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RE: Help measure AO-40 spin rate

The method I use is the "Doppler wobbling" method, that needs a PC + sound
card + FFT software
(CoolEdit or Spectrogram, for example).

1) record an audio sample on the hard drive

2) run a frequency vs time analysis and play with zoom, FFT size, etc to get
a nice readable sine Doppler curve picture 

4) measure the time elapsed between as much as peaks you can and divide it
by the number of peaks, just to improve the measurement accuracy.

(JPG picture available on request).

Please note that you can also estimate the satellite squint angle by using a
simple formula:

squint angle = arcsin [ max possible Doppler variation/ measured Doppler

The max possible Doppler variation is related the distance between the
transmission antenna and the satellite rotation axis.

All the computation details appear in a paper written by G3RUH about AO-13
squint angle measurements. This  a???.txt document is stored somewhere on
the AMSAT WEB site  (sorry, no WEB access here at the present time).


Jean-Louis F6AGR

 > -----Message d'origine-----
 > De: Richard D. Burgan [mailto:wc8j@raex.com]
 > Date: jeudi 25 janvier 2001 12:40
 > À: amsat-bb@AMSAT.Org
 > Objet: Re: [amsat-bb] Help measure AO-40 spin rate
 > Hi All,
 > I have two ways of measuring the spin.  The first is entirely
 > manual
 > and the second involves recording the signal amplitude on a
 > strip
 > chart and counting the peaks over time.  I'll describe the
 > manual
 > system since I feel its the most accurate.
 > I listen to the audio from my 2.4 GHz receiver in USB mode. 
 > (The
 > same signal goes to the 400 bps psk modem)  I use head phones
 > so I
 > can hear very clearly.  The signal has a distinct
 > Swish-swis-swis-Swish-swis-swis repetitive pattern caused by
 > the
 > side lobes of the antenna pattern.  This is an amplitude
 > modulation
 > and it can be seen on your S-meter if you set your receiver
 > AGC to
 > fast.  It may help to watch the S-meter.  I simply count the
 > number
 > of repetitive pattern groups for about two minutes and
 > calculate to
 > spin rate.  I use a stop watch and time from the first Swish
 > until
 > the first one after two minutes has passed while marking each
 > Swish
 > on a piece of paper.  Then I calculate the spin rate.  I
 > repeat the
 > process three times.  If the three measurements are very close
 > I
 > average them.  If they differ I re-measure.
 > Example:  Date 1/22/2001 01:36 EST 
 > 2 minutes 5.43 seconds - 37 Revs. (Swish's)
 >  or that is Date 1/22/2001 06:36 UTC 
 >   37 Revs / 125.43 seconds = 0.295 Revs / Second
 >  or 0.295 * 60 = 17.69 Revs / Minute = 17.69 RPM
 > Please note:  I HAVE JUST PROVED MYSELF WRONG!  I claimed that
 > my
 > measurement showed that AO-40 had slowed from 17.6 to 17.2
 > RPM.  I
 > was wrong due to my counting error.  I went back and got the
 > paper
 > that I recorded the marks and times on and recounted and
 > recalculated the results for the above example.  Originally I
 > counted 36 and not 37 counts.  I'll try to be more careful in
 > the
 > future.  Sorry I mislead all of you.  I'm glad I had the lab
 > notebook!
 > 73's de Rich Burgan @ WC8J
 > ----
 > Dave Guimont wrote:
 > > >It appears that the command team has started magnatourqing at
 > > >perigee to slow the spin rate.  Yesterday the spin had slowed
 > > >from 17.6 to 17.2 RPM just after perigee.  Also, the WOD data
 > > >showed that the battery voltage was lower around perigee,
 > > >possibly indicating that the magnets were on.
 > > >
 > > >If you can here the bird why not measure the spin rate and
 > > >post it here?  Even if you cannot copy telemetry you can count
 > > >the number of spin modulation cycles per unit time and post it
 > > >here.  (The spin rate indicated by telemetry has been wrong
 > > >since December.)
 > > Hello Rich,
 > >
 > > How can you tell which frequency variation to zero in on?
 > > The indications I get on my psk decoder are rapid, and variable.
 > > I try to get on every pass I can...and will be happy to count if
 > > I know how!!
 > >
 > > >It might even be helpful to the command team to get early
 > > >indication of spin changes here.
 > >
 > >             73, Dave wb6llo@amsat.org
 > >                     Disagree: I learn....
 > ----
 > Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
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