SCPC Satellite Channel Unit
Models SCU-2R, SCU-2RB, SCU-2P, SCU-2PE,
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The SCU-2 is a stand alone, single channel per carrier (SCPC) satellite channel unit capable of transmitting and receiving voice or data on an RF frequency pair. The SCU-2 comes in both rack-mountable ("R") and portable ("P") versions. The P model is designed for use in harsh outdoor environments.
SCU-2s can implement independent point-to-point links (with carrier inhibited between calls), or can be configured in a multi-node, multi-channel network. At either or both ends of a link, the SCU-2 can be connected to external telephone office or station equipment through its 2Wire interface. This interface passes voice band data such as facsimile as well as audio. Access authorization during call setup can be used to prevent intrusion into an SCU-2 network.
When configured in a network, an SCU-2 designated as the "Hub" continuously transmits to one or more "Remotes". A Hub side caller receives a second dial tone to prompt for identification of the called Remote, and only this Remote will ring. All other Remotes remain inhibited from transmitting for the duration of the call.
When the network has access to more than one pair of RF channels, the grade of service performance of single channel Circuit Sharing can be enhanced by using Alternate Channel Selection. This allows a Remote to initiate (but not receive) a call on up to seven alternate channels automatically whenever its primary channel is in use. Upon call completion, the Remote automatically returns to its primary channel in order to be able to receive calls.
The P model is usually part of a portable satellite terminal, and configured as a Remote. The Hub's transmit carrier (between calls) or an external pilot is used to drive a signal strength indicator to assist in pointing the Remote's antenna. The P model also has an interface for monitor and control of the RF Transceiver.
When acquiring a signal within the selected frequency acquisition range, the SCU-2 will automatically reject all spurious signals encountered. The assigned satellite channel bandwidth therefore need not include common sources of drift in the wanted and adjacent channel carriers, and can be as little as the 5 kHz bandwidth of the SCU-2 transmit signal.
Audio transmission is by SSB with pilot tone for AFC. When speech is transmitted, companding and other processing is used to make the transmitted signal nearly constant in amplitude and to suppress noise. This mode is called constant average power SSB (CAPSSB). When secure telephone, facsimile, or other voice band data is being transmitted, "CAP" processing is inhibited to minimize distortion. This mode is called Transparent SSB (TSSB).
2400 bps differential minimum shift keying (DMSK) is used to transmit user data between RS-232 data ports, and during call set up. Rate-½ coding can be selected, giving a user data rate of 1200 bps.
The SCU-2 instruction codes have been designed for ease of use, and are password restricted beyond the operator level. A Remote's transmit carrier level and other parameters are remotely programmable from the Hub. Far end satellite loop back testing of a Remote can be performed by a Hub operator. These capabilities minimize the skill level required of the Remote operator.
As its modem and control processing are implemented in software, the SCU-2 can readily be adapted to other narrow band SCPC communications applications or specifications. For example, it has been configured as a 1200 bps BPSK Remodulator for dual satellite operation of Inmarsat-C.
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