[amsat-bb] Re: ARISSat telemetry and Doppler tuning
Alan P. Biddle
APBIDDLE at UNITED.NET
Thu Aug 18 06:59:58 PDT 2011
That behavior is common around TCA, Time of Closest Approach. At that time
everything is changing rapidly, and slight errors in QTH position, system
clock, and Keps can be magnified. I have seen a few hundred Hz shift at
that time with new satellites whose Keps have not fully converged. The same
if the system clock is off a bit. As you probably know, the basic Windows
internet time calibration facility can be tweaked, but it is better to
replace it with one such as Meinberg. Finally, always have the latest Keps.
Updating with SATPC32 is easy and only takes a few seconds.
It is unlikely to be a factor on 2 meters, with a fast computer, but you can
try increasing the update calculations in SATPC32. Look on the CAT page.
You will see tick boxes for 1X, 5X and 10X. Try 5X and see if it makes any
difference. Note that this setting is not sticky.
Actually, your results show you have just about everything nailed.
From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
Behalf Of Ronald G. Parsons
Sent: Thursday, August 18, 2011 8:35 AM
Subject: [amsat-bb] ARISSat telemetry and Doppler tuning
Last evening, after having several days experience capturing ARISSat
telemetry, I sat at my radio and watched carefully how SatPC32 and
ARISSatTLM played together. I had settled on a tuning correction value of
-540 HZ to get the CW signal to center around the CW tuning point as the
Doppler correction was handled by SatPC32.
The approach of the satellite seemed normal. but as the satellite passed by,
I noticed the CW signal average "position" to be creeping upward, needing a
reduction of the tuning correction by about 40 Hz to -500 Hz. This lasted
for a couple minutes and then the CW signal started creeping down and I had
to go back to my -540 Hz value.
The first explanation I could think of is that the Doppler correction
computed by SatPC32 from my position and the keps was in error. But of
course, it could have been a change in my radio (unlikely as it had been on
for hours) or a change in the frequency of the CW signal from the satellite.
I'll keep watching this and wonder if others have noticed something similar.
Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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