Objective   The development of marine ecology, marine science, and marine economy is inseparable from marine observation data, and the requirements for marine data have developed from sea surface data to ocean profile data. Ocean profiling is an important basis and means to study the three-dimensional profile distribution of global ocean optical and biological parameters, as well as ocean carbon and energy cycles. At present, ocean profile elements (temperature, salinity, optical parameters, etc.) mainly rely on in-situ detection and airborne detection, and the lack of global-scale, high-efficiency, and high-precision remote sensing observation methods has become a bottleneck problem for comprehensive understanding and fine perception of the ocean. At present, the traditional multi-payload cooperative observation or satellite-ground linkage observation mode can obtain multi-element information in time and region, but the integrated detection of temperature, salinity and depth in the ocean profile is still blank, and the inversion of non-homologous, non-homogeneous and non-simultaneous detection data of underwater optical biological parameters and physicochemical parameters is consistent and accurate, which cannot effectively obtain the three-dimensional structure of the ocean to meet the needs of scientific research.

  Methods   An active and passive composite optical remote sensing system for multi-element detection of ocean profile is proposed, as shown in Figure 1. Based on the lidar equation calculation, the application indicators of the decomposition system, such as depth, backscatter measurement error, temperature, salinity error, etc., are calculated and calculated, and the energy, line width, optical system aperture, focal length, field of view, detector selection of the optical system of photoelectric reception and detection are confirmed through simulation analysis and calculation, as shown in Table 3. The grating primary mirror realizes the compression of light, and the diameter-thickness ratio of ULE material is 200∶1 to realize the lightweight of the system. According to the incident wavelength, incident angle, grating constant, the acceptable field of view angle of the off-axis three-mirror and the relative position of the grating primary mirror and the off-axis three-mirror primary mirror, the on-orbit deployment accuracy requirements were analyzed. The optical system ensures that the field of view, transmittance and energy concentration of each channel are more than 90% through simulation design, so as to ensure that the core components meet the system demonstration indicators.

  Results and Discussions   The design of the multi-element detection payload system for the spaceborne ocean profile is completed. The laser spectral band is designed as the optimal ratio output of 486 nm and 532 nm multi-wavelength integration, and the photoelectric receiving and detection system selects 1m×5m super-large aperture foldable grating primary mirror, and after simulation analysis, the detection system can realize the homology detection ability of multiple elements such as ocean depth of 100 m, temperature, salinity and backscattering coefficient, and the ability collection capacity is increased by 5 times under the same volume envelope condition. The detection system can achieve a spaceborne orbit of 400 km, a measurement depth of ocean water profile of not less than 100 m, a vertical resolution of better than 0.5 m, a backscatter measurement error of less than or equal to 25%, a temperature profile error of less than 0.5 ℃, and a salinity profile error of less than 1 psu.

  Conclusions   Based on the requirements of the multi-element detection task of the ocean profile, through the analysis and demonstration of the system indicators, a new type of spaceborne near-shore profile multi-element detection technology scheme was proposed, which adopted active and passive composite detection, the laser energy and the spectral synchronous detection system were integrated, the ultra-large aperture foldable optical primary mirror was used to ensure the efficient collection of weak underwater signals, and the laser spectral channel was used to obtain all elastic and inelastic scattering spectra, and the scattering signals of molecules and particles in water were identified and decoupled with high precision. In this way, a more accurate backscattering coefficient of LiDAR can be obtained, which has the ability to detect multiple elements such as underwater temperature, salinity, depth, and optical backscattering coefficient, and the data has good same-origin and same-domain simultaneity.