that the sensor data arise from elemental dipoles at each spatial location r, represented by a three-dimensional vector such that r = [rx,ry,rz]T. The orientation of each source was defined as the vector d(r) = [dx,dy,dz]T, where dx, dy, and dz are the angles between the moment vector of the source and the x-, y-, and z-axes, respectively. We defined lmζr as the output of the m-th sensor that would be induced by a unit-magnitude source located at r and pointing in the ζ direction. The column vector lζ(r) was defined as lζr=l1ζr,l2ζr,⋅⋅⋅,lMζrT. The lead field matrix, which represents the sensitivity of the whole sensor array at r, was defined as L(r) ≡ [lx(r),ly(r),lz(r)]. The lead field vector for a unit-dipole oriented in the direction d was defined as l(r), where l(r) ≡ l(r)d(r).
