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 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. Three examples are shown below. Some meteor echoes are bright, others are faint.

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 2.Register then log in 3.Go to « My countings » at Select one of the two sets of data (station BEOTTI from 00H00 to 01H00 UT on 15/03/2015 or station BEHUMA from 08H00 to 09H00 UT on 07/10/2011) by clicking on the campaign name.

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 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. This test will help us evaluate this number using two sets of data (an « easy » one with barely no plane echoes & a « complicated » one with many plane echoes) and various test samples. 2.Manual counts will be used as test cases to assess the automatic detection algorithms that we develop