Reconstructing meteoroid trajectories using forward scatter radio observations and the interferometer from the BRAMS network

When meteoroids hit Earth's atmosphere molecules, they leave a trail of plasma behind. This region, composed of free electrons and positively charged ions, is capable of reflecting radio signals. The analysis of such signals along the meteoroid path can be used for various scientific purposes: quantification of the electron line density, analysis of the thermosphere properties, characterization of the meteor ablation process, etc. To achieve these objectives, the meteoroid trajectory needs first to be determined. The reflection on the plasma trails is usually assumed to be specular, which means that the radio wave is reflected only at a given point along the meteoroid trajectory. For forward scatter systems, the position of this specular point depends on the trajectory on the one hand, and on the position of both the emitter and the receiver on the other hand. Using non-collocated receivers, one obtains several specular points along the trajectory. The receivers will thus detect the reflected signal at different time instants on a given trajectory. In this work, we propose a method that aims at reconstructing meteoroid trajectories using only the time differences of the meteor echoes measured at the receivers of a forward scatter radio system, such as the BRAMS (Belgian RAdio Meteor Stations) network. The latter uses the forward scatter of radio waves on ionized meteor trails to study meteoroids falling in the Earth's atmosphere. It is made of a dedicated transmitter and 42 receiving stations located in and nearby Belgium. Given that all the BRAMS receivers are synchronized using GPS clocks, we can compute the time differences of the meteor echoes and use them to find the meteoroid trajectory. Assuming a constant speed motion, the position (three degrees of freedom) and the three velocity components have to be determined. This inverse problem is non-linear and requires the definition of a target objective to minimize. Two different formulations are compared: the first one is based on the minimization of the bistatic range while the second one uses a forward model, which defines the trajectory as being tangential to a family of ellipsoids whose loci are the emitter and each receiver. A Monte-Carlo analysis is performed to highlight the sensitivity of the output trajectory parameters to the input time differences. The BRAMS network also includes an interferometer in Humain (south of Belgium). Unlike the other receiving stations, it uses 5 antennas in the so-called Jones configuration (Jones et al., 1998; Lamy et al., 2018) and allows to determine the direction of arrival of the meteor echo to within approximately 1°. In that case, the problem becomes much easier to solve because the interferometer gives information about the direction of a reflection point. The benefits brought by such a system regarding the accuracy of the trajectory reconstruction are highlighted. The post-processing steps allowing to extract meteor echoes from the raw radio signals are described. An approach to properly filter out the direct beacon signal is introduced. Indeed, each receiver detects a more or less strong direct signal coming from the transmitter. This signal does not contain any information about the meteor path since it simply propagates through the atmosphere and is not reflected on the meteor trail. Knowing that the BRAMS transmitter emits a continuous cosine wave, the amplitude, the frequency and the phase are fitted in the frequency domain. The beacon signal is finally reconstructed in the time domain and subtracted. This process in illustrated in the following figure, which shows an example of spectrograms (i.e. time-frequency maps where the power is color-coded) before and after the beacon signal subtraction. The proper removal of the horizontal line at around 1005 Hz (corresponding to the direct signal) is apparent in the bottom spectrogram. Afterwards, a bandpass filter is necessary to fully exploit the echoes of the detected meteors. Indeed, the raw signal at the time of the meteor echo is noisy and can have interfering signals caused by the reflections on aircrafts. If the latter are at slightly different frequencies than the meteor echo, they produce interference beats. A windowed-sinc filter Blackman filter of high order is therefore used to remove the signal components at frequencies where the meteor echo does not appear. The time corresponding to half-peak power in the rising edge of the echo (which marks the passage of the meteoroid at the specular reflection point) is finally retrieved and the time differences are computed. To analyze the accuracy of the trajectory reconstructions, data from the optical CAMS-BeNeLux network are used. Promising results showing the reconstructed position, velocity and inclination of several meteoroid trajectories with and without the interferometer are discussed. In the following figure, an example of CAMS trajectory reconstruction obtained with our post-processing is shown. The blue line corresponds to the trajectory determined with the CAMS network, while the purple line is obtained through our analysis of the radio signals obtained at the BRAMS receivers. The reconstructed trajectory using the time differences only (method 1) is shown on the left. The trajectory obtained thanks to the combination of time differences and interferometric data (method 2) is given on the right

Reconstructing meteoroid trajectories using forward scatter radio observations from the BRAMS network

peer reviewedIn this paper, we aim to reconstruct meteoroid trajectories using a forward scatter radio system transmitting a continuous wave (CW) with no modulation. To do so, we use the meteor echoes recorded at the receivers of the BRAMS (Belgian RAdio Meteor Stations) network. This system consists, at the time of writing, of a dedicated transmitter and 44 receiving stations located in and nearby Belgium, all synchronized using GPS clocks. Our approach processes the meteor echoes at the BRAMS receivers and uses the time delays as inputs to a nonlinear optimization solver. We compare the quality of our reconstructions with and without interferometric data to the trajectories given by the optical CAMS (Cameras for Allsky Meteor Surveillance) network in Benelux. We show that the general CW forward scatter trajectory reconstruction problem can be solved, but we highlight its strong ill-conditioning. With interferometry, this high sensitivity to the inputs is alleviated and the reconstructed trajectories are in good agreement with optical ones, displaying an uncertainty smaller than 10% on the velocity and 2° on the inclination for most cases. To increase accuracy, the trajectory reconstruction with time delays only should be complemented by information about the signal phase. The use of at least one interferometer makes the problem much easier to solve and greatly improves the accuracy of the retrieved velocities and inclinations. Increasing the number of receiving stations also enhances the quality of the reconstructions

Analyse d'un projet de formation professionnelle dans le domaine des métiers verts

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La léthargie chez les Chiroptères cavernicoles d\u27Afrique Centrale

From his personal research undertaken in the subterranean field (natural and artificial cavities) in Shaba (ex-Katanga, in S.E. Zaire) and Rwanda, the author briefly defines the macroclimate of the prospected regions as well as the microclimate of the subterranean habitat (humidity and temperature). A reversible hypothermia has been noticed in the dry season only (from May till August) in eleven species of troglophile Chiroptera belonging to the following families: Rhinolophidae (7 species of Rhinolophus), Hipposideridae (only Hipposideros ruber) and Vespertilionidae (Miniopterus inflatus rufus, Miniopterus schreibersi arenarius & M.s. natalensis, Myotis tricolor). No sign of lethargy has been noticed in the Megachiroptera (Lissonycteris angolensis, Rousettus aegyptiacus leachi), Emballonuridae (Taphozous perforatus sudani), Hipposideridae (Cloeotis percivali australis) or Nycteridae (3 species of Nycteris). There could be correlations between lethargy and breeding if one takes into account the phenomena of late ovulation and delayed implantation. The entry into lethargy is not caused by the scarcity of food. It does not concern all the individuals of a colony or in various populations of a cave. The degree of humidity appears to be more important than the temperature as far as the conditions for hibernation are concerned

Modélisation socio-économique de la place Charles II

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Reconstructing Meteoroid Trajectories Using Forward Scatter Radio Observations from the BRAMS Network

When meteoroids hit Earth’s atmosphere molecules, a trail of plasma located downstream of the meteoroid is created. This region, composed of free electrons and positively charged ions, is capable of reflecting radio signals. The reflection on the plasma trails is usually assumed to be specular, which means that the radio wave is reflected only at a given point along the meteoroid trajectory. For forward scatter systems, the position of this specular point depends on the trajectory on the one hand, and on the position of both the emitter and the receiver on the other hand. Using non-collocated receivers, one obtains several specular points along the trajectory. The receivers will thus detect the reflected signal at different time instants on a given trajectory. In this work, we introduce a method that aims at reconstructing meteoroid trajectories using only the time differences of the meteor echoes measured at the receivers of a forward scatter radio system. Assuming a constant speed motion, the position (three degrees of freedom) and the three velocity components have to be determined. Two alternative formulations to solve this complex problem through non-linear optimization are compared. The first one is based on the minimization of the bistatic range, while the second looks for the tangent line to several ellipsoids. A Monte-Carlo analysis is performed to highlight the sensitivity of the output trajectory parameters to the input time differences. The application of this method to actual radio observations from the BRAMS (Belgian RAdio Meteor Stations) network is also presented. The post-processing steps allowing to extract meteor echoes from the raw radio signals are described. For comparison about the quality of trajectory reconstruction, data from the optical CAMS-BeNeLux network are used. Promising results showing the reconstructed position, velocity and inclination of several meteoroid trajectories are discussed

Evaluation d'une action de formation dans le cadre de la création d'une agence d'emploi pour personnes handicapées

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Evaluation des agences immobilières sociales

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Phase-enhanced trajectory and speed reconstruction of meteoroids using BRAMS data

In this project, we aim to reconstruct meteoroid trajectories thanks to a forward scatter radio system using a pure continuous wave (CW) transmitted signal with no modulation. To do so, we use the meteor echoes recorded at the receivers of the BRAMS (Belgian RAdio Meteor Stations) network. The latter is made of a dedicated transmitter and currently 48 receiving stations located in and nearby Belgium, all synchronized using GPS clocks. Our approach processes the signals recorded at the BRAMS receivers and uses the time delays between the meteor echoes as inputs to a nonlinear optimization solver. We compare the quality of our reconstructions data to the trajectories given by the optical CAMS (Cameras for Allsky Meteor Surveillance) network in Benelux. To do so, we solve the general CW forward scatter trajectory reconstruction problem, but we highlight its strong ill-conditioning if the only inputs are the time delays of the echoes at the receivers. To obtain a better accuracy for a large number of meteoroids, the time delays are complemented by information about the signal phase. The approach used for this work is based on the pre-t0 phase technique introduced for backscatter radars. In this project, we extend and adapt the method to forward scatter systems and we illustrate the improved accuracy that it brings on the meteoroid trajectory and speed reconstruction