Digital Selective Communications

Digital techniques promise to be a major factor in future aerospace communications systems. Voice is cumbersome, slow and redundant, and does not make efficient use of the frequency spectrum. The digital approach permits the use of very narrow bandwidths, reduces redundancy and utilizes a constant loading factor. Selective addressing means that those not concerned with a message are not bothered by it, a very important factor in reducing distraction and fatigue of crew members in high performance aircraft. This paper describes terminal equipment developed for test and evaluation by the Air Force. The AN/URA-22 Control Monitor equipment provides for transmission of up to 456,976 different four-letter selective addresses, and recognition of individual, group and general calls, each of which can be any preassigned four-letter call. Remote switching is provided through the use of two mode characters, giving up to 676 possible combinations. The fieldata code is used. Mode characters and addressor call are displayed at the receive end. The AN/URA-29 Digital Data equipment adds message capability, using both words and alpha-numeric characters to provide three word messages, followed by up to 48 alpha-numeric characters in three lines of 16 characters each. Insertion is by simple push-button matrix and the message being composed is displayed. Received messages are displayed on receipt in plain language. The composed message is retained in storage until cleared. Operation of both equipments is explained in detail.