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[aprssig] PCSAT2 Downlink Challenge

Robert Bruninga bruninga at usna.edu
Sat Jul 30 16:33:45 UTC 2005

Unattended 9600 baud fixed antenna data capture:

There are many variables at work here for optimizing data
capture unattended from weak satellite signal.  Remember
with global APRS-Intenret links, the only thing that counts
will be the SUM of everyone's efforts, so not any one station
needs to get all data.  In fact, a single fixed antenna and
because of doppler, it is impossible to get all data without
changing direction of beam or doppler.  But we can each 
optimize our fixed station to make our best contribution.
Even if only one pass a day..  There are many variables.

1) Weakest signal near horizon must have beam
2) But least doppler change is at horizon
2) Strongest signal above 30 deg, but maximum doppler change
3) 19" whip is excellent gain at 435 (7 dBi) but only for >30 deg
4) But high doppler (over 8 KHz in 2 minutes) >30 deg
5) Your latitude determines if most passes are north, south
    and your area might favor east or west.
6) Also you might live on east coast or west coast.  If on east
coast, you should have fixed antenna to favor passes to east
that no one else will hear, on west coast, favor to the west.

Best combination will be different for everyone.  Or, everyone
just gets a small part on an OMNI and APRS network puts
it all together into good total...  hummh...

>>> aprssig at ei7ig.org 07/30/05 7:01 AM >>>
> 1)   You need to replace the antennas!
No rotators here unfortunately, so a 19" Whip it will have to be  
then.  Though from reading and re-reading Bob's post, and knowing  
that these stations will be un-manned should we just leave the rigs  
on the downlink frequency and hope to receive with little or no  
doppler at the centre of the pass, or should we tune/scan between the  
frequencies most likely at the beginning/end of the pass?
> 2)   For work at 9600, you must tap the receive signal off the  
> radio's discriminator output

Ok, I'll try the linux soundmodem interface again later... I think I  
hung the soundcard driver trying 9k6 last night, i have the cables  
already made up to interface to the FT-847.

I did a bit of playing with the TH-D7 this morning and i have it  
running under Linux in Kiss mode.  Though I've come across a few web  
pages that seem to suggest that its very unreliable at 9K6 in kiss  
mode. Has anyone had any experience of this? bearing in mind we are  
only trying to catch a few packets of data would this suffice for a  
9K6 downlink?

I've got the machine time synced using ntp and this should give me  
the correct output, listen -t -p 70cm | tee PC2_$(date +%y%m%d_%H%M) 

> 3)   The TS2000 appears,
So for the EI8JA's TS-2000, kiss mode and add 19" Whip, AGW (which he  
has) and AGW monitor. Excellent, one hopefulle sorted.

> If you have a PC with sufficient "horsepower" (approximately  
> Pentium II 300 or higher) and a soundcard with a stereo line input,  
> the AGW PE  softmodem can be configured to act as TWO TNCs, one on  
> the left channel and one on the right channel, emulating something  
> like a Kantronics 9612 dual-port..   You could set one to 1200 baud  
> connected to one radio via the speaker output, and one to 9K6  
> connected to the other radio via the discriminator output.   Note  
> that you MUST have a line-level input to get the stereo input; the  
> mic input (often the ONLY input on laptops) is single-channel  
> (MONO) and can't use this feature of AGW PE.

Thats a very interesting idea, I'll put that to John, as he could  
then use both rigs one on the 'big vertical' and one on the 19" Whip,

Thanks for the reply.

de John

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