Abstract: A system for determining whether an object is within a region of interest includes a radiofrequency signal receiver outside of the region of interest configured to receive radiofrequency signals from the direction of the region of interest; and a radiofrequency transponder having a directional antenna outside of the region of interest and configured to transmit radiofrequency signals away from the radiofrequency signal receiver and towards the region of interest. Presence of an object within the region of interest causes radiofrequency signals transmitted by the radiofrequency transponder to reflect off of the object towards the radiofrequency signal receiver.