# Re: Where Does Doppler Occur?

• Subject: Re: [amsat-bb] Where Does Doppler Occur?
• From: "Pieter Ibelings" <elpieter@xxxxxxxxxxx>
• Date: Sun, 2 Dec 2001 09:58:16 -0500

```They way I look at doppler is this:

Assume there is a stationary listener and a stationary siren and the
stationary siren is putting out a 1 KHz tone. The stationary listener hears
a 1 KHz tone. If he was able to see the incoming sinewave, he would see a
peak (let's say the upper peak) of the sinewave occur every 1/1KHz or every
.001 seconds. This is just the period of the 1 KHz tone.
Now, if the listener or the siren are moving toward each other with any
relative speed, the peaks of the sinewave will no longer arrive every .001
seconds. They will arrive with a shorter period since one of them has moved
a certain distance while the next peak was being generated. This gives the
listener the impression that the frequency was higher because the peaks seem
to arrive at a shorter period (more often).
With light and RF the effects are similar except the speeds have to be much
greater due to the difference between the speed of light (300000Km/s) and
the speed of sound (.340Km/s).

The question of the day is: If AO-40 had a 1 KHz buzzer on it and there was
air around it. What would an operator hear at perigee if he is at the exact
lat and lon on the earth at which perigee occurs?

Pieter

----- Original Message -----
From: "Douglas Braun NA1DB" <doug@dougbraun.com>
To: <amsat-bb@AMSAT.Org>
Sent: Saturday, December 01, 2001 10:58 PM
Subject: Re: [amsat-bb] Where Does Doppler Occur?

> Before bringing out out too much Special Relativity to describe
> this issue, remember that the Doppler Effect also happens with
> sound waves, to which relativity does not apply.  Also, unlike
> electromagnetic radiation, sound has an absolute frame of
> reference: the air through which it travels.
>
> Doug
> NA1DB
>
> At 04:21 PM 12/1/01 , you wrote:
>
> >My question is meaningless, too, without a reference frame. In the
reference frame of the satellite (neglecting its orbital acceleration) the
frequency is exactly the frequency it was transmitted with. In the reference
frame of, say, the Sun, it's something else. And of course in the reference
frame of the ground station it's some other value. Which of these is the
"real" frequency? They are all equally real. This is starting to get
philosophical.
> >
> >For most practical purposes, we can set aside all this confusing
relativism and just pick a convenient reference frame for all our
calculations. For this example, let's pick the reference frame in which the
mass center of the Earth is fixed. (We'll neglect gravitational
accelerations due to the Sun, Moon, and other celestial bodies.) That's a
convenient reference frame for computation of the main effects on a
satellite -- satellite tracking programs use it.
>
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```

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