[amsat-bb] Re: aprs iss freq: is there anyone out there? UHF works!

Bob Bruninga bruninga at usna.edu
Tue Jul 10 12:45:47 PDT 2012

> Unfortunately, 70cm UHF doesn't lend itself to unattended 
> SatGate operations like 2m VHF does.  I tried catching the 
> ISS on UHF and only managed to hear, but not decode, one 
> packet burst.  I can't imagine anything short of a full 
> satellite station with automatic azimuth/elevation control 
> would capture many packets right now.

Actually the problem is not antenna pointing but Doppler.  With the 18 KHz
of dopper shift, you are only going to capture packets in the center 1/3rd
of the pass.  But the good news is that the ISS is ten dB closer to you at
that time.

Yes, you will not get as many packets, but they can be strong.  The problem
is that very few people are using the UHF packet digipeater, so if no one is
transmitting then there is nothing to hear.

In fact, listening on UHF (for the center of the pass) can be just fine,
especially on a 19" vertical whip.  Here are the factors:

1) due to Doppler, you are only going to decode packets above say about 30
degrees (without tuning)
2) When it is above 30 degrees, the ISS is 10 dB closer than on the horizon
3) When it is above 30 degrees the antenna pattern of a 19" whip (normally
used on 2 meters) acts as a 3/4 wave vertical for UHF and it has almost 7
dBi gain above 30 degrees 

So everything is going in your favor and even better link margins on UHF
above 30 degrees and the packets should be plenty loud (assuming you
minimize your cable loss).

The only BAD news is that the ISS spends more than 70% of all inview time
below 30 degrees.

But the GOOD news is that such an IGate station can be natennteded, and with
no moving parts and I t can be replicated by lots more people due to this
simplicity.  So instead of having only say 6 unattended APRS IGates on the
2m downlink to cover the USA, we simply need more stations. Then the APRS-IS
system will still capture all packets, though any individual station will
only hear their own smaller "cone of strength"..

So we simply need to encourage more IGates and lower "individual"
expectations while the larger quantity of IGates raises the "system

Something like that.


On 7/10/2012 10:42 AM, Lizeth Norman wrote:
> Are there any stations in South America to gate aprs packets to the
> net from the uhf freq? How does one do this? I've got a few Byonics
> TT4's from a balloon project.
> The story: I'll be driving the Panamerica Sur from Lima to Nazca Peru
> and would like to have the ability to be seen via the net.
> Norm n3ykf
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