TEXT AND IMAGES ARE OUTDATED AND WILL BE CORRECTED BY THE AUTHOR
Visualize radio meteors
The ionization cloud that arises around a meteoroid during the penetration of the atmosphere is capable of reflecting radio signals. In addition, the ionization cloud can also reflect radio signals in the afterglowing track of a larger / bright meteor. How do you visualize these ‘radio sensors’ with a radio set-up on the computer?
With this manual you will actively work to visualize reflections of radio on meteors. Do you have a working setup with an RTL-SDR dongle and an antenna in use? Then you can use the description below to make the received signal visible on your computer. You don’t have a radio setup? No problem, we take you here on the basis of the webSDR that is active at CAMRAS. This way everyone can make a radio signal visible on the computer and detect meteors.
Getting started with the webSDR from CAMRAS
The receiver we use in this manual is a so-called Software Defined Radio (SDR) receiver that is used by C.A. Muller Radio Astronomy Station (CAMRAS) in Dwingeloo. The signal is available to everyone via Internet streaming and can be accessed directly via http://websdr.camras.nl:8901
Figure 1. Frontpage of the CAMRAS webSDR streaming receivers
Settings for the webSDR
There are a few settings that you have to make here to tune the receiver to a meteor beacon:
Click at bottom left at Band: at 2m, then you activate the meteor receiver (143 to 145 MHz).
Type in the Frequency field (above Band) 143048.50 kHz.
Click on USB (right, in the second white block) to make sure that the receiver is in the correct reception mode, here is that Upper Side Band).
Now the receiver is set on the transmitter carrier of the GRAVES radar and in the audible audio you sometimes hear the soft beep of the carrier of this French radar. With the variations in the ionosphere and the amount of reflections against aircraft, for example, this carrier wave can be heard more or less well. Moreover, it sometimes seems to change a bit of frequency due to the doppler shift of the aircraft reflections.
Do you work with your own setup with an RTL-SDR dongle? Then adjust the frequency of the SDR dongle to 143.0485 MHz using the control software, just slightly lower than the frequency of the radar, and select the reception mode in upper side band (USB). Follow the same steps that we discuss below; they are identical to how we use the software with the signal from the webSDR.
The reception as described here is about the GRAVES radar in France that broadcasts at 143.050 MHz. In the webSDR you can also tune into another meteor beacon in addition to GRAVES; the signal from BRAMS. It is worth doing the same thing on the 6 meter band (you can click 6m instead of 2m) and then tune to Dourbes (49,96850 MHz) or Ypres (49,98850 MHz). These two transmitters have been specially set up for meteor reception and are also a bit closer. The transmitted signal, however, is weaker, so it gives a very similar result. Interesting is the difference in audible doppler shift. The reflections via GRAVES are at an almost three times higher frequency, which means more Doppler shift can be heard (the well-known ‘pew’ effect). At Dourbes and Ypres there is often less Doppler shift and the reflected signal usually sounds like a ‘ping’ or short whistle: ‘fuut’. Due to the shorter distance to the transmitters, these meteors could sometimes be visible at night. As mentioned earlier, in this manual we focus on the GRAVES radar that functions as ‘meteor beacon’. So adjust the webSDR back to the right frequency, if you have changed it.
The audio reception of the CAMRAS webSDR is of good quality, but the video quality of this stream is deliberately limited in connection with the internet bandwidth consumption. To get more out of it, we only use the audio signal, which we process with the free but very good program ‘Spectrum Laboratory’ or SpecLab for short. Click here to download the required software.
SpecLab has a somewhat high entry-level threshold for beginners, but if you go deeper into it, the software will certainly pay back in nice results. To help you on your way, here are two applications that allow you to use the radio signal for observing meteors: (1.) the basic setting for visually displaying the audio signal and (2.) a nice way to count the number of meteor reflections. The more complex automation and file storage are not yet addressed. If you want to look at automatic logging now, go to this overview page.
1. The basic settings of SpecLab
The basic setting of SpecLab is made so that audio signals from the receiver stream down the screen like a waterfall of audio frequencies. On the left you see the lower and on the right the higher frequencies. A signal can be recognized by the brightness and color differences. At the chosen setting, the image width will correspond to about 1000 Hz. The idea is that the signals from GRAVES can be found in the middle of the image, ie at about 1500 Hz. Meteor reflections will be visible around this. Below is a first screen image.
Figure 2. A SpecLab waterfall screen with the vertical carrier wave, an inclined satellite reflection and a horizontal meteor reflection.
You see in Figure 2 a vertical line, that is the signal from the GRAVES carrier. The receiver is set so that in this audio picture the carrier is audible at about 1500 Hz (see scale at the top). The line is apparently split at the bottom by the reflection against an airplane, which gives a second signal with a small Doppler difference. The long diagonal line to the upper left is (coincidentally) a reflection against a satellite. The segments are created by the various antenna beams of the GRAVES system, enabling a positioning and timing calculation in addition to the Doppler measurement. Finally (at 37 seconds, see left scale) the horizontal line, that is now a meteor reflection, very briefly – in time – and clearly a big Doppler shift. The frequency goes down (to the left.) Which sounds like a ‘ping’ or even a ‘piieuw’ sound when you listen to the audio signal. We must therefore have these kinds of signals.
In the image below you see an ‘overdense’ meteor reflection. You see a horizontal light blue line from 1700 back to 1600 Hz (the Doppler shift), which is the reflection against the plasma cloud around the meteoroid. Shortly thereafter, if the receiver has not yet recovered from its automatic hard tone suppression, or the automatic gain control (AGC), you can clearly see the reflection of a few seconds against the ion cloud that is still hanging. The speed of a drifting / blowing away cloud is often not high and will therefore usually hang around the (GRAVES) carrier frequency (there is little Doppler shift).
Figure 3. An ‘overdense’ meteor reflection, on the right the fast Doppler of the meteorite plasma cloud and on the vertical track a few more seconds the reflection of the ‘ceasing’ ionization cloud
How are we going to set this up in SpecLab?
First of all, we have to make sure that the audio signal that we hear from the receiver (or in our case the webstream) comes in via the audio input of the PC. With a normal radio receiver it is necessary to connect the radio sound to the microphone input of the PC with a cable. In the case of the webSDR and RTL-SDR dongle, the sound can be adjusted through the internal mixer of the sound card. This is something you must have arranged before you continue.
Once you have arranged the input of the audio sound, we can start SpecLab and wait for the program to run. Now you have to select the audio settings via the ‘Options’ field (third field in the SpecLab top bar) (this is the first choice, at the top). Do not be alarmed by the complicated screen that appears now (Figure 4). Top left – in the first input field – you choose the correct item for audio input (for example, ‘external microphone input’ if you use the sound via a cable). There is still ‘Default WAVE input’ in figure 4, so that has to change. Then click on ‘Apply’ and then on ‘Close’ and the screen will close again.
If it works properly, the waterfall screen will have to respond to the audio sound of the receiver. To check if the software is processing the audio from your PC, you can test it with a headset (with which you make Skype calls, for example). If you select the microphone as input, you can see that voice is coming in. Playing a little with this setting can not hurt to see how your computer responds to this and how SpecLab eventually gets the best signal.
Figure 4. The audio setup screen with links above the important input device field.
Then we continue with the waterfall screen. The basic setting of the waterfall screen is as follows. Figure 5 shows the top left corner of the program:
Figure 5. Top left corner of the waterfall screen.
Put the three white fields at the top left. Behind the field f1 is a button with ‘opt’. Click on it and at the first item Frequency controls choose the second option Min, Max. Now set vfo to 0; f1 at 1000 (or 1kHz) and f2 at 2000 (or 2.0kHz).
This determines the width of the waterfall screen. If audio is already coming in, you will be able to see that clearly. To make the brightness and contrast optimal, you can play with the two sliders at the ‘color palette’.
Now we still have to set the speed of the waterfall screen. To do this, click on the third field at the top: options. Select the third line: Spectrum display settings. Then a complex screen appears again. Fill in all the data in such a way that it corresponds to the next image (figure 6, especially in the red ovals) and display Apply and close again.
Figure 6. Display settings input screen.
The fineness of the display is determined by the FFT setting, which determines the ‘Fast Fourier Transform’ analysis of the audio signal. You can find it in the figure above in the second to last tab. Adjust the settings as shown in figure 7: with the field FFT input size you have to play for the best result (higher is nicer but the PC heavier).
Figure 7. FFT input screen.
If you have gone through these steps, the display could / should look like this:
Figure 8. The default setting should result in this waterfall screen. Here you can see two meteors, one with a larger Doppler shift at 21:06 and a short-ping at 21:06:15.
This waterfall screen runs from top to bottom in a minute. You can also see two meteors in figure 8: a reflection with Doppler shift (which would be audible as ‘pie-hole’) at 21:06:00 and a clear ‘ping’ at 21:06:20. The latter was so loud that the receiver pinches the signal for a moment; the AGC intervened at that moment, causing the image to turn black for a short time and come back soon.
Before we continue, it is advisable to watch and listen for a while now. You will notice that the received signal is sometimes shifted slightly to the left or to the right. You can correct this, if desired, on the reception page of CAMRAS to fine-tune the frequency with the ‘+’ or ‘-‘ buttons.
To repeat all this work every time when starting the computer is a bit bland. You can therefore save all the settings made. At the top left of the screen you can choose Save settings as the FILE option. The name SETTINGS.usr is suggested and you can accept that. If you have not changed anything else, the settings of this file are automatically loaded at the launch of SpecLab. If this does not happen, the next time you can retrieve with Load settings from the same file.
In fact, you are now ready to listen and watch radio stations for a longer period of time with your setup or via the webSDR from CAMRAS. But you can do even more.
2. Looking a little longer and counting meteors
If the image disappears after a minute, it becomes difficult to have an overview of, for example, the last half hour: does it become busier or less busy with meteors?
For this we will look a bit deeper in SpecLab. There is a possibility to switch on a writing plotter. Each line on it is plotted according to a formula. We will also choose the running speed of the plotter and other settings in such a way that the image creeps over the screen in over 50 minutes. This gives us a nice overview of what has happened in, for example, the last three quarters of an hour. Below an image that gives an idea what the plotter can do.
Figure 9. A combination of waterfall and plot screen.
You will see colored boxes at the top of the plotter image. That is the legend: Each block corresponds to a line in the plot below it. Each line has the function of the maximum of the average noise in the last 200 milliseconds in a width of 50 Hz. The color blocks therefore have a frequency indication as a name. 12 stands for 1200Hz to 1250Hz and 12.5 stands for 1250Hz to 1300Hz and so on. A total of 14 areas around 1500Hz have been recorded and are therefore plotted. The plot screen slowly moves to the left. In Figure 9 you see three meteor reflections at different frequencies (pitches) and you see that as the corresponding color of the line. The signals are indicated here with figures for clarity.
The set volume of the PC gives the average noise level (here about -74 dB). If there are significant peaks this will be up to 25 dB stronger, so up to -50dB. This is interesting to be able to make a better setting later on.
How do we get this plotter screen now? Press the second last tab, just for help: View / Windows. Now the dropdown menu appears with four groups. Choose the first item of the third group: Watch List & plot window. Once you have selected that, the line function screen, the Watch List, appears. We must complete this as shown below (figure 9). The number of lines (channels) is determined by entering the number 14 (or more, default is 16) under the Memory / Misc tab of this watch list.
Figure 10. The watch list, filled in here with the legend name (title), the noise function noise_n (expression) and in the last two columns the range we want to see (scale min and scale max).
Next we have to set the horizontal scroll speed (figure 11, appears under the fourth tab ‘horizontal’):
Figure 11. Entering horizontal details for the running speed (scroll rate) and the scale allocation (markers).
And to activate all 14 lines (channels or channels) with the right function, some manual work has to be done under the channels & colors tab. For each channel we set the line style under the Channels & colors tab to mixed and the function to Max.Value. Here the example for channel 11 (figure 12):
Figure 12. Enter the same for each channel (here 10 pieces).
Now press the plotter tab to view the result. If it does not run, start the plotter briefly in the plotter submenu that is just above the plotter tab, so outside of this entry screen.
Below is a screenshot (figure 13) of how I arranged it (with volume so that the average noise level is -70dB). Note, the chosen noise_n function is especially interesting for Doppler signals (the ‘pieuuw’ sound). For the detection of pings you can better go to the ‘peak_a’ function. However, the latter is disturbed when the carrier itself is very clearly received. A nice subject for experimenting and therefore useful for Dourbes and especially Ypres reception.
Finally, I defined a channel with a (peak (1450.1550 – avrg (1000.2000)) function so that both can be tracked simultaneously.
Finally, some screenshots of what it can look like now. The first (Figure 13) is the default setting for GRAVES. That is where I hear the most signals. However, the ‘quiet’ Ypres sometimes gives better ‘overdense’ signals because they are not disturbed by the carrier (Figure 14). Then let the waterfall screen also run over the plot (figure 14).
Sometimes there is a special catch that you immediately have in the copy-paste buffer of the computer by pressing the -prt Scr- button. Next, for example, I open Powerpoint to save the picture. This way you can always respond quickly. Figure 14 shows a special ‘overdense’ cloud with various Doppler speeds. Because Ypres gives narrow reflections, I put the waterfall screen I narrower (f1 at 1400 and f2 at 1600) with sometimes amazing results.
Figure 13. A ‘standard’ viewing screen that runs for about 43 minutes. Top left the waterfall screen with clearly three meteor signals, a lower than 1500 at 09:46:47, a higher than 1500Hz at 09:47:00 and a neutral ping between them. On the plot screen at the far right (last recorded half minute) you will find them as blue, red and in between a gray signal. Coincidentally the carrier wave of GRAVES was just as weak, so the ping was detected nicely. Furthermore, you clearly see in the plotter that the gray signal is quite disturbing when a lot of that carrier is heard.
Figure 14. A quiet picture of the beacon of Ypres. It is clear now that the gray line detects the ‘pings’ neatly (the four from the waterfall screen can also be found neatly in the last minute on the plotter, far right).
Figure 15. With a smaller set waterfall screen i’ve done a stunning catch. How will these ions clouds swirl so that this figure arises? It is a long-term reflection (25 seconds) and the Doppler shift is about 20 Hz to both sides. Corresponding speeds are about + and – 20 meters per second.
Automation to count meteor reflections
Finally. The more complex automation and file storage has not yet been discussed here. There are many possibilities for that. If you want to further extend the detection of radio meteors, you can look at this page for the possibilities.
In addition, you can count the meteor reflections and log with the software Colorgramme. You can make an overview of the observed reflections from your location in monthly plots via the Radio Meteors Bulletin. You see on the RMOB page that more meteor observers with RTL-SDR dongles record and count meteor reflections. In short, a good chance to compare your radio observations with those of others!
Text contribution: Frans de Jong (PE1RXJ), radio amateur, amateur astronomer and volunteer at CAMRAS, the C.A. Muller Radio Astronomy Station.
