A power transmitter for wirelessly providing power to a power receiver via an electromagnetic power transfer signal; the power transmitter comprising: at least one transmitter coil arranged to generate the power transfer signal during power transfer; a retriever arranged to retrieve stored calibration data comprising a set of calibration parameters for each of a plurality of spatial positions of a calibration receiver relative to a calibration transmitter, the calibration parameters including for each spatial position of the plurality of spatial positions: a calibration power loss measure for the calibration power receiver being at the each spatial position, a calibration resonance frequency measure indicative of a resonance circuit of the calibration power transmitter for the calibration power receiver being at the each spatial position, a calibration coupling measure indicative of a coupling between the calibration transmitter and the calibration receiver being at the each spatial position; a test generator arranged to generate a test drive signal for the transmitter coil to generate an electromagnetic test signal; a test processor arranged to determine a set of test parameters in response to the test drive signal, the test parameters comprising: a test power loss measure indicative of a power extracted from the electromagnetic test signal, a test resonance frequency measure for a resonance circuit comprising the transmitter coil, a test coupling measure indicative of a coupling between the power transmitter and the power receiver, characterized by the test power loss measure, the test resonance frequency measure, and the test coupling measure providing different relationships with a position of the power receiver relative to the power transmitter whereby an interrelationship between the test power loss measure, the test resonance frequency measure, and the test coupling measure provides an additional indication of the position of the power receiver relative to the power transmitter; and further characterized by the power transmitter further comprising: a position estimator arranged to estimate the position of the power receiver relative to the power transmitter in response to a comparison of the test parameters to the calibration parameters for the plurality of spatial positions. at least one transmitter coil arranged to generate the power transfer signal during power transfer; a retriever arranged to retrieve stored calibration data comprising a set of calibration parameters for each of a plurality of spatial positions of a calibration receiver relative to a calibration transmitter, the calibration parameters including for each spatial position of the plurality of spatial positions: a calibration power loss measure for the calibration power receiver being at the each spatial position, a calibration resonance frequency measure indicative of a resonance circuit of the calibration power transmitter for the calibration power receiver being at the each spatial position, a calibration coupling measure indicative of a coupling between the calibration transmitter and the calibration receiver being at the each spatial position; a calibration power loss measure for the calibration power receiver being at the each spatial position, a calibration resonance frequency measure indicative of a resonance circuit of the calibration power transmitter for the calibration power receiver being at the each spatial position, a calibration coupling measure indicative of a coupling between the calibration transmitter and the calibration receiver being at the each spatial position; a test generator arranged to generate a test drive signal for the transmitter coil to generate an electromagnetic test signal; a test processor arranged to determine a set of test parameters in response to the test drive signal, the test parameters comprising: a test power loss measure indicative of a power extracted from the electromagnetic test signal, a test resonance frequency measure for a resonance circuit comprising the transmitter coil, a test coupling measure indicative of a coupling between the power transmitter and the power receiver, a test power loss measure indicative of a power extracted from the electromagnetic test signal, a test resonance frequency measure for a resonance circuit comprising the transmitter coil, a test coupling measure indicative of a coupling between the power transmitter and the power receiver,