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The aircraft emergency frequency (also known in the USA as Guard) is a frequency used on the aircraft band reserved for emergency communications for aircraft in distress.The frequencies are 121.5 MHz for civilian, also known as International Air Distress (IAD), International Aeronautical Emergency Frequency, [1] or VHF Guard, [1] and 243.0 MHz—the second harmonic of VHF guard—for military ...
121.5 MHz is the civilian aircraft emergency frequency or International Air Distress frequency. It is used by some civilian emergency locator beacons; however, the Cospas-Sarsat system no longer monitors the frequency. 243 MHz for NATO military aircraft emergency frequencies
121.5 MHz VHF ± 6 kHz (frequency band protected to ±50 kHz) [66] (Satellite detection ceased on 1 February 2009, [67] but this frequency is still used for short-range location during a search and rescue operation) 243.0 MHz UHF ± 12 kHz (frequency band protected to ± 100 kHz) [66] [68] (prior to 1 February 2009 – COSPAS-SARSAT Compatible)
A typical aircraft VHF radio. The display shows an active frequency of 123.5 MHz and a standby frequency of 121.5 MHz. The two are exchanged using the button marked with a double-headed arrow. The tuning control on the right only affects the standby frequency.
The civilian aircraft frequency for voice distress alerting is 121.5 MHz. Military aircraft use 243 MHz (which is a harmonic of 121.5 MHz, and therefore civilian beacons transmit on this frequency as well).
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.
In use, the RDF operator would first tune the receiver to the correct frequency, then manually turn the loop, either listening or watching an S meter to determine the direction of the null (the direction at which a given signal is weakest) of a long wave (LW) or medium wave (AM) broadcast beacon or station (listening for the null is easier than ...
Since February 2009, the Cospas-Sarsat Satellite System has not monitored the 121.5/243.0 MHz frequency; however, the signal transmitted by the Emergency was never strong enough to be picked up by satellite, and Breitling has announced that, as these frequencies will still be monitored by aviation, particularly during the localization phase of ...