[amsat-bb] Re: New Satellite Downlink?
bruninga at usna.edu
Thu Aug 26 13:14:00 PDT 2010
> I've done DSP soundcard coding before. Years ago
> I wrote a DGPS decoder, so if you need some more
> coders, I'd be happy to help.
I'm embarassed that I still have not heard the bird on 2 tries.
Its supposed to be on 149.987 MHz and you can hear it's 2.3 KHz
tone with an FM receiver... I did hear it on APL-JHU's system
also consisting of an amateur OSCAR class station... But not on
mine yet.... OOPS! Wait! I have a very sharp DCI BANDPASS
filter up there! Duh.... Ill try a different antenna this
OK, the signal is 50 baud modulated with + and - 60 degree phase
shift on the 400 MHz carrier (it can be switched to the 149.987
MHz carrier, but is not there currently). The wave form of the
modulation is as follows:
To transmit a "1" the carrier is phase shifted +60 degrees for
about 2 ms, then shifted -60 deg for about 2 ms and then brought
back to carrier with no phase shift for about 6 ms, then it is
shifted - 60 deg again for 2ms and then +60 for 2ms and then
brought back to no shift for another 6 ms. The "0" is the
opposite pattern of shifts. This process provides a clock
signal at twice the bit rate. The actual exact period of each
symbol is 19.7 ms.
The beginning of each exact 2 minute (6103 bit block) is marked
with 0111111111111111111111110, that is 23 "1"'s with a leading
and ttrailing zero.
> On Wed, 25 Aug 2010, Robert Bruninga wrote:
> > Possible new AMSAT Application?
> > We may have access to two old TRANSIT navigation satellites
> > a 50 baud downlink at 149.985 (and 400 MHz). (presently
> > over in the mid afternoon). My problem is, coming up with
> > meaningful application to use them for communications that
> > capture the interest of students, hams or volunteers in
> > of education, public service or emergency comms or just
> > 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
> > to write it)...
> > The total useful message capability is about 500 bytes
> > transmitted every 2 minutes (at 50 baud). The uplink is
> > specialized and can ONLY BE DONE from one (or two) very
> > commmand stations. These satellites of course were the
> > Navy Navigation satellite system (also called OSCARS) and so
> > message would be in-place of the normal navigation data. SO
> > a sense, this is a downlink BROADCAST application. Since
> > radio is two way, I'm stumped for applications.
> > The total message capability of 500 Bytes can contain one
> > ARRL bulletin, or 20 APRS position/status reports, or say 20
> > so APRS text messages, or say 50 "callsign exchanges" or
> > even 1 thumbnail image... but what's the application?
> > Even if we allow say, INTERNET link to the command station
> > "anyone" to contribute to the twice per-day upload, then
> > everyone's receiver application can receive them... For
> > So Im looking for ideas. All I can come up with so far is:
> > 1) ARRL Bulletins? (I don't even know how often ARRL sends
> > bulletins...)
> > 2) Navy/Army/AF MARS broadcast bulletins...
> > 3) Internet message in-to-command-upload-to message RF
> > Two stations do this to each other and it counts as a
> > QSO?
> > 4) ...
> > Every scenario of interest usually begins with the much
> > value of UPLINK from the individual field station, not
> > downlink.. Hence I am stumped.
> > HUMMH... Maybe purely educational? If the software can run
> > any PC with a sound card connected to any scanner... Then
> > school can use it as a satellite downlink signal of
> > What kind of thumbnail image can fit in 500 bytes? Send in
> > picture and get it downlinked on a given day?
> > Etc..
> > Will need a DSP volunteer to write the sound card decoder.
> > Bob, WB4APR
> > _______________________________________________
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> of the author.
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