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.
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.
In order to detect both precise peaks of the surface and the bottom, in this study, we separated the sea waveforms into three sub-types, such as extreme shallow-water, shallow-water, deep-water after the land and sea waveform classification. Then an algorithm based on FFT was devised and the results were tested on actual airborne LiDAR measurements from a case study.
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