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Re: Digest Number 149

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  • Mike Jacobs
    Well, I really was trying to avoid writing a lecture on whether you can use a scanner to recieve signals from a microwave communications system. In fact, I
    Message 1 of 6 , Nov 2, 1999
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      Well, I really was trying to avoid writing a lecture on whether you
      can use a scanner to recieve signals from a microwave
      communications system. In fact, I had written something like Dave
      had and then discarded it in favor of a more curt reply :) the point i
      was making is that a casual listener (such as implied in the original
      post) cannot use a consumer style receiver to listen to telephone
      calls transmitted over microwaves.

      Even if you had the facilities to listen to calls on an analog FDM
      microwave system as Dave mentioned, the biggest problem is how
      to know which of the hundereds of multiplexed channels to listen
      to, and when to listen, since long-distance trunks are dynamically
      allocated on a call by call basis. In this case, a nailed-up private
      line channel would actually be less secure than a dialup line, since
      an adversary could determine over time which channel to monitor.
      Of course any sensitive user would have to encrypt for real privacy,
      just as today.

      As far as the INMARSAT calls go, I am not familiar with any details
      of the systems, so I am guessing that they must be FM if people
      can monitor them. But you would only get one side of the
      conversation since the satellite signal wouldn't cause an image to
      be received.

      By the way, Dave, it was nice to hear about your lovely family. My
      best wishes to all of them.


      Mike Jacobs, N3YAV
      Antenna and RF Engineering Laboratory
      Penn State University
      State College, PA
    • Dave Emery
      ... For what is worth INMARSAT uses uplinks in the 1.6 ghz range. That is L band, not KU band by the way. Older INMARSAT (INMARSAT standard A) on ships used
      Message 2 of 6 , Nov 2, 1999
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        On Sun, Oct 31, 1999 at 07:08:14PM -0500, Ken Hoehn wrote:
        > From: Ken Hoehn <khoehn@...>
        >
        > Yes and no, Mike.
        >
        > You cannot receive them directly.
        >
        > It is VERY possible to receive them as images when near the TX
        > equipment.
        >
        > Folks I know have spent HOURS lying on the deck of a cruise ship near
        > the INMARSAT antenna listening to the calls from the ship on a scanner.
        > Ku band....microwave at it's best.
        >
        For what is worth INMARSAT uses uplinks in the 1.6 ghz range.
        That is L band, not KU band by the way. Older INMARSAT (INMARSAT
        standard A) on ships used regular nbfm modulation and can be easily
        received on an ordinary ultra wide coverage scanner of the sort widely
        available to hobbiests (R-7100s, R-8500s and many others). The
        downlinks at 1.5 ghz can actually be copied directly from the satellite
        with a medium size dish or loop yagi array and a suitable low noise
        amplifier.

        More modern INMARSAT (standard B and M) ship to shore radio
        links use vocoded digital modulation on Vitirbi FEC encoded OQPSK SCPC
        carriers. No encryption is used. Larger modern cruise ships are
        more likely to use the digital format since the rates are significantly
        cheaper for digital than analog fm (the carriers run lower power
        and more fit on the transponder).

        None of this applies to terrestrial Telco microwave links
        which are for all practical purposes 100% digital these days. Signals
        from these digital links look just like white noise to an analog scanner
        or radio. No audio can be heard at all, not on images and not
        on spurious responses or other anomalies. Recovering audio requires
        recovering a very fast (140 megabit or so) bit stream from the digital
        signal and demultiplexing out the 64 kbs stream corrosponding to a
        particular channel and then decoding that.

        > >
        > > From: "Mike Jacobs" <mwj116@...>
        > >
        > > You can't listen to telephone calls transmitted via microwaves
        > > using a scanner or cellphone.
        > >
        >
        Right indeed.

        --
        Dave Emery N1PRE, die@... DIE Consulting, Weston, Mass.
        PGP fingerprint = 2047/4D7B08D1 DE 6E E1 CC 1F 1D 96 E2 5D 27 BD B0 24 88 C3 18
      • UK Monitor
        For those interested in monitoring Inmarsat take a look at :- http://www.geocities.com/ResearchTriangle/System/5140/index.html ... Snip
        Message 3 of 6 , Nov 5, 1999
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          For those interested in monitoring Inmarsat take a look at :-

          http://www.geocities.com/ResearchTriangle/System/5140/index.html


          >From: "Mike Jacobs" <mwj116@...>
          >Reply-To: coldwarcomms@onelist.com
          >To: coldwarcomms@onelist.com
          >Subject: Re: [coldwarcomms] Digest Number 149
          >Date: Tue, 2 Nov 1999 23:08:17 -0500

          Snip
          >As far as the INMARSAT calls go, I am not familiar with any details
          >of the systems, so I am guessing that they must be FM if people
          >can monitor them. But you would only get one side of the
          >conversation since the satellite signal wouldn't cause an image to
          >be received.
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