Sentinel-3 is a newly launched satellite implemented by the European Space Agency (ESA) for global observation. The Ocean and Land Colour Imager (OLCI) sensor onboard Sentinel-3 provides 21 band images with a fine spectral resolution and is of great value for ocean, land and atmospheric monitoring. The two platforms (Sentinel-3A and -3B) can provide OLCI images at an almost daily temporal resolution. The coarse spatial resolution of the 21 band OLCI images (i.e., 300 m), however, limits greatly their utility for local, precise monitoring. Sentinel-2, another satellite provided by ESA, carries the Multispectral Imager (MSI) sensor which can supply much finer spatial resolution (e.g., 10 m and 20 m) images. This paper introduces a new fusion framework integrating spatio-temporal-spectral information for downscaling Sentinel-3 OLCI images, which has two parts. Based on bands with similar wavelengths (i.e., bands 2, 3, 4 and 8a for Sentinel-2 and bands Oa4, Oa6, Oa8 and Oa17 for Sentinel-3), the four Sentinel-3 bands are first downscaled to the spatial resolution of Sentinel-2 images by applying spatio-temporal fusion to Sentinel-2 MSI and Sentinel-3 OLCI images. Then, to take full advantage of all 21 available OLCI bands of the Sentinel-3 images, the extended image pair-based spatio-spectral fusion (EIPSSF) method is proposed in this paper to downscale the other 17 bands. EIPSSF is performed based on the new concept of the extended image pair (EIP) and by exploiting existing spatio-temporal fusion approaches. The framework consisting of spatio-temporal and spatio-spectral fusion is entirely general, which provides a practical solution for comprehensive downscaling of Sentinel-3 OLCI images for fine spatial, temporal and spectral resolution monitoring.

Sentinel-3 是由欧洲航天局 (ESA) 实施的用于全球观测的新发射卫星。Sentinel-3 上的海洋和陆地彩色成像仪 (OLCI) 传感器可提供 21 个波段的图像，具有精细的光谱分辨率，对于海洋、陆地和大气监测具有重要价值。这两个平台（Sentinel-3A 和 -3B）可以提供几乎每天的时间分辨率的 OLCI 图像。然而，21 波段 OLCI 图像（即 300 m）的粗略空间分辨率极大地限制了它们在局部精确监测中的应用。ESA 提供的另一颗卫星 Sentinel-2 携带多光谱成像仪 (MSI) 传感器，该传感器可以提供更精细的空间分辨率（例如 10 m 和 20 m）图像。本文介绍了一种新的融合框架，它集成了用于缩小 Sentinel-3 OLCI 图像的时空光谱信息，它有两个部分。基于具有相似波长的波段（即 Sentinel-2 的波段 2、3、4 和 8a 以及 Sentinel-3 的波段 Oa4、Oa6、Oa8 和 Oa17），四个 Sentinel-3 波段首先被缩小到空间分辨率通过将时空融合应用于 Sentinel-2 MSI 和 Sentinel-3 OLCI 图像的 Sentinel-2 图像。然后，为了充分利用 Sentinel-3 图像的所有 21 个可用 OLCI 波段，本文提出了基于扩展图像对的空间光谱融合（EIPSSF）方法来缩小其他 17 个波段。EIPSSF 是基于扩展图像对 (EIP) 的新概念并通过利用现有的时空融合方法来执行的。由时空和时空融合组成的框架是完全通用的，