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# Re: ao40 today

• Subject: Re: [amsat-bb] ao40 today
• From: Phil Karn <karn@xxxxxxxx>
• Date: Tue, 2 Jan 2001 17:57:34 -0800
• In-reply-to: <003c01c07354\$8f654640\$5b3e0c18@smateo1.sfba.home.com>(kswaggart@home.com)

```>I think the +/1 200 Hz drift might be due to the rotation of the spacecraft.

Let's compute what it should be.  Assume the 2.4 GHz antenna is offset
1 meter from the spin axis, the spacecraft is spinning at 17 RPM, and
you're looking at it from the side (squint angle = 90 deg)

(17 RPM / 60) * 2 * pi = 1.78 radians/sec

x 1m radius = +/- 1.78 meters/sec peak linear velocity

1.78 m/sec / (c/f) = 1.78 / (3e8/2.4e9) = 1.78 / (0.125 m) = +/-14.2 Hz.

Again, that's worst case.  Reports are that the signal is inaudible at
squint angles much less than 90 degrees, so the maximum observable FM
deviation is less than this. It might still be readily audible,
especially when the signal is tuned low in the passband. But it's
certainly nowhere near +/-200 Hz. If it's actually that high, the
cause must be elsewhere.

Phil

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