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Re: Geosynchronous Satellites



Jon ... and all concerned.... (with Obvious answers to innocent
questions...)
I am willing to try to compile a FAQ for each SAT of significance,  I am
willing
to make them available online and will pay for the cost to do so.

Your part will be to ACTIVELY participate to insure the data is correct by
putting up with (quarterly?) reviews of the said subject matter?

I will not take credit for publication and will require copy left rules that
I will post for review that will require all re-publication (i.e. on CDROM)
or electronically to donate to AMSAT.

So If you are willing... Send me every Tidbit you have... and I
will compile them as best I can.

If others are willing; I will work as a coordinator and collection point
to compile a FAQ for amateur Sat's from other authors.  Which would
speed the process....

BTW... I've already saved Jon's message below.... as I do all nuggets
I find on the BBS.

For me this would be the best leaning experience I could have and to give
something back to the AMSAT community (and all amateurs) that I admire
and strive to grow into.

Enjoy your holidays!

KG4ITU@amsat.com
"Take a walk in a world of imagination" -- Willie Wonka
----- Original Message -----
From: "Jon Ogden" <na9d@mindspring.com>
To: <amsat-bb@AMSAT.Org>
Sent: Saturday, December 23, 2000 7:40 PM
Subject: [amsat-bb] Geosynchronous Satellites


> OK all,
>
> In response to the reply postings people have made regarding
geosynchronous
> birds, I've gotten ripped by many of you who think my responses have been
> too harsh.  So to show that I am reasonable, I will AGAIN post the same
> information that many people have posted time and time again.  We really
> need a FAQ that is sent to each new person on the BB.
>
> OK: Specifically we have a question of a multi-channel FM bird in
> Geosynchronous orbit above the USA.
>
> Let's break this down into multiple components:
>
> First the multi-channel FM part:
>
> 1.) FM carriers require the FULL power of the satellite while operating.
> Unlike SSB or CW, the duty cycle is 100%.  If a satellite has one 5 Watt
> downlink, then that is doable.  But as soon as you make say 6 downlink
> channels, the power is then up to 30 watts! Assuming an efficiency of 75%,
> the total power consumed by the bird is 40 Watts.   This sort of power
> budget is not available in a bird the size of AO-27.  And with all the
> auxiliary hardware, 75% efficiency is likely high.
>
> 2.) FM can only capture and retransmit one signal at a time.  In a
> geosynchronous orbit, that FM receiver would capture approximately half
the
> earth.  Contrast this to the orbit of UO-14 or AO-27 where the footprint
is
> a few thousand miles.  Look at the QRM generated in the bird just during
> that short path.  If you have ever talked on that bird as it moves south,
> then you understand the problem we have with the Mexican cabbies who use
our
> frequencies illegally.  A geosynchronous FM bird would suffer from
> continuous QRM all the time.
>
> 3.) FM is not efficient as a mode in terms of propagation.  FM is
inherently
> more wide band than SSB or CW.  You may transmit 25 watts on SSB and 25
> watts on FM but the SSB signal will get you farther.  CW will be even
> better.  This is because as you increase the bandwidth your power is
spread
> out over a wide range of frequencies.  Hence your power density decreases.
> This is the reason why weak signal and EME types use SSB/CW and not FM.
>
> 4.) FM has a bandwidth of approximately 25 KHz per channel.  The
philosophy
> for such a "geosynch" bird would end up meaning that normal dual band
> mobiles would be usable.  After all, the original poster is trying to
think
> up of a way for an "easy" bird for "everyone" in the USA only.  If you
have
> a 6 channel bird, then the total downlink bandwidth would be a MINIMUM of
> 150 KHz and probably more like 300 KHz since you couldn't stack the
channels
> up right next to each other.  According to the ARRL bandplan, the two
meter
> OSCAR bands are 144.3 to 144.5 (new) and 145.8 to 146.0  But I don't think
> that the 144.3 to 144.5 part is really used as 144.39 is APRS.  So with
> "traditional" two meter frequencies we have 200  KHz of bandwidth and with
> the entire two sub bands we have 400 KHz.  A geosynchronous FM bird would
> take up almost all the OSCAR band in two meters rendering it useless for
> other birds.
>
> OK, now onto geosynch:
>
> 1.) A geo-synch bird is usable by only one half of the earth.  Those not
in
> the footprint would not get to use the bird.  Part of the reason why we
have
> satellites is that it is a multi-national effort.  Many countries
> contribute.  AMSAT-NA does not have the resources to spend money on a bird
> that would be usable to just those in the Western Hemisphere.  Never mind
> the political ramifications of alienating all the other member societies.
> It would be the death of AMSAT-NA to support something like this.
>
> 2.) Much more uplink and downlink power are needed with such a bird.  The
FM
> LEOs work well because they are at an altitude of a few hundred miles.  A
> geosynch bird is around 23,000 miles.  A BIG, BIG difference - nearly 2
> orders of magnitude.  Considering that the RF field falls of inversely
> proportionally to the distance between two objects, it follows then that
two
> orders of magnitude in power will be needed to get the same channel link
> margin.  So if a LEO bird uses an FM power of 5 watts, then approximately
> 500 Watts would be needed to give the same result with a geosynchronous
> bird.  The same goes for the uplink as well.  Some of this could obviously
> be made up with increased antenna gains, but still, it makes the power
> budget unseemly.
>
> 3.) Geosynch birds need to have some sort of propulsion system on board to
> keep them in their orbital slots.  Therefore, they are much more
> sophisticated than the "AO-27" type.  You just couldn't do it.
>
> 4.) I don't know for sure about this, but it may be hard to get a
> geosynchronous orbital slot.  There are only so many of them and who knows
> if we'd even be able to get one while high $$ paying commercial birds
could
> use it.
>
> 5.) You need a much larger launch vehicle to put a geo-synch bird into
> orbit.  This increases cost.
>
> In the end, I suppose that if a group of hams decided to put their own
money
> into an FM geosynchronous bird, they could do so.  However, to put it on
> 2m/70 cm would be nothing but stupid as for the reasons explained above
and
> to use the microwave bands would require a massive power budget that would
> be much more wisely put to use in a linear transponder SSB/CW satellite.
>
> There, let's put this one to rest, OK.  No more questions about
> geosynchronous birds.
>
> 73 and Merry Christmas,
>
> Jon
> NA9D
>
> -------------------------------------
> Jon Ogden
> NA9D (ex: KE9NA)
>
> Member:  ARRL, AMSAT, DXCC, NRA
>
> http://www.qsl.net/ke9na
>
> "A life lived in fear is a life half lived."
>
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>

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