Technology
Spacecraft
Ka-band Spectrum
One way to increase the amount of information or data per
unit of time transmitted by a satellite is to use a higher
radio frequency. Until ACTS, the Ka-band frequency was virtually
unused - the majority of communication satellites operate
in lower frequency bands called the C- and Ku- bands. Ka-band
is desirable because its higher frequency allows wide bandwidth
applications, smaller spacecraft and ground terminal components,
and stronger signal strength. The multiple transmitters, therefore,
can process several gigabits of data per second. Ka-band is
the first frequency with enough bandwidth available to carry
simultaneous services ranging from multiple voice communications,
high data rate computer connections and teleconferencing.
High frequency also implies short wavelengths. The millimeter
wavelength Ka-band signals are easily degraded by rain, a
problem known as rain fade or rain attenuation. In designing
ACTS, detailed studies of 75 years of weather patterns, rain
amounts and effects on lower frequency radio signals were
made. As a result, ACTS was engineered with higher signal
strength and digital technology that incorporated ways to
compensate for rain fade such as error codes and slowing the
transmission rate. Interestingly, the digital technology on
ACTS (known as time division multiple access or TMDA) is,
in principle, identical to that used in terrestrial cellular
systems.
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