The threat posed to the earth's atmosphere by anthropogenic activity is now well documented. A basic requirement for understanding and predicting the effect of changes to the natural - and delicate - chemical balance of the atmosphere, are precise measurements of its composition on a four-dimensional basis, i.e. latitude, longitude, altitude, and time. As part of the NASA Upper Atmosphere Research program, the ATMOS (Atmospheric Trace Molecule Spectroscopy) experiment has been designed to address some of theSe requirements. Its primary function is to record high resolution infrared solar spectra in the occultation mode from a space platform. The presentation will include an overview of the ATMOS instrument and a review of its performance and results gathered during its first flight as part of the Spacelab 3 Space Shuttle mission (April 29 through May 6, 1985). Prospects for future ATMOS sorties will also be discussed in the context of other program efforts.

Measurements of atmospheric trace gases have been performed during the first Spacelab mission on board the Space Shuttle. The principle of the observations is infrared absorption spectroscopy using the solar occultation technique. Infrared absorption spectra of NO, CO, CO2, NO2, N20, CH4 and H2O have been recorded using the Grille spectrometer developped by ONERA and IASB. From the observed spectra, vertical profiles for these molecules have been derived. The present paper summarizes the main results and compares them with computed vertical profiles from a zonally averaged model of the middle atmosphere. The scientific objectives of the second mission, Atlas 1, planned for 1990 are also presented.

A conceptual design is presented for the Stratospheric Aerosol and Gas Experiment III (SAGE III) instrument which is currently being proposed for the Earth Observing System (Eos). SAGE III is designed to monitor globally the vertical distribution of stratospheric aerosols, ozone, water vapor, nitrogen dioxide and temperature by measuring the extinction and scattering of solar radiation in the 0.3 to 1.6 μm range through the earth's atmosphere. Solar radiation is reflected from a flat scanning mirror into a Cassegrain telescope, which forms an image on the entrance aperture of a grating spectrometer. The aspheric grating forms a stigmatic, flat field image on a silicon CCD array which serves as the detector for eight of the SAGE III channels. An interference filter in front of the array sorts the dispersed energy so that the desired order (either first, second, or third) reaches the focal plane. Pixels are grouped electronically to form the desired spectral bandpass of each channel. The ninth channel is taken from the grating zero order and uses a InGaAs PIN photodiode detector. A two axis gimbal system is capable of tracking (in azimuth) and scanning (in elevation) the sun through the earth's atmosphere when in the solar occultation mode, and then performing earth limb scans between occultation events. Data is sampled at 64 times per second and digitized to 14 bit resolution. Wavelength and radiance calibrations can be performed on-orbit. SAGE III has evolved from the highly successful Stratospheric Aerosol Measurement II (SAM II), SAGE, and SAGE II programs.

By means of automatic multichannel balloon-borne spectrometers the spectra of solar radiation have been measured at altitudes up to 30-32 km in the range of 0.4 to 12.0μm under different spectral resolutions. On the basis of the experimental data obtained using the procedure elaborated by the authors the profiles of ozone concentrations in atmosphere at altitudes of 15-25 km have been defined, as well as the concentrations of water vapour and methane at altitudes up to 30 km. Vertical and spectral dependencies are evaluated for total and aerosol attenuation of radiation within the range of 0.4 to 12.0μm at altitudes up to 30 km. The origin of the transparence properties of atmosphere is discussed.

Increasing concern is being expressed about the impact of anthropogenic emissions on the present and future state of the lower atmosphere: urban & regional pollution; acid rain; stratospheric ozone depletion; climatological effects of the increase in "greenhouse" gases; etc. There is a clear need for instrumentation to study these questions on a global scale. Accordingly, we describe the concept of a cryogenic infrared (600 - 3450 cm-1; 2.9 - 16.7 μm) imaging Fourier transform spectrometer for observations of the troposphere and lower stratosphere from near-Earth orbit using natural thermal emission and reflected sunlight (when appropriate). The system, called the Tropospheric Emission Spectrometer (TES), has been accepted by NASA for Phase B definition studies leading to flight on the second polar platform of the Earth Observing System (Eos) in 1998.

The development of a passive sensor for the remote detection of gases, which is based on an interferometer, is described. Unlike conventional Fourier Transform Spectroscopy (FTS), the interferometer is scanned over a very limited range of optical delays and the presence of gas is deduced airectly from the Fourier domain. Signal-to-noise calculations and laboratory experiments, using nitrogen dioxide (NO2) with visible light, are presented. These results demonstrate that this new technique has the advantages of higher sensitivity, faster scanning and a compact optical design.

The Earth Radiation Array (ERA) represents a new instrument under development for climate research and global monitoring of the earth radiation budget. The ERA contains a mosaic array of detectors which measures the outgoing radiation of the earth in all directions without the need for mechanical scanning. The goal is to eliminate all moving parts and obtain highly calibrated measurements for up to one solar cycle (11 years) without being susceptible to mechanical failure. Over set intervals of time, the angular measurements are integrated up to produce total reflected and emitted flux values from each target area on the earth. To achieve the required sensitivity while maintaining a flat spectral response, an improved dual cavity detector with digital control loop has been developed. The detector is self calibrating to maintain high precision over the life of the instrument. Prototype detectors and a complete set of electronics with a microprocessor controller are currently being fabricated and tested by Space Instruments, Inc ., under contract to NASA/GSFC. Early measurements indicate that the new detectors are more than an order of magnitude more sensitive than previous versions. The design concept for the complete ERA and the status of the hardware development will be presented, along with key test results obtained to date.

A new airborne imaging spectrometer has been developed by the Jet Propulsion Laboratory to exploit many earth resources. The apparatus utilizes 210 adjacent spectral bands ranging from 0.40 to 2.45 um. During the first measurement campaign in the summer 1987, the spectrometer was flown over a site containing both hydrothermally altered and unaltered rocks well exposed at the surface. The related radiometrically corrected image set has been available to the investigators. Using the software developed at our Institute, a set of images of the site in any of the 210 spectral bands or the spectral signature of any single pixel can be displayed. In addition particular attention is paid to evaluate the data accuracy and the atmospheric effects utilizing the LOWTRAN 6 computer code. First results of rock outcrops classification are presented and discussed.

The detection and description of subsurface land-based voids has become of increasing importance to the military, civil and industrial communities. These voids can represent caves or shelters where military personnel can be hidden. In the civil community, they can represent collapsing infrastructure such as voids surrounding buried utility pipes or voids in concrete structures such as airport taxiways. In the industrial community, these can represent sink holes forming beneath existing and planned structures. Land-based and airborne computer enhanced infrared thermographic techniques have been shown to provide accurate location and description of these hidden voids. These techniques also have the ability to survey large areas quickly and efficiently. This paper reviews our initial paper presented to the International Society of Optical Engineers (SPIE) in October, 1983 and contains an update of our experiences, both successes and failures, of several large-scale void detection projects.

Present-day problems of investigation and preservation of the environment, the increasing effect of the outer space radiation on the activity of man, problems in medicine and solar energetics require increased accuracy of measurements of the illuminance from direct and scattered sun light.

This paper deals with a new instrument under development at the CNES (the French Space Agency) : a Fourier Transform spectro imager. Based on the principle of the Michelson interferometer, it will perform on-board satellite high spectral resolution remote sensing. This instrument gives images in which each pixel is, in fact, a set of about 200 points : each pel is the whole spectrum of the observed point on Earth. After a description of the spectro-imager, we present the solution brought to the problem of transmission of these "vector" images. Obviously, the straight transmission implies a very high bit rate (estimated at 126Mbits/s). So we apply techniques of signal processing to the signals of each pixel (interferograms) : we model the interferogram by an AutoRegressive process, then we get a set of parameters to be coded. We show in the paper that the most efficient set is defined by : the p first points of the interferogram, {y(n)}n=0,...,p-1,p partial correlation coefficients {ki}i=1 and the model error {e(n)}n=p,...,N(N = 200, p = model order = 10). After these pfocessings, we reach a compression ratio ofd with a very good spectral fidehty. In conclusion, we present several examples of actual reflectances and their compressed versions, exhibiting the very low degradation of the spectra obtained even with high compression ratio.

Global concerns over the atmospheric increase in carbon dioxide have lead to the study of the sources and sinks responsible for regulating that increase, and to the conclusion that the ocean is a major sink for atmospheric carbon dioxide. One aspect of the uptake of atmospheric carbon dioxide by the oceans is the flux of carbon through the surface mixed layer into the deep ocean, controlled in part by the fixation of carbon by marine phytoplankton through the photosynthetic process of primary production. The estimation of oceanic phytoplankton biomass, and the primary production which leads to that biomass on regional and global scales, is the focus of efforts using both in situ measurements and remote sensing techniques. This paper surveys the present state of our understanding of the estimation of marine primary production by remote sensing techniques, through the modeling of the physiological mechanisms of carbon assimilation by phytoplankton, and the relationship of these techniques to in situ estimates. This model requires measurements of surface pigments and a determination of the incident solar irradiance as a measure of the energy available for photosynthesis.

This paper describes a design for the Medium Resolution Imaging Spectrometer (MERIS), which has been proposed as a candidate core instrument for the European Polar Platform scheduled for launch in 1997. The instrument has a spectral range of 400 to 1050nm, a swath width of 1500km, spectral resolution of 1.25nm and spatial resolution of 260m. The instrument has applications in the fields of marine, land and atmospheric research.

The "Reflective Optics System Imaging Spectrometer" (ROSIS) is a compact, programmable imaging spectrometer based on a CCD matrix detector array. The optics of the system is made of reflective components only. The present horizontal resolution is 500 pixels across-track for a one-camera system with 32 selectable channels out of 85 within the spectral range 430 - 850 nm. The spectral mode permits all 85 channels to be read using every third across-track pixel. The instrument has been designed specifically for the monitoring of water colour and of natural chlorophyll fluorescence in order to derive concentrations of phytoplankton chlorophyll and other pigments, suspeded matter and yellow substance, and to map their horizontal distribution in the sea. However, its high spectral resolution (of ≤ 5 nm) and flexibility also permits new applications in the remote sensing of atmospheric parameters such as cloud-top height and vegetation monitoring. An airborne prototype of ROSIS is being jointly developed at present by MBB, GKSS and DFVLR. The instrument concept, the data processing system and the scope of application is discussed.

Two active optical techniques have been developed to measure the spatio-temporal structure of small scale waves with wavelength in the range of 2mm to 20 cm. One technique, the imaging slope gauge (ISG), is based on light refraction at the water surface and records im-age sequences of the wave slope at a maximum area of 1 m2. With the other technique, the reflective stereo slope gauge (RSSG), the water surface is illuminated from above by a monochromatic light source. Two CCD-cameras take stereo image sequences of the specular reflexes returned by the water surface. The wave height is calculated from the parallax between the right and left images with a fast multi-grid algorithm. This technique is also suitable for measurements in the field. The new instruments have successfully been used in the world's largest wind/wave facilities (Delft Hydraulics, The Netherlands) in the frame of the international VIERS-1 project. This project inves-tigates the physical mechanisms of radar scatterometry. Besides our research groups the VIERS-1 participants are the Physics and Electronics Laboratory of the Organization for Applied Research (FEL-TNO), Technical Univ. of Delft, the Royal Netherlands Meteorological Inst. (KNMI), Delft Hydraulics (WL), and the Directorate Tidal Waters of the Department of Harbors and Public Works (RWS-DGW). Two-dimensional wavenumber spectra, two-dimensional slope and one-dimensional height distributions calculated from the data of the two instruments are discussed.

Recent developments in remote sensing technology using 2 dimensional array detectors have resulted in a growing family of sensors called imaging spectrometers. Two instruments built in Canada have the very high spectral resolution and sensitivity required to image water colour variations due to such phenomena as the blue-green absorption of certain fish and of marine plants; and the solar stimulated in vivo fluorescence from the chlorophyll pigment molecule. This paper briefly describes the Fluorescence Line Imager (FLI), built in 1983 for the Canadian Department of Fisheries and Oceans, and a smaller, more flexible, second generation instrument called the Compact Airborne Spectrographic Imager (CASI), first tested in the summer of 1988. Examples of spectral and spatial image data over a number of different targets are shown and the use of this technology for mapping coastal sites is discussed.

The Eos era will provide the Earth System Science remote sensing community with a wealth of new information on the interactions between the atmosphere, land surface, and oceans. High spectral and spatial resolution images provided by instruments such as the High Resolution Imaging Spectrometer (HIRIS) on the Eos-1 platform will be a key component of the Eos data set. We anticipate that the improved understanding of the Earth provided by Eos will create the need for even greater flexibility and measurement capabilities than represented by the current generation of sensors. The spectral flexibility, polarization selecting properties, and simplicity of acousto-optic filters (AOTF's) suggest an important application of these devices to future imaging spectrometers. AOTF's are compact, robust, solid-state crystals which provide rapid (millisecond), electronically-controlled spectral tunability in both states of polarization. In this paper, we will discuss concepts for spaceborne AOTF-based imaging spectrometers.