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Re: 3Dec 2000Z pass on 2400MHz

At 01:25 PM 2000-12-03 -0800, you wrote:

>I was able to copy telemetry with P3T starting at 2006z and my 2m IF indicated
>2401.318.7.  The frequency did not appear to be stable, forgot to turn on 
>the log,
>and don't think I got any valid CRCC.  Is the instability due to my old 
>The signal was quite strong, and cut-off occurred at 2009z with an 
>indicated MA of
>25.4 on IT150 with the infamous set #3.  Elevation here in San Diego was 
>about 2°, and a
>"swing check" with my 30" dish indicated the AZ of 250° appears to be good.
>My antenna "looks" right at the ocean...
>I'd sure like some comments on the instability, as well as my frequency at 

Hi Dave,

You have experienced "wobulation" a normal and often useful physical 
phenomenon.  As the S/C rotates, at higher squint angles the S-band antenna 
is actually moving toward and away from you in a cyclic fashion.  The 
doppler shift associated with this motion causes the frequency to vary a 
few 10's of Hz.  In fact the "wobulation" is proportional to the sine of 
the squint angle, spin rate (RPM), and distance of the antenna from the 
center of rotation.

Wobulation was used VERY successfully to directly measure squint angle on 
AO-13, and hence determine ALON/ALAT. In  G3RUH's old AO-13 articles in the 
AMSAT archive is one called (paraphrasing), "Measure Squint Angle Directly" 
which describes this approach.  In all likelihood, we will use this 
technique to occasionally measure squint on AO-40 while it is in spin mode.

Obviously, wobulation will not be a problem at low squint angles or in 
3-axis mode.

There is another factor to consider with the S-band antennas on AO-40.  At 
high squint angles, they are probably occulted at least partially as the 
S/C rotates by the 400N engine exhaust nozzle/bell.  This may produce some 
periodic QSB.

  Stacey E. Mills, W4SM    WWW:    http://www.cstone.net/~w4sm/ham1.html
   Charlottesville, VA     PGP key: http://www.cstone.net/~w4sm/key

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