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.
Ron W5RKN
Ron,
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.
73s,
Alan WA4SCA
-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Ronald G. Parsons Sent: Thursday, August 18, 2011 8:35 AM To: AMSAT-BB 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.
Ron W5RKN _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Ron,
Alan's points are spot on. That being said---I see something similar to what you are seeing and "blame" the Keps :)
In the first few days, the manual freq. correction at TCA was over 1,000Hz (seems like I had to shift it down, so -1,000) as compared to AOS and LOS...which were "on frequency."
It has gotten much better over time. It was off -500Hz, and now it's under -100Hz at TCA--so my thought is that the Kep elements are to blame, not a drifting satellite transmitter, etc.
73!
Mark N8MH
On Thu, Aug 18, 2011 at 9:59 AM, Alan P. Biddle APBIDDLE@united.net wrote:
Ron,
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.
73s,
Alan WA4SCA
-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Ronald G. Parsons Sent: Thursday, August 18, 2011 8:35 AM To: AMSAT-BB 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.
Ron W5RKN _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
I found it quite noticeable last night too, I'm 150 miles or so North of Ron's QTH. In my experience, the marker on ARISSatTLM was moving down rapidly as the satellite approached. After TCA (which was at about 68 degrees elevation, I believe) it reversed back to "normal". I was using the latest keps that I obtained from Space Track yesterday afternoon. I did find my PC clock to be off (ahead) by just less than one second, too. For what it's worth. My thoughts given what you all have written, would be that errors in all three (my location, my clock, and the keps) as Alan mentioned, probably contributed to it. On a typical linear satellite pass with a 2 meter downlink, I have never noticed it. I imagine that the effect is amplified in a perceptive sense, by sitting and watching the signal on a graph with pretty good resolution and not being distracted by copying the CW or working a voice QSO.
73, Jerry N0JY
Ron,
Alan's points are spot on. That being said---I see something similar to what you are seeing and "blame" the Keps :)
In the first few days, the manual freq. correction at TCA was over 1,000Hz (seems like I had to shift it down, so -1,000) as compared to AOS and LOS...which were "on frequency."
It has gotten much better over time. It was off -500Hz, and now it's under -100Hz at TCA--so my thought is that the Kep elements are to blame, not a drifting satellite transmitter, etc.
73!
Mark N8MH
On 8/18/11 6:59 AM, Alan P. Biddle wrote:
Ron,
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.
As everybody knows, even tiny errors in the Keplerian mean motion (MM) will accumulate over time and put the satellite ahead of or behind where it really is. That will show up as a large frequency error when Doppler is changing rapidly at closest approach, but a small (or no) error near AOS and LOS when Doppler isn't changing rapidly anyway.
So instead of manually tuning the radio to compensate for unpredicted Doppler, why not manually tune the element set? I recommend manually adjusting the "mean anomaly (MA) at epoch" until the satellite is on frequency. This parameter says where the satellite is within its orbit relative to perigee at the epoch time, so changing it slides the satellite forward and backward along its orbit. If this is indeed the main error in the element set, then correcting it should cause the satellite to track normally even through closest approach.
This has the added advantage of correcting your antenna pointing as well as your radio tuning.
This is probably better than tuning the mean motion (MM) as it would be difficult to distinguish between the two during a single pass. The MM is the time derivative of MA, so the effect on orbital position from a given change in MM depends on how long it's been since the epoch; the older the eleset, the greater the resulting change in position from a given change in MM. A given change to MA always moves the satellite by the same amount regardless of how long it's been since the epoch.
Properly adjusting the MM separately from the MA would require careful Doppler measurements of a number of orbits.
-Phil
One thing I've discovered which makes things go more smoothly is to reduce the minimum Doppler tuning interval.
I am using SatPC32 V12.8a. There are two parameters in the CAT menu -- the Speed which I changed to 5x (which is not sticky) and the minimum Doppler tuning step for SSB which I set to zero (it will be sticky if you remember to click Store).
With these two changes, although the Doppler correction may drift off, it does so very smoothly, which I assume allows the decoding of the BPSK data to be more accurate.
Ron W5RKN
On 8/20/11 6:40 AM, Ronald G. Parsons wrote:
With these two changes, although the Doppler correction may drift off, it does so very smoothly, which I assume allows the decoding of the BPSK data to be more accurate.
Yes, the closer you keep it the better it'll work. Errors of up to +/- 50 Hz are fairly negligible, but beyond that it starts to degrade. The demodulator searches +/- 200 Hz in 100 Hz steps around a nominal carrier frequency of 1500 Hz. I should probably have made it search a wider range but with that much mistuning the sidebands would start to be clipped off by the edges of a typical SSB filter.
participants (5)
-
Alan P. Biddle -
Mark L. Hammond -
n0jy＠lavabit.com -
Phil Karn -
Ronald G. Parsons