Page 144 - Intro to Space Sciences Spacecraft Applications

131
Communications
c. Will we still be in contact with the spacecraft when it flies beyond the
farthest reaches of Pluto? (apluto = 5.9 x lo9 km, epluto = 0.25)
4. An FM radio station wishes to transmit baseband frequencies from 20
to 20,000 Hz in order to broadcast good quality music. The station
wants to transmit digitally (by first converting the baseband signal to a
digital signal) using byte-sized digital words.
a. Determine the number of Q-levels the signal may be assigned values
from.
b. What is the minimum sampling frequency (and associated sampling
period) that may be used?
c. Determine the bit period, the transmission rate, and the minimum
carrier frequency that might be used to transmit the music.
d. Can the current frequency range assigned to FM broadcasting be
used by this station? (Explain.)
5. The Army and Marine Corps would like to have the capability for satel-
lite communications with ground forces using a portable transceiver.
Such a radio would have a small (1 meter) diameter parabolic dish
antenna with a low efficiency (0.5). Maximum transmitted power would
be 2 watts. The soldier would like to be able to transmit to a geostation-
ary satellite down to 5 degrees elevation angle (v = 5" [described in
Chapter 61, klmt-max = 41,126.7 km). The satellite has a 5-meter diam-
eter parabolic dish (efficiency = 0.9) and uses deep space as a heat sink
to keep the receiver cooled to 75°K. Up-link frequency is 30 GHz and
worst case (\v = 5") incidental losses are 4 dB. Consider the antennae as
being able to boresight on each other during communications, and deter-
mine the minimum signal-to-noise ratio the satellite will have to be able
to handle for a successful link. While showing your calculations for the
link budget, put a box around the following parameters: