<Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2009) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2009) (Learn how and when to remove this template message) </Td> </Tr> <P> The Marine Radiotelephone Service or HF ship - to - shore operates on shortwave radio frequencies, using single - sideband modulation . The usual method is that a ship calls a shore station, and the shore station's marine operator connects the caller to the public switched telephone network . This service is retained for safety reasons, but in practice has been made obsolete by satellite telephones (particularly INMARSAT) and VoIP telephone and email via satellite internet . </P> <P> Short wave radio is used because it bounces between the ionosphere and the ground, giving a modest 1,000 watt transmitter (the standard power) a worldwide range . </P>

Which statement is true concerning the nature of radiotelephone sensors