The GSM is the basic underwater transceiver unit.   It has manual mode only with PPT (push to talk) transmission and automatic receiving.   It operates on only one channel.  

• Wireless ultrasonic unit
• Features a digital driver for vocal message equalization
• Double transceiver channel
• Automatically activated once submerged in water, receiving also automatic
• System type H-SSB

Click "here" to view communications specifications chart