Funcube 1 Beacon Frequency Behaviour Coming Out Of Eclipse ?

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Funcube 1 Beacon Frequency Behaviour Coming Out Of Eclipse ?

Postby G7III » Sun Aug 17, 2014 9:01 pm

Hi Folks,

Putting this one in the Welcome area, as it really doesn't fit in the others... I

I have notice that it seems that as Funcube-1 comes out of eclipse, the beacon *rises* in frequency (while it is on *approach* to me)

I discovered this, as I actually adjust for doppler, but I saw it actually go *up* my waterfall display, at what seemed to be quite a rate. I had a BPf of +/-1500Hz, but had to double that, just to keep it in the pass band! I fear that it's actually changing quickly enough during that timeframe, that the Linux decoder is having issues, as even when I increased the bandwidth of the filter to compensate, I don't tend to get very many decodes on an eclipse -> daylight pass, even post the beacon going full power (which is very obvious...)

I'm guessing it's temperature related, and I don't expect anyone to fix the Linux decoder (I can go fiddle and adjust filter B/W's and offsets etc), but can anyone confirm the behaviour I see ? Has anyone characterised it, and worked out what the extra frequency shift is precisely ?


73s

Iain
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Re: Funcube 1 Beacon Frequency Behaviour Coming Out Of Eclip

Postby wa8sme » Mon Aug 18, 2014 11:11 am

Iain,

This is the analysis of the frequency corrections that I had to make with respect to the Doppler correction. I believe your assessment is correct that the drifting is due to the rising and lower of the temperature as the bird comes in and out of eclipse. The two plots reflect the two cases of the bird warming and cooling.

Mark, WA8SME
Attachments
Freq Departure from Doppler.jpg
Freq Departure from Doppler.jpg (41.81 KiB) Viewed 4038 times
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Re: Funcube 1 Beacon Frequency Behaviour Coming Out Of Eclip

Postby G7III » Tue Aug 19, 2014 7:01 pm

Thanks Mark. Looks like it's roughly 100Hz per minute on the night time pass, so approx 1.6667Hz per second.

I've hackeed up a quick bit of python script to add to my doppler adjustment, so will test it next time the bird comes out of eclipse over here, and see if I can decode a few more packets


73s

Iain
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Re: Funcube 1 Beacon Frequency Behaviour Coming Out Of Eclip

Postby g4bbh » Sun Sep 07, 2014 11:40 am

Funcube-1 has always been difficult to correct for doppler due to the tability of it's oscillator. However I notice that UKUBE-1/Funcube-2 is much more stable. Makes me wonder if the oscillator used in Funcube-2 is of a different design to Funcube-1. Anyone out there know?
Dick G4BBH
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Re: Funcube 1 Beacon Frequency Behaviour Coming Out Of Eclip

Postby g0mrf » Fri Nov 14, 2014 4:11 pm

Hello Dick
The two oscillators are essentially identical. As is the oscillator on QB50PC-1. The difference may be due to the increased mass of UKube causing the temperature to vary less dramatically throughout the orbit.
I notice that $50Sat, which is even smaller than FUNcube-1, has temperature extremes in eclipse that prevent the battery working

Thanks

David G0MRF
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