Time is taken into account in this model whereby the assumption is made that dipole magnitudes are evolving slowly with regard to the sampling frequency [4,15]. For a measurement taken at time t, assuming that D^t−1 and D^t may be very close to each other means that the orthogonal projection of D^t on the hyperplane ED^t−1⊥ perpendicular to D^t−1 is 'small'. The following nonlinear equation is obtained:
