Determining the speed of a meteor using the phase of the Fourier spectrum : application to BRAMS data

BRAMS (Belgian RAdio Meteor Stations) is a network using forward scatter of radio waves on ionized meteor trails to detect and characterize meteoroids. It is made of a dedicated transmitter and of 50 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. One of the receiving stations is an interferometer using the Jones configuration.Since there is no information available on the range traveled by the radio wave, the only data that can be used to reconstruct the meteoroid trajectory and speed is the time delays measured between the time t0 of appearance of the meteor echo at various receiving stations (assuming specularity of the reflection).  The problem is ill-posed since a small error on the measurement of t0  (of the order of a few ms) can lead to a strong error on the reconstruction (Balis et al., 2023). Therefore there is a need for additional constraints that can come e.g. from pre-t0 phase measurements at some of the stations. Indeed, in Balis et al. (2025), it is shown that these measurements systematically improve the reconstruction, sometimes by an order of magnitude.Here, we study another possibility to constrain the speed of the meteoroid by looking at phases computed using the Fourier transform (FT) of the meteor echo. This method was proposed for back scattering meteor radars by Korotyshkin (2024) but is applied here for the first time to a forward scatter system using BRAMS data. The method will be explained in detail, emphasizing the modifications we introduce due to the forward scatter geometry on one hand, and the fact that BRAMS transmits a CW signal instead of pulses on the other hand. The advantage of the method is to increase the signal-to-noise ratio by combining samples from the entire meteor echo. Hence, in principle, it can be applied to fainter meteor echoes. We present preliminary tests on BRAMS data with the aim in the future to incorporate the constraints put on the meteoroid speed into our trajectory solver.References :Balis, J. et al., Reconstructing Meteoroid Trajectories Using Forward Scatter Radio Observations From the BRAMS Network, Radio Science, 58, 2023.  https://doi.org/10.1029/2023RS007697 Balis, J. et al., Enhanced meteoroid trajectory and speed reconstruction using a forward scatter radio network: pre-t0 phase technique and uncertainty analysis, submitted to Radio Science, 2025 Korotyshkin, D., Radio meteor velocity estimation based on the Fourier transform, Advances in Space Research 74, 4134-4145, 2024. https://doi.org/10.1016/j.asr.2024.06.080