With the development of hyperspectral and high-resolution ocean color satellite remote sensors, such as hyperspectral imager for the coastal ocean (HICO), advanced hyper-spectral imager (AHSI), and ocean color instrument (OCI), the existing above-water automatic observation systems cannot meet the application needs for on-orbit calibration and in-situ verification of these sensors. For example, CE318-SeaPRISM and radiation measurement sensors with enhanced spectral resolution (RAMSES) sensors can only verify multispectral remote sensors, as they have a lower spectral resolution of about 10 nm. However, the hyperspectral remote sensors have a spectral resolution of about 5 nm, which can capture more subtle spectral characteristics of water bodies. Therefore, an above-water hyperspectral radiometer with higher spectral resolution is needed. In addition, the separate radiometric calibration of radiance and irradiance radiometers may also introduce uncertainty of R_rs (remote sensing reflectance). To address these challenges and obtain high-precision hyperspectral apparent optical properties data of water bodies, we put forward a design scheme of an acquisition system with three-channel synchronous observation, same dispersion optical path design, and same system calibration. We propose a water apparent optical property acquisition system (WAOPAS), which can provide technical support for high precision on-orbit calibration and product authenticity verification of ocean color remote sensors.

We develop a novel three-channel hyperspectral acquisition system for ocean color remote sensing based on the principle of above-water measurement. To achieve high spectral matching consistency among the three radiometers, we use the same dispersion acquisition unit and different front optical system designs. We also implement shutter synchronization and wireless remote transmission technologies to enable the synchronous and rapid multiple acquisition of sea surface radiance (L_surface), sky radiance (L_sky), and sea surface incident irradiance (E_s), which can cope with the complexity and variability of the marine environment. Furthermore, we utilize GPS positioning and tracking technology to automatically adjust the observation geometry and avoid sun glint. We apply an automatic integration time design to automatically adjust parameters according to the environmental light intensity and water transparency, adapting to the diversity of the marine environment. To enhance the stability and reliability of the instrument, we leverage an integrated instrument housing and multiple protection design. To improve the accuracy of radiometric measurement, we adopt a near-synchronous radiometric calibration scheme of radiance and irradiance, which can be traced back to the National Institute of Metrology of China (NIM). Finally, we conduct a comparison experiment with HR-1024i and RAMSES outdoor to verify the accuracy of the measurement system.

The WAOPAS is calibrated and tested in the laboratory. It has a spectral range of 350-900 nm, a spectral resolution better than 3 nm, and functions of automatic observation geometry adjustment, automatic gain integration time, data remote transmission, and automatic preprocessing. It can realize unattended observation in all weather conditions. The radiance and irradiance meters have the same spectral range and sampling interval, and the maximum difference in resolution is 0.26 nm, ensuring the spectral matching of the measurements. The radiance and irradiance are calibrated by the same calibration system and a near-synchronous calibration method, decreasing the remote sensing reflectance measurement uncertainty of 0.34%-0.83% (ratio coefficient K=1). The outdoor comparison experiment with international mainstream measurement instruments preliminarily verifies the accuracy and feasibility of the measurement.

We present WAOPAS that synchronously and rapidly measures sea surface radiance, sky radiance, and sea surface incident irradiance. The main features of WAOPAS include: 1) the same dispersion optical path design that ensures consistent spectral range and resolution; 2) a near-synchronous radiometric calibration of radiance and irradiance that can be traced back to the NIM, which significantly reduces R_rs measurement uncertainty; 3) the shutter synchronization and wireless remote transmission technology that enables simultaneous data acquisition by three radiometers;4) the GPS positioning and tracking technology that automatically adjusts observation geometry and avoids sun glint; 5) an automatic gain integration time design that adapts to different light intensity and water transparency; 6) an integrated instrument housing and multiple protection design that enhances stability and reliability. We evaluate WAOPAS by comparing it with HR-1024i and RAMSES in outdoor experiments and find high measurement accuracy. For the measurement and application of water objects, we conduct a continuous observation experiment at the Dongpu Reservoir in the western suburbs of Hefei. Due to space limitations, the process and results of residual item correction, data quality control, and high-resolution satellite authenticity inspection will be reported in another article.