[amsat-bb] Trying to understand downlink corrections

[W3AB/GEO]

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/GEO​

On Jul 10, 2018, 12:43, 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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>of
>AMSAT-
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>_______________________________________________
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>  
>_______________________________________________
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>to all interested persons worldwide without requiring membership.
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