V1OOPS
I thought 121.5 as an ELT frequency had been superceded years ago in favour of 243MHz which is now being superceded by 486(?)MHz since UHF has better ionispheric penetration for automatic satellite reception.
In fact a signal on 121.5 will also transmit on 243 MHz as the first harmonic. Not sure if it works the other way around though. (the techno buffs out there can expand if they like) 243MHz is generally termed as the military distress frequency.
Both these frequencies are interrogated by the COSPAS SARSAT system but with a reduced degree of accuracy over 406MHz. Usually about 20 KM from satellite fixing as opposed to about 3Km on 406. No identifying data is transmitted in the signal of 121.5 / 243 however the 406 MHz ELTs and EPIRBs transmit an identifying code which is registered with the SAR authority of the vessel or aircrafts home port. They also have the capability of being coupled to onboard GPS units for precise position notification.
The satellite system will cease interrogation of the 121.5 / 243 MHz signals from 2009 however for SAR homing, these frequencies will probably remain in use. Consequently the 406 MHz EPIRBS have the ability to simultaneously transmit on these frequencies also.
Taken from
"Guidelines for participating in the COSPAS - SARSAT system"
Interference in the 406 MHz and 121.5 MHz bands limits the performance of the Cospas- Sarsat System by preventing receipt of distress beacon transmissions in regions where interfering transmitters are operating. Participants should endeavour to detect, locate and turn off these interfering signals in their national areas of responsibility, as described in ITU
Recommendation ITU-R SM.1051.
As AirNoServicesAustralia said earlier, if people start transmitting on 121.5 it could cause problems. The satellites only interrogate ELTs / EPIRBs but a stong voice modulated signal might block out a weaker signal from a beacon.[