Sea surface skin temperature (SST_Skin) is the seawater temperature at a depth of about 10 μm. Satellite infrared radiometers can be employed to obtain long-time series of SST_Skin data over a large area. However, as their accuracy can be easily affected by the environment, it is necessary to adopt field data to verify their data accuracy. When obtaining field data, the spatio-temporal mismatch of the sea-sky field of view will introduce uncertainties to the measurement. Thus, we propose an SST_Skin correction method, which utilizes a circulating water-film device designed, a non-cooled infrared thermal imager, and corresponding algorithms to calculate sky radiation. Compared with applying an infrared radiometer to measure sky radiation, this method can indirectly obtain the sky radiation distribution over a larger range. Additionally, the infrared thermal imager simultaneously observes the circulating water-film device and sea surface to realize the synchronous measurement of sky radiation and sea surface radiation. Therefore, this method can reduce the error introduced by the spatio-temporal mismatch of the sea-sky field of view and lower the measurement equipment cost, thus obtaining high-precision SST_Skin.

The infrared thermal imager operates in the 8–14 μm spectral bands. Therefore, when the imager measures the SST_Skin, the total radiation received in the corresponding band is composed of two parts, including radiation emitted by the sea surface and downward radiation from the sky reflected by the sea surface. Thus, after knowing the received radiation, if the sea surface emissivity and the sky radiation can be obtained, the accurate SST_Skin can be inverted. According to the empirical formula, the sea surface emissivity is 0.97994 when the observation angle is 45°. To obtain the sky radiation distribution and reduce the instrument cost, we design the circulating water-film device whose surface can form a smooth water film with uniform temperature. The cold skin effect is the phenomenon where the seawater temperature at the sea-air interface is lower than that of the deeper seawater. In the circulating water-film device, the surface layer of the water film and the water body below the surface layer are fully mixed through the water circulation. Finally, the differences between the skin temperature (depth of about 10 μm) and the water body temperature are eliminated to accurately measure the water-film skin temperature. Based on knowing the true skin temperature of the water film and the measured value of water-film skin temperature by the thermal imager, the sky radiation can be obtained by combining it with the radiation characteristics of the water film. After calculating the sky radiation, the influence of the sky radiation on SST_Skin measurement can be removed by the sea surface emissivity to obtain accurate SST_Skin data. Based on this theory, combined with the circulating water-film device and the thermal imager, we design two SST_Skin correction schemes. Meanwhile, a comparison scheme that employs the sky radiation measured by the infrared radiometer to correct the measured SST_Skin value is formulated to verify the correction effect of the designed two schemes.

In indoor simulation experiments, a background radiation simulation panel is leveraged to simulate the sky with uniform brightness temperature distribution. After correction, the thermal imager's measurement error of water-film surface temperature decreases from 0.203 K to 0.005 K (Fig. 6). In outdoor experiments, when the sky radiation varies in time and space, the designed two schemes are better than the comparison scheme in correcting measurement of water surface temperature, indicating that the two schemes can reduce the error introduced by the spatio-temporal mismatch of the sea-sky field of view (Fig. 10 and Table 1). Before algorithm correction, the average value of water surface temperature measurement error is -0.503 K. After the correction with Scheme 1, the error is -0.081 K. After the correction with Scheme 2, the error is -0.041 K.

We propose an SST_Skin correction method by the circulating water-film device and the infrared thermal imager, which can reduce the error introduced by the spatio-temporal mismatch of the sea-sky field of view. In the circulating water-film device, the water-film skin temperature can be accurately measured by the thermometer through the water circulation. This method employs the thermal imager to measure the skin temperature of the water film and seawater simultaneously and corrects the measured SST_Skin value through the differences between the real and measured values of water-film skin temperature. As a result, the influence of sky radiation on the SST_Skin measurement can be removed and accurate SST_Skin data is obtained. In indoor simulation experiments, after correction, the thermal imager's measurement error of water-film surface temperature decreases from 0.203 K to 0.005 K. In outdoor experiments, after correction, the thermal imager's measurement error of water surface temperature decreases from -0.503 K to -0.041 K. The experimental results show that the proposed method can improve the measurement accuracy of water surface temperature when sky radiation is variable in time and space, and reduce the measuring equipment cost. In the future, this system will conduct on-site SST_Skin measurement comparison experiments with an infrared sea surface temperature autonomous radiometer (ISAR). We hope to study the influence of sky radiation on the temperature measurement accuracy of the rough sea surface to further improve the proposed method, providing a new technical approach for obtaining field SST_Skin data to verify satellite remote sensing data.