They all listen continuously on either 121.5 or 121.5 and 243 MHz and all use a frequency comparison squelch system that constantly adjusts the receiver for maximum usable sensitivity. It will respond to an ELT that is damaged or has no modulation, as well as a carrier signal produced by a stuck microphone. An adjustable integrating timer in the unit, usually set to 10 minutes, prevents alarm trips on voice conversations or ELT tests. The base station units are intended for fixed locations such as a home, office airport or harbor where people are present. The mountain top units are built for installation at radio repeater sites and generate an alert through the radio system.
Monitors can greatly assist rapid response to both accidents and false alarms.
The Santa Barbara Monitor System is a summary of over 9 years of operation of a group of these receivers. This monitor is usually left running in its desk stand and connected to AC power. When the alarm activates, the speaker comes on. It can also trigger an optional auto-dialer for remote reporting of the alarm by telephone. A mute button disables the speaker and causes the dial light to flash brightly as a reminder that the signal is still being received.