Measure and exploitation of multisensor and multiwavelength synergy for remote sensing: 1. Theoretical considerations

A synergetic scheme refers to an algorithm that simultaneously or hierarchically uses the observations of two or more spectral ranges in order to obtain a more accurate retrieval than the independent retrievals put together. This study is composed of two companion papers; this first part introduces some theoretical considerations. The goal of this study is, first, to identify the various forms of synergy for remote sensing applications. Simple linear models are used to introduce concepts such as additive, unmixing, indirect, or denoising synergies. The second objective of this paper is to develop a methodology to measure, in real-world applications, these different synergies. For this purpose, some experiments are conducted using the classical information content analysis which is often used in the context of assimilation or to design new instruments. This technique is tested on a real-world application where the microwave and infrared observations from the Atmospheric Microwave Sounding Unit-A, Microwave Humidity Sounder, and Improved Atmospheric Sounding in the Infrared instruments are used to retrieve the atmospheric profiles of temperature and water vapor over ocean, under clear-sky conditions. This approach will show its limitation to measure synergy and stress the need for other tools. In the companion paper, statistical retrieval schemes will show their potential to measure and exploit existing synergies, for the same application.