[amsat-bb] Re: Galileo interference on L band

Bruce Rahn brahn at woh.rr.com
Fri Sep 22 14:27:43 PDT 2006

John B. Stephensen wrote:
> Part of the concern about using L as the primary digital uplink is the 
> fact that the ground stations will be high duty cycle emitters. BPSK 
> has a very low crest factor and one of the uses for a 256 kbps link is 
> streaming video, so it will be very much like an ATV repeater. Given 
> the equatorial orbit, Eagle will also be closer to the horizon than 
> previous amateur HEOs.
> Even a restriction similar to the one in place for U uplinks in areas 
> of the U.S. (1 kW EIRP) would make high-speed uplinks unavailable. 

Thank you for bringing this point to my attention...through my neglect 
of things I have lost track of the dominance the digital mode has taken 
in this project.  You are correct that this signal format is a high duty 
cycle one.  My thoughts of where Eagle was heading have been more 
aligned with the desires of the membership expressed in the survey 
results presented in the September/October 2004 issue of "The AMSAT 

I'm going to ask some hypothetical questions here which I really don't 
expect you or anyone to answer.  They are more food for thought than 
anything else.

-  As part of the system engineering process, were other bit rates and 
modulation schemes considered which would mitigate potential 
interference problems?

-  What percentage of the user base (AMSAT-NA members) would be 
disenfranchised if digital video were eliminated because of its high 
duty cycle requirements and the potential for causing interference to 
other spectrum users?

-  In the aforementioned survey results, the surveyed members indicated 
their highest preference was for analog modes followed in second place 
by digital.  Has the user mindset shifted to digital over analog?  If 
not, or unknown, are the spacecraft resources being fairly partitioned 
and allocated to support analog users?  What percentage of the user 
community will be using digital video and text messaging?

In a private exchange with Mr. Sanford, I expressed my concern that the 
user community was not being represented by a strong 'user advocate' at 
critical design meetings.  'Designers are not users and users are not 
designers' but both camps must be fairly represented to achieve harmony 
and consensus between the two.  Bringing a strong 'user advocate' into 
the design process would be a win-win situation for both the user 
community as well as the design community.  Users would feel someone is 
directly addressing their operational concerns and the 'user advocate' 
could be the one defending decisions rather than occupying the time of 
the designers in addressing these concerns. 

I believe in the 20 plus years I have been an AMSAT member history has 
demonstrated that the 'if we build it they will come' approach has not 
worked well.  Had it been successful, the organization would have more 
resources in terms of members and dollars than we could deal with.

Respectfully -- Bruce

Bruce Rahn

Wisdom has two parts:
1.  having a lot to say; and
2.  not saying it!

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