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Minor issue with packet uplink on 145.990





One of the minor issues with QRM on the ISS
packet uplink on 145.990, is the repeater input
on 146.010.

There is a Repeater pair on 146.010 input with
output on 146.610 (in the USA)
Most repeater channels in the USA have approximately
200 repeaters on each channel.
(maybe some one at the ARRL can come up with
the current channel count for this frequency)

On Earth, we do not have a problem with 200
repeaters sharing the same channel.  This is because
of the term Terrain Shielding.  If you place two
repeaters 200 miles apart, there is a 99.99% chance
they will not cause each other interference.
(assuming 2-meters and no ducting)
The curvature of the earth, trees, hills, etc.
help block the signals.  Terrain shielding helps
us load up a channel and reuse it over and over again.

ISS and Terrain Shielding:
ISS is at an altitude of approximately 220 miles (400~km).
This altitude gives ISS a LINE-OF-SITE range of 1500 miles (2000~km).
When ISS is over South Dakota, its radio range covers the USA coast
to coast.  It is even possible for a station in Boston Mass to Digi
to a station in California.  Once ISS is above your horizon
there is no terrain shielding.  
Any station operating on the ISS uplink channel improperly
can inadvertently cause serious interference (qrm).

Adjacent Channel Interference:
We all know, that you can not run 2 FM repeaters in the same
town with a 10k channel separation.  Even a 15k channel separation
does not work.  
A FM voice (5k deviation) need 20k of channel spacing on EARTH!  
In space, it is a whole nother story.
In space, you must also add to your channel spacing the doppler 
frequency correction.  In this case, it is 3.6k, so lets round to 5k
that brings us to 25k.  Now lets add another 5k to help prevent
the FM capture effects from hearing the repeater inputs.  
That puts us at 30k channels.

After 12 years of experiment with Mir, we came to the conclusion
that 25k channel spacing was minimum to avoid qrm from terrestrial
stations or other satellites and 30k channels spacing was ideal.
For the Mir space station, we compromised on 145.985 FM simplex.
This put Mir 25K away from the 200 repeater inputs on 146.010.

In my opinion, a small percentage of the QRM being received by
ISS on 145.990 is caused by adjacent channel interference caused
by the repeater inputs on 146.010.  There is not much we can do
about the QRM at this time.  If we do installed a filter
to notch out the qrm from 143.xxx, we can look into adding
an extra filter for 146.010.

good luck,

knowledge is greater than QRO

Miles, 
www.marex-na.org
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