Page 126 - Intro to Space Sciences Spacecraft Applications

11 3
Communications
We can define a term known as thefree space loss (Ls) which repre-
sents the losses due to the spreading out of the signal through space
between the transmitter and receiver:

4xR
Ls=(h)
(5 - 7)

Incorporating the free space loss term, our equation simplifies to:

Finally, this expression usually incorporates a term that takes into
account a multitude of other signal power losses due to factors such as the
atmosphere, ionosphere, electromagnetic interference, and the like. This
term is called the incidental loss term (L,) which, as a loss term, appears
in the denominator of the above equation. The final form of this equation
is called the link (or power) budget which gives a value for the power
received by a station at a distance R from a transmitter:

In many cases this calculation is made using decibel values which would,
for the above relationship, be written as:

where each term is either specified in or converted to dB using equation 5-3.

Noise. The amount of power received may still not determine if the receiv-
er will understand the transmitted information, which is required for suc-
cessful communications. The reason for this is the presence of noise which
can interfere with the transmitted signal. There are many sources for this
noise including the transmitting and receiving equipment as well as natur-
al causes such as the sun, earth, atmosphere, and even other celestial bod-
ies. The amount of noise a source produces is represented by the source’s
equivalent noise temperature (Teq). For our purposes, we will assume that