Sky Background Light Research Articles

An EEMD-SVD method based on gray wolf optimization algorithm for lidar signal noise reduction

ABSTRACT Atmospheric lidar is susceptible to light attenuation, sky background light and detector dark current during detection, which results in a lot of noise in the lidar return signal. In order to improve the SNR and extract useful signals, this paper proposes a new joint denoising method EEMD-GWO-SVD, which includes empirical mode decomposition (EEMD), grey wolf optimization (GWO) and singular value decomposition (SVD). Firstly, the grey wolf optimization algorithm was used to optimize two parameters of EEMD algorithm according to moderate values: the standard deviation Nstd of adding Gaussian white noise to the signal and the number NE of adding Gaussian white noise. Secondly, the mode components obtained by EEMD-GWO decomposition are screened and reconstructed according to the correlation coefficient method. Finally, the SVD algorithm with strong noise reduction ability was used to further remove the noise in the reconstructed signal, and the lidar return signal with high SNR was obtained. In order to verify the effectiveness of the proposed method, the proposed method was compared with empirical mode decomposition (EMD), complete ensemble empirical modal decomposition (CEEMDAN), wavelet packet decomposition and EEMD-SVD-lifting wavelet transform (EEMD-SVD-LWT). The results show that the noise reduction effect of the proposed method was better than that of the other four methods. This method can eliminate the complex noise in the lidar return signal while retaining all the details of the signal. In fact, the denoised signal is not distorted, the waveform is smooth, the far-field noise interference can be suppressed and the denoised signal is closer to the real signal with higher accuracy, which indicates the feasibility and practicability of the proposed method.

New-Type Shearing Interference Detection System Based on Double-Grating Structure for Suppressing the Skylight Background.

To overcome the influence of the daytime skylight background on long-distance optical detection, a new type of shearing interference detection system was proposed to improve the detection performance of the traditional detection system for finding dark objects such as dim stars during the daytime. This article focuses on the basic principle and mathematical model as well as the simulation and experimental research of the new type of shearing interference detection system. The comparison of the detection performance between this new-type detection system and the traditional system is also carried out in this article. The experimental results show that the detection performance of the new type of shearing interference detection system is significantly better than that of the traditional system, and the image signal-to-noise ratio of this new-type system (about 13.2) is much higher than that of the best result of the traditional detection system (about 5.1).

Automatic Inhomogeneous Background Correction for Spatial Target Detection Image Based on Partition Processing

High-resolution imaging with wide field of view (FoV) ground-based telescopes is often affected by skylight background and noise due to the detector, resulting in an inhomogeneous background. In this paper, we propose an improved method for spatial image non-uniformity correction based on partition processing. First, an evaluation metric is introduced to evaluate the partition size and automatically iterate a suitable partition value for different scenarios based on the different operating conditions of the telescope. Then, we iteratively calculate the mean and variance in each partitioned region to extract the background of each partitioned region. Finally, after applying bilinear interpolation to the background extracted from each region, the inhomogeneous background is obtained and removed from the original image. The experiments on the simulated and real images show that the proposed method can effectively remove the inhomogeneous background of spatial images and meet the requirements of the real-time processing of high-resolution images under long exposure conditions.

A Novel Lidar Signal-Denoising Algorithm Based on Sparrow Search Algorithm for Optimal Variational Modal Decomposition

Atmospheric lidar is susceptible to the influence of light attenuation, sky background light, and detector dark currents during the detection process. This results in a large amount of noise in the lidar return signal. To reduce noise and extract a useful signal, a novel denoising method combined with variational modal decomposition (VMD), the sparrow search algorithm (SSA) and singular value decomposition (SVD) is proposed. The SSA is used to optimize the number of decomposition layers K and the quadratic penalty factor α values of the VMD algorithm. Some intrinsic mode function (IMF) components obtained from the VMD-SSA decomposition are grouped and reconstructed according to the interrelationship number selection criterion. Then, the reconstructed signal is further denoised by combining the strong noise-reduction ability of SVD to obtain a clean lidar return signal. To verify the effectiveness of the VMD-SSA-SVD method, the method is compared and analysed with wavelet packet decomposition, empirical modal decomposition (EMD), ensemble empirical modal decomposition (EEMD), and adaptive noise-complete ensemble empirical modal decomposition (CEEMD), and its noise-reduction effect is considerably improved over that of the other four methods. The method can eliminate the complex noise in the lidar return signal while retaining all the details of the signal. The signal is not distorted, the waveform is smoother, and far-field noise interference can be suppressed. The denoised signal is closer to the real signal with higher accuracy, which shows the feasibility and the practicality of the proposed method.

Polarization reflection distribution characteristics of wakes on the sea surface.

Research on the polarization reflection distribution characteristics of wakes on the sea surface can provide a theoretical basis for ocean wake target detection and has important research value in the field of ship and underwater moving target monitoring. The Kelvin wake model and the Cox-Munk model are used to describe a wake on a rough sea surface. Considering the atmospheric Rayleigh scattering and the reflection characteristics of a rough sea surface, a visible spectrum band wake polarization characteristic model based on the Stokes vector and Mueller matrix is established to explore the polarization reflection distribution characteristics of wakes on the sea surface under skylight background at different wind speeds, wind directions, and sun angles. A simulation is done of the airborne polarization reflection imaging of wakes on a rough sea surface. The results show that under the determined observation angle, the polarization distribution characteristics of wakes on a rough sea surface are mainly related to the angle of the sun. The polarization contrast of simulated wakes in typical scenes is acceptable, and it is feasible to detect sea wake targets by the polarization method. The analysis and simulation of the wake polarization characteristics model can provide a theoretical basis for ocean wake target detection.

Development of the spectrum transmittance measurement system for large-size optical elements of Cherenkov telescope

The Cherenkov and fluorescence telescopes are used to detect cosmic rays in many large-scale experiments. Certain large-size optical elements, such as quartz glasses with window and filters, are installed in front of cameras as the window to seal the cameras and minimize the impact of the night sky background light. The transmittance of those optical elements in Cherenkov and fluorescence telescopes is therefore vital for the measurement of the number of photons reaching the camera. The uniformity is an important parameter of transmittance at different wavelength over the whole window surface. It is difficult to ensure the uniformity of the window material when it is produced, especially for the size as large as 1 m2. The measurement of the uniformity of the transmittance of filters at different positions on the window, and over a large optical spectrum, is not a trivial task and therefore a dedicated system has been developed. The large-size filter transmittance measurement system can measure the spectrum range from 250 nm to 900 nm. A two dimensional motorized device is used to carry the system and the accuracy of the system is up to 0.01 mm. In addition, the measuring system can measure the transmittance at an incident angle ranging from 0° to 44°. The relative standard deviation can be reduced to less than 1% by alternately measuring with and without samples. The transmittance spectra of the neodymium glass standard sheets and quartz glass JGS2, of size 876 mm × 862 mm × 4 mm have been measured with this system and compared. The result of neodymium glass indicates that the system can measure the transmittance spectrum precisely, up to 1% uncertainty. The results of the JGS2 quartz glass demonstrate that our system is capable of measuring the optical parameters of very large sample with a precision at the level of 1% over a wide spectrum range.

Characterizing the aerosol atmosphere above the Observatorio del Roque de los Muchachos by analysing seven years of data taken with an GaAsP HPD-readout, absolutely calibrated elastic LIDAR

ABSTRACT We present a new elastic LIDAR concept, based on a bi-axially mounted Nd:YAG laser and a telescope with HPD readout, combined with fast FADC signal digitization and offline pulse analysis. The LIDAR return signals have been extensively quality checked and absolutely calibrated. We analyse seven years of quasi-continuous LIDAR data taken during those nights when the MAGIC telescopes were operating. Characterization of the nocturnal ground layer yields zenith and azimuth angle dependent aerosol extinction scaleheights for clear nights. We derive aerosol transmission statistics for light emitted from various altitudes throughout the year and separated by seasons. We find further seasonal dependencies of cloud base and top altitudes, but none for the LIDAR ratios of clouds. Finally, the night sky background light is characterized using the LIDAR photon backgrounds.

A fast muon tagger method for Imaging Atmospheric Cherenkov Telescopes

The Cherenkov Telescope Array (CTA) will be the next major observatory for Very High Energy gamma-ray astronomy. Its optical throughput calibration relies on muon Cherenkov rings. This work is aimed at developing a fast and efficient muon tagger at the camera level for the CTA telescopes. A novel technique to tag muons using the capabilities of silicon photomultiplier Compact High-Energy Camera CHEC-S, one of the design options for the camera of the small size telescopes, has been developed, studying and comparing different algorithms such as circle fitting with the Taubin method, machine learning using a neural network and simple pixel counting. Their performance in terms of efficiency and computation speed was investigated using simulations with varying levels of night sky background light. The application of the best performing method to the large size telescope camera has also been studied, to improve the speed of the muon preselection.

Research on the new detection method of suppressing the skylight background based on the shearing interference and the phase modulation.

In order to effectively suppress the effect of the skylight background on the detection performance of the optical telescope and make it have the advantages of the all-time operation and the high star-magnitude detection, a new detection method based on the shearing interference and the phase modulation is proposed in this paper. This method utilizes the discrepancy of the spatial coherence between the target (smaller size) and the skylight background (larger size). By adjusting the shearing amount reasonably, the target light can form the interference fringes, while the skylight background cannot. Then, the phase modulation technique is used to form the moving interference fringes over the detector. Afterward, the periodical electrical signal is formed by the photoelectrical conversion of the detector. Finally, the weak signal detection equipment can be used to extract the target signal from the skylight background noise. This paper introduces the scheme design and the theoretical analysis of the new detection method and also presents the detection performance simulation of this new method.

Field experiment and analysis for free-space laser transmission characteristic in turbulent path based on MWIR and NIR

It is hard to achieve low bit error rate (BER) and high-quality signals in free-space optical (FSO) communication systems due to atmospheric link interference. For the sake of seeking solutions, we proposed an experimental scheme of free-space laser communication based on middle-wave infrared (MWIR) and near infrared (NIR) for obtaining atmospheric transmission characteristic in 1 km turbulence path. Compared with NIR waveband, MWIR-based scheme is more suitable for atmospheric FSO communication since it owns higher anti-interference ability for atmospheric attenuation, turbulence and skylight background noise. The experimental results may offer a significant reference for FSO communication in atmosphere link.

Sodium lidar observation over full diurnal cycles in Beijing, China.

Due to the severe interference from strong solar background light on the received signal, daytime ground-based lidar observation of the sodium (Na) layer is challenging. In this paper, a Na lidar permitting full-diurnal-cycle observation of the metal Na layer over Beijing, China (40.5°N, 116°E) was reported. In order to suppress the skylight background during daytime effectively with less signal losses, a dual-channel Faraday filtering unit was implemented in the lidar receiver. Based on the diurnal Na lidar system, a good number of continuous observational results that lasted more than 120 h with good signal-to-noise ratio were obtained, demonstrating its reliability. Considerable variations within the Na layer during the day were revealed, especially on the layer top and bottom side. In particular, strong sporadic Na layer (Nas) events that occurred during the daytime of 3 consecutive days were also captured with complete evolution process. These observational results showed the advantages of the diurnal Na lidar for investigating the metal layer photochemistry and dynamics in the mesosphere and lower thermosphere region. Na layer observations over the whole diurnal cycle not only benefit the improvements of current theoretical models, but also can allow for a specialized analysis of Nas that occur in the daytime and provide valuable observational support for investigating the rapid production and disappearance mechanisms of Na atoms.

The extraction of spot signal in Shack–Hartmann wavefront sensor based on sparse representation

Several techniques have been used with Shack–Hartmann wavefront sensors to determine the local wave-front gradient across each lenslet. While the centroid error of Shack–Hartmann wavefront sensor is relatively large since the skylight background and the detector noise. In this paper, we introduce a new method based on sparse representation to extract the target signal from the background and the noise. First, an over complete dictionary of the spot signal is constructed based on two-dimensional Gaussian model. Then the Shack–Hartmann image is divided into sub blocks. The corresponding coefficients of each block is computed in the over complete dictionary. Since the coefficients of the noise and the target are large different, then extract the target by setting a threshold to the coefficients. Experimental results show that the target can be well extracted and the deviation, RMS and PV of the centroid are all smaller than the method of subtracting threshold.

Joint observation results of Na layer and ionosphere in Wuhan during the Total Solar Eclipse

During the total solar eclipse on July 22, 2009 in Wuhan, the joint observation test of Na layer and ionosphere was conducted by using the daytime observation atmospheric lidar and the GPS ionosphere detector. The results show that the full width at half maximum (FWHM) of Na layer density slightly narrowed during the total solar eclipse and broadened after the eclipse, while the height of Na peak slightly decreased in the eclipse and increased after the eclipse. These implying that Na layer changes reflect the rapid process of sunrise and sunset. The ionosphere total electron content (TEC) and the sky background light noise also presented an obvious fluctuation characteristic with the changes of solar irradiation during the process of total solar eclipse. The difference lies in that the changes of FWHM of Na layer atoms are much slower than that of ionosphere, the reason for this might be that the Na layer, after being disturbed by the total solar eclipse, will generate a series of complicated photochemical reactions and momentum transport processes, and then recombine the Na atoms. The Na atoms to be detected by the lidar need a lag process, which rightly conforms to the theoretical simulated results.

Simulation, Measurement and Spectral Analysis of Night Sky Light Background

Spectral measurements of the night sky provide important information about natural light sources and light pollution. Analysis of spectral data enables identification of interesting events occurring in the atmosphere. We describe the used method and the technical solution, as well as first obtained results. Photometric and spectral simulations of the night sky light were performed under laboratory conditions. The results were compared with measured data of the whole night sky with respect to location and timing. The outdoor measurements provided spectroscopic data for examining the light pollution and night sky light background.

全固态589 nm激光器及其钠导星激发亮度

A LD side-pumped 589 nm small-scale all-solid-state laser with an intra-cavity sum-frequency structure was developed and the experiments for exciting the brightness of a sodium guide star by the laser were described. The numerical simulation method was employed to analyze and design the cavity length and the position of a sum-frequency crystal, and it shows a smaller size and higher conversion efficiency. A synchronous power system in time domain was developed to compensate the delay between pulses of the two fundamental lasers. Special films were coated on the sides of the splitter mirror to obtain linear polarized laser as well as to increase the conversion efficiency and the thermal stability. Moreover, etalons and special coating methods were employed to suppress the oscillation of 1338 nm spectral line. Finally, a dedicated launch telescope and a receive system were used for the generation and observation of sodium guide star. Test results show that the laser offers an output power more than 8 W, a repetition frequency of 5 kHz and the linewidth less than 3.5 GHz, meanwhile, its conversion efficiency is up to 2% corresponding to the 808 nm pump power. Under the sky background light of Changchun, the brightness of the guide star excited by the laser in winter experiments is about 10 Mv.

Research on the calculation method of sodium laser guide star magnitude

This paper briefly introduces a complete sodium laser guide star experimental system first. A lot of guide star images have been obtained. A method of real time comparison with a known natural star is adopted to calculate the V-band magnitude of the guide star. Grey value image data and photon-count image data are both used in the calculation. Moreover, the photon-count image data is converted to return flux to verify the theoretical magnitude of the known natural star. The influence of a narrow band optical filter on the calculating process is analyzed and emphasized. Finally, a simple discussion is made on the accuracy of the calculation result. The preliminary result shows that, under the condition of sky background light of Changchun city, the brightness of the guide star excited in summer by a 589 nm laser which has an average power of 5.2 W and a bandwidth of 4-5 pm is about 14.05 Mv .

An Analog Level 1 Trigger Prototype for CTA

The Cherenkov Telescope Array (CTA) project aims to build the largest ground-based gamma ray observatory based on an array of Imaging Atmospheric Cherenkov Telescopes. The CTA will implement a multi-level trigger system to distinguish between the gamma ray-like induced showers and the background images induced by night sky background light. The trigger system is based on looking for coincident detections among pixels (level 0 trigger), clusters of pixels (level 1) or telescopes (level 2). In this article one prototype of a Level 1 trigger system is presented, capable of working with different Level 0 strategies and different trigger region sizes. At the same time it complies with all the requirements specified by the CTA project, specially the most critical ones as regards noise, dynamic range, latency and power consumption. All these features make the presented system very suitable to be used by the CTA telescopes.

The implementation of adaptive optics wavefront spot extraction on FPGA

The skylight background and the noise generated by the detection device system in adaptive optical system have a great impact for the spot detecting precision. In order to eliminate their affects, a wavefront spot signal extraction system is designed and implemented in this paper, and the effect of the extraction and the precision are tested in the indoor adaptive optics precision tracking system. The experiment results show that the design is better in extracting the target spot; the centroid accuracy after processed is higher. In the other hand, the processing time delay is small, which can meet the adaptive optics system’s real time requirement.

The Implementation of Adaptive Optics Wavefront Spot Extraction on FPGA

The skylight background and the noise generated by the detection device system in adaptive optical system have a great impact for the spot detecting precision. In order to eliminate their affects, a wavefront spot signal extraction system is designed and implemented in this paper, and the effect of the extraction and the precision are tested in the indoor adaptive optics precision tracking system. The experiment results show that the design is better in extracting the target spot; the centroid accuracy after processed is higher. In the other hand, the processing time delay is small, which can meet the adaptive optics system’s real time requirement.

偏振滤波白天抑制天光背景作用分析

偏振滤波白天抑制天光背景作用分析