On Wed, 14 Jul 1999, angel rodriguez wrote:
> I have found two URL's in the last hour and one of them is nice.  why
> would each station need it's own GPS if I could get the signals from
> the sensor to a main station either using a VCO a transmitter/receiver
> and Larry's demodulator then the three or five stations into one of Larry's
> AtoD and sdr for later processing??  

You could try this, but there will likely be some processing and other time
sensitive operations done at the recording site if you want to transmit
digital data.  If you are transmitting analog data directly from the sensor
then keeping time at the central station is okay, but keep in mind that
there may be time delays in this process as well that could possibly affect
the accuracy.  For an array time-keeping is extremely important if anything
is to be done with the data.  I think it is safer to have each site keep
time (that is what I would do).  This also makes each station more
independent...  

> To keep the power requirments down at
> the remote stations it would be nice to have as little electronics as
> possible.
> If the costs and complexity of the telemetry stuff is tooo much I might
> be willing to lay cable to each of the remote station. It would be
> nice to keep this experiment as simple as possible and still get
> usefull data.

Angel,
   You will run into a whole lot more complexities and problems by laying
cables (and probably expense if you are doing things on a kilometer scale!). 
The only way to avoid signal loss or distortion using cables is by digitizing
at the sending end of the line, which brings you back to the same situation as
before.  This is a classic problem which has been aching for a better solution
for over 100 years.  Contrary to popular belief, electric signals do not
simply zip from one end of a cable to the other at the speed of light with
arbitrary accuracy.  This is due to the finite resistance, capactitance and
inductane in metal wires due to intrinsic defects in chemistry that make it
non-ideal.  The actual equation describing the transmission of a signal is
given by the transmission line equation, which is a second order differential
equation similar to the one for the damped seismometer.  To transmit signals
you have to set the unit capacitance and inductance of your line to a
specified value using actual electronic rigs.  This is the reason why we can't
build phone modems faster than 56Kbps.  The unit capacitance and inductance is
set to frequencies corresponding to the human voice...  The only way around
this problem is to use fiber optics, which is becoming less expensive over
time but is still costly to install. 

John Hernlund
E-mail: hernlund@.......
WWW: http://www.public.asu.edu/~hernlund/

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