Magnetic Vector Gradient Compensation for Survey Aircraft

Abstract:

A magnetic survey aircraft undergoes small changes in its rotational attitude. The roll, pitch and yaw angles are the appropriate measures of this attitude (Fig 1). The changes cause manoeuvre noise. This manoeuvre noise derives from the metal on the survey aircraft 1) acting as a permanent magnet, 2) inducing a magnetic response while moving through a magnetic field, and 3) also inducing time dependent eddy currents from the skin of the aircraft.

We propose a new statement of the compensation problem. Equation 1 uses an array of weights, which are simply scaled from the Euler angles. The first term of Equation 2 is the curl term for just the conductive current, out of phase and the second term is the standard magnetic field term in phase.

The measurement system is assumed to be spatially insensitive, and considers the three orientation variations in time. Geologists want the magnetization direction for each magnetic anomaly. This is a proxy for the emplacement date of the geological source of the magnetic anomaly. This requires high quality vector gradient measurements.

Confusion occurs around vector gradients and standard scalar measurements. The current practise of using 3 or 4 TMI sensors at the wingtips, nose and tail leads to the creation of an approximation for the gradient vector by differencing and dividing by the separation distances. This is not the B field but a poor linear differential approximation, leading to more ambiguities.

Vector gradient measures suffer from manufacturing tolerance errors, so B field components also report in the raw gradient measures. It is for this reason that efforts to actually measure vector gradients directly have failed to date.

Faced with this set of mixed gradient terms, the challenge is how to untangle the B field, gradients and Euler angles. What can be done to address the problem? Some suggestions:

• Exponential decay for the vertical terms, rather than the current harmonic treatment
• Third order orientation terms for the B field third order tensor can be added
• Induced magnetic response terms in compensation have both a gradient and a B field component
• Common mode rejection in the design

Leliak P., 1961, Identification and evaluation of magnetic field sources of magnetic airborne detector MAD equipped aircraft: IRA Transactions on airspace and navigational electronics, Vol 8, P.95-105