[amsat-bb] ISS Packet Frequency?

Scott scott23192 at gmail.com
Tue Apr 25 01:00:12 UTC 2017

Thanks for the feedback, Graham!

I wasn't concerned with demod or decoding packets on this particular night;
but rather to run a wide-band capture with no doppler correction to allow
the transmitted signal to paint the doppler curve on my display to identify
the center crossing at closest approach.  That does a number of things for
me like identify the transmitted frequency (at least as received by my
hardware), verify my TLE's, and to get a visualization of how much doppler
shift is occurring between AOS & LOS.

It does interest me quite a bit to try to characterize differences between
those who are seeing better performance -vs- those who are not, so you're
right on to mention hardware.  We can't generalize, but I see "some"
evidence since the freq change that the group having better luck might be
those with equipment like the FT-817 and the folks using the excellent
Kenwood HT's.  But it goes without saying that with such a large sample
group, a VERY large number of people could step forward with minimal
equipment reporting good success as well.  From my own testing I can
observe that I decode way more packets with an SDRPLay than with a cheap
mobile rig or generic RTL-SDR.  I know that sounds obvious (that superior
hardware gives superior results) but that's not my point... rather that it
just "feels" like that wasn't so much the case with the previous 2-meter

I could easily be wrong but when it seems like some experimentation might
provide some useful info to improve everyone's enjoyment of the resource, I
have a hard time leaving it alone!


On Mon, Apr 24, 2017 at 8:33 PM, Graham <planophore at aei.ca> wrote:

> Scott,
> I had a look at the two noted images. The first thing that I noticed was
> that the mode was set in both cases to wide band FM not narrow band FM. If
> you are trying to copy ISS packet using WBFM that might be the source of
> your problem. It could be that the new radio on the ISS has a somewhat
> lower deviation than the old radio.  The second image (Spectravue) does
> indicate that your receive might be a bit high in frequency.
> If you are not using your SDR to try and copy the ISS then perhaps you
> should state what equipment you are in fact using; I may have missed it
> from an earlier post.
> I have been having good luck copying the ISS using my FT-817. Sometimes I
> use a dual band VHF/UHF collinear vertical and sometimes a small yagi, the
> yagi works better but the vertical still works OK too. I don't adjust for
> doppler, I just set the FT-817 to 145.825. Doppler on 145.825 for a typical
> pass is about +/- 3.5 kHz at least according to GPredict.
> cheers, Graham ve3gtc
> ===========================================
> On 2017-04-24 22:20, Scott wrote:
>> Hi everybody.
>> This is essentially a request for what I believe the scientific folks call
>> "peer review"...
>> I continue to try to understand why none of my equipment has been
>> receiving
>> the ISS digipeater very well since the switch back to 2 meters, despite
>> the
>> fact that the same hardware does well on terrestrial packet and in the
>> case
>> of 145.825 in particular, I have even received packets from PSAT since
>> re-configuring for VHF reception on my Raspberry Pi / RTL-SDR iGate.  So,
>> my point is that it isn't that NOTHING is working on 2-meter packet or
>> 145.825 here.  And as I've mentioned in the past, I hear from others with
>> similar stories so we're not sharing hardware or location.
>> This evening there was a favorable pass here and I see on ariss.net that
>> several stations were digipeated while the ISS was over the U.S.  Great
>> news!
>> I monitored that pass in receive-only mode with the same type of TXCO
>> version-3 RTL-SDR that I use for my iGate.  The results were very strange
>> and I hope incorrect.  I certainly would appreciate it if someone might
>> repeat my test when time permits.
>> Here is an image showing my SDR tuned to the local NOAA Weather Radio on
>> 162.475.  By all appearances, my frequency display is accurate in the VHF
>> band.
>> https://www.dropbox.com/s/w1dll06xwinteap/NOAA-Wx-Radio-on-freq.png?raw=1
>> ... without changing any settings other than frequency, the following
>> image
>> shows a Spectra-Vue plot of my reception of the packet transmissions from
>> the ISS on this evening's pass:
>> https://www.dropbox.com/s/s46o3pi87yk9t2h/spec-view.png?raw=1
>> ... I took a ballpark stab at the center point of closest approach and
>> unless I did something awfully wrong, it's roughly 5 KHz above the
>> expected
>> 145.825 frequency.  If by any chance this is accurate, it explains why
>> I've
>> been seeing poor reception here.
>> (On the other hand, if my observations are completely wrong and flawed,
>> this makes just over 1000 times that I've looked foolish!)
>> Anyway, if anyone is setup to make similar observations, I would imagine
>> that we would all like to know if there is a frequency issue up there.  If
>> not, then of course I'm sorry to tie up the mailing list!
>> 73!
>> -Scott,  K4KDR
>> Montpelier, VA  USA

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