Radio Meteor Observing

Introduction

[Note: previous versions of this page mentioned analogue TV and specialist radio hardware using an RS232 interface. This technology is now largely obsolete but the older page can still be found here]

In the past you would have needed specialist radio ham equipment and to have been pretty good at electronics to build a radio detection kit.

However technology has advanced and it is now possible to buy relatively cheap USB based radio recievers like the one shown here. You still need some basic computer skills to configure the software and some DIY skills to build the antenna, but it is much simpler than it once was.

 

So why not give it a go!

Why do it?

Radio meteor observing allows continuous meteor observations to be made regardless of the weather or daylight. The ‘observer’ is also sitting inside a dry, warm house and doesn’t need to risk frostbite to observe. Those of us used to December and January meteor spotting will appreciate that!

Counts can also be very high. Although it can’t differentiate between shower meteors and sporadics, radio systems are often capable of picking up meteors invisible to the human eye or even to cameras. For example radio sometimes shows the southern Delta Aquarids (rich in faint meteors) out-performing the Perseids (rich in bright meteors) and during December 2019’s Geminids I detected over 800 meteors in a nine hour window as shown below!

The heatmaps produced also help identify shower timings  – in this case the Geminids can be pretty easily picked out from the map as being centred around the morning of the 14th – but ther’s also an interesting secondary peak in the evening.

 

The Science Bit

As explained in the Meteor Section’s booklet “Observing Meteors“, a meteoroid entering the Earth’s upper atmosphere excites the air molecules, producing a streak of light – the meteor – and leaving a trail of hot ionised air behind it. Hot ionised air is more reflective to radio waves, and so if the meteor passes through a strong radio beam, reflections from the ionisation trail may be detected using a radio receiver.

There are two main radio observing methods, back-scatter and forward-scatter. Backscatter is commonly called RADAR, and is the realm of professionals because you need a radar station. Forward scatter is more practical. This is where the transmitter and receiver are in separate locations – often hundreds of kilometres apart – and the signal is scattered forward to the receiver from a meteor somewhere between the two as shown in the diagram a little further down this page.

This webpage provides some suggestions for how you can make, record and report such forward-scatter radio meteor observations. There is some jargon here but if you come across a term you don’t understand, or simply need more advice about choosing, setting up or operating please e-mail meteor@popastro.com.

Equipment

Receiver:  Once upon a time you would have needed a ham radio kit and electronics experience to try this but it is now possible to buy a cheap USB dongle radio reciever to do the job. There are two types in common use: Funcube dongles are reliable but cost about £130 from their website; RTLSDR dongles are a newer but cheaper technology, costing about £15-20 from everyone’s favorite online retailer.  This guide will talk about the latter as its more within the reach of amateurs and importantly because I have set one up myself (using a Nooelec RTLSDR dongle).

Antenna: The antenna needs to match the frequency you are hoping to receive. You will also need to position it so it is pointing towards the transmitter, and place it as high up as possible above the ground (mine is in my loft). Note that you don’t need clear line-of-sight towards the transmitter – a few trees or house roofs won’t matter though a big hill would.

Computer: You don’t need an expensive PC for this. My system is running on a five year old laptop running Windows 10 home.  However you will need to leave it running 24 hours a day so consider how noisy it will be and how you will get power to it. 

 

Choice of Target Transmitter

For forward-scatter detection to work, the reciever and transmitter need to be far enough apart but not too far. The diagram below explains why – the meteors are detected at an altitude of about 100km, and both you and the transmitter need to have a line of sight to them. If you’re too far apart the meteors will be below the horizon of one or the other. On the other hand if you’re too close together, the signal from the transmitter itself will be directly audible and might drown out the echoes.  

If you’re in the UK or northern Europe, you’re in luck – there is a space-radar station at GRAVES in France which is ideally placed about 700km from central UK. This broadcasts at 143.050Mhz and is located just outside Dijon. 

If you’re in other countries you may need to look around a bit I’m afraid. If your country still has analogue TV, a station about 700-1500 km away would be ideal, otherwise look for a VHF radio beacon or possibly even an FM station around 100 to 600km away. NB: if you are in another country and have a successful rig operating, please send me details of the transmitter so i can mention it on this page.

 

Radio-Horizon.jpg
Vertical section through the transmitter-receiver axis

 

No matter what transmitter you listen for, it needs to be broadcasting 24 hours a day in all directions so you get maximum chance of a meteor passing through the beam. Coastal TV stations often broadcast towards the land only so may not be ideal but still worth a try. 

The choice of transmitter will affect your antenna in two ways: firstly the antenna needs to be ‘tuned’ to the frequency of the transmitter (don’t worry, this is easy and I’ll explain shortly); secondly of course it affects where you point the antenna (I’ll cover this later too).

Making the Antenna

We are going to make what’s called a “Three Element Yagi” antenna. This consists of three parallel metal elements called the Director, Reflector and Active element on an insulating support with the Active element connected to the aerial cable.

You will need a 3 metre length of 15mm copper pipe, a length of 50x50mm timber at least 650mm long, a 1m length of 15mm electrical square-section mini-trunking (the type with a top that pulls off), a length of coaxial cable to go from your antenna to your PC and a ‘pigtail’ connector to attach the coax to the USB dongle’s antenna socket. Check the dongle to see what connector it requires before buying the pigtail. You’ll also need a couple of small self-tapping screws with washers, and some wood screws to attach the trunking to the wood. Note that the Coax should be RG-58 as it has the right electrical characteristics. Ordinary TV Aerial cable will work but be less sensitive.

 

Earlier on I mentioned “tuning” the antenna to match the transmitter. We do this by cutting the elements to specific lengths and placing them just the right distance apart. The instructions below are therefore designed for one specific frequency – that of the GRAVES transmitter at 143.050 MHz. For other frequencies the lengths and spacings are different so if you’re using a different transmitter please contact meteor@popastro.com for advice.

So here’s how to make your Antenna:

  1. Cut the copper pipe into four: 1076mm, 836mm, and two of 479mm.
  2. Cut the mini-trunking into three: two 25cm and one 50cm. Discard the tops.
  3. Draw three lines across the wooden boom at right angles. The first should be 100mm from one end, the next 265mm from the first mark and the last 235mm from the second. Label them R, A and D respectively.
  4. Screw the short pieces of mini-trunking to the boom at R and D. Use a set-square to get them exactly at right angles to the boom.
  5. Screw the long piece of mini-trunking on at A, again exactly at right angles.
  6. Drill two small holes in the ends of the shortest lengths of pipe and screw in the self-tapping screws with two washers each. This is where you’ll attach the coaxial cable.
  7. Clip the longest length of pipe into at R and the medium length into at D, making sure they are central.
  8. Clip the two short lengths of pipe into each side of A, with the screws in the middle. Leave a 15mm gap between the ends.
  9. Strip about 30mm from one end of the coaxial cable. Attach the core to one screw and the braiding to the other, then cover the two screws and the end of the cable in silicone sealant to keep water out.

Your finished article should look like the image below.

Installing the Antenna

The antenna should be as high up as you can get it, with the elements horizontal and the wooden boom pointing towards the transmitter but angled up a bit.

To work out the angle remember that the antenna needs to point at a place 100km above the ground, half way between you and the transmitter. You can work out the exact angle with some trigonometry but roughly speaking, if the transmitter is 200km away, angle up 45 degrees, 500km away, angle up 22 degrees, 700km away, 17 degrees, 1000km away, 13 degrees.

Install your antenna and run the coaxial cable down to your PC. Remember to ask before boring holes in the walls or ceiling!

Attaching the Dongle to the Antenna and PC

Use the pigtail connector to attach the coaxial to the antenna socket on the dongle.  The USB dongle can be plugged directly into the USB port of a PC or Laptop however I recommend against this: PCs are terrible sources of electronic noise – from CD drives, from screens, from mice and keyboards, from components inside the box etc. This can play havoc with sensitive radio measurements so get round this, buy a 1 metre long USB extension cable and put the dongle on the end of this. Locate it safely away from your PC where there is less chance of noise.

Receiver Setup

Next we’ll set up the reciever software. You will need the following:

SDR# or HDSDR – software defined radio programmes. I used SDR#
VBCable – virtual audio cable software to connect the radio software to the analyser.
Spectrum Lab – to capture and view the meteors.
The Spectrum Lab config file from here (if you have any problems contact me).
And optionally – Colorgramme to draw pretty heatmaps and submit data to RMOB.

Tuning the Radio

The funcube dongle comes with its own interface software you can use for this, but for the RTLSDR devices you have two options, either SDR# or HDSDR. Either will work but these instructions are for SDR# (pronounced SDRSharp in case you wondered).

First Install Spectrum Lab and VBCable and create folders c:\spectrum and c:\spectrum\screenshots if they don’t already exist. These folders will store the logs and screenshots of meteor detections.

Then Install SDR#. This doesn’t have an installer so just unzip the package into a folder on your computer and create a desktop shortcut to SDRSharp.exe.

Installing the right USB Driver

Unfortunately the default USB driver that comes with Windows doesn’t work properly for these devices yet (as of 2020 anyway). So you’ll need to update it yourself.

Open the SDR# folder and run install-rtlsdr.bat to download the required driver.  Then run Zadig.exe which is also in the SDR# folder. Click ok if prompted to allow the app to make changes to your computer.

From the Zadig options menu, select List All Devices.  In the dropdown select “Bulk in, Interface (Interface 0)”. The Driver box should now show RTL2832UUSB with a green arrow pointing to WinUSB. Take care to select the right interface, otherwise you might disable your mouse or webcam!

Click Replace Driver, and when its done the screen should look like the screenshot below, with the WinUSB driver on both sides. Now you can exit Zadig.

NB: if this all sounded a bit worrying, have a look at this webpage which explains in more detail  – https://learn.adafruit.com/getting-started-with-rtl-sdr-and-sdr-sharp/driver-fix

Setting up the Software

Okay we are now ready to set the software up. Start SDR#, set the source to RTL-SDR (USB) and click the cog button. Select your device, set the sample rate 1.024MSPS and tick the Tuner AGC box. Now close the settings dialog.

Tune to an FM station, click the Play button and you should hear the radio. Adjust the offset, range and contrast sliders on the right till you can see the signal but don’t worry if the sound is crackly, the antenna isn’t designed for FM. If you can’t pick up anything check your antenna connections and FM frequency.

Next change the frequency to 143.048.000, i.e. 2kHz below GRAVES’ frequency. Expand the Radio tab, select USB (upper side-band) and set bandwidth to 4000 Hz. You should hear hissing noise.

Once you have picked up an FM channel you have the radio working. Click Stop, expand the Audio tab and set the audio output to Cable Input. Click Play again, and leave SDR# running. You won’t hear anything now, because the sound is being sent into the “virtual audio cable”. You can double check everything is ok by closing SDR# and re-opening it – it should have saved all your settings.

Now we can start to capture some meteor data. Start Spectrum Lab. Click on File/Load Settings From, and load the config file you downloaded. This is based on one developed for the BAA some years ago but has been updated a bit by me.

Click on Options/Audio settings and set the Input Stream to Cable Output. You should now once again hear a noisy hiss and have a scrolling display as shown below.

Click the Signal Gen button on the left to turn on a test signal which should show up as a blip or line on the screen at 1500Hz. Adjust the Color Palette sliders till the test signal stands out well against the background then turn off the test signal again. Ideally you want the background to be mostly black but with blue speckles of noise on it.

What Happens now?

That’s it – you are now set up to detect meteors!

When a meteor is detected you’ll hear a “ping” on the audio, and see a bright dot or line on the display, fairly close to the 2000 Hz line with a label below it as shown in Figure 6, which shows several detections. A screenshot like the one below (showing four or five events) will be saved in c:\spectrum\screenshots and the event logged in c:\spectrum. You can turn off the PC volume if it gets annoying, the system will keep running.

Don’t despair if you don’t get anything at first – I detected nothing at all for weeks! The best time to test is during a busy meteor shower such as the Geminids or Quadrantids. Check your cables and antenna alignment, get your antenna as high up as possible, play with gain in SDRSharp and the Color Palette in Spectrum Lab and of course if you have any questions please get in touch.

What to Record and Report

The Spectrum Lab config file you downloaded earlier is in fact all set up to help you record relevant data. It will save the information into c:\spectrum, creating a new log file for each day and a separate file for each month in a special “RMOB” format, as well as screenshotting each meteor. You can load the RMOB files into Colorgramme to produce “heatmaps” showing the meteors per hour for a whole month (as shown at the start of this article) and these can be submitted to the RMOB organization. If you want more information about this please contact me or the RMOB organizers.

If you’re not using my config file, you should record the number of meteors per hour, or per ten minutes if activity is high. Times should be recorded in UT of course, based on the start-time of the reporting period. Do not adjust your set while recording as it might alter sensitivity.

You can also examine the duration and shape of the ‘pings’. Strong traces may indicate a fireball, and in principle you can work out details of the speed and deceleration from the shape and frequency of the trace. This is a quite advanced technique however!

Please also also report any unusual radio propagation conditions, or interference, that you notice.

In addition to the counts and the date and time information, each report should include the following information:

  • The observer’s name and observing location, preferably giving both the latitude and longitude, and the name of the nearest town or city, and county if in Britain.
  • The type of receiver used.
  • The frequency observed at.
  • The antenna type, the azimuth bearing of its horizontal direction in degrees east of north, and its elevation in degrees above the horizontal.
  • The type of any pre-amplifier used (none if using SDR#).
  • The observing system used – e.g. SpectrumLab, Meteor v8.0, etc.
  • Any other remarks, such as interference or equipment problems.

Once your report is ready, send it by e-mail to meteor@popastro.com. You may also wish to submit reports monthly to the Radio Meteor Observations Bulletins (RMOBs).

Further Reading

Books:

  • “Meteor Science and Engineering”, by D W R McKinley, McGraw-Hill Book Company Inc, 1961. Despite its age, this is still considered a classic reference by many radio meteor observers.
  • “Proceedings of the Radio Meteor School 2005”, edited by C Verbeeck & J-M Wislez, International Meteor Organization, 2006. This details many of the technical aspects and physics of radio meteor observation and analysis. Although it concentrates on the use of back-scatter, which is easier to analyse the results from, forward-scatter is covered too, while the physics involved are the same for both.
  • “Amateur Radio Astronomy”, by John Fielding, ZS5JF, Radio Society of Great Britain.
  • “Meteor Burst Communications”, by Jacob Z. Schanker, Artech House.
  • “Meteor Astronomy” by Sir Bernard Lovell, Clarendon Press, 1954. Now old, but a good technical read as to how radio meteor astronomy got underway.

Webpages

  • RMOB – Set-up to collect and publish monthly radio meteor data in 1993, this continues in the same format, and provides a means of contacting other enthusiasts to swap data and ideas for observing and analyses.
  • Radio Meteor Observatories On-line – Contains “live” radio meteor data published on the Internet.
  • Colorgramme/HROfft software example – A guide to the use of the Colorgramme & HROfft software.
  • CMOR intensity plot Canadian Meteor Orbit Radar (CMOR) daily intensity. Be aware that although the timestamp regularly updates and the constellations move across the sky, the intensity are only updated once a day.

Extensively rewritten by Mark McIntyre, 2019-2020, to take into account changes in technology of both transmission and reception systems. Original page By David Entwistle (with updates by Tony Markham, Aug 2014 and Paul Sutherland, Oct 2017)