Page 377 - Satellite Communications, Fourth Edition

The Space Link 357

The major source of loss in any ground-satellite link is the free-space
spreading loss [FSL], as shown in Sec. 12.3.1, where Eq. (12.13) is the
basic link-power budget equation taking into account this loss only.
However, the other losses also must be taken into account, and these are
simply added to [FSL]. The losses for clear-sky conditions are

[LOSSES] [FSL] [RFL] [AML] [AA] [PL] (12.12)
The decibel equation for the received power is then

[P ] [EIRP] [G ] [LOSSES] (12.13)
R
R
where [PR] received power, dBW
[EIRP] equivalent isotropic radiated power, dBW
[FSL] free-space spreading loss, dB
[RFL] receiver feeder loss, dB
[AML] antenna misalignment loss, dB
[AA] atmospheric absorption loss, dB
[PL] polarization mismatch loss, dB

Example 12.4 A satellite link operating at 14 GHz has receiver feeder losses of
1.5 dB and a free-space loss of 207 dB. The atmospheric absorption loss is 0.5 dB,
and the antenna pointing loss is 0.5 dB. Depolarization losses may be neglected.
Calculate the total link loss for clear-sky conditions.
Solution The total link loss is the sum of all the losses:

[LOSSES] [FSL] [RFL] [AA] [AML]
207 1.5 0.5 0.5
209.5 dB

12.5 System Noise
It is shown in Sec. 12.3 that the receiver power in a satellite link is very
small, on the order of picowatts. This by itself would be no problem
because amplification could be used to bring the signal strength up to
an acceptable level. However, electrical noise is always present at the
input, and unless the signal is significantly greater than the noise,
amplification will be of no help because it will amplify signal and noise
to the same extent. In fact, the situation will be worsened by the noise
added by the amplifier.
The major source of electrical noise in equipment is that which arises
from the random thermal motion of electrons in various resistive and
active devices in the receiver. Thermal noise is also generated in the