3. Telemetry. The telemetry transmitter receives the status signal that develops from the encoding of sensor
signals within the satellite. Encoded signals represent samples of temperature, power supply voltage and
current, signal strength at the communications receiver and transmitter, solar sensors output, and pressure in the
hydrogen peroxide gas system. The telemetry transmitter raises the power of the 136-MHz FM signal and sends
it to the ground stations by way of the diplexer-hybrid combination and the turnstile antenna. The dashed lines
in the diagram represent the signal flow from the command channel to the orientation system (control jets), and
the signal flow from sensors in the orientation system to the telemetry channel. This is part of the feedback
circuit for the operator in the ground control terminal.
4. Security. The circuit shown in Figure 1-10 is vulnerable to interference. Signals that happen to be similar
in structure to authorized command signals may cause the jets in the satellite to activate. This is very
undesirable because the result may be a change in the spin axis orientation which will, in turn, cause a change in
the antenna field pattern coming toward the Earth. It is also undesirable in that the satellite may be forced into a
different orbit whereby it will change position from its intended synchronous "park." One method of
overcoming these difficulties is to include a sequential decoder in the satellite which responds only to specially
coded signals from the ground terminals.
5. Communications. The block diagram of the communications circuit is shown in Figure 1-11.
a. Signal processing. The communications antenna projects from the top of the satellite and is used for
simultaneous reception of the uplink signals and transmission of the downlink signals. Uplink signals are 7,360
MHz (f1a,) from ground terminal A and 7,260 MHz (f2a) from ground terminal B. Both signals simultaneously
pass through the hybrid and diplexer into the transponder receiver. A communications oscillator signal
heterodynes with the two uplink signals to produce the downlink frequencies of 1,815 MHz (f1b) to ground
terminal B and 1,715 MHz (f2b) to ground terminal A.
b. Beacon signal. In diplexer 2, a beacon oscillator signal of 1,820 MHz (f5) joins with the two downlink
communication signals. The composite of all three signals is amplified in the transponder transmitter which
raises the power level of the composite signal. The amplified signal is fed back to the hybrid-diplexer 1
combination and then to the communications antenna where it is radiated to the two ground terminals.
SS0031
1-36
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