[amsat-bb] Doppler measurment of TCA

[Armand SP3QFE]

Hi Jim, Hello Everyone,

Jim as you wrote... at TCA, the Doppler shift is equal to zero.

You can try to find this point directly from your screen when the 
frequency is equal. However, it is worth to remember that the 
transmitter on the satellite has a shift (the VFO) or your frequency 
scale in your SDR is not perfectly calibrated.

For that reasons, I suggest to write down as much as possible points: 
the value of frequency and its time. After that, you can plot the 
diagram, which should be more or less similar to "S" (or it's mirror). 
The shape of the "S" depends on the maximum elevation of the satellite 
for your location and sometimes it can be straight line "/" or "\".

Then on the plot, you can very easily find the symmetry center of this 
plot. This point is for the TCA.
PS. For straight line you should not have the antena obscurations to 
find "the center".

73, Armand SP3QFE

On 2018-11-27 01:47, Jim White wrote:
> Wednesday's launch of a large batch of satellites seems an opportunity
> to try the Doppler shift method of matching a satellite to a set of
> keps.
> 
> In reading a few blogs and posts it seems the method is to use an SDR
> and GNURadio tools to determine the time of TCA for a downlink signal.
> Then use a tracking program to provide the TCA time for several sets
> of keps. And finally to match the observed/calculated TCA with one set
> of keps.  Can anyone provide a pointer to a GNURadio flow diagram that
> includes a tool to calculate TCA time from the signal received during
> a pass?  I'm pretty new to GNURadio so finding such resources is still
> a bit of a mystery for me.
> 
> Jim
> 
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