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400 bps tlm (once again)



Dear friends,

A few historical and technical datas about pskdem and tlmdec programs.
The modem software comes from work back in late 97 when I tought it
would be pretty to have telemetry onboard the next sputnik. Having
recognized the ruggedness of the modulation scheme used on ao-10 and
ao-13, I began to work on a generator and a demodulator.
The specs made it that it should be *very* easy to receive the telemetry
on a FM receiver with the bitstream modulating a 1200Hz soundcarrier.
Tests proved that the choices were good and we implemented that on the
shitnik transmiter with a low power 8031. That's the reason why the
demodulator was implemented on DOS, to ease the job even on the slower
computer only equiped with w3.1 and a very standard sound recorder.
Since the sound sub carrier was 1200 Hz, there was no need to enter a
central frequency. So after the wav file was recorded with the proper
name, it was only necessary to double click on the pskdem program (which
was then named puzzle after the game it was intended to support). An
HTML file was then automaticaly generated including an applet that
showed bargraph values of the 8 channels and the accompanying text in a
window.
Meanwhile, with p3d in mind, the soft modem was designed to have full
provision for Doppler tracking and performances as close to the theory
as possible. Both works were simultaneously conducted.
Two test generated were build, one with equal probability noise, which
is quite close to that of a FM demodulator, and one with AWGN for
'normal' modulation scheme. (the one which is available on download on
amsat.org).
Extensive tests has shown that the soft modem is close to less than 1.2
dB of the theoretical results as some of you may have noticed while
decoding. The tracking loop has an acq. range of +- 100 Hz while the
lock range is only limited by the sample frequency , the bandwith of the
receiver and the distorsion of the waveform resulting thereoff. An om
succeded in decoding with a center frequency as low as 800 Hz, leaving
barely one sideband of the signal.
As p3d launch was closing in, we decided to release the soft modem 'as
is' or so, in order to keep the philosophy of simplicity that made the
original design. But we got the decoding values of the tlm very late and
that's why we turned to the Victor Kudielka's C source. May he be
thanked here. It was just the matter of adding crc to check the frames
the modem was not doing.
Puzzle was the name we intended to give to sputnik 99 (beatnik). The
game was very simple : each day a different message was sent on the
400bps beacon. That message was made of two parts : an 8 channel
telemetry, some temps and voltages and an URL that pointed on a site
where it was necessary to find clues. As 15 different messages were to
be sent, it was a 15 days game wich would have allowed everybody around
the world to find all the clues and come to the solution of the game.
You all know how it ended in jeopardy.
Why didn't we release unlimited version ?. Simply because a satellite is
expensive to build and that we tought a good software was more valuable
to om's than cardboard satellite. So that's why the windows version will
be sold for 10$.
Release is expected for tomorrow evening (CET). With that you will get
the full unlimited software, using the same powerful algorithm than the
DOS version, and soon (a week or so) the window display of tlm values
with possibly export to spreadsheet and compatibility to p3t. Bernard
F6BVP shall compile the C source of the DOS version to LINUX and release
it soon after the (very small) changes are made.
The DOS version, whith its (cumbersome, I admit, but it works almost
every time you try, isn't it?) simplicity will soon be released in
unlimited version. Donations to amsat-f welcome.
All donations and licence fees will be entirely used for the SATEDU
project, fully dedicated to Hams and education.
A few words on it. SATEDU includes a FM transponder and linear
transponder on 144 and 2400 MHz as well as an education payload similar
to that of sputnik 18. Telemetry will be the same as the one intended
for shitnik including about 160 channels, a parametrable voice recorder
and moreover will give direct but limited control of the satellite's
attitude to the students (or hams, why not ?) in order to play with
basic space mechanics and earth's magnetic field.
Have been quite long so forgive me those who aren't interested in. (and
forgive grammar faults, I assume word spelling ones)
Best 73'.
-- 
--------------
Ghislain RUY
F1HDD
AMSAT FRANCE
--------------


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