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satellite passbands

Discussion in 'Electronic Design' started by Philip Newman, Oct 8, 2003.

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  1. I would like a definition on what is meant by satellite passbands in reagard
    to communication systems

    thanks,

    Phil
     
  2. Allot of satellites contain linear transponders. These are basically
    wide banded frequency convertors, they listen on one band of frequencies
    and retransmit on another offset from the listening frequency by a fixed
    amount. Usually the input frequency is in a different RF band than the
    output. Low earth orbit satellites travel so fast (with respect to the
    surface of the Earth) that doppler shift is a big problem. Rather than
    trying to adjust transmit frequency to account for the doppler, the
    transponder is wide enough to allow for some drift due to doppler.

    One amateur radio satellite that I used to talk thru listened on 2mtr
    VHF and transmitted back on 10mtr HF. The transponder was about 20khz
    wide IIRC. This allowed several SSB conversations to occur
    simultaneously without interference to one another.

    michael brown
     
  3. thanks. another question that has cropped up, are the definitions for
    x-band, c-band etc

    what are the frequency ranges for these? is there any difference for UK or
    US? I am in the UK so those would be preferable, so far I have:
    X-band: 520GHz
    S-Band: ?
    D-Band: 660-692GHz
    C-Band: 430-510GHz

    thanks

    Phil
     
  4. I'd assume these definitions to be global.
    AFAIK, the
    S Band is 2.5 GHz,
    X Band is at 10GHz,
    Q Band is at 35GHz
    W Band is at 90GHz

    Rene
     
  5. Hi,

    Take a look at this -

    http://www.packer.dsl.pipex.com/images/
    radar_band_designations.gif

    Don't forget to string it all together. This won't give you
    the exact band edges but will give you the general idea. I once
    read the reason behind the use of these particular letters but
    can't now remember what it was.


    Cheers - Joe
     
  6. Ken Taylor

    Ken Taylor Guest

    Hmmm, a satellite doesn't have a wide passband to get around Doppler shift -
    in the case of simple satellites, the Doppler is ignored because, as you
    say, there's enough room for all (or because interference can be tolerated).
    Passband is broad so as to allow for the number of carriers to be
    transported. Also, a highly inclined geostationary satellite can exhibit
    significant Doppler shift which requires correction if the bandwidth of the
    carrier is low enough that the Doppler shift is significant wrt it.


    Ken
     
  7. no_one

    no_one Guest

  8. If a LEO satellite contained a repeater listening on a specific
    channelized frequency, it would be nearly impossible to use without some
    kind of computerized compensation at the transceiver tweaking the
    transmit and receive frequencies accordingly. The passband eliminates
    some of this fuss, until you run off the end of the passband anyway. ;-)

    I'm curious as to how a stationary satellite might experience
    significant doppler shift problems. With virtually no relative motion
    to the land based station, how could it happen? Now with a LEO at
    1000km altitude traveling at >30,000 km/h, doppler is significant and
    constantly changing. You can't really ignore that. ;-)

    I made allot of QSO's on RS-10/11 before it croaked. It really took
    some practice to be able to communicate with someone effectively,
    especially when a good pass was good for less than 15 minutes of talk
    time. It's kinda ironic that most of the QSO's were with a friend of
    mine that only lived about 2 miles from me. It's a real hoot though the
    first time you actually hear your voice coming back delayed. ;-)

    michael
     
  9. Ken Taylor

    Ken Taylor Guest

    'Geostationary' is a relative term - they are not 'stationary'. The amount
    of drift in the orbit varies - some of the Inmarsat satellites (which I work
    with) have several degrees of inclination, which can cause several kHz of
    Doppler drift. With small (<~10kHz) carriers, this is very significant. The
    corrections are done in the ground stations (as ditinct from the consumer
    end of the link).
    I only tried on AmSat a few times - I heard myself coming through a few
    times, but never had a QSO. :)

    Cheers.

    Ken
     
  10. OK I knew that the "stationary" satellites moved around a bit, but what
    I didn't know was that carriers in the khz range were used on
    satellites.
    Which satellites did you do it on? I never really messed with the ones
    that required 440 transmit ability or circularly polarized antennas. My
    amsat setups were quite crude as I lived in an apartment at the time.
    ;-) The Russian RS satellites were the only ones I played with, but
    they were a blast. You had to really stay on top of things to make sure
    you didn't lose the other guy as the doppler was really pronounced.

    Satellites and chasing weather balloons (google on south texas BLT's)
    before they all started carrying GPS, were probably the most fun I've
    had in amateur radio. Of course working russians while only running
    five watts into a 300' high kite antenna is pretty neat too. ;-)
    WARNING: don't try this at home, I'm not responsible for others idiotic
    behavior, only my own. ;-D
     
  11. Ken Taylor

    Ken Taylor Guest

    Crikey! I *think* it was one of the Russki ones, but it was mid-eighties, so
    my memory isn't too flash on the subject. I could hear myself whistling
    through it despite just whip antennas at 440 and 144. One of my mates was
    driving through a valley in a fairly remote part of Australia and could hear
    the beacon for the satellite clear as a bell as he drove through, but once
    he came out of the valley it faded into the noise. Lots of nice effects out
    there! :)
    LOL! That was something we always wanted to do, but the Air Force used our
    area as a low-altitude training area for many years after getting F111's, so
    it was out of the question.

    Cheers.

    Ken
     
  12. The letters were assigned out of sequence for military usage so the
    enemy would have a harder time figuring out what was going on. Of
    course, this was in tube days when even UHF was difficult to use
    reliably. Only key people knew what the operating frequencies were, and
    you didn't have good LNAs, accurate frequency counters, or a microwave
    spectrum analyzer you could carry aboard an airplane to look for the
    Enemy's radio signals. The early military HF and VHF radios used regen
    and super regen receivers that emitted enough noise they could be
    tracked by the enemy. The poor quality also allowed the use of
    broadband jammers to wipe out an entire range of signals to keep the
    enemy from talking to other units.
     
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