Patent GB-679081-A (GB)
Title: Apparatus for measuring magnetic declination and dip abroad aircraft
Inventor: COGNE RENE
IPC: G01R33/02
Filed: 1950-11-17
Abstract: 679,081. Magnetic measurements. OFFICE NATIONAL D'ETUDES ET DE RECHERCHES AERONAUTIQUES. Nov. 17, 1950, No. 28123/50. Class 37. [Also in Group XL (b)] Declination and dip are measured aboard an aircraft at O, Fig. 1, by sighting a telescope on a star S, and 'maintaining its alignment automatically, and maintaining the magnetic axis of a fluxgate magnetometer automatically in the direction of the earth's magnetic field OH. The telescope, magnetometer and a gyroscope for indicating the zenith direction OZ are all mounted in gimbals and the angles HOZ, α are measured. The angle HOZ is the complement of the dip angle i and the angle a between the planes ZOS, ZOH, gives the declination D by subtracting z, the known azimuth of the star at the time of measurement, therefrom. As shown, Fig. 2, the telescope 213 is fixed to a frame 212, and, as described in U.S.A. Specification 2,444,933, is associated with a pick-up device 214 which generates two voltages proportional to the linear deviation of the star image from the focus of the telescope in two perpendicular directions in the focal plane. These voltages control respectively servomotors 217, 218 which rotate the forked frame 207 about the fixed axis 201 and rotate the frame 212 about the axis 210-211 to keep the telescope sighted on the star. The frame 212 supports by gimbals a gyroscope 229 and a frame 232. The gyro axis always remains vertical, i.e. the zenith direction, and through a link 243 constrains the frame 232 to lie in a vertical plane. This frame 232 carries by means of further gimbals embodying servomotors 235, 239 a fluxgate magnetometer comprising three wound cores 246, 247, 248 perpendicular to each other. Of these, the orienting cores 247, 248 control respectively the servomotors 235, 239, which, as disclosed in U.S.A. Specification 2,488,389, bring the measuring core 246 into alignment with the earth's magnetic field. The angle α is then indicated by a pointer 253 rotatable with the shaft 236, 233<SP>1</SP> of the servo 235, and the dip angle i, is indicated by the pointer 255 rotatable with the rotor of the servo 239. The strength of the earth's field is measured by the intensity of the second harmonic voltage induced in the magnetometer measuring winding 249, this voltage being filtered out and actuating a D.C. ammeter through an amplifier and rectifier, Fig. 3 (not shown). Second embodiment, Fig. 4. The telescope 413 is here mounted on the frame 401 by gimbals embodying servomotors 417, 418 which are controlled as in Fig. 2 to maintain the telescope sighted on the star. The three cored magnetometer 446-448 is likewise mounted by gimbals, and the measuring core 446 is maintained in alignment with the earth's field by servo-motors 435, 439 embodied in the gimbals and controlled as in Fig. 2. Parallel motion linkages constrain members 445, 459 to remain parallel to the core 446 and the telescope axis respectively. The gyroscope 429, indicating the vertical, is also gimbal mounted in the frame 401 and through a link 434 retains shafts 425, 426 in the vertical. These shafts carry synchronous motors 427, 428, the vertical rotor shafts of which drive arcuate arms 431, 432 which generate hemispheres at the centres of which are located the universal joints 452, 460 of the members 445, 459. The member 445, Fig. 5, carries a lamp 463 and a photocell 464 separated by a screen 465, light being reflected on to the cell once per revolution of the arcuate arm 431 by the concave reflecting surface thereof. The photocell pulses are applied to the plates 466-467 of a cathode-ray tube 470 to produce a " Y " deflection, the time base being synchronized with the speed of rotation of the arcuate arms 431, 432. The arm 459 similarly produces a " Y " deflection of the cathoderay tube beam, and the angle α is given by the distance between the deflection produced by the magnetic field arm 445 and the star direction arm 459. The dip angle i is the angle between the arm 445 and the vertical and is derived by means of a rod 471, Fig. 6, shaped so that if drawn on the hemisphere generated by rod 431, L = #/2 - k#, where L is the longitude, k a constant and # the latitude. A further " Y " deflection is then produced on the CRT by the rod 471, the displacement between the pulses produced by the rods 431, 471 being proportional to i.