It has now been eight years since I have retired my Hy-Gain DX-88 antenna declaring it dead for good. I was standing with a broken base element for the second time and I decided not to spend any more time and money on it but try a new antenna, a Butternut HF2V covering 160, 80, 40 and 30 meters.
Back in August 2010 I ran a brief experiment using GstInputSelector to switch between various video sources. You may have noticed that it was using Theora encoder while most of my other DVB experiments used H.264 encoder in MPEG-TS container. The reason for this was that I could not make x264enc work in the pipeline used for the video switcher.
As I mentioned in my previous post, my “optimizations” of the multi-mode receiver code caused a sudden loss in performance.This was a big surprise because what I did was to replace two filters with only one, which I would expected to yield a performance gain and certainly not a loss. What happened at the same time was that the sample rate in the demodulators went from 50 ksps to 250 ksps and it was the responsibility of the demodulators to downsample this to 50 ksps. I suspected that this might have cause the increased CPU load and I have set up a simple experiment to confirm it.
I have had this idea of using my webcam for digital video transmission for quite some time now. Capturing and processing video from UVC webcams has been a routine for a long time and I have had great success with Logitech webcams (the 9000 series) that have great UVC support. I still had a problem though with finding a good way to interface the GNU Radio transmitter and receiver to the video processing pipeline implemented in Gstreamer.
In my experiment with receiving packet radio from the ISS I used a named pipe to create a real time interface between the GNU Radio receiver and the packet decoder multimon. I decided to try this trick for sending video in to and out of GNU Radio and it works! The following experiments were implemented and executed on the 27th and 28th of July with some minimal preparation on the 26th.
Thanks to Hannes DG1GGH of ESOC, we now have an idea what kind of precision is required for the Phobos flyby experiment. The answer is in the comments to this very informative and interesting post on the Mars Express blog from today. According to Hannes, they are expecting the influence of Phobos to cause a maximum deviation of 400 mHz from the nominal Doppler shift. Yes, that’s milli-Hertz, not mega-Hertz.
Obviously, this is much lower than what I had in mind when I first read about the experiment two days ago. I was thinking more along tens, maybe hundreds of Hz. Had I known this in the beginning I probably wouldn’t have considered the experiment at all. In fact, I wasn’t even aware of that measuring with such accuracy and precision was possible. Not that 400 mHz is anything special on it’s own – I can easily create a tone generator that is precise within 10 mHz. But in this case we have to keep in mind that this 400 mHz is to be considered together with an 8.4 GHz signal coming from 117.5 million kilometers away. That’s many decades in the same equation!
For the simple setup I was considering this constraint would require me to keep all oscillators in the system stable within a fraction of this limit. I could not have have achieved that with a 7.15 GHz oscillator mounted outdoors on the antenna. I don’t know the stability of the Kuhne LNC, but I am used to see “1 parts per million” in similar cases. 1 ppm @ 7 GHz is … you see my point 😉
The second error source in the chain is the USRP and the RF front end board itself. Despite all good intentions, it is still a system built of consumer grade components and millihertz precision is probably not what people have in mind when creating such devices. But I have put it on my list to measure the accuracy and precision using a good signal reference. More on that later.
Have a nice weekend!
July 30, 2009 at 1722 UTC, space shuttle Endeavor (STS-127) deployed the Atmospheric Neutral Density Experiment (ANDE) consisting of two satellites, Pollux and Castor. The Deployment happens just after 5:00.
You can read more about the ANDE mission here.