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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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