<p>BRAMS (Belgian RAdio Meteor Stations) is a network using forward scatter of radio waves on ionized meteor trails to study meteoroids. It is made of a dedicated transmitter and of 42 receiving stations located in or near Belgium. The network started in 2010 but has recently been extended and upgraded.</p> <p>The transmitter emits a circularly polarized CW radio wave with no modulation at a frequency of 49.97 MHz and with a power of 130 W. Each receiving station uses a 3-element zenith pointing Yagi antenna. The first stations used analog ICOM-R75 receivers and a PC. Since 2018, new improved stations have been installed using digital RSP2 receivers, a GPSDO and a Raspberry Pi, providing better dynamic, sensitivity and stability.</p> <p>A vast majority of the meteor echoes detected by BRAMS are specular, which means that most of the power of the meteor echoes comes from a small region along the meteoroid path centered on the specular reflection point, a point which is tangential to a prolate ellipsoid having the transmitter and the receiver as the two foci. This puts important geometrical constraints on whether a specific meteoroid trajectory can be detected or not by a given receiving station since the position of the reflection point must fall within the so-called meteor zone.</p> <p>As a consequence, for meteor showers, the observed activity based on the raw counts of meteor echoes recorded by a BRAMS station is modulated by the position of the radiant throughout the day and does not truly reflect the real activity of the shower.  A possibility to correct these raw counts is to compute the so-called Observability Function (OF) introduced by Hines (1958) and further developed by Verbeeck (1997). This OF contains a geometrical part which provides the location of potentially observable meteor trails at a given moment and for a given station, and another part which takes into account which fraction of these trails will actually be detected by the receiving station.  Indeed, whether a meteor echo will be detected at the station also depends on the sensitivity of the receiving chain, on the power transmitted and on the ionization at the reflection point, the latter depending on the initial mass of the meteoroid.</p> <p>We will describe how the geometrical part of the OF is calculated and will provide results for several receiving stations of the BRAMS network to emphasize the importance of the geometry. We will also describe how we take into account important characteristics of the system to determine the sensitivity of the receiving chain such as the gains of the antenna in the direction of the meteor echoes.  Finally, we will apply the OF to the raw counts of a few main meteor showers (e.g. Perseids, Geminids, Quadrantids) obtained from the Citizen Science project, the Radio Meteor Zoo, that we have developed since 2016 in cooperation with Zooniverse (https://www.radiometeorzoo.be).</p> <p> </p> <p>Hines, C., Can. J. Phys., 36, 117-126, 1958</p> <p>Verbeeck, C., Proceedings of the International Meteor Conference, Apeldoorn, the Netherlands, 122-132, 1996</p>

<p>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.  </p> <p>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. </p> <p>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. </p> <p>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. </p> <p>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. </p> <p>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. </p> <p> <img src="" alt="" width="1047" height="566" /></p> <p>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.  </p> <p>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. </p> <p> </p> <p><img src="" alt="" width="1087" height="668" /></p>

<p>BRAMS (Belgian RAdio Meteor Stations) is a network using forward scatter of radio waves on ionized meteor trails to study meteoroids. It is made of a dedicated transmitter and of 44 receiving stations located in or near Belgium. The transmitter emits a circularly polarized CW radio wave with no modulation at a frequency of 49.97 MHz and with a power of 130 W. Each receiving station uses a 3-element zenith pointing Yagi antenna. The first stations used analog ICOM-R75 receivers and a PC. Since 2018, new improved stations have been installed using digital RSP2 receivers, a GPSDO and a Raspberry Pi, providing better dynamic, sensitivity and stability. <br />Recently, several methods have been developed to reconstruct trajectories from meteor echoes recorded at several BRAMS stations. These methods rely on time delays between meteor echoes, pre-t0 phase measurements, and sometimes information from a radio interferometer, or a combination of all the methods. This has opened the possibility to use the BRAMS network to determine the Mesosphere and Lower Thermosphere (MLT) wind speeds using data coming from a large number of meteor echoes.<br />In this work, we will present the status of the BRAMS network and discuss how BRAMS data can be used to determine MLT wind speeds.  Using a forward scatter system with a very large number of stations allows to increase the number of detections, to increase the altitudinal coverage, and to relax the homogeneity assumption.  Simulations will be considered to estimate the impact of the meteoroid trajectory reconstruction uncertainties (in particular the uncertainty on altitude of the specular reflection point) on the wind speeds retrieval.  We will discuss which temporal and spatial resolutions of the MLT wind field measurements can be achieved.  We will finally discuss several upcoming upgrades of the network and their potential impact on this work.  </p>

In 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.