Summary Notes on Hydrographic Office Magnetic, Gravity, and Depth 



Measurements 



A. Project MAGNET - Airborne Magnetic Surveys (F. B. Woodcock) 



The Vector Airborne Magnetometer (VAM) system is one whereby 

 four parameters are measured directly: (1) Direction of the magnetic 

 meridian (magnetic heading, MH), (2) inclination (I) of the magnetic 

 field vector, (3) azimuthal angle of a celestial body (RB), and (4) total 

 intensity of the magnetic field (F). The first three (angular) measure- 

 ments are made either in, or at right angles to, a "horizontal" plane 

 as defined by a viscous damped pendulum gimballed about the trans- 

 verse and longitudinal axes of the survey aircraft. The fourth measure- 

 ment, magnetic intensity, is made with respect to a precisely controlled 

 electric current which is "standardized" by comparison at the USC&GS 

 Magnetic Observatory, Corbin, Virginia. The "standard" current is 

 reproduced on survey operations by correlation with unsaturated stand- 

 ard voltage cells. The data, MH, I,RB,and F, are recorded as functions 

 of time (GMT) accurate to about ±1 second. 



The VAM measurement of the three angles is accomplished by 

 means of a synchro system wherein the unknown angle, x, is established 

 in a synchro control transmitter which, in turn, is connected electri- 

 cally to a remote synchro control transformer. The latter is manually 

 positioned to an angle, X, which is an integer multiple of 5°. The 

 transformer a.c. output voltage is then determined by (x-X), the 

 algebraic sign here indicating the relative phase of the output with 

 respect to the input current to the control transmitter. The indexing 

 mechanism for the control transformer is calibrated to an accuracy 

 of about ±0.03 degree for each value of (x-X) = to establish accurate 

 base lines. The residual values, (x-X) equal to or less than 5°, are 

 detected from the a.c. voltage output and recordedto an accuracy of about 

 ±0.03 degree on a recorder which is calibrated to about ±0.03 degree. 

 In addition, the recorded data is scaled to an accuracy of about ±0.03 

 degree. Thus, the combined angular error should not exceed about 

 ±0.12 degree with respect to the pendulum-established reference system. 



The VAM measurement of magnetic intensity is accomplished by 

 passing a precisely controlled electric current through a detector 

 (saturable inductor) which is servo-oriented parallel with the magnetic 

 field vector. The "neutralizing" current is adjusted manually to obtain 

 a value, N, in precise 50-gamma steps (1 gamma = 10" 5 oersted), 

 thus establishing a base line. The detector a.c. output voltage is an 



Encl. (3) to Code 5401-BEO/bah memo of 23 Jul 1958 



B-10 



