Leonids 1999 Observing Campaigns
Radio observation campaign
On 17 November 1999 a team of ESA scientists were to listen to the sound of Leonid meteors as they silently sweep across the night sky.
Instead of using cameras or simply observing with the naked eye, their aim was to try an experiment using new digital signal processing to make audible the impact of the myriads of shooting stars as they hit the Earth's upper atmosphere.
During the nights of 16 - 18 November, as the annual Leonid meteor shower reached its peak, the group from ESA's Solar System Division at ESTEC, was to be glued to a radio receiver and their computer screens.
Listening to these radio signatures of the meteors was one of the popular ways of observing them. All over the world amateurs were to be using radio techniques to observe the Leonids during the forthcoming encounter.
"But we will be trying something different", said fellow team member Trevor Sanderson. "We will detect the minute changes in the signal frequency caused by the motion of the meteor trail in the upper atmosphere, using advanced signal processing techniques," explained Sanderson. "We will also try to turn these small changes into an audible signal so that we can hear a "ping" or some such sound every time a meteor hits the atmosphere."
"The technique employed relies on measuring the frequency of the signal after it has been reflected by the trail of ionisation left by the meteor. The motion of the meteor trail due to the upper atmosphere winds changes the frequency of the reflected signal due to the Doppler effect. The change is so small that we have to use advanced signal processing techniques to detect it."
"We will analyse the signals in real time, and also record them on digital audio tape so that we can analyse them later," said Lebreton. "We're also hoping to put the recording on the Web so that everyone can hear the sound of shooting stars!"
"I got interested in the Leonids after I saw last year's fireballs which came the night before the predicted peak. I was fortunate enough not to trust the predictions and was up early on the morning of 17 November to observe the fireball display in the Dutch sky!" explained Jean-Pierre Lebreton.
"We were inspired by a new method developed by a UK communications expert Peter Martinez. We will be using his software, as well as software developed by the Centro de Observação Astronómica no Algarve in Portugal", said Sanderson.
So why were they going through all this trouble?
"We're hoping to complement the optical observations that are planned by some of our colleagues," said Lebreton. "And we wanted to do something that would be of interest to the public". One of the advantages of radio observing is that meteors can be detected when skies are cloudy or during daylight. Radio observing has some advantages at night, too. The human eye can only see shooting stars brighter than 6th magnitude, but radio methods can detect meteors that are at least 5 times dimmer.
The Doppler method can be tried by anyone with a good shortwave receiver and a PC. Suitable software and a description of the method can be downloaded from the related link to "instructions for Doppler experiment", which includes a description of the method.
This software uses the sound card of the PC to analyse the signal. All that is needed is a connection from the headphone output of the receiver to the PC's sound card input. Download the software, install, read the help file and you are ready to go. Tune to a station around 500 km or so away. Switch to SSB mode, and start the software. All you need now are the Meteors!
A simpler experiment
For this you use your FM receiver with an external aerial. Try to find a station a long way away (that's the difficult bit, as usually a nearby station gets in the way). Under normal circumstances the transmission should be difficult or impossible to detect, but when a meteor intervenes the signal jumps over the horizon and a brief fragment of the transmission can be heard. Depending on the type of transmission, it might sound like a tone, a fragment of music or voice, or simply noise. Contact lasts for as long as the meteor train persists, usually from 100 milliseconds to a few seconds.
For further information visit the "Radio Observations of Meteors" section at the International Meteor Organisation (IMO).
What's so special about this new radio experiment?
To try to illustrate the difference between the two main methods for meteor radio detection - between a simple echo monitoring experiment and one which performs a frequency analysis of the echo - Jean-Pierre Lebreton performed the following simple experiment:
"I recorded on a simple tape recorder the sound of a scooter (motor + horn) approaching and passing by me. I made two similar recordings. The two consecutive recordings have been transformed in a single audio *.wav file."
"I then analysed the wav file with an audio analysis programme and the result of the analysis is shown in the image below. The used software is available on-line and can be accessed through the related link 'Spectrogram Software'.
"The top plot shows the simple time sequence of the recorded sound. The bottom plot shows the spectral analysis of the signal. The motor sounds for the first 4 seconds; then the horn sounds between 4.5 and 8.5 seconds. The scooter passed in front of me at 7 seconds."
"The horn spectrum is rich in harmonics. Note the change in tone (frequency) at about 7 seconds Before then the scooter was approaching me; after that the scooter was going away from me.
"The experiment is repeated in the second part of the Figure.
"You can clearly note that the frequency increases when the source is approaching the listener (me) and the frequency decreases when the source moves away from the listener. Note the big frequency change around the time of the velocity direction change with respect to the listener.
"Measuring the relative change in frequency before and after passing by me, you can deduce the speed of the scooter (try it and you should get about 9 ms-1, assuming a sound speed of 330 ms-1)."
See also 'A novel experiment to demonstrate doppler shifting'.