LEARNING EVENT 14: ACTIVE-REPEATER -ORBITAL SATELLITE COMMUNICATIONS
An active-repeater communications satellite differs from a passive satellite in that the former is equipped with
radio receivers and transmitters and other apparatus to receive, store, amplify, and retransmit messages. It
serves as a real-time repeater of a store-and-forward repeater, or both, depending on the devices used.
1. Real-time and store-and-forward modes of operation.
a. Real-time repeater. A real-time repeater is a combination of a microwave receiver, converter, and
transmitter in one unit. It receives a signal, converts it to a different frequency, and retransmits it at increased
strength instantaneously. In other words, as fast as the message is received, it is immediately sent out on a
second frequency. There is no time delay involved with this system. The bandwidth of the transmitted signal is
identical with that of the received signal. The message received at the ground receiving station can be
immediately demodulated with no delay in the terminal receiving process. The rate of any pulse signal received
by this method remains the same as the rate in the transmitted signal. For example, if the sending
teletypewriters are operating at 60 WPM, the telegraph signal pulses at the output of the receiving terminal can
be applied directly to receiving teletypewriters operating at the same rate of speed (60 WPM). With this
method, one ground station communicates with another ground station by using the real-time satellite as a
microwave repeater.
b. Store-and-forward repeater. A store-and-forward repeater receives a signal, stores it, and retransmits it
on reception of a prearranged code signal. It can receive information at a high rate of speed, record it, and
retransmit it again at a high rate of speed. The ground station originating the messages must process them with
suitable recording devices so that the messages can be transmitted to the repeater at high speed. When the
prearranged code signal causes the repeater to "unload" its messages at high speed, the receiving station must be
capable of recording the messages at an equivalent high speed. Before the messages can be demodulated by the
terminal equipment, the recording tape must be "played back" at a rate which the terminal station teletypewriter
equipment can handle. The system must have a very broad bandwidth to allow for the transmission of the
sidebands that are developed in the transmitter. Tape recording equipment does not have to be synchronized as
long as the speed of the tape recorders can be adjusted at the terminal equipment to match the teleprinter
operating speeds. In other words, the tape may be run over the system at a rate corresponding to many hundreds
of words per minute, but when the received tape is run through the reproducing cycle, it must operate at a speed
so that the terminal correctly interprets the message traffic and the printers correctly reproduce the messages.
c. Selection of type of repeater. If a choice is to be made between the facilities of either of the two
methods just described, a code signal can direct the satellite to bypass the recorder, thus making it a real-time
repeater in place of a store-and-forward repeater.
d. Antenna tracking. The antenna tracking systems for both passive and orbital active-repeater satellites
are similar in their operating characteristics.
SS0031
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