[amsat-bb] Re: New Satellite Downlink?

STeve Andre' andres at msu.edu
Wed Aug 25 14:24:25 PDT 2010

On Wednesday 25 August 2010 12:27:00 Robert Bruninga wrote:
> Possible new AMSAT Application?
> We may have access to two old TRANSIT navigation satellites with
> a 50 baud downlink at 149.985 (and 400 MHz). (presently coming
> over in the mid afternoon).  My problem is, coming up with any
> meaningful application to use them for communications that would
> capture the interest of students, hams or volunteers in support
> of education, public service or emergency comms or just plain
> fun...
> The downlink can be heard on an OMNI antenna (though I would
> suggest a 3/4 wave (55") vertical) and could be decoded by a
> simple software only application with a sound card. (someone has
> to write it)...
> The total useful message capability is about 500 bytes
> transmitted every 2 minutes (at 50 baud).  The uplink is very
> specialized and can ONLY BE DONE from one (or two) very special
> commmand stations.  These satellites of course were the original
> Navy Navigation satellite system (also called OSCARS) and so the
> message would be in-place of the normal navigation data.  SO in
> a sense, this is a downlink BROADCAST application.  Since ham
> radio is two way, I'm stumped for applications.

Well, I'm not sure how many applications there are for this, but it
could be fun to try some stuff.

Way way back hundreds of years ago in the 70's I wrote some code
to take English text and crunch it down and transmit it over a modem.

I won't say the following is reasonable, but at 50 baud the little link
needs all the help it can get. ;-)

A lookup table can be made for about 65,000 of the most commonly
used words plus various technical stuff.  A message can then it
converted into a series of 16-bit offsets into the table of words,
taking 2 bytes (octets) per word.  Printing out words takes the
stream of data, does a lookup for each 16-bit quantity, prints
that word plus a space, and goes on.

A word like "communications" which is 14 bytes becomes two and
is thus a win, but "a" "I" and the like is a loss.  There could be an
escape sequence to provide for the literal transmission of a word
not in the 65,000 lookup table, and one could also be added for
upper casing of the next word, etc.

Doing this, you can transmit 250 words from the lookup table
each minute, fairly faster than squirting out raw ASCII.

Since you'd likely need a decoder no matter what the transmission
is, the 65,000 word table is stored on the client side.  Hilarity
will probably ensue when someone doesn't update their table
after a big change, and gets slightly demented messages till
they update their code.

Thinking about what to transmit...  Possibly space weather
transmissions?  CMEs and such are something that has world
wide impact.

Well, that, or national lottery scores.

STeve Andre'
wb8wsf  en82
Disease Control Warden
Dept. of Political Science
Michigan State University

A day without Windows is like a day without a nuclear incident.

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