It is known to use perturbations in the ambient static DC electric field between the surface of the Earth and the ionosphere, caused by a moving target object, which is either dielectric or conducting (resistive), in order to provide information about the movement of that object. No excitation field needs to be created locally, since the mere presence of the moving target object will perturb the ambient electric field and produce a local variation in this electric field at a detection site for the target object.
In these arrangements, a number of sensors, usually two per axis, have been employed to infer the location of a single moving object within a detection area defined by the positions of the sensors, as well as to monitor object velocity and movement for target tracking.
The prior art thus discloses the use of the ambient static (DC) electric field due to the Earth to detect movement of a target object, but such prior art has the significant drawback that the target object must be moving to be detected.
Furthermore, it has been found that the known techniques suffer from significant limitations when dealing with multiple targets, in terms of the complexity of signal acquisition and processing, and more importantly in terms of accurately discriminating between the different objects.
US2008/0255779 also discloses a method of detecting anomalies in an ambient AC field, using an AC sensor placed in such ambient AC field to generate a signal representative of the field, and processing the signal to monitor for anomalies.