1.If I want to use a radio to send commands and a wirwless receptor(put on the pixhawk), can the conmunication happens when the robot is underwater？(The water is 1 meter deep)
2.If I cannot do that, then if I want to use a joystick to directly connect to the pixhawk using usb line, how to connect and how to let the commnication happens?
2.4ghz radios and receivers will not work underwater.
You need to have digital communications with a laptop. This can be achieved with a raspberry pi and ethernet cable, or the BR serial tether interface boards.
-Jacob
Hi WYK,
No, radio signals do not travel through water well.
You can connect to the Pixhawk directly with USB and then connect to it with a joystick in QGroundControl. Please see the ArduSub instructions at ardusub.com.
-Rusty