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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