Sky Sounds

What is a meteoroid?
It is a solid object of natural origin that orbits around the Sun, and which is between 30 microns and one metre in size.

What is a meteor?
It is the phenomenon that occurs when a meteoroid crosses the atmosphere at high speed. It includes the emission of light, heat, sound, ionization, etc. Meteors brighter than the planet Venus are called bolides. Meteors are often referred to in a non-scientific context as "shooting stars".

What is a meteorite?
It is the remains of a meteoroid that overcomes the meteor phase without completely disintegrating in the atmosphere and reaches the ground.

What is a "star shower"?
It is a group of meteors produced by a set of meteoroids with a common origin and very similar orbits.

What is radar?
Radar is an acronym for "radio detection and ranging".It is a system based on the emission and reception of electromagnetic waves that allows the measurement of positions and speeds of static or moving objects.

How can radar be used to detect meteors?
The ionised trail of a meteor reflects the electromagnetic radiation emitted by a radar station. A receiving station can pick up this reflected radiation. This is called a radar echo.

What is the radar light curve of a meteor?
This is a graphical representation of the radar signal level reflected by a meteor wake, as a function of time.
What is a spectrogram?
It is a graphic representation of the level of the radar signal reflected in the meteor wake that is picked up at the receiving station, as a function of time and frequency. In this case, the frequency is on the horizontal axis, the time is on the vertical axis, and the signal level is represented by a colour code.

What is the Doppler effect?
It is the change in the apparent frequency of a wave produced by the relative motion of the source and the observer. In our case, the meteoroid moves at high speed in relation to the radar transmitting and the receiving stations. At the receiving station a frequency variation due to the motion of the meteoroid can sometimes be seen, which can help to calculate its speed and trajectory.

What types of echoes do we detect?
We mainly detect meteor echoes, which are of various types:

Underdense echoes: produced by very small meteors. They last less than a second. The signal rises very quickly and falls approximately exponentially. Their radar light curve has a very characteristic narrow peak shape.

Medium duration overdense echoes: produced by slightly larger meteoroids than the previous ones. They last a few seconds. The rise in the signal is also very fast, but after a while it can rise more. The fall is exponential.

Long duration overdense echoes: produced by larger meteoroids. They last from several tens of seconds to a few minutes. Its light curve shows strong oscillations.

Meteoroid echoes: in some medium and long duration overdense echoes, the echo of the meteoroid producing the wake can be recognized. At the beginning of the light curve a high, short peak appears, and a Doppler effect is seen in the spectrogram.

But it is not all meteor echoes, we also detect other events:

Artificial satellites: their echoes last several seconds, have a large Doppler shift and a not very high signal level.

Lightning: when there is a storm, lightning is detected. They are not reflections of the radar signal, the lightning emits electromagnetic radiation with a high bandwidth and short duration in the VHF range. They are easy to identify and eliminate from our records.

Artificial interference: due to electrical and electronic devices in the surroundings of the receiving antenna.

How big are the meteoroids we detect?
Around 80% are about the size of sand grains, with a mass of a few milligrams. These meteoroids disintegrate completely when they pass through the atmosphere and produce wakes with low levels of ionisation (subdense) and echoes that last less than a second.

Some of them are a little larger, like a pea or a marble. They produce bright fireballs and their wake produces echoes that last for a few seconds.

Far less frequent are the larger ones, with a size of several centimeters and a mass of a few hundred grams. They produce brighter fireballs than the full Moon, and their strongly ionised wakes reflect the radio signal for minutes. They can survive the trip through the atmosphere. Our work can help to find the places where meteorites fall.

How many meteors does a radio station detect?
It depends on the receiver sensitivity and the antenna gain. A station with a low-cost SDR receiver and a home-built Yagi antenna can register more than 200 meteors a day in periods of low activity and exceed 1,000 meteors on the days of maximum heavy showers (Perseids, Geminids, Quadrantids).

What do I need to set up a detection station?
Having a transmitting beacon, such as GRAVES in the Iberian Peninsula, you can set up a meteor detection station with few resources: an antenna that can be manufactured with materials available in any DIY or plumbing shop, a low-cost SDR receiver, a PC that does not need many resources or a Raspberry PI , and the detection software, for which there are several open source and free use options. If you are encouraged to build one and need help, please, feel free to contact us.