[amsat-bb] FO-29 and AO-73 Antenna Patterns and Crosslinks

Zach Leffke zleffke at vt.edu
Tue Oct 20 15:58:24 UTC 2015

     First off, I'd like to say I had a blast at the 2015 AMSAT 
Symposium.  It was a lot of fun and I know that the rest of the VT team 
really appreciated the warm welcome from everyone.

Drew and I got to talking at one point after the Banquet and he 
re-kindled an interest in something for me.  He posted a while back 
about how the downlink for AO-73 is in the passband of FO-29's uplink, 
and that the PSK can be heard at times when monitoring FO-29's 
downlink.  I'd like to do a bit more analysis using STK to predict when 
an 'accidental crosslink' might occur allowing a telemetry decode of 
AO-73 via FO-29.

Right now I'm using a simple model for all of this where I'm treating 
each satellite as a single point in the sky.  The first question I'm 
asking (via STK simulation profiles) is when can the two "points in the 
sky" see each other when not blocked by the curvature of the earth.  The 
next question I will ask of the great and glorious STK is when can the 
two "points in the sky" see each other AND when can the point in the sky 
that is FO-29 see the point on the ground that is the VT Ground Station.

When the second question yields accesses in time when the two spacecraft 
can see each other and FO-29 can see the VTGS, then I have a window to 
schedule the VTGS for this experiment.

The next trick will be accounting for doppler between the two spacecraft 
(via range rate info from STK for the two points) to determine where 
exactly in FO-29's uplink passband the AO-73 signal will fall.  Then 
account for the Linear inverting transponder offsets, then account for 
doppler between FO-29 and VTGS.  Also, the signal spectrum will be 
inverted due to the transponder, but we can "re-invert" in our GNU-Radio 
flowgraph to account for that.

End result should be an access window at VTGS along with a predicted 
doppler curve for tuning at VTGS, ultimately getting us to an AO-73 tlm 
decode via FO-29.

My question for the group concerns the antenna patterns for FO-29 and 
AO-73.  I'm trying to plan ahead for refining the access windows for 
when to actually conduct this experiment by doing a bit more than treat 
each satellite as a "point in the sky."  Does anyone have any info on 
the actual antenna pattern for FO-29?  Along with this, I remember 
reading somewhere that FO-29 has an attitude control system.  Is this 
system still operational and if so, what is the "target attitude."  I'm 
trying to get a sense of the antenna patterns for the two spacecraft and 
their orientations to identify "sweet spots" where antenna patterns 
might align positively.  On the other hand it will be good to know when 
we have an access window but one of the spacecraft is in the null of the 
antenna pattern of the other spacecraft.  For AO-73 I'm assuming simple 
whip antennas.  I know it doesn't have a positive attitude control 
system, but if memory serves I think it has a bar magnet and the spin 
from the anodized bars imparted on it that should give me a sense of its 
orientation at particular times.

Thanks in advance!
-Zach, KJ4QLP

Research Associate
Ted & Karyn Hume Center for National Security & Technology
Virginia Polytechnic Institute & State University
Work Phone: 540-231-4174
Cell Phone: 540-808-6305

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