For example, Japanese Patent Application Laid-Open Publication No. 2009-105600 (Patent Document 1) discloses a position estimating apparatus that includes a sheet-like signal transmission device and two communication devices placed at given positions near the sheet-like signal transmitting device. According to this position estimating apparatus, a first communication device transmits a presence confirmation request signal to a second communication device, the second communication device having received the presence confirmation request signal transmits a presence confirmation response signal to the first communication device, and the sheet-like signal transmitting device estimates the position at which the second communication device is placed, based on a time that has passed from transmission of the presence confirmation request signal by the first communication device to reception of the presence confirmation response signal by the same.
Japanese Patent Application Laid-Open Publication No. 2007-127529 (Patent Document 2) discloses an apparatus including electromagnetic wave transmitting means, a measurement subject, and detecting means that detect signal strength. According to this apparatus, the detecting means detect the signal strength of a synthesized wave signal created by synthesizing an electromagnetic wave transmitted from the transmitting means and a reflected wave resulting from reflection of the electromagnetic wave by the measurement subject, and the distance of the measurement subject is calculated from the detected signal strength.
Japanese Patent Application Laid-Open Publication No. 2001-60905 (Patent Document 3) discloses a position detection method for a system including a receiver and a transmitter having two antennas. According to this position detection method, the transmitter transmits electromagnetic waves with a changed interference pattern from two antennas, as an interference wave, the receiver receives the incoming interference wave and feeds back information of the strength of a received electric field to the transmitter, and the transmitter detects the position of the receiver based on the feedback information from the receiver.