A self-contained crawler assembly is engageable to a stringer and is adapted to position a drill bushing along the stringer for performing machining operations. The spacer includes a layer of compressible material for insertion between the component assembly and the housing. After the start of the transmission of the message information from the first antenna, transmission of the message information from the first antenna to the receiving unit terminates and transmission of the message information from the second antenna in the transmitting station to the receiving unit is initiated.