Page 142 - Intro to Space Sciences Spacecraft Applications

Communications
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boosts the signal up to the desired transmitter power (PT) and delivers this
to the antenna from which the electromagnetic signal (XIt1) will propa-
gate. The channel represents the medium through which the signal travels
between stations. (For a radio station, this medmm would be the atmos-
phere between the station and your car. The channel for telephone or cable
TV is the line coming in your house.)
On the receiving end, the antenna picks up the signal (with its associat-
ed noise) from the channel. Since the power level received is usually quite
low compared to that which was transmitted, the signal is delivered direct-
ly to an amplifier to boost the signal strength. The front-end electronics
attempt to filter out some of the signal noise before delivering the signal
to the demoduZator; which recovers the baseband signal from the carrier
wave frequency. (Note: As was mentioned earlier, the receiver electronics
contribute significantly to the overall equivalent noise temperature of a
communications link. To minimize this contribution, many receiving sta-
tions cool their front-end electronics, some down to near absolute zero
using liquid nitrogen, to improve the received signal-to-noise ratio).
Finally, the back-end electronics reproduce the transmitted information in
its original form (whether by speaker, TV tube, teletype, etc.). It is here
that the digital-to-analog (D/A) converter will reconstruct the original sig-
nal if it was first digitized before transmission.

REFERENCES/ADDITIONAL READING

Pritchard, W., Suyderhoud, H., and Nelson, R., Satellite Communication Sys-
tems Engineering, 2nd ed. Englewood Cliffs: Prentice-Hall, Inc., 1993.
Feher, K., Digital Communications, Satellite/Earth Station Engineering.
Englewood Cliffs: Prentice-Hall, Inc., 1983.
Roden, M., Analog and Digital Communication Systems. Englewood
Cliffs: Prentice-Hall, Inc., 1979.
Gagliardi, R., Introduction to Communications Engineering. New York:
John Wiley & Sons, 1978.

1. A ship at sea wishes to communicate with a shore station over the hori-
zon using its HF radio. Using the F2 region of the ionosphere for a
“reflection” point between the stations, it is determined that the inci-