SELCAL System

SELCAL System

• The word SELCAL is derived from the term selective calling and the SELCAL DECAER is an instrument designed to relieve the pilot and co-pilot from continuously monitoring the aircraft radio receivers.

• The Selcal decoder is in effect an automatic monitor which listens for a particular combination of tuner which is assigned to the individual aircraft. Whenever a properly coded transmission is received from a ground station, the signal is decoded by the Selcal unit which then given a signal to the pilot notifying him that a radio transmission is being directed to the aircraft. The pilot can then listens to his receiver and hear the message. 

• Ground station is equipped with tone transmitting equipment cell individual aircraft by transmitting two pairs of tones which will key only an airborne decoder which is not to respond to the particular combination of tones. When the proper tones are received, the system operates an external alarm circuit to produces a chino, light, buzz, or a combination of such signals.

• When the ground operator wishes to contact a particular aircraft by means of the Selcal unit, he selects the four tone code which has been assigned to the aircraft. The tone code is transmitted by a radio-frequency wave, and the signal can be picked up by all receivers tuned to the frequency used by the transmitter. The only receiver which can respond to the signal and produce the alert signal for the pilot is the receiver and decoder which has been out for the particular combination of tone frequencies.

• Each decoder requires four Vibersponder reads to establish the code for the unit. The reads respond to audio frequencies and cause the generation of a voltage which is processed by the unit. The resulting voltage is used to operate the alarm circuits.

Vibresponder  reads are manufactured with tone frequencies from 31.6 to 977.2 Hz.