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Using OQPSK




I've been reading Phil and others comments regarding a potentially
new amateur satellite with great interest.  
While working in the satellite business put a damper on my enthusiasm
to go home and get on the birds, I do enjoy hte dialog here and
find it interesting to see the cross-pollenation of commercial
and amateur ideas and practices. 

With regard to Phil's comments regarding modulation schemes,
 it should be said that ORBCOMM (my employer) successfully uses
OQPSK on our low data rate subscriber links in a TDMA scheme.
 Our earth-to-space links are in the VHF band (148-150 Mhz) at
2.4 kbps, with the user terminals transmitting no more than 450
msecs per burst at no more than 5 watts RF output from the box.
 In all cases that I am aware of, the antenna is one type of
terrible antenna (1/2 wave whip) or another (flat plane).  We
do make the link.  One reason OQPSK (and SDPSK) were chosen,
I'm told, was that the technology is known and simple, and from
a business persepctive, easily designed and inexpensively reproduced...important
in business!

The satellites transmit to the users in the 137-138 Mhz band
at 4.8 kbps.  We're not really bent-pipe.  The user sends to
data to the satellite, which re-transmits it as part of a 56
kpbs datastream on fixed frequencies in-band to a gateway earth
station.  The reverse path is the same, as everything goes through
the data switch (read: billing center) before being distributed.


While our protocols are proprietary, I'll try and dig up what
I can on the link budgets and other related technical material
should it be of any value.  ORBCOMM been through some significantly
tough times for about a year now...we're coming out of Chapter
11 bankruptcy with a new set of owners, and a significantly smaller
(80) staff than at this time a year ago (570).  But the technology
works! 

Eric W3DQ
Washington,DC
w3dq@amsat.org





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