating the position of fine tuning plunger FT. In general, the range D can be expressed as:- D =  ¢ # (N+#+p) # = modulation wavelength N = whole number of wavelengths 8 = fraction of wavelength p = phase factor, = 0 in above arrangement. Thus for two wavelengths # 1  and # n  D = ¢# 1  (N 1 +# 1 ) = ¢# n  (N n +# n ) and N 1  #n/#n-1[(N 1  - N n ) + (# 1  - # n )] - # 1    # 1  is made zero, by putting # 1  = 2ft., and adjusting the variable path device until a null output is indicated at P.I. The reading on vernier VR then gives the fractional part of the distances. Therefore N 1  = #n/#n-# 1  [(N 1  N n ) - # n ] The invention allows the determination of N 1  in ascending orders of magnitude by arranging the modulation wavelengths such that #n/# n  - # 1  has successively ascending values as follows:- Thus for range of 4867.315 ft., the successive measurements may be tabulated as follows: the required multiplication ratio of frequency multiplier FM, the method of local oscillator feed, and the means of determining # n  and what it represents after a zero reading has been obtained on indicator PI by adjustment of variable path length device VP:- With # = # 1 , fine tuner plunger FT, is varied until one cycle of change from zero to zero is observed on indicator P.I. Scale C then gives # 5 , i.e. Thus D=# 1 +# 2 +# 3 +# 4 +# 5 =4867.315 ft. In the arrangement of Figs. 1 and 2, the electric field vectors E in the cavity M are radially aligned, and the Z axes of the K.D.P. crystals are aligned therewith, the X axes of the crystals X 1 , X 2  and X 3 , X 4  being aligned with the polarization plane of the filters P1 or P2 respectively. Figs. 1a and 2a (not shown) show an arrangement with the electrical field vector axially aligned and only two crystals (one per go and return path) in use.