QUESTION:
In which radio frequency band is the normal link between Galileo and earth?
The one you chose,is the best to avoid "noises" and other sources of
interference?

ANSWER from Jim FH Taylor on March 5, 1997:
The frequency of the radio signal from Galileo is approximately 2295 MHz.
For some additional detail about our radio communications, including
frequencies, there is a FAQ at
http://www.jpl.nasa.gov:80/galileo/newfaq4.html#radio

As the telecommunications analyst on the Galileo flight team, I was
answering a question someone else asked about how they might be able to
receive Galileo's signal on a radio astronomy telescope.  In the FAQ,
you'll see that we had a second radio frequency, 8415 MHz, ready if we
could have used the high gain antenna.  But I'll talk about only the 2295
MHz frequency here.  That's the one that our current mission, called the
"S-band mission", uses.  S-band is another name for the frequency band that
includes 2295 MHz.

By the way, "MHz" stands for megahertz.  The unit MHz used to called
megacycles or megacycles/sec.  One MHz is one million cycles/sec.  Perhaps
you know that an AM radio broadcasting station might be at 1000 kHz (they
might say "1000 on your AM dial"), which is 1 MHz.  An FM station might be
at 99.9 MHz ("fm 100" might be that station's name).  So you can see that
the  Galileo signal is many times higher in frequency than even an FM
station.    

The second part of your question, about whether we chose our frequency on
the basis of noise, is a very good one.  The short answer is: no.  The
Galileo radio equipment is made up of flight spares (extra units) that were
built for the Voyager spacecraft.  Voyager was launched toward Jupiter in
1977.  The Voyager radio was built for specific Deep Space channels near
2295 MHz.  To avoid having to change and retest this already built spare
equipment, Galileo simply adopted the same channels as Voyager.  We had to
do an analysis to prove to the Voyager project that we would not interfere
with their signal.  If our spacecraft transmits on the same channel as
Voyager, can you think of a reason why the signals from the Galileo would
not interfere with Voyager?  Hint: there are two Voyager spacecraft, and
*they* transmit on the same channels!

Now, why did Voyager choose an S-band (2995 MHz) channel?  It's because all
current Deep Space missions transmit in one of three general bands: S-band,
X-band (8415 MHz), and with the newest missions, Ka band (32,000 MHz).  The
NASA Deep Space Stations, in Southern California, near Madrid, Spain, and
near Canberra, Australia, that Galileo uses can receive in each of these
bands, but only in these bands.  It's the same way that all FM broadcast
stations have to be on a channel in the FM band that can be received on
your radio. 

The S-band and X-band regions were chosen on the basis of relatively low
noise and relatively low attenuation (signal weakening) through the earth's
atmosphere.  The wavelength of a radio signal is in inverse proportion to
its frequency.  X-band wavelengths are shorter than S-band, and Ka-band
wavelengths are shorter yet.  There is a broad region of frequencies that
includes S-band that has the least amount of noise due to the earth's
atmosphere and attenuation from rain.  There are other noise sources, such
as noise emitted from the sun, from planets such as Jupiter, and from the
Galaxy itself.  These noises are somewhat lower at X-band and Ka-band than
at S-band.  

Two other simple factors enter into the design of spacecraft radios: these
are the power efficiency and the equipment size.  Generally, S-band
equipment is more efficient (requires less spacecraft power for a given
amount of radio output power) than X-band, and X-band more efficient than
Ka-band.  NASA has technology programs underway to improve the efficiency
at higher frequencies.  Also, generally, some radio equipment sizes (for
example, antenna diameters or the dimensions of the wave guides that carry
signal to the antenna) are proportional to the wavelength.  So Ka-band
equipment is smaller than X-band.  This is important when every gram of
mass is counted in a spacecraft design.

In summary, the Galileo radio frequencies in use came from the state of
Deep Space spacecraft and tracking station technology that existed in the
early 1970s when the equipment was designed.  While we take account of the
noise levels in our band, the choice was not primarily driven by noise
levels.  

Something to think about, far beyond Galileo:  we are beginning to think
about spacecraft communication systems that use light waves instead of
radio waves.  The "transmitter" would be a laser, the transmitting and
receiving "antennas" would have lenses like telescopes, and the "receiver"
would be a detector of photons.  What do you think might be some of the
factors in the design of an optical communications system?