Survival radios are carried by pilots and search and rescue teams to facilitate rescue in an emergency. They are generally designed to transmit on international distress frequencies. Maritime systems have been standardized under the Global Maritime Distress Safety System. Civil and military organisation's utilized different frequencies to communicate and no infringement on either sector would take place. For emergencies involving civilian aircraft, the radio frequency used is VHF 121.5 MHz and for military aircraft incidents, the frequency used is UHF 243 MHz.
The use of radio to aid in rescuing survivors of accidents at sea came to the forefront after the sinking of the RMS Titanic in 1912. Lifeboats were equipped with spark gap transmitters such as the Marconi Type 241, c. 1920.[1][2] These operated using Morse code on 500 kHz, the international distress frequency at the time. This frequency had the advantage of long range due to ground-wave propagation and was constantly monitored by all large ships at sea after the Titanic's sinking. However, due to its wavelength of 600 meters, a long antenna was required to achieve good range. Long wires on the order of 1/4 wavelength held up by kites or balloons were often used. Spark-gap continued to be used in lifeboats long after the technology was banned for general communication.
During World War II, Germany developed a hand-crank 500 kHz rescue radio, the "Notsender" (emergency transmitter) NS2. It used two vacuum tubes and was crystal-controlled. The radio case curved inward in the middle so that a user seated in an inflatable life boat could hold it stationary, between the thighs, while the generator handle was turned. The distress signal, in Morse code, was produced automatically as the crank handle was turned. An NS2 unit was captured by the British in 1941, who produced a copy, the Dinghy Transmitter T-1333. Britain gave a second captured unit to the United States, which produced its own copy, the SCR-578. United States Army Air Forces aircraft carried the SCR-578 on over-water operations. Nicknamed the Gibson Girl because of its hourglass shape, it was supplied with a fold-up metal frame box kite, and a balloon with a small hydrogen generator, for which the flying line was the aerial wire. Power was provided by a hand cranked generator. The transmitter component was the BC-778. The frequency was 500 kHz at 4.8 watts, giving it a range of 200 miles (300 km; 200 nmi). Keying could be automatic SOS (including the 4-second long dash for autoalarm), or manual. Crystals for frequency control were a scarce item for the U.S. during the war and the SCR-578 was not crystal-controlled.
A post-World War II version, the AN/CRT-3, which added a frequency in the 8 MHz range, was in use by ships and civil aircraft until the mid 1970s.[3]
The use of aircraft for search and rescue in World War II brought line-of-sight VHF radios into use. The much shorter wavelengths of VHF allowed a simple dipole or whip antenna to be effective. Early devices included the British Walter, a compact single vacuum tube oscillator design operating at 177 MHz (1.7 meter wavelength), and the German Jäger (NS-4), a two-tube master oscillator power amplifier[4] design at 58.5 and, later, 42 MHz.[5] These were small enough to include in life rafts used on single-seat fighter aircraft.[citation needed]
Post-war designs included the British Search And Rescue And Homing beacon (SARAH) beacon made by Ultra Electronics, used in the location and recovery of astronaut Scott Carpenter after his Mercury space flight,[6] the U.S. AN/URC-4 and the Soviet R 855U. These operated on the aircraft emergency frequencies of 121.5 and 243 MHz (2.5 and 1.2 meter wavelengths).[citation needed]
After a light plane with two U.S. congressmen on board went down in 1972 and could not be found,[7] the U.S. began requiring all aircraft to carry an Emergency Locator Transmitter (ELT) that would turn on automatically in the event of a crash. Initially these units sent beacon signals on the 121.5 MHz aircraft emergency frequency. These are being phased out in favor of ELTs that use a 406.025 MHz signal, which can be picked up by the Cospas-Sarsat international satellite system for search and rescue. Each 406 MHz beacon has a unique digital ID code. Users are required to register the code with the Cospas-Sarsat, allowing inquiries to be made when a distress signal is picked up. Some advanced models can transmit a location derived from an internal GPS or GLONASS receiver. Maritime practice has shifted from rescue radios on 500 kHz distress frequency (which is no longer officially monitored) to the Global Maritime Distress Safety System, which includes use of the Cospas-Sarsat system and other measures, including radar transponders and hand-held marine VHF radios.[citation needed]
There are many other types of emergency locator beacons that do not use the 406 MHz Cospas-Sarsat system, including man-overboard beacons that transmit Automatic identification system beacons and Avalanche transceivers.[citation needed]
Military organizations still issue pilots and other combat personnel individual survival radios, which have become increasingly sophisticated, with built-in Distance Measuring Equipment (DME), Global Positioning Satellite receivers, and satellite communication. In slang terms "PRC" radios were called a "prick" followed by the model number, "Prick-25," and "URC" radios were called an "erk." United States military survival radios include:
