# Re: Predicting Doppler Shift

• Subject: Re: Predicting Doppler Shift
• From: "Kenneth J. Ernandes" <n2wwd@xxxxxxxxxxxxxx>
• Date: Thu, 12 Mar 1998 08:02:51 -0500

```Matt -

The easiest way to compute Doppler shift is using vectors.  The orbit
propagation algorithm takes the Keplerian data and converts it to position
and velocity vectors as a function of time.  These vectors are in an
inertial (non-rotating; non-accelerating) frame-of-reference.  You can do
a simple coordinate transformation to respresent position and velocity
in an Earth-fixed (i.e., rotating) coordinate frame.

You can then compute a slant range vector (observer to satellite) as a vector
subtraction (satellite position vector minus observer position vector).  If
you normalize the slant range vector, you have unit direction vector
pointing from the observer to the satellite which gives you a basis for
computing the azimuth and elevation antenna pointing angles.

Once you have position and velocity in an Earth-fixed frame-of-reference,
you can compute the velocity with respect to any observer on the surface
of the Earth (i.e., range rate).  This is done by computing the dot product
of the satellite's velocity in the Earth-fixed frame with the normalized slant
range vector.  The range rate is the relative speed between the satellite
and the observer used in the standard Doppler shift formulas.

BTW, vector computations may sound intimidating, but if you use cartesian
(X,Y,Z) reference frames, they're straight forward.

73, Ken N2WWD
n2wwd@amsat.org
http://www.mindspring.com/~n2wwd

At 00:42 3/12/98 -0500, you wrote:
>  Name here is Matt the call is AB8AF. I have a guestion regarding
> doppler shift, I would like to set up my tracking software to recognize
>
> the amount of shift at a given time. I need to get attitude latitude
> and longitude for RS-12 or any OSCAR sat. available. Any suggestions ?
> Thanks for your help!  de Matt AB8AF
>
>
Ken Ernandes
n2wwd@amsat.org
http://www.mindspring.com/~n2wwd

```

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