Monday, June 26, 2017

Meteor Detection: Some Initial Results

I've analyzed the results of a 24-hour run of the detection system from a few days ago and the results are promising.

The analysis was the visual inspection of 1,542 screen caps taken at 55 second intervals

The columns represent meteor counts in a given hour; for example, column 0 show that about 150 meteors were counted between midnight and 1 A.M. Because I tried to count only things that I felt were clearly meteor signals the above diagram represents an underestimate of meteor activity. Even with that, this is a fair replication of the diurnal meteor cycle. More days of data would be better, of course.

This was performed using my attic antenna monitoring Canadian station CITO, channel 3, at 61.26MHz.

When I went to repeat this it was evident that something had changed with the signal. Meteors were now producing a strange oscillating siren signal that appeared as interwoven sine waves on the waterfall chart.  I noticed that was showing an anomalous signal as well, and shortly after that switched frequency to 55.24MHz, that of a channel 2 station (probably CHBX in Sault Ste. Marie, Ontario).

I've now switched to that same frequency and, using Argo, am collection another day's worth of data.  Argo presents what is essentially a highly zoomed version of the SDR# waterfall, but with a couple of advantages. Argo makes clear that when I use the channel 2 frequency that I'm monitoring several stations at once--there appear to be slight offsets in the carrier frequencies. It also makes it easy to see when a signal is due to a reflection of a local channel 2 from aircraft. (I'll post some diagrams showing this soon.) The Argo strip chart allows me to do a screen cap every eight minutes, which will make counting meteors much easier.

I'll post more as I start analyzing results and gain a better understanding of what Argo is showing me.


And it's clear tonight! I should be able to get the first half of my mosaic done! Yay!!!

Tuesday, June 20, 2017

Meteor Detection Using Canadian TV Stations

Last time I wrote about the problems I was having with a meteor detection system that relies on National Weather Service weather radio stations. The problems were so substantial that I indicated I'd be looking into using Canadian TV station signals as the basis for meteor detection. I've now done that in at least a preliminary way, and the outlook is much more promising.

Good introductions into this method are found here and here. In brief, there are a handful of Canadian TV stations that continue to broadcast powerful analog video carriers at frequencies that are near optimal for meteor detection.  Because these stations are located near the U.S.-Canada border, it's possible for most of the eastern and midwestern U.S. to use them for meteors.

There are only a couple of differences between this method and the one using NWS stations:
  • The antenna. Not only must it be designed for the lower frequencies, but because you will be listening to a specific station you can use a directional antenna. One catch, though: With the switch to digital TV, lowband VHF antennas are becoming more difficult to find.
  • Sidechannels. If you've watched, you probably have noticed that it's tuned to 61.26MHz and not the nominal 61.25MHz. This is because transmitters have added side channels at +/1 0.01MHz in order to reduce interference between stations. Why 61.26 was chosen by MeteorsLive instead of 61.24, I don't know [Edit 6/21/17--now I do...see the next post.]
 Here are the stations that can be used:

Canadian TV stations useful for meteor detection (with channel number)

The nice thing about these is that they're all far enough away that aircraft arriving at and departing from the nearby airport can't cause enhanced reflections. At the same time all of them (except Channel 2 in Sault Ste. Marie) are far enough away that aircraft flying at flight level 410 also can't cause spurious reflections.

My antenna is an old FM/TV multi-element antenna hanging in the attic of a wood frame house. It's directed about 40 degrees east of north. I'm initially monitoring the same station in Timmins, Ontario, used by , which is about 61 degrees east of north. The misalignment probably isn't significant given the antenna design.

I've surveyed the available channels, and only 4 and 6 did not show evidence of any meteor-like signals. Channel 4 in Sydney, Nova Scotia, is beyond the 1400 mile limit and can't provide single-reflection meteors. Channel 6 in Medicine Hat, Alberta lies at a right angle to the axis of my antenna.

Radio path between my position and Channel 3, CITO-TV, near Timmins, Ontario

I'm happy to say that Channel 3 at 61.26MHz is producing a substantial number of strong pulses that resemble those suggested to be reflections from meteors. It actually seems to be working, and working well.

The question now is: Are the meteor-like signal pulses really of meteoric origin, or is some other process at work?  The only way I know to answer this is statistically. Will meteor counts match the known diurnal variation? Will meteor counts become enhanced around the times of notable showers?

(In case you're asking why this couldn't be confirmed using an all-sky camera or some other visual means, you've got a point. A light-amplifying camera set up and pointing toward the transmitter might pick up a few meteors. In my case that camera would be pointing over the Twin Cities, and the light pollution would ruin any chance of picking up anything other than a bright fireball. Worth a try, yes, but not in my plans.)

At this moment I'm running the system, doing screen caps of SDR#'s waterfall display at regular intervals. I'm going to run it for 24 hours and analyze the data to compare frequency and time of day. I'll probably have the results later this week.

I'll do the same at the time of the Perseid shower in mid August.

Stay tuned!

Monday, June 19, 2017

Radio Meteor Detection

The June 2017 issue of the Astronomical League Reflector had an interesting article about someone's participation in the Radio Astronomy Observing Program. What caught my eye in Dr. Alex Vrenios' description of his efforts was his attempt at detecting meteors using forward scattering.

"Seeing" meteors using distant FM station signals scattered off ionized trails of meteors is nothing new.  I've never given it a serious try because the FM spectrum in the Twin Cities is almost saturated. It's difficult to find a frequency that isn't taken by a local station. Dr. Vernios solves this problem by using National Weather Service weather radio stations. NWS stations operate at seven different frequencies, and their coverage is designed to not interfere with other stations. It's usually the case where only one station comes in clearly. Tune to a station frequency that doesn't serve your location and you'll probably get only static, which is perfect.

The downside of using NWS frequencies is that they are not particularly good at scattering from meteors. Compared to an optimal frequencies like TV channel 2 (55.25MHz, more about that in another post) they reflect only about 1/25th as well. That's a whopping 14dB drop in potential received signal. NWS stations also tend to be very low power and run only 1.0 or 0.3 kW. (That's compared to a typical 50 or 100 kW for TV stations.)

That said, the approach seemed innovative and worth a try.  Also in its favor is the extremely low cost of the needed components.  Here's what is needed:

  • A crossed dipole antenna that's built to receive NWS frequencies. 
  • An RTL-SDR USB dongle. This is a simple device that turns your computer into a digital tuner. It has excellent frequency resolution and sensitivity.
  • SDRSharp software for contolling the dongle and presenting the results. It's free!
  • Chronolapse freeware for performing timed screen captures of the SDRsharp display.
The antenna is easy to make if you don't plan on requiring it to be weatherproof. The dongle is available from Amazon, and the needed software is downloadable. The entire project can be assembled in one or two days, but finding the optimal settings for SDRSharp may take longer. I'll have more on that later, but for the moment let me show you what the results look like.

To start, here is a map of the surrounding NWS stations of interest with their frequencies. My location is indicated by the red circle.

Next, here is a typical FFT power spectrum and display of power levels shown in a "waterfall" diagram.

You will want to click this to see it full scale. The top portion of the diagram shows instantaneous power as a function of frequency; the seven NWS frequencies are marked.  During daytime three stations can be heard: The one serving my area, loud and clear; A 300W station in Norwood/Young America, poor; a 1000W station at Clearwater, very poor. A signal is evident at 162.4MHz, but no voice is discernible. That leaves three frequencies (162.4, 162.45, and 162.525MHz), none of which has any signal evident. However, as you can see, there has been a short burst of signal at 162.525MHz.

Another example:

This shows an approximately seven second burst at an otherwise silent frequency.

Note that in both cases only one frequency showed a burst, suggesting that the cause was something local to the transmitting station or the path between it and the receiver.

Bursts like these appear to happen frequently during the daytime. However, there are also longer and weaker bursts that show more complex structure. Are those cause by meteors or some other phenomenon?

Sadly, it appears most of the bursts are being caused by reflections from local aircraft. After using the system for a while it became obvious that the bursts were coinciding with air traffic departing from the nearby airport. Recording the waterfall chart all night showed that there were no bursts overnight until a little after 5 A.M., when departures starting taking place.

That's circumstantial evidence. But there is more: I zoomed the frequency scale to see what was happening in the way of Doppler shifts:

Most of the bursts looked just like this, with a significant Doppler shift that diminishes over time as  an aircraft passes. The magnitude of the shift indicates a speed of about 200 mph, which is consistent with a departing, climbing commercial jet.  A meteor would not show this kind of signature.

So the NWS-based system won't work for me unless I can reject the aircraft signals. This is a problematic task.  There's a chance a system like this would work if it wasn't operating a few miles from an airport, but I'll have to leave that to others to discover.

For now I'm moving on to trying out the method utilizing Canadian TV stations that broadcast video carriers at more optimal frequencies. More on that next time.


One more piece of the mosaic imaged (but not yet processed). There's an outside chance I'll get it half finished this year. 7 down, 9 to go:


Oh, and I'm pretty much done lambasting Trump for a while. It's not that he's matured or gotten any smarter or wiser--he's still a spoiled child with the mind of a dirty old man. It's just that there's apparently no changing him. The Republicans will use him to their advantage as long as possible, then either ignore or discard him based on their own limited self interest.

The best way to deal with him and toadies like Jason Lewis is to educate yourself on the issues, support candidates who run against them, and to VOTE.

Monday, June 12, 2017

Virgo Cluster Mosaic Progress

Last year I talked about creating a Virgo Cluster mosaic that would be suitable for poster-sized printing. Progress has been slow thanks to the abysmal weather we've "enjoyed" this spring, but it's hardly hopeless to think this won't be finished by the time of ALCON 2018. (The convention is being held here in the Twin Cities, and the notion was that a mosaic poster might be sold as a fund-raiser.)

Here's a graphic of my current progress in terms of the 4x4 grid:

Green = completed sub-image

That's six of the required sixteen sub-images done.

Here's a look at the top row stitched together using Microsoft ICE:

Not much to see in this row other than M98 at far right and M100 at middle right.

The middle two sub-frames need to be reprocessed to minimize the substantial moonlight in them. They were imaged during a first quarter moon which was nearby in the sky. This first row suggests that the finished mosaic will be the equivalent of a single 150 megapixel image taken at a focal length of about 135mm. The sub-images are taken using a Canon T2i (18 megapixels).

I wonder how long it will be before consumer-grade DSLRs are sold with 150 megapixel resolution. The new full-frame Canon 5DS has a 50.6 megapixel sensor!

There's a chance I may be able to get a couple more images done this week to put me at the halfway mark, but the Cluster's availability for imaging is fading fast as time goes on.  The next new moon will be the last opportunity to image it this year.


Next time: First attempts at meteor detection using National Weather Service weather radio stations, a home-built antenna, and an SDR dongle. And possibly more venting about the President and my pseudo-conservative, pseudo-intellectual, Congressional Representative.