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RE: P3D information



Phil KA9Q wrote [edited]:
> One goal may be to enable as many radio amateurs
> as possible to communicate directly by amateur
> satellite with the greatest flexibility, highest
> quality and lowest overall cost -- this being the
> sum of the earth segment and space segment costs.

> If this is your goal, then the case for moving to
> digital modes on microwave frequencies becomes
> inescapable. So does the need to design the necessary
> ground station equipment and make it inexpensive and
> readily available.

Well stated, Phil.

When AO-13 died I sold all me "big iron" (huge antennas, preamps,
radios, power amps, etc).  Now, as I consider putting it all back up, I
wonder "what do I want to do?" The answers are different for everyone,
but I want to do new things, that's for sure.  I am interested in the
science and engineering of it all so I will probably be most interested
in the new modes using higher frequencies.

I also see the excitement generated by Bob's APRS system, especially
when using LEO's.  He is committed to making this new mode of
communication useful and accessible to all Hams.  I think that's what
you have proposed with digital voice and data using P3D Rudak, Phil.
This is the kind of thinking that will help re-invigorate Ham Radio and
get back some of the new blood that has gone to computers and the
Internet.  I commend you for thinking "outside the box" and being
willing to stuck your head out of the trenches, knowing that you will
get shot at for your trouble.  Keep it up. Some of us are listening.

Rick
W2GPS (was WB2TNL)


-----Original Message-----
From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
Behalf Of Phil Karn
Sent: Sunday, October 08, 2000 3:14 AM
To: tlang@freeway.apana.org.au
Cc: amsat-bb@AMSAT.Org
Subject: Re: [amsat-bb] P3D information


>ability to _talk_ to each other.  Data is neat, but not as personal.
However,
>whether the voice be carried by analog or digital signals isn't so much
of an
>issue for the end user experience, as long as we can... TALK! :-)

I absolutely agree, and that's just what makes the P3D Rudak subsystem
so interesting: it'll be the first amateur satellite with a real-time
digital repeater capability fast enough to support good quality
digital voice. Being able to do that with small portable antennas
should definitely interest a lot of hams.

Sure, this has been done outside the ham bands for years in Inmarsat,
Iridum and Globalstar. But it's not quite the same as doing it for
ourselves on the ham bands.

>However, again, we are _amateurs_, with considerable investment in
analog gear,
>so unlike a commercial operation, we need to keep existing users in
mind, while
>helping them migrate to the new technologies as time and money permit
for each
>amateur.

At some point it becomes much more expensive to maintain that old
investment than to start over with something new. I believe we passed
that point in AMSAT long ago.

What baffles me the most is the accusation that I'm somehow an
"elitist" for advocating digital and microwave modes that require new
ground equipment and/or don't make full use of existing equipment.

These people apparently do not consider it even slightly elitist to
insist on modes and bands that require expensive analog radios, high
power amplifiers and enormous amounts of aluminum that can only be
legally erected on a tiny minority of homes built in the last 25
years. These are modes that only a tiny fraction of radio amateurs
have ever used or will ever be able to use. Modes that have kept AMSAT
as a small niche of a hobby that is itself rapidly becoming irrelevant
because of similar attitudes.

I believe my DSS/C-band analogy is very apt. Before digital satellite
broadcasting, only a tiny minority was able to receive satellite
television because of the large antennas required and the hassle of
installing them. Now satellite TV is completely mainstream.

Now imagine if the powers that be had insisted that DirecTV and
Echostar be backward compatible with existing C-band dishes.

To be sure, they still wouldn't have gone anywhere had they not
developed the necessary digital receiving equipment and made it widely
available at reasonable prices. And so it is with digital and
microwave modes on amateur satellites.

Every satellite system consists of two segments: the space segment and
the earth segment. Over the 20 years I've been involved (off and on)
with AMSAT, I've seen an enormous amount of work go into the space
segment. I have nothing but admiration for those who conceive, design,
build and operate amateur satellites. P3D is a technological tour de
force.  It's something that AMSAT can rightly be very proud of.

Somewhat less, but still considerable work has gone into the earth
segment. Much work has gone into automatic antenna and doppler
tracking systems, antennas, RF equipment and station integration.

Yet very little effort has gone into satellite *system* design.  By
"system design", I mean the process that asks "what are we really
trying to accomplish?" and then makes the fundamental tradeoffs
between all the components of the system to meet those goals as
closely as possible.

One goal may be to enable as many radio amateurs as possible to
communicate directly by amateur satellite with the greatest
flexibility, highest quality and lowest overall cost -- this
being the sum of the earth segment and space segment costs.

If this is your goal, then the case for moving to digital modes on
microwave frequencies becomes inescapable. So does the need to design
the necessary ground station equipment and make it inexpensive and
readily available.

Of course, there are other goals. Maximizing experimental
opportunities on the largest number of bands and modes may be more
important than maximizing amateurs' communication capabilities or
minimizing satellite or ground station costs. You may consider
educational opportunities to be paramount. Or you may consider
maintaining the existing equipment investments of the satellite
"elite" to overrule everything else.

Whatever your goals are, you should be explicit in stating them,
because there are some very real conflicts among them and ignoring
that fact won't make them go away.

Phil

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