Date: February 2, 2011
Title: Meteor Detection by Radar for the Amateur Observer
Podcaster: Steve Carter
Contact: Steve can be contacted at firstname.lastname@example.org
Description: As well as being able to visually observe meteors streaking across the night sky, it is also possible for amateurs to make radar observations of meteors. Using simple receiving equipment and a distant TV or radio transmitter, radar echoes can be received that will allow meteors to be observed throughout the day, including meteor showers that only ever occur during daylight. Radar also allows much smaller meteors to be observed than can be seen with the human eye.
Bio: Steve Carter is a professional telecommunications consultant working on the regulation of advanced services. However with outside interests that include both astronomy and radio propagation, radio astronomy offers a perfect combination.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Neil Christie. Those who can Podcast, those who can’t donate.
Hello and welcome to this edition of the 365 days of astronomy podcast.
Those eerie whistles that you have just heard emerging out of the background radio noise are the radar echoes of meteors as they enter the earth’s atmosphere.
My name is Steve Carter and I am an amateur radio astronomer from the UK. For this edition of the podcast I would like to talk to you about observing meteors by radar using relatively simple equipment.
You will already know that it is possible to visually observe meteors as streaks of light racing across the night sky. Meteors are tiny grain to pebble sized pieces of matter which burn up when they enter the atmosphere, leaving a trail of light.
But as well as visually observing meteors, it is also possible to detect them by radar. Using radar has a number of advantages over visual observations since they can be seen throughout the day or on a cloudy night. What’s more, some meteor showers only ever occur in daylight and therefore radar observations are the only way to see them, and even at night it is possible to detect much smaller meteors with radar than can be observed with the human eye.
It’s also the case that each day the rate at which meteors enter the atmosphere peaks over that part of the earth which is facing into the direction that we orbit around the sun (for exactly the same reason that you collect flies and other bugs on the front windscreen of your car and not generally on the rear or sides). For the earth the equivalent of the windscreen turns out to be that part of the earth round about where sunrise is occurring, but since it is twilight in these areas it is not ideal for visually observing meteors and so radio detection allows us to observe this peak when visual observations would be difficult.
So what makes it possible to observe meteors at radio wavelengths? Well the largest meteors reflect radio signals directly, but even very small meteors entering the atmosphere at very high speed burn up and this temporarily ionizes the air immediately around the path of the meteor, and these ionised atoms make a very good reflector of radio waves. So if we bounce radio signals off these ionised trails we can listen to the returning echoes.
This is essentially radar, that is you transmit a radio signal upwards towards the ionised trail and listen for the reflected signal coming back.
Conventionally radar works by having a transmitter and receiver located at the same place and working through a single antenna. Think of the rotating radar antennas that you can see at airports, or high on the superstructure of big ships.
However in a radar system, there is no reason why the transmitter and receiver need to be located in the same place. In fact they can be hundreds or even thousands of kilometres apart. And separating the transmitter and receiver by a large distance has a big advantage for the amateur radio meteor observer. In a collocated radar system where the transmitter and receiver are together, generally the transmitted signal has to be pulsed in order to allow the receiver to listen to the returning radio echoes during the gaps between the transmitted pulses. But if you separate the transmitter and receiver far enough then the transmitter will be hidden over the horizon and there is no chance of the receiver being able to receive the signals directly while it will still be able to receive the signals bounced of meteors high up in the atmosphere. This means that the transmitter no longer needs to be pulsed, but can operate continuously. In fact to observe meteors the transmitter doesn’t even have to be specially designed for radar work, and virtually any transmitter such as a radio or TV transmitter can be used.
So this is where it becomes easy for the amateur observer. Instead of having to build and operate a powerful radar transmitter we can make use of existing transmitters. Analogue TV transmitters are a favourite for observing meteors because they typically operate around the low VHF frequencies that are optimum for meteor observations and TV transmitters are usually very powerful and transit a virtually continuous signal 24 hours a day.
Unfortunately with the advent of digital television, many of the existing analogue television transmitters have already been closed down or will be closing down shortly, and because of the transmission techniques used for terrestrial digital television, digital TV transmitters are not really suitable for meteor detection. But many other transmitters can be used to detect meteors including VHF radio beacons, FM radio transmitters and so on. The meteor pings that you can hear at the beginning and end of this podcast were recorded using a variety of transmitters.
So how do we go about receiving the echoes from the meteors?
Well I have already said that we can use an existing transmitter, say a VHF FM broadcast station. Ideally this needs to be located a few hundred kilometres away from the observation site. Here in the UK I usually tune into transmitters that are based either in France or in Spain. This means that I cannot normally hear the signal directly since the transmitter site is way below the horizon but I can still hear the meteor echoes. It’s also important to choose a transmitter that is on a frequency that is not being used by a more local station since this will block the more distant signal.
So all we need is a suitable aerial and receiver to listen to the echoes. Firstly for an aerial we don’t need anything sophisticated, and in some circumstances a complex directional aerial would actually be a disadvantage since the meteor echoes can return from a wide range of directions. So a very simple antenna will suffice. I have received meteor echoes using nothing more than the built in extendable aerial on a portable radio, but I often just use a dipole which is essentially 2 pieces of wire each cut to ¼ of the wavelength of transmitted signal. Googling the phase “radio dipole” will show you how to build one.
For a receiver you need something that is relatively sensitive, but more importantly can be accurately tuned to the frequency of the signal that you want to receive. So a receiver with a digital frequency display is ideal. A good quality FM receiver with digital readout will suffice, but a communications receiver or high quality scanner is better since it will allow a greater choice of frequency range. Ideally a receiver that can receive Single Sideband (or SSB) transmissions is even better. The reason is the following. As you gradually tune in a radio to a transmitted signal all you will hear a change in the background noise which may either increase or decrease. If your radio has a signal strength meter this will indicate when the signal is tuned in.
But neither of these are usually particularly sensitive techniques for listening to meteors. Instead if you could simultaneously inject into the radio another signal that is on virtually the same frequency as the one you are trying to receive when there is a radar return from a meteor the two signals will mix together and what you will hear is a tone representing the difference between the two frequencies. So with no received echo from a meteor there is no signal to mix and therefore no tone, but with a meteor echo the two signals mix and you will hear a tone. This is a much more sensitive way to listen for echoes.
Furthermore, because the meteor and its ionised trail are moving, the incoming echo will be Doppler shifted and so the frequency of the tone will vary, and that why you can hear the haunting and weird changes in the tones of the echoes on this podcast.
Fortunately if you invest in a communications receiver you can select what is known as USB or LSB reception and it will automatically inject a signal into the receiver in precisely the way that you want.
So there we have it. You just need a simple aerial, a decent radio receiver, tune it to a distant radio station and wait. For most of the year, with patience, you should hear a meteor ping every hour or so, but during the meteor showers that occur throughout the year, the rate of pings will increase to a few every minute. But you will have to listen carefully since each individual meteor ping will only last anything from a fraction of a second up to a maximum of a few seconds in length depending primarily on the size of the meteor.
But if you don’t have the patience to sit and listen to the radio in the hope of hearing a ping that you might miss because your phone rings just at the wrong moment, you can record the output of the radio on a PC using any one of a number of recording applications. If you record for a period of few hours (preferably around dawn or during a meteor shower) and then visually scan through the display of the recording on your pc you will be able to see where the signal suddenly increases. This is much faster than listening to the recorded signal. You will quickly learn to discriminate between bursts of interference from heating thermostats, light switches and so on and real meteor pings.
And if you want to go one stage further you can install spectrum analyser software onto your pc which will allow you to visually display the output from your receiver and gives a much clearer way of identifying (and automatically recording) the individual echoes. A favourite download is the excellent free software called “Spectrum Lab”. Google spectrum lab or DL4YHF to find it.
So there you have it. Next time cloudy skies prevent you from observing the latest meteor shower go dig out a suitable radio and have a go. You will soon be hearing echoes like these.
I’m Steve Carter. Thanks for listening.
End of podcast:
365 Days of Astronomy
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