[ao16aprs] Re: [amsat-bb] APRS via AO-16

[Jim White <jim@coloradosatellite.com>]

LU-19 is oriented 'upside down' relative to AO-16 and the other two
Microsats.  However, while there is some distortion (relative to a dipole)
of the receive antenna pattern because the body of the satellite is large
compared to the thickness of the whip, the receivers are sensitive enough
to make that up.  We studied the patterns and uplink BER extensively for
the first three or four years of life.  All indications pointed to
interference as the almost exclusive cause of uplink problems (assuming all
transmit factors were optimized).
At one point I did some very low power uplinks tests on AO-16 when we were
working on improved software and there was no other uplinking going on.  An
EIRP of about 5 watts produced almost 100 % uplink success in the absence
of interference.  However it should be noted that many sources on the
ground produce an equivalent EIRP, and most of those are not intended for
or directed to the satellite at all.  Most of the interference sources are
south of the U.S. boarder or non-satellite U.S. amateur activity.  Two guys
talking on HTs will compete for the receiver capture with a mobile packet
station attempting to digi through the bird.
An additionally an often overlooked factor that reduces uplink success is
over deviation.  When I observe local terrestrial packet activity with a
service monitor the average deviation is about 7 KHz.  If that carries over
to the case of satellite uplinks (as I suspect is the case) it is right on
the margins of what the receiver filters will pass and would require
careful adjusting the uplink for Doppler.  The ideal deviation (again
determined after extensive testing) is 3.2 KHz.  And deviations as low at
2.5 will result in almost as good a success rate, and do not require
Doppler correction.

So I would suggest the key factors that effect uplink success are (in order
of commonality):
- Deviation to wide
- TXD to short
- Interference
- Incorrect frequency
- Asymmetrical modulation

jw
wd0e@amsat.org
jim@coloradosatellite.com


>> One possibilty for the poor southern performance is the orientation of the
>> pacsat antenna on the top of the bird and/or QRM from the southern USA
>> which disappears when the birds get north of here..
>
>Bob, if I recall correctly (hopefully someone with 1st-hand knowledge will
>correct me if I am mistaken) the magnet orientation is reveresed between
>AO-16 and LU-19, so the receiver antenna orientation is different for the
>two satellites.
>
>South American and Carribean area QRM on the uplink is, unfortunately, a
>real possibility.


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