Doppler positioning is employed using both 121.5 MHz and 406 MHz signals. Position accuracy of the 121.5 MHz signal is within an area of approximately 15-20 km radius about the transmitter. Due to the better signal integrity of the 406 MHz, its location accuracy is within about a 3 km radius.
1.6.2 Switch Operation
In a crash, an acceleration activated crash sensor (G-switch) turns the ELT “ON” automatically when the ELT experiences a change in velocity (or deceleration) of 4.5 fps ±0.5 fps. Activation is also accomplished by means of the cockpit mounted remote switch or the switch on the ELT. To deactivate the ELT, set either switch to the “ON” position then back to “ARM”.
The ELT does not have an “OFF” position. Instead, a jumper between two pins on the front D-sub connector must be in place for the G-switch to activate the unit. The jumper is installed on the mating half of the connector so that when the connector is installed, the beacon is armed. This allows the beacon to be handled or shipped without ‘nuisance’ activation (front connector removed).
NOTE: The ELT can still be manually activated using the local switch on the front of the ELT. Care should be taken when transporting or shipping the ELT not to move the switch or allow packing material to become lodged such as to toggle the switch.
In the ME406P, activation is also accomplished by any of the 5 auxiliary “G” switches at a nominal 12 “G” level.
1.6.3 Portable Function
The ME406P ELT has capabilities identical to the AF (automatic fixed) version, including crash-sensing G-switch and transmission on 121.5 MHz and 406 MHz to be used by SARSAT forces in locating the site of the emergency. The construction of the ME406P has been optimized for the auxiliary antenna, which is designed to be used in a variety of conditions and terrain. The auxiliary antenna is stowed on the ME406P ELT. The antenna can be removed from the case and attached to the BNC connector with one hand. A 6-inch tether is connected between the antenna and the ME406P ELT, along with a 3 foot lanyard and a clip for attachment to a life raft. The entire ELT system can be removed from the interior of the plane and activated at another site.
1.6.4 Self-Test Mode
Upon turn-off, the ELT automatically enters a self-test mode that transmits a 406 MHz test coded pulse that monitors certain system functions before returning to the “ARM” mode. The 406 MHz test pulse is ignored by any satellite that receives the signal, but the ELT uses this output to check output power and correct frequency. If the ELT is left activated for approximately 50 seconds or greater, a distress signal is generated that is accepted by one or more SAR satellites. Therefore, when the self-test mode is required, the ELT must be activated, then, returned to “ARM” within about 45 seconds otherwise a “live” distress message will be transmitted.
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