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Satellites and Doppler shift

  1. Oct 28, 2010 #1
    At page http://www.qsl.net/vk3jed/doppler.html" is discussed Doppler shift at various ham radio frequencies. I calculated for this Doppler shift relative velocity of their satellite as 6720 m/s. Their satellite in height of 800 km above ground and at circular orbit should have velocity of 7453 m/s (orbital speed). I thought that difference between these two values should be max 465 m/s (rotation of Earth at equator), but it is almost twice as much... Any ideas?
     
    Last edited by a moderator: Apr 25, 2017
  2. jcsd
  3. Oct 28, 2010 #2
    The 6720m/s will be the maximum velocity of the satellite relative to the receiver on ground. You have to consider the angle between satellite and receiver direction.

    For a satellite at horizon the relative velocity(neglecting earth rotation) would be 7453*cos(pi/2-asin(6378/(6378+800)))=6622.
     
  4. Oct 28, 2010 #3

    berkeman

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    Staff: Mentor

    He said their answer is bigger, not smaller, so the effect you mention is probably not what is causing the difference.
     
  5. Oct 28, 2010 #4

    berkeman

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    Staff: Mentor

    I get numbers close to yours. So if you add the two velocities, you get close to 7900m/s. Which band number are you using to say that they get twice the shift?
     
    Last edited by a moderator: Apr 25, 2017
  6. Oct 29, 2010 #5
    For any frequency in Table 1 I got same relative velocity -6718 m/s caluclated this way:

    [tex]v = - \frac{\Delta f c}{f_0}[/tex]

    For orbital speed:

    [tex]v = \sqrt{ \frac{G M}{ R + h }}[/tex]

    at altitude of 800 km i got 7466 m/s. Difference is 748 m/s. That means, I didn't get twice the shift, from shift which I suppose is correct I got twice the velocity.
     
    Last edited: Oct 29, 2010
  7. Oct 29, 2010 #6
    OK. Let's try it again. The speed the OP has calculated(6718 m/s) is the maximum relative speed between the satellite and the receiver, not the orbital speed of the satellite(7453 m/s).

    The satellite is never moving directly towards the receiver on ground. There is a height difference of 800km between both. You have to consider receiver position and velocity relative to the orbit.

    The example calculation I have offered has been for the most simple case I can think of. Having a fixed receiver placed in the orbit plane and the satellite passing the horizon. A satellite passing the zenith would have a relative speed of 0 m/s.
     
  8. Oct 29, 2010 #7
    I am starting to understand the problem... Orbital velocity is calculated fot Earths centre and receiver is 6378 km from that centre, right? But I still don't see the way to calculation you posted...
     
    Last edited: Oct 29, 2010
  9. Oct 29, 2010 #8
    Here is a sketch. :)
     

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  10. Oct 29, 2010 #9
    Thank you very much, now I understand it. I plotted Doppler shifts calculated from your formula and I got exactly same graphs as showed on that webpage.
     
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