[amsat-bb] Trying to understand downlink corrections

[Jay Cox]

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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