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[jamsat-bb:6579] 2400 MHzビーコン周波数変動


JA6BX 江崎です。表題に関してちょっと興味ある書き込みが
あったので、御紹介します。

2400MHzのビーコン周波数が不安定に聞こえることに関して、
W4SM Stacey Mills氏が、amsat-bbに「それはドプラー効果の
一種で、"Wobulation"という現象である・・・」と説明しています。
衛星のSquint angle、およびSバンドアンテナがスピン軸から
どれほど離れているかによって、スピンとともにアンテナの距離
が変化し、数十Hzのドプラー偏移が生じるためとのことです。
この現象を利用して、Squint angleを正確に測定し、ALON/ALAT
の姿勢制御に利用することはAO-13でも行われたとのことです。
また、Squint angleが小さい場合、あるいは衛星の姿勢が3軸
制御されればこの現象はなくなると言っています。

以下に原文を添付します。
73,

JA6BX   江 崎  昌 男
<ja6bx@jamsat.or.jp>


[amsat-bb:38112] 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
>converter?
>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
>cut-off.

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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