The ocean plays an important role in the global carbon cycle. Carbon is transmitted along the food chain in the form of particulate organic carbon (POC), starting from the photosynthesis of phytoplankton. Therefore, detecting the content of POC in the ocean plays a significant role in estimating the carbon sink capacity of the ocean. With the help of LIDAR detection systems, the profile concentration information of POC in seawater can be detected based on the optical property changes caused by phytoplankton. In order to satisfy the detection requirements of a LIDAR system, a detection capability with sensitivity up to the single photon level, dynamic range of 50dB, and response bandwidth of 180MHz, the echo optical signal detection and processing system is designed combining the analog detection and photon counting detection. By means of theoretical analysis, simulation analysis and experimental analysis, the analog detection channel output signal meets the following ADC acquisition requirements, and the photon counting channel can accurately discriminate the input pulse signal accurately. This echo optical signal detection and processing system is capable of providing a signal bandwidth of 180MHz and a fixed time delay without distortion, oscillation or pulse width broadening.
Laser communication technology has garnered considerable attention in recent times due to its advantageous features such as high security, wide bandwidth, and high transmission rate. Its potential for long-distance atmosphere-sea information transmission is particularly promising. In this study, we developed a laser communication system that utilized a high-altitude aircraft and an underwater platform, enabling communication distances of up to 10 km in the air and 100 m underwater. The system incorporates a high-power blue-green laser, delivering an energy of 80 mJ per laser pulse with a repetition frequency of 100 Hz. During experimental investigations, we found that the Doppler shift effect becomes more pronounced with communication time. This phenomenon becomes more pronounced when transmitting large-capacity data, as cumulative frequency errors can lead to pulses appearing in incorrect time slots, resulting in erroneous demodulation of data by the pulse position modulation (PPM) scheme. A time-slot synchronization correction algorithm was proposed for PPM demodulation. This algorithm utilized a broad time-slot modulation technique to ensure that the information pulse is positioned at the center of the time slot. Real-time predictions of clock offset error are made by statistically analyzing the laser pulse position within a given time slot. Subsequently, the clock count is corrected using the obtained error information. The proposed algorithm effectively eliminates erroneous time slots resulting from the accumulation of frequency shift errors, thereby significantly reducing the bit error rate (BER) in the transmission of large-capacity data over long distances through atmosphere-sea laser communication.
KEYWORDS: Submerged target modeling, Photons, LIDAR, Point clouds, Water, Optical simulations, Monte Carlo methods, Target detection, Refraction, 3D modeling
Airborne Lidar Bathymetry measures ocean depth by transmitting 532nm wavelength lasers and recording the return energy and moment of the laser echo, and is an important means of acquiring the topography of coastal zones and shallow seas. Since underwater targets can also reflect the emitted laser, bathymetric lidar holds promise for underwater target detection. However, most of the existing echo simulation models for laser underwater transmission are for full field-of-view (FOV) and flat terrain, but there are few echo simulation models for hovering targets with sizes smaller than the FOV. Therefore, we combined the ray tracing and semi-analytical Montecarlo methods to simulate the laser echo energy of underwater targets. The model combined the speed advantage of the semi-analytical Montecarlo method with the ability to simulate echoes of underwater targets of arbitrary shapes. In order to simulate bathymetric lidar echoes under real operating conditions, the model also included simulations of the scanning mode and surface waves. The trend of the echo energy of an underwater target within the field of view with the deviation distance from the line-of-sight axis was investigated, and the results showed that even if the underwater target is deviated from the main axis, there is still an obvious target echo due to the scattering of the laser in the water, and the echo energy is weakened with the increase of the depth. The waves can make the ideal refraction direction change, which has a great impact on the quality of the scanned point cloud.
KEYWORDS: LIDAR, Data compression, Clouds, Data storage, Image compression, Field programmable gate arrays, Data transmission, Algorithms, Airborne remote sensing
Due to the high detection sensitivity and efficiency, single photon time-of-flight distance ranging Lidar have emerged in rapid, large scale, high resolution, topographic mapping in recent years. We designed an airborne mapping Lidar system based on arrays of Geiger-mode Avalanche Photodiode (GmAPD) detectors capable of detecting a single photon, and the area coverage rates in excess of 122 km2 /hr. The Lidar system uses an eye-safe, low-power fiber laser and a 64×64 pixel GmAPD arrays capable of readout rates in excess of 20 KHz. Although large-pixel-format GmAPD detector array can obtain plenty laser point cloud in one frame, but the requirement of raw data transmission and storage rate would be particular high when the detect repetition frequency reach at 20 KHz or even more. Therefore, we proposed a lossy realtime data compression algorithm which can reduce a half of data transmission and storage rate so that the data can transmission through low bandwidth situation. By analysis the quality of point cloud, this lossy real-time data compression method had been validated works well during our airborne experiment which had carried out in Wuhu city, China. For a single mapping strip, the point cloud had a mean measurement density greater than 110 points per square meter in flat topography and 75 points per square meter in rough topography when the aircraft above ground levels (AGLs) was 1 km and velocity was 220 km/h.
Single-photon counting Lidar (SPL) systems using Geiger mode Avalanche Photo Diode (Gm-APD) arrays are more sensitive than traditional linear mode Lidar systems and are capable to detect the sparse target-returned photon less than single photon. However the single-photon sensitivity of SPL system also make it susceptible to solar background light which is a major noise source of SPL data. The relatively high noise level of SPL systems poses a significant challenges to the noise filter processing of measured data. In this paper, two image based noise filtering methods: K-Nearest- Neighbor (KNN) filtering method and Single Frame Histogram (SFH) filtering method were proposed, to reduce the noise points in Gm-APD array Lidar data. In these methods, noise points were removed through raw image data processing. We count the number of corresponding time of flight data points in single frame image and remove the noise points from signal through a predefined threshold. The noise filtering results of the two proposed methods were analyzed and compared based on raw data obtained from our 64×64 Gm-APD array Lidar imaging experiment. The noise filtered image results show that more than 90% of the noise points in single frame data has been removed. Finally, the noise filtered image data was further processed to get the cleaned 3D images. The results indicates that the proposed imagebase noise filtering methods is suitable for the noise reduction processing of our (Gm-APD) array Lidar data.
The detection of ocean profile optical parameters is of great significance to ocean science. The existing ocean passive remote sensing systems mainly focus on the detection of surface parameters. If we want to obtain the ocean profile parameters in a big area, active remote sensing technology is necessary to adopted. The difficulty of ocean active remote sensing detection system lies in the strong reflection of the sea surface, and rapid attenuation of the laser energy under the surface. Therefore, the dynamic range of the signal can reach nearly five orders of magnitude. It’s a great challenge for the performance of the detection system that undertakes the role of photo-electric conversion. Only by fully simulating the performance of the detection system can we obtain the corresponding relationship between the returned optical signal and the output electrical signal, and ensure the reliability of the subsequent signal processing. In this paper, Hamamatsu multipixel Photon Counter is selected as the detector, and the return photo profile data is provided by Ocean University of China. The photoelectric conversion process is simulated both in digital and analog mode. Meanwhile, the influences of background light, after pulse, accumulation times, laser wavelength and detection area are also simulated and analyzed. The simulation results are in good agreement with the trend of the theoretical curve. The simulation system provides a basis for accurately evaluating the photoelectric conversion process of Ocean Lidar, and guarantees the authenticity of the subsequent signal processing system.
To quantitatively analyze the influence of vehicle platform vibration on the vehicle laser radar (LIDAR) measurement accuracy, the modal, harmonic response, and random vibration are analyzed using finite-element software. On this basis, combined with the optical path structure of LIDAR, the deformation of mirror caused by vibration is analyzed. The results show that the maximum deflection of the laser output angle resulting from the mirror deformation is about 0.0001 deg, which is much less than the 0.00573 deg (0.1 mrad) that is required by the performance of the vehicle LIDAR.
The vehicle LIDAR has been widely used in the fields of three-dimensional city modeling, terrain mapping, the agroforestry ecosystem, military detection, and unmanned driving for its advantages of high resolution, strong anti-interference ability, small volume, and light weight. The measurement accuracy of vehicle LIDARs is influenced by the internal heat source of the system and the external ambient temperature to a great extent. A compact vehicle LIDAR system is designed and analyzed. Combined with the distribution characteristics of the internal heat source of the LIDAR system, the temperature distribution and thermal deformation in the LIDAR system under different external temperatures are analyzed. On this basis, the laser path deviation of the LIDAR system caused by thermal deformation is also analyzed and compared. Finally, when the ambient temperature changes from −10 ° C to 50°C, the deviation of the laser output angle is within ±0.1 mrad by optimizing the design, which meets the requirements of the measurement accuracy of the vehicle’s LIDAR.
A miniaturized underwater polarized radiation measuring instrument (MUPRMI) LiDAR system applied in detecting the polarization optical parameter profiles for shallow water has been designed. This system will be used for detecting the depolarization of laser propagating underwater. For that purpose, a 532 nm linearly polarized laser with the repetition rate of 100 Hz and per pulse of energy of 50 μJ will be used in the system. When propagating underwater, the polarization state of laser will be changed in case of collision with the particles suspended in water. The linearly polarized laser will gradually become non-polarized due to depolarization, and the depolarization degree is related to the suspended particles. In order to detect the depolarization effect of waters, two orthogonal polarization receiving channels have been assembled in the MUPRMI system. For signal receiving, a photomultiplier tube has been assembled in each of the channels. By detecting the change of polarization state, parameters of scattering particles suspended in water in the detecting area can be inverted using inversion algorithm. The MUPRMI system can be controlled by a host computer, which communicate with the MUPRMI system using ethernet communication protocol. An adjustable aperture driven by a stepping motor has been assembled in the receiving optical path. Using this adjustable diaphragm, we can control the change of receiving field of view by transmitting instructions from the host computer, and change the reception of signals from different kind waters. A ship-borne experiments have been conducted in South China Sea, results show that the deepest bathymetry of the MUPRMI system is about 9 meters with the pulse energy of 50 μJ, in South China Sea.
The airborne lidar with two segmented field-of-view (FOV) receivers was used to detect the subsurface scattering layers. Significant differences were observed in the waveforms from one channel with small FOV of 6 mrad and the other channel with larger FOV of 40-6 mrad. The larger FOV of 40-6 mrad was to provide a larger dynamic range for the deep-water signal detection. A small-angle approximation based Lidar waveform simulation model was developed, and found that these differences are owing to the narrow beam divergence of laser pulse of only 0.3 mrad. Next, an algorithm, which incorporates a waveform-decomposition technique and a lowpass digital differentiator, was then used to detect the scattering layers from both small- and large- FOV channels. The observation of scattering layer along the coastal region of Sanya Bay of China shows that, more than three thin scattering layers can be found in the same water column close to the coasts, and the maximum depth of the scattering layer detected by the large FOV Channel can be up to 35m, and internal waves can be detected from spatial distributions of scattering layer. It can be found that the airborne bathymetry lidar with segmented field-of-view receivers can also be a great tool for the subsurface scattering layer detection.
Abstract: The measurement of ocean optical parameters is an important part of ocean research. According to the transmission of the blue and green laser pulse, the lidar return signals were analyzed, and the vertical profiles of the lidar attenuation coefficient were studied with airborne polarization lidar. Simultaneously, the absorption coefficient and extinction coefficient of South China Sea were measured using AC-S. Comparing the lidar and in situ measurements, we found that the lidar attenuation coefficient is between the absorption and extinction coefficient. The correlation analyses of lidar attenuation coefficient with absorption and extinction coefficient were carried out respectively, and it was shown that they have a good correlation. Overall, the results indicated that the airborne polarization lidar is an efficient way to detect the profiles of ocean and the combination of airborne lidar and in situ measurements provide comparable and complementary information about ocean optical parameters.
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