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 time!
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.
Bob, Wb4APR
On Wed, 25 Aug 2010, 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.
The total message capability of 500 Bytes can contain one
long
ARRL bulletin, or 20 APRS position/status reports, or say 20
or
so APRS text messages, or say 50 "callsign exchanges" or
maybe
even 1 thumbnail image... but what's the application?
Even if we allow say, INTERNET link to the command station
for
"anyone" to contribute to the twice per-day upload, then everyone's receiver application can receive them... For
what?
So Im looking for ideas. All I can come up with so far is:
- 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
downlink.
Two stations do this to each other and it counts as a
two-way
QSO? 4) ...
Every scenario of interest usually begins with the much
higher
value of UPLINK from the individual field station, not downlink.. Hence I am stumped.
HUMMH... Maybe purely educational? If the software can run
on
any PC with a sound card connected to any scanner... Then
every
school can use it as a satellite downlink signal of
interest..
What kind of thumbnail image can fit in 500 bytes? Send in
your
picture and get it downlinked on a given day?
Etc..
Will need a DSP volunteer to write the sound card decoder.
Bob, WB4APR
Sent via AMSAT-BB@amsat.org. Opinions expressed are those
of the author.
Not an AMSAT-NA member? Join now to support the amateur
satellite program!
Subscription settings:
http://amsat.org/mailman/listinfo/amsat-bb