These
factors influence the signal quality
between a spacecraft and its ground control station.
Link
Budget
Downlink
signals
have lower frequencies than uplink signals for several
reasons. Satellite power is limited and low frequency
transmitters require less output power than high
frequency transmitters. Low frequencies attenuate
less in the atmosphere than high frequencies, so
the transmitter requires less power to maintain
a nominal signal quality at the receiver. Low frequency
transmitters in general are more efficient than
high frequency transmitters.
Uplink
signals are
assigned different bands than downlink signals for
frequency deconfliction.
Noise
sources
degrade signal quality. Spacecraft communications
design must account for radiation from the Earth,
planets and even stars. Electronic communication
circuitry has intrinsic noise associated with random
electron motion.
Noise
temperature
is a way of quantifying a
source noise intensity by comparison to the noise
power of a passive source in Kelvin degrees.
Noise
figure
represents the signal to noise
ratio degradation of a circuit.
Link
equation
mathematically
describes the link budget and accounts for transmitter
effective isotropic radiated power (EIRP), space
loss, atmospheric and other losses and receiver
losses.
