A system for monitoring the position of a downhole tool assembly having multiple beacons. In a preferred embodiment first and second beacons are adapted to transmit electromagnetic signals indicative of the position of the downhole tool assembly. A receiving assembly having a single antenna arrangement detects the signals transmitted from the first and second beacons. The receiving assembly processes the signals to determine the relative position of the receiving assembly to the downhole tool assembly. The determination of the relative position comprises determining a lateral offset and a distance from the downhole tool assembly.