Page 128 - Intro to Space Sciences Spacecraft Applications

Solution: Communications 11 5

Ae(,, = 11.78 m2 = 10.71 dB
Ae(,, = 353.43 m2 = 25.48 dB
GT(*) = 59,217.63 = 47.73 dB
GR(') = 1,776,528.8 = 62.5 dB
LS(*) = 1.011 x IO2' = 200.05 dB
PR(3) = 7.44 x W = -81.3 dBW
No = 4.14 x W = -143.83 dBW
PR/N,= 1.8 x IO6 = 62.55 dB
- -. _. ___ - _.. - __
Noteq (I) h = c/f = 0.05 m
(2) R and h must use same units
(3) I dB = 1.26 decimal

MODULATION

Modulation is the means by which information is impressed on elec-
tromagnetic radiation so that it may be transmitted from one location to
another. There are at least two different components to a communications
signal: the carrier wave and the baseband signal.
The baseband signal represents the information desired to be transmit-
ted. In the case of a radio station, this may be the audio frequencies (of a
few thousand hertz) of the DJ's voice or the music of a record. These are
good examples of analog signals which are the raw, continuous wave-like
frequencies of sound or light. Television stations transmit higher baseband
frequencies that are used by a TV set to illuminate the tube in a certain
way to produce a picture. Banks and businesses (and satellites) may con-
vert their baseband information (numerical transactions, written text and
pictures, remotely sensed frequencies) into digital signals (strings of ones
and zeros) which then represent the original information which may allow
improved transmission of the data.
Currier waves, as the name implies, are electromagnetic signals used to
carry the baseband information between the transmitter and receiver. Car-
rier waves are used because the baseband signal may not have sufficient
propagation characteristics for successful communications. For instance,
audio frequencies are attenuated severely by the atmosphere, limiting
their useful ranges (can you imagine radio stations that simply blasted
their music from loudspeakers all over the city?). The particular carrier
wave frequency may be chosen primarily for its propagation characteris-