Search

The design and characterisation of small and cost-effective reference black-bodysources for radiometric calibration of Fourier transform spectrometers in the midand far infrared is presented. The paper describes the optimisation of the absorbingblack-coating material and the study of two types of cavity geometry, one designedfor collimated beams and one for focused beams. These two reference sources havebeen developed for the Radiation Explorer in the Far InfraRed spectrometer cov-ering the 100-1400 cm −1 spectral range. They are currently used on-board twoprototypes for ground and stratospheric balloon based atmospheric emission mea-surements. An emissivity of better than 0.9999 is obtained for both type of cavitygeometry giving a calibration error, depending only on the temperature gradientinside the blackbody, of less than 0.5 K. Keywords: Fourier transform spectra, Infrared spectroscopy, Blackbody radiation PACS: 33.20.Ea, 07.57.Ty, 44.40.+a 1 IntroductionIn emission spectroscopy, the calibration of the observed radiance requiresthe measurement of a known source of radiation, which typically is givenby the emission of a blackbody (BB) cavity described by the Planck law ofradiation. The design of a well-characterised and reliable BB source can be

A simple and compact parallelogram mirror drive based on flexure pivots suitable for intermediate resolution Fourier transform spectroscopy is presented. The system permits 12 mm of mirror translation with a residual tilt of less than ±0.4 mrad in the horizontal plane and ±0.2 mrad in the vertical plane. A system prototype was built and fully characterized for a Fourier transform spectrometer operating on a stratospheric balloon.

This paper presents the project Earth Cooling by Water Vapor Radiation, an observational programme, which aims at developing a database of spectrally resolved far infrared observations, in atmospheric dry conditions, in order to validate radiative transfer models and test the quality of water vapor continuum and line parameters. The project provides the very first set of far-infrared spectral downwelling radiance measurements, in dry atmospheric conditions, which are complemented with Raman Lidar-derived temperature and water vapor profiles. Citation: Bhawar, R., et al. (2008), Spectrally resolved observations of atmospheric emitted radiance in the H2O rotation band.

Measurements have been performed at eight laboratories on Raman-shifted radiation from frequency-quadrupled Nd:YAG and KrF lasers. Each of these sources is a possible candidate for making differential-absorption lidar measurements of ozone in the free troposphere. The spectral purity, efficiency and divergence of the different sources are compared and explained using a simplified model of the Raman interaction. Particular attention has been given to the use of buffer gases to optimize the Raman gain and divergence. The experimental results show that the operating conditions required to optimize the output energy of each of these types of laser source can be quite different. In the case of the KrF laser, it has been shown that high energies can be generated at low active gas pressures, and that increasing the pressure increases the beam quality, but decreases the energy. (Less)

The Aerospace Leap-frog Imaging Stationary interferometer for Earth Observation (ALISEO) is a hyperspectral imaging interferometer for Earth remote sensing. The instrument belongs to the class of Sagnac stationary interferometers and acquires the image of the target superimposed to the pattern of autocorrelation functions of the electromagnetic field coming from each pixel. The ALISEO sensor together with the data processing algorithms that retrieve the at-sensor spectral radiance are discussed. A model describing the instrument OPD and interferogram center is also discussed, improving the procedures for phase retrieval and spectral estimation. Images acquired by ALISEO are shown, and examples of retrieved reflectance spectra are presented.

In this paper we describe a new instrument named ALISEO (Aerospace Leap-frog Imaging Stationary Interferometer for Earth Observation), which belongs to the class of the “stationary interferometers”. ALISEO has been selected as the main payload for a novel Italian Space Agency (ASI) optical mission based on a micro-satellite (MIOSat) with steering capability. The mission is constituted by a micro-satellite equipped with advanced payloads, among which there are an extensible telescope, a MEMS Mach-Zehnder non-imaging interferometer, and ALISEO. MIOSat is expected to provide the scientific community with the technological demonstration of this new remote sensing technology.

ALISEO (Aerospace Leap-frog Imaging Stationary Interferometer for Earth Observation) belongs to the stationary interferometers representing a promising architecture for future Earth Observation (EO) sensors due to their simple optical layout. ALISEO has been selected by the Italian Space Agency as the principal payload for a new optical mission based on a micro-satellite (MIOsat). Payloads planned for MIOsat are an extensible telescope, a high-resolution panchromatic camera, a Mach-Zehnder MEMS interferometer, and ALISEO. MIOsat is expected to provide a platform with pointing capability for those advanced sensors. ALISEO operates in the common-path Sagnac configuration, and it does not employ any moving part to generate phase delay between the two rays. The sensor acquires the target images modulated by a pattern of autocorrelation functions: a fringe pattern that is fixed with respect to the instrument's field of view. The complete interferogram of each target location is retrieved introducing relative source-observer motion, which allows any image pixels to be observed under different phase delays. Recent laboratory measurements performed with ALISEO are described and discussed in this paper. In order to calibrate the optical path difference (OPD) of raw interferograms, a set of measurements have been carried out using a double planar diffuser system and several coloured He-Ne lasers. Standard reflectance tiles together doped with Holmium and Rare Earths have been used for validating the wavelength calibration of the instrument and proving the reliability of the reflectance retrieving procedure.

A cryogenically operated laser diode spectrometer (COLD) for the airborne measurement of carbon monoxide is described. The instrument is designed, and fully qualified, for operation on a high-altitude aircraft and the scientific mission is the in situ measurement of trace gases in the upper troposphere and lower stratosphere. Sensitivities achieved so far during in-flight operation are a few ppbV with a time resolution of 4 s, coupled with a good reliability. Airborne data, obtained by COLD during research campaigns in Australia and Brazil in the frame of international projects, are also presented to demonstrate COLD in-flight performance.

An old-fashioned technology such as the "searchlight profiler", which was used in the 1960s for the early profiling of atmospheric backscatter, recently modified for its use with lasers and CCD cams1, has been revisited in the telemetric-LIDAR described here. The instrument is intended for the low-cost monitoring of urban aerosols, with a limited useful range of 100-200 meters. A modulated CW laser beam is used to probe the atmosphere, and a refractive telescope is used to collect the backscattered light on a photodiode array. Like in a telemetric system, the distance between the laser beam and the telescope axis determines the range resolution and the distance of the measurement volume corresponding to each pixel of the array. The system was used in conjunction with in-situ PM10 instruments for several months of continuous operation in Prato and Florence (Italy). The comparison between LIDAR-derived and conventional PM10 measurements is shown.

The spectrally-resolved characterization of the atmospheric emission has a fundamental role in the study of the Earth radiation balance, and only a measurement performed in a wide spectral range enables us to separate the contributions to the radiative balance due to the different altitudes, constituents and physical phases. The REFIR-PAD (Radiation Explorer in the Far InfraRed-Prototype for Application and Development) Fourier transform spectroradiometer can perform a characterization of the broadband radiative signature of clouds and aerosols, with the only limitation of the need of low levels of water vapour like those that are obtainable in high altitude stations during winter. The capabilities of this kind of measurement have been assessed in a series of test campaign performed in Tuscany during the winter of 2006/2007 when atmospheric emission spectra have been acquired in various transparency conditions and the evidence of transparency in the far-infrared region below 600 cm -1 has been demonstrated. REFIR-PAD operates in the spectral range extending from 100 to 1400 cm -1 with a resolution of 0.5 cm -1 , using room-temperature detectors and optics and a compact, misalignment-compensated design. The instrument, developed at IFAC-CNR, Florence, has been successfully deployed in several campaigns, both in the ground based zenith-looking geometry and in the nadir-looking balloon borne configuration. The operating spectral range of the REFIR-PAD spectroradiometer encompasses great part of the atmospheric emission spectrum, from the relatively unexplored far-infrared region below 400 cm -1 , dominated by water vapour rotational band, to the atmospheric transparency window, where a number of atmospheric instruments are already operating and can provide intercomparison data.

A polarization lidar operating at 532 nm was converted into an automatic, polarimetric lidar capable of measuring the entire Stokes vector of backscattered light and its derived quantities. Among these quantities, circular and linear depolarizations were studied as tools for investigating the presence of anisotropic scattering media. Isotropic scatterers show a simple relationship between linear and circular depolarization, a relation that we confirm theoretically and experimentally. Deviations from this relation, which are possible in the presence of anisotropic scatterers such as horizontally oriented ice plates when they are observed with a slant lidar, were studied both numerically and experimentally.

The first spectral measurement of Earth’s emitted radiation to space in the widebandrange from 100 to 1400cm −1 with 0.5cm −1 spectral resolution is presented. The mea-surement was performed from a stratospheric balloon in tropical region using a Fourier 5 transform spectrometer, during a field campaign held in Brazilin June2005. The instru-ment, which has uncooled components including the detector module, is a prototypedeveloped as part of the study for the REFIR (Radiation Explorer in the Far InfraRed)space mission. This paper shows the results of the field campaign with particular atten-tion to the measurement capabilities of the prototype. The results are compared with 10 measurements taken by IASI-balloon (Infrared Atmospheric Sounding Interferometer –Balloon version), aboard the same platform, and with forward model estimations. Theinfrared signature of clouds is observed in the measurements. 1 Introduction The observation of the upwelling emission in the full relevant spectral range, from the

In recent years the Italian Space Agency funded a feasibility study for an aerospace imaging interferometer for Earth Observation. The instrument, named ALISEO (Aerospace Leap-frog Imaging Stationary Interferometer for Earth Observation), belongs to the class of the so called "stationary interferometers," and it operates in the common path Sagnac configuration not employing any moving parts to generate phase delay. ALISEO acquires the target image as modulated by a pattern of autocorrelation functions of the energy coming from each target's location. The complete interferogram is retrieved introducing relative source-observer motion. In order to calibrate the optical-path-difference (OPD) axis of the raw interferograms, a set of measurements have been carried out illuminating a double planar diffuser system by He-Ne lasers operating at different wavelengths. Standard reflectance tiles together with diffusers doped with Holmium and Rare Earths have been used to verify the wavelength calibration of the instrument. Finally, a procedure for raw data pre-processing aimed at retrieving at-sensor radiance spectra is developed taking into account the issues of dark signal subtraction, spectral instrument response compensation, effects of vignetting, and Fourier back-transform algorithm.

In preparation for a possible space mission, a breadboard version named REFIR-BB of the Radiation Explorer in the Far Infrared (REFIR) instrument has been built. The REFIR is a Fourier-transform spectrometer with a new optical layout operating in the spectral range 100-1100 cm(-1) with a resolution of 0.5 cm(-1), a 7-s acquisition time, and a signal-to-noise ratio of better than 100. Its mission is the spectral measurement in the far infrared of the Earth's outgoing emission, with particular attention to the long-wavelength spectral region, which is not covered by either current or planned space missions. This measurement is of great importance for deriving an accurate estimate of the radiation budget in both clear and cloudy conditions. The REFIR-BB permits the trade-off among all instrument parameters to be studied, the optical layout to be tested, and the data-acquisition strategy to be optimized. The breadboard could be used for high-altitude ground-based campaigns or could be flown for test flights on aircraft or balloon stratospheric platforms. The breadboard's design and the experimental results are described, with particular attention to the acquisition strategy and characterization of the interferometer. Tests were performed both in laboratory conditions and in vacuum. Notwithstanding a loss of efficiency above 700 cm(-1) caused by the poor performance of the photolithographic polarizers used as beam splitters, the results demonstrate the feasibility of using the spectrometer for space applications.

In this paper, mid-infrared spectrometer based on a liquid nitrogen cooled lead-salt diode laser, emitting at 2190 cm/sup -1/ for the in-situ measurement of stratospheric nitrous oxide is developed. The system operates unattended for several hours under changing ambient conditions: during flight, the pressure is ranging from 1000 down to 30 mbar, temperature ranges from +70/spl deg/C to -90/spl deg/C. Typical flight measurements are performed between 10000 and 22000 m of altitude and at an aircraft speed of 160-190 m/s.

In this paper, compact near-infrared distributed feedback diode-laser, emitting at 1.651 micron for the measurement of atmospheric methane profiles on an airborne platform is developed. The detection technique is based on two-tone frequency modulation spectroscopy. The calibration procedure for the analyser with respect to pressure changes during the flight mission is also discussed.

In this paper we present some laboratory measurements obtained by a new imaging interferometer. This instrument has been derived from the so called “stationary interferometers,” which do not employ any moving part to optically scan the instrument field-of-view. The device acquires the image of an object superimposed to a fixed (stationary) pattern of autocorrelation functions of the energy coming from each pixel. The interference pattern, constituted by a system of vertical fringes, is scanned by moving the observed target with respect to the imaging device. In order to calibrate the optical-path-difference axis of the raw interferograms, we have executed a set of measurements employing a He-Ne laser source spread by a pair of planar diffusers. The dependence of the optical-path-difference values on the source spectral content has been addressed performing a set of measurements after filtering a 600W halogen lamp with interference filters of 10nm bandwidth. We have described the procedure of pre-processing of the acquired data to retrieve the spectrum of at-sensor radiance (dark signal subtraction, spectral instrument response compensation, effects of vignetting and Fourier transform algorithm). Some hints are given about the use of this instrument from airborne platforms for remote sensing of the Earth.

A new solar spectral irradiometer that operates in the visible and near-infrared spectral ranges has been developed. This instrument takes advantage of a new concept optical head that collects the light that impinges on a hemispheric surface, thus improving the instrument angular response with respect to traditional devices. The technical characteristics of the instrument are investigated and detailed, and its radiometric calibration, performed by means of a Langley-like method, is discussed. A new simplified theoretical model that accounts for the diffuse irradiance observed in an optically thin plane-parallel atmosphere has been developed to improve the fit of the irradiance diurnal evolution. An alternative polynomial parametric representation of monochromatic diffuse irradiance evolution has been attempted, but satisfactory results were not obtained from the fitting of experimental data. The new instrument could be useful to carry out remote-sensing validation campaigns.

The fundamental component of a stationary interferometer is a beam-splitter semi-transparent plate that provides phase-delay between the two interfering rays. The phase-delay changes with varying the incident angle of the entering ray, thus producing the entire interference pattern while moving the device over the surface of the observed target. Due to their technical characteristics these interferometers can reliably be adapted for aerospace remote sensing applications. Their ability to produce the interference of the incoming radiation over a broad spectrum of optical wavelength together with the possibility to accommodate the spectral resolution by changing its optical aperture and the sampling step make these instruments interesting for Earth remote sensing. This paper describes the results coming from laboratory experiments and numerical simulations carried out in order to investigate the use of the static interferometers for remote sensing purposes.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In the present paper a Lidar (light detection and ranging) system is described. It allows high-speed data acquisition of Lidar signals and meteorological parameters by means of standard instrumentation. The Lidar, controlled by a computer, utilized two laser sources (Nd: Yag and ruby), a modified Ricthey-Chretien telescope and signal detection, storage and real-time monitoring equipment. The whole system was installed in a container for field experiments.

A room-temperature broadband FTS was designed for the characterization of the Earth outgoing flux in the FIR spectral range with a resolution of half wavenumber. Laboratory characterization and field measurements (ground-based and balloon) are shown.

International audience; The first spectral measurement of Earth's emitted radiation to space in the wideband range from 100 to 1400 cm-1 with 0.5 cm-1 spectral resolution is presented. The measurement was performed from a stratospheric balloon in tropical region using a Fourier transform spectrometer, during a field campaign held in Brazil in June 2005. The instrument, which has uncooled components including the detector module, is a prototype developed as part of the study for the REFIR (Radiation Explorer in the Far InfraRed) space mission. This paper shows the results of the field campaign with particular attention to the measurement capabilities of the prototype. The results are compared with measurements taken by IASI-balloon (Infrared Atmospheric Sounding Interferometer – Balloon version), aboard the same platform, and with forward model estimations. The infrared signature of clouds is observed in the measurements.