Other satphone systems use satellites in in which only a few satellites are needed, but these cannot be used at high latitudes because of terrestrial interference.
The information signal is used to the carrier, varying some aspect of the carrier wave, impressing the information on the carrier.
Information such as sound is converted by a transducer such as a to an electrical signal, which modulates a produced by the.
Or the radio link may be , a bidirectional link using two radio channels so both people can talk at the same time, as in a cell phone.
Sometimes the transmitter is implanted in the animal.
It operates a network of about 47 carrying radio receivers, which detect distress signals from anywhere on Earth transmitting on the international Cospas distress frequency of 406 MHz.