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[aprssig] Field Day ISS APRS Contacts? Doppler

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Thu Jun 21 22:49:39 UTC 2012


The satellite is transmitting on 437.550.  When it is approaching you, 
you need to listen (RX) higher at 437.560, but TX lower at 437.540.  
That way the 10KHz increase will move your 437.540 to 437.550 at the 
satellite and you'll hear the satellite's increased 437.550 at 437.560.  
Yep, he had it right when RX/TX is relative YOU as the satellite is 
approaching.  You have to listen higher but transmit lower when the 
satellite is approaching and reverse that when the satellite is flying away.

Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32

On 6/21/2012 6:38 PM, david vanhorn wrote:
>
> Perhaps I misread, I thought Bob was saying that as the sat 
> approached, the RX doppler was higher than nominal, AND at the same 
> time the TX was lower.  The doppler effect does not know which end(s) 
> are moving, it is only about relative motion, so as the distance is 
> decreasing, doppler shift is always up  and as distance is increasing 
> doppler is always down, regardless of who is speaking.
>
> RX 437.560  TX 437.540  Approaching
>
> So the sat is far away, and the relative velocity between it and me is 
> high.
> So my TX gets upshifted by some amount as seen by the sat. (matches 
> earthbound experience with train horns and such)  but so does the sat 
> TX, appearing higher on the dial than it would if we were both at rest 
> with respect to each other.  The above has me looking for the sat tx 
> frequency LOWER than nominal, when the doppler effect will be pushing 
> the sat tx frequency higher.
>
> How could it be different?
>
> The table makes no sense to me.   The frequencies should be the same 
> for RX and TX, just shifting through the appropriate memory as the sat 
> approaches , and then recedes.
>
>
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