Re: Digest Number 149
- 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
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
- On Sun, Oct 31, 1999 at 07:08:14PM -0500, Ken Hoehn wrote:
> From: Ken Hoehn <khoehn@...>For what is worth INMARSAT uses uplinks in the 1.6 ghz range.
> Yes and no, Mike.
> You cannot receive them directly.
> It is VERY possible to receive them as images when near the TX
> 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.
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
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.
> >Right indeed.
> > From: "Mike Jacobs" <mwj116@...>
> > You can't listen to telephone calls transmitted via microwaves
> > using a scanner or cellphone.
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
- For those interested in monitoring Inmarsat take a look at :-
>From: "Mike Jacobs" <mwj116@...>Snip
>Subject: Re: [coldwarcomms] Digest Number 149
>Date: Tue, 2 Nov 1999 23:08:17 -0500
>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