only practical means of communicating with moving vehicles, such as ships or aircraft. Also, radio communication can span great distances in any or all directions. It is the most practical system to use for sending information to many points, as in broadcasting to large numbers of ships or aircraft. Modern aircraft use radio equipment as navigational aids. Navigation aids consist of many types and are of varying complexity. They range from simple radio direction finders to complex navigational systems. Some systems use computers and other advanced electronic equipment to solve navigational problems automatically. The Aviation Electronics Technician (AT) rating normally maintains communications and navigational equipment. AIRBORNE COMMUNICATIONS EQUIPMENT Several means of radio communications are in use today. Some of these radio communications methods are: !    Radiotelegraphy—The transmission of intelligible coded radio-frequency waves as Morse code. !    Radiotelephony—The transmission of sound intelligence  (voice,  music,  or  tones)  by continuous radio-frequency waves. !    Radiofacsimile—The  transmission  of  still images (weather maps, photographs, sketches, and so forth) over a radio-frequency channel. !    Radioteletype—The transmission of typewrit- ten messages over a radio-frequency channel. !    Radiotelevision—The transmission of a rapid succession of images (still or moving) over a radio-frequency channel. Airborne communications equipment usually con- sists of equipment that can use either or both radio- telegraphy or radiotelephony. Radiotelegraphy and radiotelephony are called Morse code and continuous wave (CW) voice communications, respectively. Long-range Communications Airborne long-range communications sets normally operate in a band of frequencies from about 3 MHz to 30 MHz.  Frequencies within this band are called the HF or high-frequency band. Radio frequencies within this band have characteristics that make them highly useful. The radiated waves transmitted along the surface of the earth bend around objects in its path. In addition, radio waves that are transmitted skyward bounce off the ionosphere and return  to  earth  at  extreme  distances  from  the transmitting station. This allows the waves to travel extremely long distances. Most long-range communications sets are designed for both voice and CW (Morse code) operation. It is often necessary to have a long antenna for long-range communications. A weighted antenna wire (trailing wire antenna) is installed in some large aircraft. The wire is reeled out to provide an antenna of the desired length. Short-range Communications Short-range airborne communications sets operate in the frequency range from about 30 MHz to 3 GHz. The lower portion of this band is the very-high-frequency (VHF) band; the higher portion is the ultra-high-frequency (UHF) band. The VHF/UHF frequency bands have transmission characteristics that differ from those frequencies in the HF band. Radio waves transmitted at these frequencies travel in a straight line. This limits the transmission to line-of-sight. VHF/UHF communications sets are called line-of-sight communications sets. Radio waves at these frequencies normally do not return to earth. Therefore, VHF/UHF transceivers are mainly used for air-to-air and air-to-ground contact in close range operations. Landings and takeoffs are typical situations using air-to-ground VHF/UHF transmissions. Special situations exist where VHF/UHF equipment is involved in long-distance com- munications. An example of this is the network of remote-controlled transceivers installed along the airways system in the United States. Pilots of aircraft traveling the airways can talk directly to controllers in distant aviation activities. A system of telephone lines and relay stations connect the remote transceiver sites. The radio part of the transmission takes place over a relatively short distance. NAVIGATIONAL EQUIPMENT Modern naval aircraft use a lot of navigational equipment. Radio receivers and transmitters are used to handle signals that determine bearing and/or distance. The tactical air navigation (TACAN) system, Global Positioning system (GPS), and navigation computer systems  are  discussed  briefly  in  the  following paragraphs. 7-14