ADVANCED IMAGING INTERFEROMETERS AND SPECTROMETERS: NEW PERSPECTIVES IN EARTH OBSERVATION Alessandro Barducci, Donatella Guzzi, Cinzia Lastri, Paolo Marcoionni, Vanni Nardino, Ivan Pippi CNR – IFAC Via Madonna del Piano 10, 50019 Sesto Fiorentino, ITALY Tel.: +39 0555226301, Fax: +39 0555226348 ABSTRACT Recent advancements in spectroscopic technologies are paving the way to the adoption of new optical configurations for the development of laboratory and aerospace hyperspectral imaging spectrometers. Linear variable filters, Fabry-Perot instruments, immersed gratings, new FTS configurations, non-trigonometric multiplexing devices are examples of new spectroscopic approaches available for the development of new remote sensing instruments. In this paper emerging spectroscopic technologies are reviewed and compared. Our analysis shows the high potential of immersed gratings and imaging interferometry. 1. INTRODUCTION Hyperspectral images acquired by aerospace optical sensors are often utilized for the solar system exploration as well as to derive useful information about the Earth and the state of natural resources. Advancement of global change studies takes advantage from such type of data, which are providing a large amount of spectral data for each ground location observed. On the other hand, the growing demand for higher accuracy data and remote sensing products is pushing forward the precision and the likelihood of computer models describing various environmental processes, while asking for more accurate correction procedures. Novel calibration, correction, and validation methods would take into account the various effects of different noise sources [1 - 3], the necessity of improved radiometric calibration of data [4], and an enhanced modeling of atmospheric effects [5 – 8]. In remote sensing, spectroscopic techniques are widely adopted to gather sets of images covering a continuous spectral range that can span from the visible up to the thermal infrared. It is assumed as a good choice covering a broad spectral range (e.g. 700 nm in the VNIR, or 7 μm in the TIR) with several independent spectral channels with bandwidth from 10 nm (in the VNIR) until 0.5 μm in the TIR. This architecture, based on dispersive spectrometers, has remained unchanged for the past 30 years in which the Earth remote sensing was growing up. Recently the use of different system configurations such as linear variable filters, Fabry-Perot spectrometers, immersed gratings, and Fourier transform spectrometers have been investigated. These emerging techniques can improve traditional remote sensing products, or even create new applications. This paper describes the main differences between dispersive and interferometric techniques applied to Earth observation, pointing out both advantages and disadvantages. The use of different concept designs will be presented and discussed. 2. COMPARISON OF NEW SPECTROSCOPIC TECHNIQUES In this section we give a basic definition of the main spectroscopic techniques, and a brief discussion of their properties. 2.1 Fourier Transform Spectrometry Fourier Transform Spectrometry (FTS) was initiated by Fizeau, Michelson and Morley in their famous works concerning the interferometry [9]. In these works interferometers were employed in order to assess the width of selected spectral lines or to retrieve qualitative spectral information concerning the source. After the work of Fellgett [10 - 13] a great effort was devoted to retrieve full spectral information stemming from an interferometric measurement. This complex purpose was finally achieved in the 60’s, when the first computing machines became available for general research initiatives. Nowadays, Fourier transform imaging spectrometers can be developed using the “common path” (e.g. the Sagnac triangular ray-path) or the “tilted” configuration (e.g. the traditional Michelson interferometer). Examples of imaging interferometers are the Fourier Transform Hyperspectral Imager (FTHSI), and the Aerospace Leap-frog Imaging Stationary interferometer for Earth Observation (ALISEO). FTHSI was the first spaceborne imaging interferometer, operated on board of the Air Force Research Laboratory (AFRL) – U. S. Department of Defence (DoD) technological satellite MightySat II.1 (Sindri P99-1). ALISEO, instead, has been chosen by _____________________________________________________ Proc. ‘Hyperspectral 2010 Workshop’, Frascati, Italy, 17–19 March 2010 (ESA SP-683, May 2010)