Tag Archives: radio

Meteor showers and different types of meteor trails

Okay, so the Perseid meteor shower has passed and I didn’t detect as many meteors as I expected, so I did a little bit of an internet search and found that the Perseid meteor shower is actually a Northern Hemisphere shower. The radiant (the point from which meteors seem to originate from) doesn’t get as high up in the sky for us and thus the Southern Hemisphere does not  get as many meteors. On top of that, they were coming from the North, while my antenna is pointing South so this seems to explain why I detected no difference in my meteor count.

However, I found a website which gives a list of meteor showers for the Southern Hemisphere, so I will be referring to this from now on and see if what I am getting is matching with what is written here.

Now, how do I know that my radio signal is reflecting off a meteor trail?

According to the research paper, “Forward scattering of radio waves off meteor trails, 1995” by Jean-Marc Wislez, in classical theory of meteor scatter, there are two extreme cases of meteor trails that can occur – the “underdense” meteor trail and the “overdense” meteor trail. When a meteoroid enters our atmosphere, the atoms in the atmosphere become ionised, thus creating a trail of free electrons and ions. It is the free electrons that contribute the most to the scattered signal. If the line density (i.e. the number of electrons per unit of length of the meteor trail) of ionisation is low, then the incident wave penetrates the meteor trail and the electrons absorb the energy of the wave. This makes them oscillate and thus they re-emit the radio wave in all directions. The radio signal that is scattered in this way is said to have scattered off an “underdense” meteor trail. If the line density of ionisation is high, then we assume that the meteor trail acts like a metallic cylinder whose radius is much larger than the wavelength of the radio wave. This approximation assumes that the incident waves scatter off the surface of the cylinder since the radio waves cannot penetrate the central part of the trail. This type of trail is called an “overdense” meteor trail. However, the “metallic cylinder” approximation has flaws, as it does not take into account any low density part of the meteor trail, and it assumes that the power of the scattered signal in the perpendicular direction to the meteor trail is directly proportional to the radius of the cylinder.

Here is a simulation of a power-time graph, based on theory, of an underdense and a short overdense meteor trail (taken from the “Forward scattering of radio waves off meteor trails” paper):

Power-time graph of an underdense and a short overdense meteor trail. Reference: Forward scattering of radio waves off meteor trails - Jean Marc Wislez

Power-time graph of an underdense and a short overdense meteor trail. Reference: Forward scattering of radio waves off meteor trails – Jean Marc Wislez

And here are examples of some of my observations:

As it can be seen in the graphs, the power-time graphs of the underdense and short overdense meteor trails almost perfectly match the predicted simulations. The only difference is that some of the fluctuations that I got could be due to the signal itself as I am using an FM radio signal, whereas in the research paper they used a constant amplitude signal. However, these signals are both quite short so I think it’s safe to assume that in these two moments the signal was very close to being constant in amplitude and thus they seem to fit the theory.

an underdense and short overdense meteor trails

Now, there is also a third type of power-time graph that can happen, and this meteor trail is called a long overdense trail. In the research paper that I keep referring back to, there isn’t a simulation to predict this type of meteor trail, however it was detected and this is the observation that they made:

Power-time graph of a long overdense meteor trail. Reference: Forward scattering of radio waves off meteor trails - Jean Marc Wislez

Power-time graph of a long overdense meteor trail. Reference: Forward scattering of radio waves off meteor trails – Jean Marc Wislez

And here is an example of a long overdense signal that I detected. As it can be seen,
all three types of graphs seem to fit predictions or observations that have already been made and thus I conclude that what I am detecting really are meteors. The only one that is difficult to say that it was reflected off a meteor trail for sure is the long overdense meteor trail as it lasts much longer than the others, and thus the signal itself can fluctuate in power as it is an FM radio signal. It is very difficult to remove the actual signal fluctuations to compare to the long overdense meteor trail observation.

A long overdense meteor trail

I should also point out that these examples I showed are the “perfect” examples I found. There are signals that I have detected whose power-time graphs aren’t exactly as what is predicted, but still follow the similar patterns. The variations in the graph could be due to the fact that my signal is not constant in amplitude, but also these simulations count on ideal conditions and make approximations and assumptions that do not always happen in real life. Meteor trails can be in any size and shape, depending on the size of the meteor, its speed, direction and also wind, therefore the radio wave reflections can be quite different from these ideal predictions.

 

Update

Last week I wrote about how I didn’t have remote access to the computer in Ardmore anymore because of the storm. That has now been restored – I asked our IT service to help me, and it works now and everything is back on track.

As I mentioned in a previous blog post, I thought that my originally chosen frequency of 101.7MHz may have been too weak to detect and so I decided to change to a stronger frequency. This is showing up some results! Currently I am tuned into 91.3MHz, which is the ZM radio station in Christchurch. The way that the software records the signal is that it records the whole bandwith that I can see on my screen (from about 90.8MHz to approximately 91.4MHz), and not just the 91.3MHz frequency. That means that while I’m recording 91.3, I am also recording 91.0, which is ZM in Auckland (ZM plays current pop music), and 91.4 which is Radio Concert (which is classical music) in the Waikato area, as my antenna can easily pick up the radio stations from there.

What is interesting, is that sometimes, when I detect a signal at 91.3MHz, there is also a peak that shows up on the 91.2MHz frequency.
To help explain better, here is a screenshot from a signal that was recorded on 07/21/2015  at 04:14:07.

A screenshot of one of the signals I detected at 91.3MHz

A screenshot of one of the signals I detected at 91.3MHz

Here, we have the Auckland ZM station on the left, at 91.0MHz, and we have the Waikato Radio Concert station on the right at 91.4MHz.  In between, there is a signal at 91.3MHz, which is ZM in Christchurch (and I will explain why I’m so sure of it shortly), but there is also an unknown signal at 91.2MHz. Looking at the list of radio stations in NZ, I saw that a possible radio station at this frequency is Radio Concert in Nelson. Looking at the map of NZ, we can see that Nelson is between Auckland and Christchurch and so it is possible that if a meteor (or something else) happened to fly between us and Nelson, it could have reflected both frequencies from Nelson and Christchurch. Although, I still have to prove this to myself geometrically, rather than assume that it can happen.

Okay, so at first I was a little bit skeptical, as it seemed a little bit too good to be true – that I can actually detect two frequencies that have reflected off something in our atmosphere seems a bit lucky. However, these signals are so strong that you can actually hear the music. So when I played the signal I detected on 91.3MHz, I heard a part of a Miley Cyrus song that was recorded, and when I tuned into the 91.2MHz, I heard classical music. This was my initial confirmation that I have detected ZM from Christchurch and Radio Concert from Nelson. Now I needed confirmation that what I recorded actually came from those stations. Assuming that all transmitters of the same  radio station play the same stuff at the same time (except maybe the ads), I listened to the 91.0MHz frequency I recorded, (which, as I said before, is ZM in Auckland) and it was the exact same part of the song  that was recorded on 91.3. Then, I tuned into the 91.4MHz frequency, (which is Radio Concert in Waikato) and I heard the part of the piano that sounds exactly the same as what was recorded on 91.2MHz.

I checked the list of all the radio stations again, and saw that there is no other ZM station in NZ that is being broadcast at 91.3MHz, and no other Radio Concert station in NZ that is being broadcast at 91.2MHz. All of these reasons led me to conclude that I have actually recorded two signals from the South Island, that have been reflected off something in our atmosphere because they are both too far away to be received directly.

This still seems too good to be true to me. It just happened, by luck, that I can record all 4 radio stations and compare the recorded signals to the actual signals that I definitely know were being broadcast at the time of recording.

I have emailed one of the radio stations to confirm that they play the same songs at the same time everywhere in the country. Now, what I have left to do is to actually figure out how to interpret these signals.

How do I find out that these signals were reflected off a meteor? Well, that’s a post for next time 🙂

Set back due to the storm

As most people would know now, there was a big storm on Saturday, which also hit the Ardmore area. There was even a picture on the NZ Herald of a couple of light aircraft being flipped over from the wind in the Ardmore airport. I was actually planning on going out there that day to look into changing some things with the equipment to see if it improves the detection of signals, but needless to say, the weather changed my plans. Since then, I haven’t been able to remotely access the computer, so I was left in the dark for the rest of the weekend about whether there was any damage to my antenna or any of the other equipment.

Taken from the NZ Herald article Ardmore Airport. Photo / Matt Archer

Taken from the NZ Herald article
Ardmore Airport. Photo / Matt Archer

After my classes on Monday, I made the decision to go there to see if everything was okay. Not knowing if my antenna was still in one piece stressed me out, so I needed to see for myself if there was any damage, calm my thoughts and decide what my next step should be. Luckily, everything seemed to be working and the antenna is still in its place! However, there was a problem with the computer and internet access, because the storm caused a power cut which lasted a few hours, and the back-up power source for the internet ran out of battery during that time, so it didn’t start up again after the power came back on. I restarted it, but I still couldn’t manage to access the computer remotely from my laptop.

I sent an email to the person who helped me set up the computer in the first place and hopefully he’ll be able to find some time over the next couple of days to go there and help me fix the problem.

I’ve been thrown off a little bit by the storm, but I couldn’t predict something like this. I just have to be positive and hope that although it’s a little set back, it seems to be something that can be fixed quickly and easily, so I’m hopefully still on track and will have everything going to plan soon!

My Honours Project

Hi there! So since this is my first blog post, I should probably introduce myself before I write anything else.

I’m Ana and I’m currently doing my Honours degree in physics. I have just graduated with a Bachelor of Science, majoring in physics and maths and decided to pursue an academic career in physics.

My project for this year is to detect meteors using forward scattering of radio waves off meteor trails. Instead of building a dedicated radio receiver specific for this project, I decided to use Software Defined Radio (SDR). Basically, this means building an antenna, connecting it to the SDR and setting up a software to detect a radio wave that I normally wouldn’t detect as the transmitter is beyond the horizon. When a meteor enters the atmosphere, it creates a trail of ionised air, the radio wave reflects off this trail, and thus it can be detected.

Since this project was started from scratch, I divided my main goal into smaller goals:

  • choose a frequency that I was going to look for
  • buy a receiver
  • find a computer
  • find the software I would use
  • build an antenna
  • set it all up
  • enable remote access to the computer

The system is set up at Ardmore Field Station, in South Auckland so the remote access was necessary so that I wouldn’t have to drive out every day. I had help from a couple of technicians in the physics department with getting the materials for the antenna and with building it, and I had help from IT services when it came to setting up the computer, installing the software and enabling the remote access. At the moment, I am also getting help from my dad with the plugins that had to be added to the software to record the signals, as the currently available plugins online weren’t what I needed.

Here are a couple of progress photos from when the antenna was being assembled:

Putting the elements together

Putting the elements together

One of the technicians working on the dipole

One of the technicians working on the dipole

The dipole

The dipole

The final set up of the antenna

The final set up of the antenna

Currently, as my project stands, I chose the frequency 101.7 MHz from Christchurch, the receiver is Airspy and I am using the software SDR# to detect and record any signals that my antenna picks up at that frequency. So far, I have detected some signals, but these don’t seem to have the power spectrum that a signal that has been reflected off a meteor should have. Based on my readings and what other people have done in the past, there are a few power spectra that I should get depending on how big the meteor is. However, I’m either not interpreting the signals correctly, or these haven’t been reflected off any meteor trails. For reference, please look up “Forward Scattering of Radio Waves off Meteor Trails” by Jean-Marc Wislez, or look at this website since its information mostly came from that research paper.

I started to think that maybe the power at which my frequency is being emitted isn’t strong enough to be detected upon reflection, so I have just tuned into another frequency that’s stronger and I’m waiting to see if my antenna picks anything up. From what I’ve read so far, I’m expecting the biggest meteor count in the hours between midnight and 7am, and the least count should happen in the evening hours (Refer to this website).

The main aim at the moment is to have this whole set up working and detecting by the time the Perseids meteor shower happens, which is only a few weeks away. The peak this year will be at around the 11th of August. I will know that my set up is working if even before the meteor shower I am getting the expected power spectrum, and the expected rate of meteors (most over night and little during the day). The Perseids should be the biggest confirmation of my set up working and it should also be when I collect most of my data to analyse and write about in my dissertation!

I’ll follow up with more posts with updates and/or if I remember anything important I’ve done that I forgot to mention.

Until next time,
Ana