Non-oriented cesium sensors for airborne magnetometry and gradiometry

Abstract

Optically pumped magnetometers are characterized by an optimal angle between their optical axes and the direction of the magnetic field they are sensing. Departure from the optimal angle causes a shift in the Larmor frequency with a corresponding error in the scalar value of the magnetic field being measured. To minimize this error, magnetometers are conventionally either mounted in multiple sensor clusters such that the errors tend to cancel, or they are mechanically oriented to maintain the optimal angle with respect to the magnetic field vector. Recent cesium vapor magnetometers using a split-beam technique have a sufficiently flat error characteristic that they can be flown in a non-oriented or "strap-down" configuration. This configuration has advantages with respect to conventional methods in terms of reduced size and weight and of greatly reduced cost. This paper describes two fixed orientations for a particular split-beam magnetometer and calculates the allowable maneuver envelope for all dip angles from 0 to 90 degrees. It is shown that the residual orientation errors can best be handled by the conventional type of magnetic interference compensation model that must, in any case, be implemented in digital form for high-sensitivity magnetometry or for any type of gradiometry. The National Aeronautical Establishment (NAE) of the National Research Council of Canada has flown strapdown magnetometers in a three-axis gradiometer array in a Convair 580 for several years. Results for the entire normal maneuver envelope of the aircraft, including 30 degree bank turns, have equalled or surpassed those obtained with oriented magnetometers. Several typical maneuver compensation results are presented that gave root-mean-square (rms) (one-sigma) residual errors as low as 0.03 gammas (G) for total field and 3.5 mgammas/m(mg/m) for lateral gradient.