TUTORIAL BRAMS ZOO
What is a meteoroid?
A meteoroid is a solid object
moving in interplanetary space,
of a size considerably smaller
than
an
asteroid
and
considerably larger than an
atom
(IAU
definition).
Meteoroids travel around the
Sun in a variety of orbits and
with velocities ranging from ∼ 11
to ∼ 72 km/s. Sometimes they
are in a collision orbit with Earth
and enter our atmosphere. Most
meteoroids are tiny pieces of
dust.
What is a meteor?
A meteor (or "shooting star“) is
the
visible
phenomenon
resulting from the passage of a
meteoroid into the Earth's
atmosphere. It typically occurs
between altitudes of ∼ 120 and
∼ 80 km.
What is a meteorite?
A meteorite is a solid piece of
debris which has survived the
passing of a meteoroid through
the Earth’s atmosphere and fall
on the ground. It is considerably
smaller than the size of the
initial meteoroid. Only large
meteoroids can give rise to
meteorites which are therefore
quite rare.
Ionisation trail
When a meteoroid enters the Earth’s atmosphere,
it also creates a trail of ionisation (made of ions
and electrons) along the trajectory behind it. In
first and good approximation, this trail is more or
less a straight line.
Forward scattering of radio waves
• This trail (red line) can
temporarily reflect a radio
wave (yellow line) sent by a
transmitter on the ground.
• If a receiver is tuned to the
frequency of the transmitter, it
can receive a signal with a
duration lasting from a fraction
of a second up to a few
seconds : we talk about
meteor echo.
• Forward scatter means that
the receiver is not located at
the same place as the
transmitter.
Forward scattering of radio waves
• The duration of the meteor
echo is roughly dependent on
the size of the meteoroid : the
bigger the meteoroid, the
longer the reflected signal.
• Most meteor echoes last only a
fraction of a second.
• The analysis of the signal can
provide a great deal of
information on the meteoroid
such as mass, speed and
trajectory.
The BRAMS project
BRAMS (Belgian RAdio Meteor Stations) is a project of the
Belgian Institute for Space Aeronomy (BISA) using forward
scattering of radio waves off meteor ionisation trails to
detect and study the meteoroid population above
Belgium and surroundings.
The BRAMS project
BRAMS comprises 1 dedicated transmitter and ∼ 30 receiving stations spread all over
the Belgian territory (blue dots). Each receiving station uses a 3 elements Yagi antenna
and a commercial ICOM receiver.
TX
• 49.97 MHz
• 150 Watts
• cw wave
+
BRAMS data : spectrograms
BRAMS data are usually presented under the form of a spectrogram which provides
the frequency content of the received signal as a function of time.
A typical spectrogram is shown below : frequency is along vertical axis and spans 200
Hz while time is along horizontal axis and spans 5 minutes. Power of the signal is color
coded. Red means very large power. Blue is noise.
The horizontal signal is the direct signal coming from the transmitter. The
spectrogram is built in a way that the 200 Hz range is centered on this
signal.
The long duration signal is a reflection of the radio wave on an airplane
flying above Belgium.
The short-lived signals are meteor echoes. They appear mostly vertical.
They are due to tiny dust particles and make the bulk of meteor echoes
detected by BRAMS. A few examples are shown below inside the red
circles. Some meteor echoes are bright, others are faint. Note that the
signal can be discontinuous but if it is along the same vertical line, consider
it as a single meteor echo.
Meteor echoes can also have a longer duration. They are produced by
larger meteoroids. Difficulty is that their shape in spectrograms can be very
complex and take multiple forms. Examples are given below and in the
next 2 slides.
Parasitic signals can also appear in spectrograms. They can be of artificial
(e.g. an on-off switch) or natural origin (e.g. solar flares).
They can be easily discriminated from meteor echoes as they span the
whole 200 Hz range in the spectrogram while meteor echoes only span a
fraction of it. Below is an example of a very bright parasitic signal.
Problem when many airplane echoes ….
The spectrogram becomes « crowded » with many echoes superimposed
Airplane echoes can be complex too
Echo due to a plane changing direction. Note that the signal can be
discontinuous (sometimes the power received is below the noise level)
What do we expect from you?
Draw rectangles around potential meteor echoes
How can you do that?
1. Go to http://brams.aeronomie.be/zoo
2. Click on the button « Start counting »
3. Register first then log in
4. Start counting. NB : so far only spectrograms from the station
BEOTTI from 00H00 to 01H00 UT on 15/03/2015 are available
First reset the zoom of your browser
For example, on Firefox, press CTRL + 0 or go to View > Zoom > Reset
Click and drag to draw a rectangle around a meteor echo.
Try to draw the smallest possible rectangle and try to
include the whole meteor echo. Do not worry if the rectangle
also includes e.g. a plane echo.
Double click in the rectangle if you made
a mistake and want to remove it
What to do when two meteor echoes are very
close to each other?
• When there is a connection between the
« two » meteor echoes, draw only one
rectangle around the whole echo.
• In case of doubt, consider only one
meteor echo. Nobody knows for sure if
two meteor echoes are overlapping or if it
is a single but complex meteor echo.
How do I move to next/previous spectrogram?
Use the two yellow arrows on each side of the spectrogram
How will this help us?
1. For the BRAMS zoo project, we need to estimate the
number of times a given spectrogram must be counted
by participants. Read more details at
brams.aeronomie.be/zoo
2. Manual counts will also be used as test cases to assess
the automatic detection algorithms that we develop