Silvère Gousset, Juana M. Rodrigo, Etienne Le Coarer, Hélène Ehrhardt, Eric Stadler, Zoltán Hubert, Yves Magnard, Laurence Croizé, Yann Ferrec, Florence de la Barrière, Roland Domel, Christophe Coudrain, Remi Gouyon, Mathieu Chambon, Philippe Perrault, Jumana Boussey, Cécile Gourgon, Marie Panabière, Nadine Gerges
The Space CARBon Observatory (SCARBO) European program aims at assessing the monitoring of anthropogenic emissions of Green House Gas (GHG) with the uncovered goal of a sub-daily revisit of the Earth at an affordable cost. One of the main project scopes is the feasibility study of a hybrid constellation including both high accuracy reference missions (Copernicus CO2M or CNES MicroCarb mission) and 24 small-satellites onboarding innovative miniaturized payloads. The key GHG-sensor of the smallsat constellation is the NanoCarb concept, an unprecedently kilogram-class Fourier Transform imaging spectrometer. We are reporting here some preliminary experimental results from a demonstration airborne campaign. A low cost, 2-bands prototype designed for CO2 and CH4 measurement has been developed then integrated over a Falcon-20 from SAFIRE, jointly to SPEX aerosol sensor from SRON. During October 2020, we flew over Spain, Italy, and towards Poland from Francazal airport in Toulouse, France. Even if we did not have the opportunity to flight over a powerplant, a lot of data has been acquired and are being processed. After presented the instrument, mission and data products, we assess the data quality and the reliability of the model. We derive finally an expected sensitivity over CO2 and CH4 columns according to the background respectively about 1.5-2.5% and 5%. We finally demonstrate the operability of this first TRL5 prototype of NanoCarb.
The goal of a cophasing sensor (CS) is to measure the phase disturbances between the sub-apertures or inside each aperture of a telescope. Three CSs are currently studied at ONERA. A first CS for Earth imaging is based on phase diversity on extended sources (cf companion paper by L. Mugnier). A second CS for intercalibration uses phase retrieval on a point source. The third CS for nulling interferometry (“DWARF”, for the ESA/DARWIN mission) is based on similar algorithms. To test performance of these CSs, ONERA has defined and integrated a multipurpose bench, “BRISE”. Its main features are the simultaneous imaging of a point source and of an extended source, the minimisation of absolute and differential disturbances, the use of any aperture configuration and the generation of pure calibrated piston/tip/tilt aberrations on three sub-apertures by a dedicated PZT-based device. Preliminary experimental results are consistent with numerical simulations and confirm nanometric performance.
Hyperspectral imaging from small unmanned aerial vehicles (UAVs) arouses a growing interest, as well for military applications as for civilian applications like agriculture management, pollution monitoring or mining. This paper establishes a quick state of the art of cameras of which the capabilities in small-UAVs embedded campaigns have been demonstrated. We also introduce a novel compact hyperspectral camera operating in mid-infrared spectral range embeddable on small UAVs. This camera combines birefringent interferometer for size reduction and cooled imaging optics for a better signal noise ratio. The design of a first prototype and first results from a ground-based measurement campaign, of which the goal was an optical concept validation, is also discussed. Finally, we present the design modifications for the small-UAVs-embeddable version.
Recent developments in unmanned aerial vehicles have increased the demand for more and more compact
optical systems. In order to bring solutions to this demand, several infrared systems are being developed at
ONERA such as spectrometers, imaging devices, multispectral and hyperspectral imaging systems. In the field
of compact infrared hyperspectral imaging devices, ONERA and Sagem Défense et Sécurité have collaborated
to develop a prototype called SIBI, which stands for "Spectro-Imageur Birefringent Infrarouge". It is a static
Fourier transform imaging spectrometer which operates in the mid-wavelength infrared spectral range and
uses a birefringent lateral shearing interferometer. Up to now, birefringent interferometers have not been
often used for hyperspectral imaging in the mid-infrared because of the lack of crystal manufacturers, contrary
to the visible spectral domain where the production of uniaxial crystals like calcite are mastered for various
optical applications. In the following, we will present the design and the realization of SIBI as well as the first
experimental results.
SYSIPHE is an airborne hyperspectral imaging system, result of a cooperation between France (Onera and DGA) and
Norway (NEO and FFI). It is a unique system by its spatial sampling -0.5m with a 500m swath at a ground height of
2000m- combined with its wide spectral coverage -from 0.4μm to 11.5μm in the atmospheric transmission bands.
Its infrared component, named SIELETERS, consists in two high étendue imaging static Fourier transform
spectrometers, one for the midwave infrared and one for the longwave infrared. These two imaging spectrometers are
closely similar in design, since both are made of a Michelson interferometer, a refractive imaging system, and a large
IRFPA (1016x440 pixels). Moreover, both are cryogenically cooled and mounted on their own stabilization platform
which allows the line of sight to be controlled and recorded. These data are useful to reconstruct and to georeference the
spectral image from the raw interferometric images.
The visible and shortwave infrared component, named Hyspex ODIN-1024, consists of two spectrographs for VNIR and
SWIR based on transmissive gratings. These share a common fore-optics and a common slit, to ensure perfect
registration between the VNIR and the SWIR images. The spectral resolution varies from 5nm in the visible to 6nm in
the shortwave infrared.
In addition, the STAD, the post processing and archiving system, is developed to provide spectral reflectance and
temperature products (SRT products) from calibrated georeferenced and inter-band registered spectral images at the
sensor level acquired and pre-processed by SIELETERS and Hyspex ODIN-1024 systems.
Sysiphe is an airborne hyperspectral imaging system, result of a cooperation between France (Onera and DGA) and
Norway (NEO and FFI). It is a unique system by its spatial sampling -0.5m with a 500m swath at a ground height of
2000m- combined with its wide spectral coverage -from 0.4μm to 11.5μm in the atmospheric transmission bands. Its
infrared component, named Sieleters, consists in two high étendue imaging static Fourier transform spectrometers, one
for the midwave infrared and one for the longwave infrared. These two imaging spectrometers have very close design,
since both are made of a Michelson interferometer, a refractive imaging system, and a large IRFPA (1016x440 pixels).
Moreover, both are cryogenic and mounted on their own stabilization platform which allows at once to actively control
and independently measure the line of sigh. These data are useful to reconstruct and to georeference the spectral image
from the raw interferometric images. Sysiphe first flight occurred in September, 2013.
The SYSIPHE system is the state of the art airborne hyperspectral imaging system developed in European cooperation.
With a unique wide spectral range and a fine spatial resolution, its aim is to validate and quantify the information
potential of hyperspectral imaging in military, security and environment applications. The first section of the paper recalls the objectives of the project. The second one describes the sensors, their implementation onboard the platform and the data processing chain. The last section gives illustrations on the work in progress.
Imaging static Fourier transform spectrometers are well-adapted instruments for remote sensing. However, the path from the raw images to the spectral images is quite long, thus multiplying the potential noise sources. In this article, we propose to review these perturbation sources. We first recall how detector or photon noise on the interferogram affects the estimated spectrum. Then, we focus on inhomogeneities of the focal plane array. Although these inhomogeneities are deterministic, they generate noise when they are ill-corrected. Finally, we describe defects due to errors that may occur during the image processing step. We also give some avenues to limit the impact of these noises.
KEYWORDS: Cameras, Control systems, Line of sight stabilization, Optical imaging, Target acquisition, Imaging systems, Target designation, Detection and tracking algorithms, Filtering (signal processing), Digital signal processing
In an airborne optical imaging system, a key function is to command and control the observation direction or line of sight whose aim is to track various targets during a determined period. Indeed, the optical images will be affected by the residue of pointing.
Moreover, the airborne environment adds complementary difficulties on the line of sight control. The Line of sight command is composed of three phases : the "designation", the "hanging" and the "tracking" phases. Each of one is characterized by a specific control law. The first one allows to place the instrument line of sight following the provisional target trajectory. The second control law is optimized for the target acquisition and the third one is dedicated to track the target. The acquired imagery allows, after validation of the known target and/or rallying it by human intervention, to calculate an angular deflection for measurement of tracking error. According to the scientific objective of the imaging system, various types of targets could be observed. So the angular deflection measurement is calculated by barycentric or images correlation methods. This information is injected into the second control law which will be substituted, without unhooking, to the first performed for designation. The line of sight of the imaging system is realized with a gyro-mirror for the fine pointing in front of a camera and an independent mechanical framework, supporting the camera and the gyro-mirror. This pedestal offers to the instrument a wide angular field of view but a coarse pointing. These elements individually controllable are dimensioned for the design and realization for the control law. This paper presents each station to study needs for the definition and the realization of the control law for an airborne optical imaging instrument. This paper also describes an approach of the harmonization of the lines of sight of different instruments pointing the same target.
Onera has designed and developed an scientific airborne infrared measurement system. This system is constituted of a supervisor computer and two scientific instruments (a cryogenic IR multiband camera and a cryogenic IR spectro-radiometer). This article presents the different elements of the system and focuses on the design of the cryogenic IR camera. The IR camera design involves instrument control, data acquisition, IRIG time stamping, target acquisition and tracking. This article highlights also the communication design using two Ethernet networks linking the elements of the experimental measurement chain.
A Shack-Hartmann wave-front sensor has been used to characterize non-isotropic turbulence simulated in a transonic wind-tunnel. Wavefront measurements have been obtained for a large number of turbulent conditions.
The phase 2-D power spectra exhibit standard Kolmogorov -11/3 power law but also -17/3 power law in the transverse direction, which appears to be a new characteristic for such turbulent flows. Results are further discussed in terms of the various simulated turbulent parameters.
This paper presents new concepts for a Fringe Sensor Unit (FSU) optimized for high accuracy and low flux operation. This concept has been studied for the VLTI/PRIMA instrument in the H (and K) bands. To optimize both photon use and accuracy, an efficient spatial achromatic discrete modulation is chosen. For optical path difference measurements, most of the photons are used in a single polychromatic quadrature while the adjustable remaining part is dispersed for simultaneous group delay tracking. Integration time can be very short since no moving device is used. This FSU can also be turned to a classical two quadratures FSU if needed, for differential delay or visibility measurements. Optical designs for these FSUs are proposed. These simple designs are also very well suited to future space instruments. Theoretical performance and simulation results are finally given and compared to other existing devices.
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