[amsat-bb] Trying to understand downlink corrections

[k6vug at sbcglobal.net]

 A big thank you for all the replies !  
 
W3AB/GEO, 
Just to clarify, SatPC32 has the latest TLEs and does all the work during the pass. However, at AOS I have to apply a small correction to the downlink frequency (via SatPC32's feature) in order to zero beat the CW beacon. After that, it tracks nicely throughout the pass. So my question was (a) why do I need the correction and (b) why is the correction different for different satellites. 
 
Jay, 
Thanks, I did calibrate it as much as SDRsharp would allow and it seems to be dead-on for ground VHF transmissions.  Assuming it was still off by a little, I thought it would be off by the same amount for all satellites. 
 
Alan, 
Thanks for the detailed explanation, the temperature based drift was my first guess due to the size limitations. I had not thought of the component degradation and esp., seasonal changes and the PC clock.   
 
Pedro, 
Thanks for the TLE updating tip.  I guess TLEs change to keep up with orbit drift, etc. right?  
 
Again, thanks everyone, this is fascinating and it looks like there is no way to "Set-it-and-Forget-it"...  
So, for now, I'm making a list of downlink corrections for the satellites that I'm playing with and that have a CW beacon.  
Some day, if I get my hands on some SDR code I may look into adding auto-zero-beat like FLDIGI or DM780 or CWGET. 
 
 
Best 73s, 
Umesh 
k6vug 
 



    On Tuesday, July 10, 2018, 12:59:21 PM PDT, W3AB/GEO <w3ab at yahoo.com> wrote:  
 
 Umesh,

Are you asking about doppler corrections that must be made during the operation or a fixed frequency before you start?

___
Sent from my two way wrist watch
73 de W3AB/GEOOn Jul 10, 2018, at 12:43, Jay Cox via AMSAT-BB <amsat-bb at amsat.org> wrote:
Don't forget your SDR could be off by a few parts per million in frequency if you haven't calibrated it to a known frequency source.  At 435 MHz, being off by 3 parts per million would translate to a 3*435 =1305 Hz or 1.305 KHz offset from the actual intended frequency of reception.
Jay F. CoxKG5BZW
"Knowledge can only exist if doubt is possible."  -- stolen from an amazon.com review of "On Certainty:" by by Ludwig Wittgenstein. 

    On Tuesday, July 10, 2018, 1:43:09 PM CDT, Alan <wa4sca at gmail.com> wrote:  
 
 Umesh,

There are two primary satellite issues.  First, the published frequencies
are usually nominal values obtained during ground checkout. Over time due to
normal component aging, radiation, loss of voltage regulation, etc they can
drift.  

The second is the temperature effect you mentioned.  You will find that the
beacon and transponder calibration will change slightly with the spacecraft
temperature as it goes through periods of eclipse and full illumination.
There is also a slight seasonal shift which you can see.  This is most
prominent in CubeSats which have very little thermal inertia.  Also, the are
tradeoffs to be made in terms of power consumption, radiation resistance,
and performance.  AO-7 also drifts due to the lack of a battery to stabilize
the voltage.  

Another issue is your station computer clock.  They can drift rather badly,
and the operating system may not do a very good job of syncing them with a
reliable time standard.  There are some free programs which do an excellent
job available.

73,

Alan
WA4SCA




<-----Original Message-----
<From: AMSAT-BB [mailto:amsat-bb-bounces at amsat.org] On Behalf Of
<k6vug at sbcglobal.net
<Sent: Tuesday, July 10, 2018 12:28 PM
<To: AMSAT BB <amsat-bb at amsat.org>
<Subject: [amsat-bb] Trying to understand downlink corrections
<
<Greetings,
<I have been using a SDR dongle to receive the downlinks of SSB satellites.
The
<combination of SatPC32's Downlink Correction feature and the SDR's visual
<display helps to zero beat the CW beacon exceptionally well.  The Lat/Lon
is
<good to 6 decimals.  However, I find that the downlink correction needed
is
<quite different for different satellites. Could it be due to rounding off
errors or
<due to temperature variations at the satellite itself ?   Maybe this has
been
<explained earlier, but I wonder why ?
<
<Obviously, the more I learn, shows how less I know...  Thanks for any
pointers
<!
<
<73!
<Umesh
<k6vug
<
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