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@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@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@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@amsat.org] On Behalf Of <k6vug@sbcglobal.net <Sent: Tuesday, July 10, 2018 12:28 PM <To: AMSAT BB amsat-bb@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 < <Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb