Observation 2396103: GRIFEX (40379)

Regarding Observation 2396103

My analysis of this observation is that our rotator could not keep up with azimuth changes near zenith, causing the antennas to be misaligned and the drop in signal strength in the middle of the obs. Additionally I am wondering if the difference in signal strength in the 1st and 2nd half is due to polarization changes - any other ideas?


I don’t believe you lost signal due to rotator speed… I have a G5400B and it has never failed to keep up with overhead passes.

Plus, the surrounding noise signal drops out, too… which wouldn’t be the case if you just failed to keep up with the satellite. Could it be that the back lobe of the antenna is picking up a strong signal that is desensing the SDR? I also seem to remember that OZ9AEC saw similar results from a station that he overlooked and offered a beer to the first person who could guess why it was happening. That thread is in this forum, somewhere…


Oh, I was mistaken… it had to do with # of decodes and not signal strength on the waterfall. Here’s the thread I was talking about:

It seems to me by looking at the plot that it is necessary to pass the north, I have the same concern on some of my observations in which in addition to passing well above (80-90°) it is necessary to pass the north (to go to the maximum to the north then to go on the other side to the maximum to the north is almost 360°) and thus in spite of the speed it has big difficulty to coordinate the elevation and the azimuth to point the satellite.
I hope I’m understandable.

This is one of my example https://network.satnogs.org/observations/2187733/
The signal loss is visible at t-150s when the rotator need to pass north + turn a lot to follow the sat in high elevation


Why does the background noise also reduce, then? For example, the terrestrial signal on the right almost complete disappears for ~25 seconds. This should not be the case if the loss of the satellite signal was only because the rotator couldn’t keep up.

It all depends where that noise is coming from, and how it interacts with the antenna pattern.

In the case of obs 2187733, I’m wondering if the rotator hit an endstop and had to rotate completely around to continue to follow the sat.

I had this issue a lot on my station, and ended up writing some code which intentionally pre-sets the rotator position into an overwind region before the pass, so the rotator moves ‘out’ of the overwind region during the pass: https://github.com/darksidelemm/satnogs-unwinder/
However, this will only work on rotators that report an ‘absolute’ position (e.g. can report a value outside of 0-360 when in overwind) via Hamlib.

That’s exactly the case, my rotator knows that it has arrived at 0° and therefore it is at its virtual limit at 0° and has to go around to go to 359°, and what I meant is that this action “go around” + the action of having to move quickly the azimuth and elevation due to the high elevation pass, cause a loss of the satellite
(The simple “go arround” alone can also sometimes cause a long loss of the satellite, but in the case of a high pass it completely “disorients” the antenna. )

Maybe when antenna is pointed to the zenith the terrestrial signal in not anymore in the 30° lobe of the antenna.

What kind of rotator are you using, and does it have any overwind capability?
If your rotator and cabling loom can handle it, enabling up to +/- 180 degrees of overwind can help reduce these kinds of events a lot.

I’m using my own rotator which is similar to the SPX-02 rotator, in fact I think that I have enough cable length to go from -180° to 360°
I just haven’t had time to make the necessary firmware upgrades to get it past the 0° limit yet.