Orientation sensor for downhole tool

A downhole drilling apparatus incorporating an orientation sensor (24) is disclosed. The orientation sensor is mounted to a housing (16) for providing a signal representing the orientation of the housing, and is connected to a first transceiver coil (26). A second transceiver coil (30) and a series of magnets (32) are mounted to a rotary shaft (18). As the shaft rotates relative to the housing, passage of the magnets past the first transceiver coil generates a voltage to provide electrical power to the sensor, and electromagnetic coupling between the first and second transceiver coils enables signals from the sensor to be transmitted to a measurement while drilling tool mounted on the rotary shaft.