Compensation of Earth's Field Variations by Field Controlled Rubidium Oscillator

The east-west variations of the earth's magnetic field over the working volume of the NBS precision solenoid are being canceled out by a closed loop system featuring a field controlled rubidium oscillator. The rubidium oscillator and solenoid are sufficiently separated in distance and do not affect each other's operation. Each system is enclosed in three-dimensional Helmholtz coils. Vertical and north-south components of earth's field are canceled. An east-west bias field is provided for rubidium oscillator operation at 10 kHz. The output of a phase sensitive detector locked to the 10 kHz frequency standard provides current for the electrically series connected east-west Helmholtz coils at both locations. The magnitude of this current is proportional to the phase difference of oscillator and reference. The full range of compensation is 100 nT. The rate of compensation is limited by a time constant of 0.002 sec, an adequate rate for most disturbances. To the extent that the two separated components of the closed loop system can be oriented directionally the same at any given time, the earth's field east-west variation is compensated exactly.