[amsat-bb] SO-67 back on air!

Jan-Albert Koekemoer jan.koekemoer at sunspace.co.za
Mon Jun 21 07:04:52 PDT 2010



After much delay we are now in a position to re-activate the SA-AMSAT payload
schedules on a regular basis again!  


During the downtime we conducted several experiments in order to investigate
possible alternatives for the "cut-off" problem experienced by many at the
end of last year / beginning of this year.  The cut-off is caused by SW
running on piggyback electronics that continues to gate the received audio
onto the transmitter for about 3s after a specific ground transmission has
ended, whereafter the received audio is cut off.  The SW then waits for the
next ground transmission with a valid "transponder" tone embedded, before
gating the audio onto the transmitter again.  So if a second transmission
starts within the 3s "tail" period, it will be cut off when the 3s expires.
Listen to this mp3 recording as an example:  http://tinyurl.com/38vvg3o 


Unfortunately the piggyback electronics does not have reprogramming
functionality, so we are not able to change the SW.  We did however manage to
implement a mechanism on the satellite transponder (used by the piggyback)
that should be of some assistance when using the payload:  the PTT of the
transmitter now STAYS ON for the duration of the payload activation (i.e. 15
min), regardless of a valid "transponder" tone received.  Note that this does
not get rid of the "cut-off" problem, nor of the requirement to embed a valid
tone when you want to use the payload - see detailed explanation below.


The following example depicts the ideal usage scenario of the payload:

1.       As the satellite comes over the horizon, an unmodulated carrier
("quieting") will be received by all on UHF

2.       Person A starts to transmit on VHF (with valid "transponding" tone
embedded in the transmission) and the transmission is relayed on UHF

3.       When Person A releases his/her PTT, the satellite will continue to
transmit what it is receiving (usually this will be "noise") for a further
±3s.  Nobody should start to transmit during this time!

4.       At the end of the ±3s period, everybody will again hear the
unmodulated carrier.

5.       NB: only when this unmodulated carrier is heard should Person B
start to transmit.  Listen to this mp3 recording as an example of valid
usage:  http://tinyurl.com/32rjbxm 


I suppose this will take some getting use to - at least the system has a
built-in "punishment" mechanism for the impatient! ;-)


Please refer to previous posts on general voice quality optimization when
using the SA-AMSAT payload on SO-67.  


I will request Gladys Magagula at the Satellite Application Centre to obtain
regular schedules from the regional coordinators as soon as possible.  We
will start off with day passes only, transmit on 2.5W and see how things go.







Jan-Albert Koekemoer, PrEng

Sun Space & Information Systems (Pty) Ltd.

Stellenbosch, South Africa

www.sunspace.co.za <http://www.sunspace.co.za> 

+27 21 880 8117 (tel)

+27 83 461 6643 (cell)

+27 21 880 1703 (fax)


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