A time‐frequency method for detecting VHF underdense meteor signals
Underdense meteor echoes observed using very high frequency (VHF) radar can be accurately modeled as a single complex damped sinusoid in additive white Gaussian noise. The normalized damping coefficient is expected to be between 0 and 0.3 for a VHF meteor return based on the modeled ambipolar diffusion rates near 90 km and a radar operating between 30 and 50 MHz. Current meteor echo detection routines operate either in the frequency domain, where it is difficult to detect highly damped signals, or in the time domain, where it is difficult to detect narrow band signals. An added difficulty is that typical approaches require a priori knowledge of the noise variance which can bias the performance of these estimators. In this paper, a time‐frequency waveform detector is proposed to address these problems. By normalizing the signal power and power spectral density in the time and frequency domains, respectively, a detector that is invariant to the noise variance can be implemented. The characteristics of a damped sinusoid in the time and frequency domains are exploited to construct the detector. The threshold for the proposed detector is only a function of of the time series length, and is stable with respect to a range of damping coefficients and noise levels. The time‐frequency waveform detector exhibits superior performance to the conventional energy or power detectors when the signal of interest is highly damped or the noise variance is unknown. A derivation of the time‐frequency waveform detector, comparison with the energy and power detectors and numerical results demonstrating the effectiveness of this detector are presented.
A new method for the measurement of meteor speeds: The pre‐to phase technique
In this paper we describe a new technique for measuring the speeds of individual meteors based upon the variation of the phase received in specular reflections from the meteor train prior to the meteoroid reaching the point of closestapproach to the observation site. The technique is characterized by high precision, high yield, and is less subject to selection effects than the techniques used heretofore. About 75% of observed meteors are able to have a speed determined, compared with only 10% previously. It has application to the measurement of the velocities of meteoroid streams and the distribution of speeds of sporadic meteors. The latter is vital to the modeling of the response of HF/VHF radars to meteor backscatter [Elford, 1964; Thomas et al., 1988], as well as having astronomical implications. Results of observations with our narrow‐beam VHF radar are presented. Data collection carried out during the epoch of the θ Ophiuchid meteor shower, whose radiant passes through the response function of the VHF radar, renders speeds which are in accord with the previously known speed for that shower. The speed distribution of sporadic meteors is determined and compared with the distributions obtained in previous radar and optical programs
High-latitude tidal behavior in the mesosphere and lower thermosphere
Abstract The high-latitude structure of the mean winds and tides is described in this paper using climatologies prepared from radar data during the Atmospheric Tides Middle Atmosphere Program. The monthly evolution of the amplitude and phase of the tides is discussed. Comparison between the southern and northern hemispheres indicate that the diurnal tide is stronger in the southern hemisphere and that the antisymmetric diurnal tidal modes are dominant. The semidiurnal tide is larger than the diurnal tide. The vertical wavelength structure is significantly different between the southern and northern hemisphere. Comparisons with recent tidal models show several discrepancies.