Frequency drift
Question I am also using the Kiwi to decode QRSS signals. In the attachment, you can see the steady decode of the CQ beacons on 40 meters...rock-steady. Then suddenly at 0200Z the signals start to drift down by about 20 Hz and restablize. Then at 0300 they start to return to original.
WSPR spots show the same behavior.
I am using the GPS with default cable and default antenna. I have the antenna hanging vertically, resting outside the window of my office.
GPS position in wikiamap.org is accurate. Any suggestions on how to investigate further?
Thanks, Mark WD4ELG
WSPR spots show the same behavior.
I am using the GPS with default cable and default antenna. I have the antenna hanging vertically, resting outside the window of my office.
GPS position in wikiamap.org is accurate. Any suggestions on how to investigate further?
Thanks, Mark WD4ELG
Comments
Also, please do this after the Kiwi has been running for a long time (like at least 1 day) so the az/el graph has a good cyan-colored history of the sat tracks.
Is the GPS really oriented like that? With its base on the side of the structure and the "top" of the antenna not facing toward the sky?
More accurately, what compass bearing is the flat top of the GPS antenna pointed towards?
Are there other buildings in the vicinity?
Where I have my antenna, I can see a shadowing effect of the two houses closest to the antenna, towards the E in this image:
Also - the local environment where the KiwiSDR itself is located, are there strong temperature fluctuations through the night? If the GPS signals are flaky and intermittent, and there are gross temperature chances of the KiwiSDR itself, then the drift offset previously calculated will be invalid for the different temperature times, and that would easily explain the frequency drift you're seeing.
Attachments:
https://forum.kiwisdr.com/uploads/Uploader/0a/3f7c20d19b8308a66ea416dd661d32.jpg
John, the GPS antenna is oriented like that...since I can't get satellites to the north, I figured I would try orthogonal to the south. Attached GPS screenshot. This is with the unit running for 3 weeks.
Cathal, no temperature fluctuations, no sunlight on the unit and plenty of AC air to keep the room cool.
I am including another screenshot showing the long-term capture of the 40 meter slow-speed CW beacons...0730 UTC shows the behavior with correction at 0830 UTC, and it starts again at 1550 UTC.
Attachments:
https://forum.kiwisdr.com/uploads/Uploader/cd/6668f2d4a693881195ec36a144ae7d.jpg
https://forum.kiwisdr.com/uploads/Uploader/79/250b4fbd276ee57e50e8965f27fb5b.jpg
The gradual transition shape might be the averaging that is applied to the GPS corrections so the frequency doesn't jump around. Although I thought I had set the time constant a bit shorter than that. It is a compromise between smoothing noisy GPS time/position results and dampening sudden temperature changes (e.g. Kiwi in garage when the sun goes down, door is opened letting hot air into room Kiwi is located in etc.) However, the noisy GPS issue is now essentially eliminated with Christoph's Kalman filter.
I have thought about having a long-term graph on the admin page showing GPS reception and clock correction details to help answer some of these questions.
Specifically in your case: The GPS tab shows 142 hours of runtime. That's 5.9 days. 6.2k fixes over that interval is very small given fixes occur every two seconds in the best case. That would have been 255k corrections in 142 hours, so your Kiwi was only correcting 2.4% of the time. This also lends support to the argument that the short-term excursions are when GPS correction is occurring.
The az/el also shows very limited sky coverage which means the GDoP is low which in turn means the quality of the corrections is low (although, again, I think Christoph's Kalman filter / Single point positioning (SPP) makes this issue moot).
So, unless you can improve your antenna placement I think you might be better off to disable clock corrections (setting on admin "config" tab) and do a one-time calibration to WWV (see here: http://kiwisdr.com/quickstart/index.html#id-cal). And then check the calibration periodically. [Anyone remember the good old days of checking ham rig calibration using the harmonics from a 100 Kc xtal calibrator? Am I really that old? Jeeze..
I'm sure I've seen that level with a broken coax (the shield corroded to white powder).
I really would check that antenna and feed, also move it away (shield) from WiFi routers, motors or DECT phones.
There are other threads on GPS antenna placement but first off I'd do my best to put it out horizontal on a metal plate (about the size of a CD/DVD), ideally more than about 20cm from the wall. If it has to be on a window try a plate behind it.
Here I use a car GPS repeater to get the antenna up at gutter level, Kiwi GPS antenna and repeater TX in a can close to the Kiwi.
73 Stu
Attachments:
https://forum.kiwisdr.com/uploads/Uploader/fe/4908a7dce3202fa54bd6daa17fc144.png
Here is the new implementation of my GPS antenna. If this does not get the better reception, then I will go "old school" using the method John described and I remember using in my early ham days (back about the time the lightbulb was invented LOL).
What are other folks doing for their KiwiSDR antenna setups?
I do have a workshed with power, out in the back yard. I wonder if I should just shift the unit to there....
Is this an active GPS antenna? Any suggestions on a replacement unit?
Thanks
Remember that a sat is not actually being received until the "subframe 54321" blocks light up with colors. The stuff to the left may look like a sat is being received but often it's just the acquisition software desperately trying to pull the PRN-coded signal out of the noise and do a tracking lock. For example, there are no sats being received in the 4th post of this thread.
Instantly I am now picking up more satellites.
Speaking of GPS... I have the one outside high gain antenna feeding my Thunderbolt 10 MHz reference as well as three KiwiSDR receivers. This is probably going to make some RF purists cringe... but here's how I do it:
The Thunderbolt directly feeds the antenna so it can power it. I have four capacitors (0.047 uF?) tapped off that, each going to an F connector. These feed the KiwiSDRs and provide DC blocking. Ugly, but as you can see above, it works. The 4th output is in case I get another KiwiSDR
This thread is super informative and helpful. I hope you will organize the graphics and supportive text here and add this to the official documentation stack for the Kiwi. While I have a fairly good update rate on my Kiwi, I now understand why the legs on my sat tracks are so short. This is good material for someone like me, who is not knowledgeable in GPS.
Mike N8OOU 73
Also, obligatory: https://xkcd.com/1356
The mountains do that for me
Good for GPS(?), but bad for DX.
However, I own a GPSDO, (Navstar only, no Galileo) which never archives specified accuracy. Maybe due to lack of visible satellites, or reflections, I don't know.
The ADC clock frequency shown in the IQ display window is the default value:
```
./init/clk.h:#define ADC_CLOCK_TYP (66.665900*MHz) // typical 20 degC value
```
My guess is that on the admin "config" tab the option "Enable GPS correction of ADC clock?" has been switched to "No".
I say this because on the IQ extension, as you show, the manual "Fcal" adjustments are shown as opposed to the usual message saying "GPS is correcting". This in combination with the fact that the "Stats" tab is showing plenty of sats with good reception (i.e. the Fcal adjustments will be shown when a Kiwi is first started up until GPS reception is established and GPS correction starts occurring).
And lastly, due to the large offset of (presumably) 15 MHz WWV it looks like a manual calibration has not been performed, or performed improperly.