A geographic tracking system with minimal power and size required at the mobile terminal collects observation data at the mobile terminal and forwards the data to a processor, which calculates the position. The component is made by a method which comprises: forming, in a mould, surfaces complementary to the working and neutral surfaces of the component; and moulding the component on these surfaces of the mould. The supporting field is a two-dimensional or three-dimensional magnetic quadrupole field so that the supporting force is independent of a position of the dipole. The particles are made of a copolymer of styrene and methyl methacrylate. At least some of the effects of the stimulus are removed from the outputs produced by the various carrier frequencies to produce two or more filtered outputs.