Radiotelephone

In spite of the name, radiotelephony systems are not necessarily connected to or have anything to do with the telephone network, and in some radio services, including GMRS,[2] interconnection is prohibited.

This would include systems fitting into the category of two-way radio or one-way voice broadcasts such as coastal maritime weather.

The most common method of working for radiotelephones is half-duplex, operation, which allows one person to talk and the other to listen alternately.

They may use simple modulation schemes such as AM or FM, or more complex techniques such as digital coding, spread spectrum, and so on.

For example, airband radiotelephones used for air to ground communication between pilots and controllers operates in the VHF band from 118.0 to 136.975 MHz, using amplitude modulation.

To help direct messages to the correct recipients and avoid irrelevant traffic on the network being a distraction to other units, a variety of means have been devised to create addressing systems.

The receiver will be programmed to respond only to a unique set of tones in a precise sequence, and only then will it open the audio circuits for open-channel conversation with the base station.

In practice many selcall systems also have automatic transponding built in, which allows the base station to "interrogate" a mobile even if the operator is not present.

Features like this, while very simple, are one reason why they are very popular with organisations that need to manage a large number of remote mobile units.

The Marine Radiotelephone Service or HF ship-to-shore operates on shortwave radio frequencies, using single-sideband modulation.

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.

These allow them to install, service, and maintain voice-only radio transmitter systems for use on ships and aircraft.