There are two kinds of signal buoys, light signal buoys and radio signal buoys.
The light signal buoy is a hollow floating body with a signal lamp at the top, which sends out a circular flashing light signal to indicate the position of the sunken ship. There is a signal buoy at the bow and stern of the submarine deck, which is placed in the hull wall together with the transmission device.
The signal buoy is connected with the rescue capsule on board by cable. The length of the cable is greater than 15% ~ 25% of the submarine's limit dive depth. The cable is tied to the buoy to supply power to the signal lights and telephones in the buoy. The signal lamp cable passes through the interrupter in the submarine cabin, giving out a circular flashing light, which is easy to be found. After the surface rescuers appear, they can take out the telephone in the signal buoy and talk to the crew on board.
According to the situation in the boat, the cable can provide emergency lighting power for the submarine.
Radio signal buoys are equipped with short-wave radio stations and transmitting antennas. In order not to let the enemy know the location of the sunken ship, the radio signal buoy has no direct contact with the wrecked submarine, but drifts with the waves at sea, and informs the sunken location of the wrecked submarine with coded signals. Our own rescue forces can go to the rescue.
The buoy will sink by itself after a certain period of time.