# AO40 tracking at extreme angles: squintdoppler

• Subject: [amsat-bb] AO40 tracking at extreme angles: squintdoppler
• From: "nn0v" <nn0v@xxxxxxxx>
• Date: Fri, 26 Oct 2001 23:30:50 -0500

```I was just tracking it for fun even with the xponder and rudaks off;

the doppler plot to the right of the demod spectrum in ao40rcv is shwoing a
z shape,
the hz/sec indicator is bouncing between +15.3 and -15.5 hz,
valid crc is 28 out of 176(16%), signal is bouncing 13 db, with each rev,
5.76 rpm approx,
the doppler from the edge is very pronounced squint at this131 translating
to a doppler of 7.67 mph to and fro from whatever is sweeping the rf around.
You got gravity on board that thing now!

Well just calculating out loud: lets see what we got here.
Doppler=veloc*freq/c-Freq/c where c=983,516,160mph.
Diameter to generate a speed of 7.67mph where it rotates 5.76 per minute:
7.67mph=11.2493 fps 0.096 revs per sec. and 10.4166 seconds per rev
circumference thus 117.18 feet; circumference=2piR.
circumference=2pi*r   so radius is 18 feet...that isnt possible , so what
are we seeing here:
Some kind of  knife edge difraction moving along the straight parts of the
satellite ?
So the doppler of the signal is made worse by the shape of the satellite
at this sharp angle.like whipping the antenna around or something. It would
be nice to be
able to measure phase shift vs rotation for various satellite shapes to try
to find
a more ideal combination.

There's plenty of signal from the beacon, its  above noise floor even at the
bottom of the dip.
Well this doesnt add up well, and when the satellite is at better squints I
dont think the doppler
zig zags are more than a few tenths of a hz. Now that I know what I'm
looking for
I'll watch for this when its at smaller squint angles  and see what the
antenna looks like in other cases.
That must be why nasa always has an s band omni mounted on a stick that juts
away from the
satellite a few feet, so the edges wont whip  the doppler around like that,
cause they hate the
phase mod it does to the carrier.

I guess the triple legged starfish shape would explain this doppler zigzag
shape.