Page 127 - Intro to Space Sciences Spacecraft Applications

Introduction to Space Sciences and Spacecrajl Applications
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the receiving equipment is the major source of noise and that its noise tem-
perature will be known or given. The resulting noise is given by:

(5-10)

where k = Boltzmann's constant (k = 1.38 x W/"K Hz) and bw rep-
resents the bandwidth or the range of frequencies the receiver has been
designed to pick up. Less noise may be experienced by using a small oper-
ating bandwidth, but in all practical cases the desired information signal
is spread out over a range of frequencies dictating what the minimum
bandwidth for a receiver must be.

Signal-to-Noise Ratio. Successful communication systems (known as
links) are designed to operate under all expected conditions of losses and
noise. The measure of this is given by a receiver's signal-to-noise ratio,
which is given by equation 5- 11 :

PR
SIN = - (5-11)
NO
A receiver will have a minimum signal-to-noise ratio specified for
acceptable operation. If the received power decreases or noise increases
such that the actual signal-to-noise ratio is less than the minimum speci-
fied for a receiver, the information in the carrier signal may not be intelli-
gible or the carrier signal may not be detected at all.

Example Problem:

A geostationary communications satellite has a traveling-wave-
tube (TWT) amplifier with a maximum power output (PT) of 9 W
and transmits via a 5 m diameter parabolic dish antenna with an effi-
ciency of 0.6. The antenna is boresighted on a ground station using
a 30 m diameter parabolic dish antenna (p = 0.5). The receiver band-
width is 1 MHz wide around the 6 GHz down-link frequency, and
the receiving components are cooled to maintain an equivalent tem-
perature of 300 OK. Slant range (distance between antennae) is
40,000 km and the incidental down-link power losses (L,) are 1 dB.
Determine the signal-to-noise ratio of the receiver.