An Entropy-Based Anti-Noise Method for Reducing Ranging Error in Photon Counting Lidar

Photon counting lidar for long-range detection faces the problem of declining ranging performance caused by background noise. Current anti-noise methods are not robust enough in the case of weak signal and strong background noise, resulting in poor ranging error. In this work, based on the characteristics of the uncertainty of echo signal and noise in photon counting lidar, an entropy-based anti-noise method is proposed to reduce the ranging error under high background noise. Firstly, the photon counting entropy, which is considered as the feature to distinguish signal from noise, is defined to quantify the uncertainty of fluctuation among photon events responding to the Geiger mode avalanche photodiode. Then, the photon counting entropy is combined with a windowing operation to enhance the difference between signal and noise, so as to mitigate the effect of background noise and estimate the time of flight of the laser pulses. Simulation and experimental analysis show that the proposed method improves the anti-noise performance well, and experimental results demonstrate that the proposed method effectively mitigates the effect of background noise to reduce ranging error despite high background noise.

Effects of Vertical Nonlinearity on the Superconducting Gravimeter CT #036 at Ishigakijima, Japan

One of the characteristic features of the gravity recordings produced by the superconducting gravimeter CT #036 at Ishigakijima, Japan, is that it indicates gravity increase when a typhoon (hurricane) approaches the island. Since we are trying to detect small gravity signals associated with the long-term slow slip events in this region, it is very important in the interpretation of the observed data whether such gravity changes are of natural or instrumental origin. In this paper, we investigate whether or not nonlinearity in the sensor of the superconducting gravimeter is responsible for this phenomenon. Here we take the same theoretical approach as taken by Imanishi et al. (2018) which investigated the effect of coupling between horizontal and vertical components of the gravity sensor in order to understand the noise caused by the movements of a nearby VLBI antenna. From theoretical and experimental approaches, we prove that the gravity increase observed by CT #036 at the times of high background noise level can not be explained by instrumental effects such as the nonlinearity in the vertical component or the coupling between horizontal and vertical components of the gravity sensor. This implies that the observed gravity increases are real gravity signals of natural origin.

Caller ID for Risso's and Pacific White-Sided Dolphins

Climate change affects the distributions of marine mammals1, and some temperate water species are spreading northward into the Arctic Ocean2, 3. Tracking expanding species is crucial to conservation efforts and using automatic detectors and classifiers to track the locations of their vocalizations could help. Risso’s (Gg) and Pacific white-sided (Lo) dolphins were documented spreading poleward2 and make very similar sounds, making it difficult for both human analysts and classification algorithms to tell them apart. Variational Mode Decomposition (VMD) has provided both an easier visualization tool4 for human analysts and offers promising capabilities in separating call types of similar spectral and temporal properties. Here we show a new visualization tool and feature extraction technique using VMD that achieves 81.3% accuracy, even when using audio files with faint signals and high background noise levels and without context clues. Because not all dolphins whistle5–7, being able to distinguish between just their pulsed signals is important for tracking them using as many files as possible from under-sampled areas of the ocean. Automating the VMD method and expanding it to other dolphin species that have very similar pulsive signals will lead to a faster understanding of ecosystem dynamics under a changing climate than can currently be achieved.

A new method for estimating the correlation of seismic waveforms based on the NTFT

Summary We propose a new correlation function called the similarity coefficient (SC) based on the normal time-frequency transform (NTFT) to evaluate the similarity between two nonstationary seismic signals as a function of the delay time. The SC is defined in the time-frequency spectrum of the NTFT, and the instantaneous phase and amplitude of each frequency component in a signal are employed to calculate the SC. Our simulation experiments demonstrate that the SC method can effectively recognize similar signals compared to the conventional normalized cross-correlation coefficient (NCC) under high background noise conditions. The SC presents good robustness in identifying similar signals and performs well in the case of an extremely low signal-to-noise ratio (SNR), which makes it suitable for detecting weak seismic signals concealed by noise. As a real application case, we use the SC method to detect quasi-Love (QL) surface waves. QL waves are scattered Love waves and are important indicators for lateral anisotropic gradients in the upper mantle. We detect the QL waves at 21 stations deployed across Japan after the December 23 2004 Mw 8.1 Macquarie earthquake by using the SC method. Obvious QL waves are observed at 19 stations, and we locate the Love-to-Rayleigh scatterers by applying the delay times between the QL and main Love waves. Our results show that the QL wave scatterers were mostly generated in two areas: Mariana subduction and Papua New Guinea. The observations of QL waves suggest the presence of lateral gradients in anisotropy beneath those two areas. The spatial distribution of the 13 scatterers in the Mariana subduction zone agrees well with the Mariana Island Arc, and we infer that the Mariana slab may have melted and coupled with the surrounding mantle at depth.

Rinne Test Results: How Badly Can We Be Mistaken?

Objective To establish the extent to which sound amplitudes delivered by a vibrating tuning fork change around its long axis and to evaluate whether such differences in amplitude might change the results of the Rinne test. Study Design Experimental measurements. Setting Laboratory setting. Methods Setup I: a vibrating tuning fork was handheld and manually rotated around its long axis next to a sound recording device (the simulated ear) in order to record sound amplitude data at a full range of angles relative to the device; files were split into segments in which sound amplitude changed: A (from a maximum to a minimum) and B (from a minimum to a maximum). Setup II: a vibrating tuning fork was machine-rotated, and the angle of rotation, along with the sound amplitude, was automatically recorded through a single full rotation. Results The angles of 0° and 180° (which equate to the established best practice in Rinne testing) were associated with the highest sound amplitudes. All other angles decreased sound amplitude. The greatest decrease in amplitude was recorded at 51° and 130°. This difference ranged from 9.8 to 34.7 dB, depending on the initial amplitude. Conclusion The outcome of a Rinne test can be affected if attention is not paid to the precise angle at which the tuning fork is held relative to the ear. The potential of this effect will be greater when high background noise or patient hearing loss requires that the tuning fork be vigorously excited to obtain high sound amplitudes.

A technical description of the Balloon Lidar Experiment (BOLIDE)

The Balloon Lidar Experiment (BOLIDE) was the first high-power lidar flown and operated successfully on board a balloon platform. As part of the PMC Turbo payload, the instrument acquired high-resolution backscatter profiles of polar mesospheric clouds (PMCs) from an altitude of ∼ 38 km during its maiden ∼ 6 d flight from Esrange, Sweden, to northern Canada in July 2018. We describe the BOLIDE instrument and its development and report on the predicted and actual in-flight performance. Although the instrument suffered from excessively high background noise, we were able to detect PMCs with a volume backscatter coefficient as low as 0.6×10-10 m−1 sr−1 at a vertical resolution of 100 m and a time resolution of 30 s.

A technical description of the Balloon Lidar Experiment BOLIDE

The Balloon Lidar Experiment (BOLIDE) was the first high-power lidar flown and operated successfully onboard a balloon platform. As part of the PMC Turbo payload, the instrument acquired high resolution backscatter profiles of Polar Mesospheric Clouds (PMCs) from an altitude of ∼38 km during its maiden ∼6 day flight from Esrange, Sweden, to Northern Canada in July 2018. We describe the BOLIDE instrument and its development and report on the predicted and actual in-flight performance. Although the instrument suffered from excessively high background noise, we were able to detect PMCs with a volume backscatter coefficient as low as 0.6 × 10−10 m−1 sr−1 at a vertical resolution of 100 m and a time resolution of 30 s.

Decision making in the face of a deadly predator: high-amplitude behavioural thresholds can be adaptive for rainforest crickets under high background noise levels

Many insect families have evolved ears that are adapted to detect ultrasonic calls of bats. The acoustic sensory cues indicating the presence of a bat are then used to initiate bat avoidance behaviours. Background noise, in particular at ultrasonic frequencies, complicates these decisions, since a response to the background may result in costly false alarms. Here, we quantify bat avoidance responses of small rainforest crickets (Gryllidae, Trigoniinae), which live under conditions of high levels of ultrasonic background noise. Their bat avoidance behaviour exhibits markedly higher thresholds than most other studied eared insects. Their responses do not qualitatively differ at suprathreshold amplitudes up to sound pressure levels of 105 dB. Moreover, they also exhibit evasive responses to single, high-frequency events and do not require the repetitive sequence of ultrasonic calls typical for the search phase of bat echolocation calls. Analysis of bat and katydid sound amplitudes and peak frequencies in the crickets' rainforest habitat revealed that the cricket's behavioural threshold would successfully reject the katydid background noise. Using measurements of the crickets' echo target strength for bat predators, we calculated the detection distances for both predators and prey. Despite their high behavioural threshold, the cricket prey still has a significant detection advantage at frequencies between 20 and 40 kHz. The low-amplitude bat calls they ignore are no predation threat because even much louder calls would be detected before the bat would hear the cricket echo. This leaves ample time for evasive actions. Thus, a simple decision criterion based on a high-amplitude behavioural threshold can be adaptive under the high background noise levels in nocturnal rainforests, in avoiding false alarms and only missing detection for bat calls too far away to pose a risk. This article is part of the theme issue ‘Signal detection theory in recognition systems: from evolving models to experimental tests’.