Abstract: A data communication system is provided for communicating with an underground instrument package positioned on a drilling apparatus. The drilling apparatus has an insulated section between first and second conductive sections. The system includes a power supply connected to the first conductive section of the drilling apparatus and to the ground. A receiver measures current flowing through the power supply. An electrical path is provided between the first and second conductive sections of the drilling apparatus. A switch provided in the electrical path opens and closes the electrical path in response to the data generated by the instrument package.