A radio-controlled submarine is a scale model of a submarine that can be steered via radio control. The most common form are those operated by hobbyists. These can range from inexpensive toys to complex projects involving sophisticated electronics. Oceanographers and military units also operate radio-controlled submarines. Radio transmission through water As the conductivity of a medium increases, the more a radio signal passing through it is attenuated. High frequencies are also attenuated more than low frequencies, and tend to reflect off the surface of the water more. As is well known communication with military submarines uses very low frequency electromagnetic radiation for this reason. Military frequencies are well below allocated hobby radio control bands, but the lowest hobby bands - typically around 27MHz/40MHz - can penetrate several feet of water at short distances, typically less than 45 meters. Penetration at these frequencies is easier in fresh water, but are difficult to impossible in sea water. Modern radio control sets using the 2.4GHz band penetrate water very poorly, and are thus not used by serious divers. For underwater radio to work, even at these frequencies, the receiving aerial needs to be completely insulated from the surrounding water. Plastic covered wire provides adequate insulation - the aerial need not be held in an airtight container - but the cut end of such wire must be sealed against water ingress. Depending on water conditions, positive control can be maintained at perhaps 3 meters depth. Since control of model submarines may not be reliable at all times, such models usually carry a variety of apparatus intended to prevent model loss. Fail-safe systems which detect loss of signal and command the submarine to surface, or pressure sensors which limit the depth attained, may be used. Such specialist complexity usually makes a model submarine an expensive item compared to a model surface boat. Professional or military remote-controlled diving equipment may be controlled via a tether, or by using sound signals. Quite commonly, such equipment has onboard computers which allow autonomous operation following a set path, so continuous communication with the controlling base is not necessary. The advent of small cheap computers such as the Raspberry Pi or Arduino has allowed model submariners to emulate their professional brethren and provide autonomous control in situations where radio transmission or adequate visibility is lacking. Hobby Submarines Dynamic diving Models with dynamic diving are positively buoyant, and will remain on