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Re: Doppler lesson



Good Evening Gunther,

I too was puzzled by seeing just what you described.  I copied some range
and Doppler calculations into Excel and plotted them out which cleared
things up for me.

The key point to remember is that the Doppler shift is based on the relative
speed between the two objects.  What you observed was the shift falling
rapidly to a large negative value as the satellite reached its maximum speed
away from you.  As the satellite climbs from perigee to apogee the velocity
relative to you decreases.  I had to toss a ball into the air a few dozen
times to get this idea fixed in my head.  This decreasing velocity results
in the Doppler shift gradually moving up from its maximum negative value.
The total shift will be still be negative resulting in a lower observed
beacon frequency.  Just not as low as near perigee.

I hope this helped


Lee-KU4OS
----- Original Message -----
From: "Gunther Meisse" <gjmouse@neo.rr.com>
To: <amsat-bb@AMSAT.Org>
Sent: Monday, September 10, 2001 21:40 PM
Subject: [amsat-bb] Doppler lesson


> Set me straight!
>
> I always thought that the frequency increased as the object approached and
> fell as the object went away.
>
> Well, I picked up the bird (AO-40) tonight at about MA 6 or so. The beacon
> freq was falling fast as the bird started to streak away, as I would
expect.
> Then at about MA 15-20 I suddenly noticed that the freq was going UP and
> continues at this time (MA-26). Nova continues to say that the range
> continues to increase (About 28,500km), thus I would think that the freq
> should continue to fall until apogee, at which time the range would start
to
> fall and the freq would start to again rise.
>
> Is there something I don't understand, or am I seeing a massive drift in
my
> UEK-3000 DC? I leave my Downconverter on all the time and it has been rock
> stable. I think I have noticed this before but it just hit me tonight.
>
> 73 from your old DAAAAAAA -BBer
> Gunther Meisse
> W8GSM
> AMSAT LM# 594
>
> I have a feeling I am going to be embarrassed by the answer.
>
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