Raymarine Wireless Mast Rotation Transmitter - T221This unique transmitter compensates for the movement of a rotating mast to ensure that you get the most accurate wind readings at all times. Operating as a wireless device on a Micronet network the Mast Rotation Transmitter compares compass information from the boat with its own information and corrects the wind angle coming from a standard wind transmitter at the mast head. Features: No wires Solar powered Light weight