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

Bill Ress bill at hsmicrowave.com
Fri Sep 22 16:02:59 PDT 2006

To All,

Finally, leaving Galileo "off the table" right now, I believe Bruce's post
gets to the core of what is causing the concern among members. I would hope
the Board and the design team gives Bruce's questions due reflection.

The designers and the Board must look to the users for their direction. If
users want 145 up and 29 down, and that will satisfy their satellite
communications needs, then by golly that's what you put in space even though
it isn't technically cute or fashionable. They're the main source of
donations (I think unless there are a few well heeled donors with deep
pockets driving the ship). In short the Board should be working at the
behest of the membership.

Where does all this high duty cycle, high bandwith 256 kbps digital video
and text messaging come from anyway? Are we trying to be a cell phone
company in space? It neat for sure, but what percentage of our members can
make the technical and financial investment for this specialized mode. (This
mode wasn't even proposed in the 2004 survey - as read it)

How can I put this nicely? Well, I can't so here goes.

AMSAT should be building satellites that meet majority user requirements not
build satellites for the technical ego's of a few.

Ouch - there I said it!

Bruce's suggestion for "user advocates" is good but the organization might
well find that it is just another impediment to doing their design work. But
no need! The user's spoke in 2004. Perhaps a new user survey is in order -
though judging from the comments on the -bb, the opinions have not likely

Regards...Bill - N6GHz

-----Original Message-----
From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org]On
Behalf Of Bruce Rahn
Sent: Friday, September 22, 2006 2:28 PM
To: John B. Stephensen
Cc: AMSAT BB; K3IO at verizon.net
Subject: [amsat-bb] Re: Galileo interference on L band

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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