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. 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.
The Gibson Girl 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 Type T1333. 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 . 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.
VHF era 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 design at 58.5 and, later, 42 MHz. These were small enough to include in life rafts used on single-seat fighter aircraft. 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, 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).
Automated beacon systems After a light plane with
two U.S. congressmen on board went down in 1972 and could not be found, 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. 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. ==U.S. Military survival radios==