Gerd Ulbrich

APEX: current status of the airborne dispersive pushbroom imaging spectrometer

Over the past few years, a joint Swiss/Belgium ESA initiative resulted in a project to build a precursor mission of future spaceborne imaging spectrometers, namely APEX (Airborne Prism Experiment). APEX is designed to be an airborne dispersive pushbroom imaging spectrometer operating in the solar reflected wavelength range between 4000 and 2500 nm. The system is optimized for land applications including limnology, snow, and soil, amongst others. The instrument is optimized with various steps taken to allow for absolute calibrated radiance measurements. This includes the use of a pre- and post-data acquisition internal calibration facility as well as a laboratory calibration and a performance model serving as a stable reference. The instrument is currently in its breadboarding phase, including some new results with respect to detector development and design optimization for imaging spectrometers. In the same APEX framework, a complete processing and archiving facility (PAF) is developed. The PAF not only includes imaging spectrometer data processing up to physical units, but also geometric and atmospheric correction for each scene, as well as calibration data input. The PAF software includes an Internet based web-server and provides interfaces to data users as well as instrument operators and programmers. The software design, the tools and its life cycle are discussed as well.

Refractive-index changes caused by proton radiation in silicate optical glasses.

We have studied experimentally, by using a differential interferometric technique, the effect of proton radiation on the refractive index of commercial (Schott) silicate crown glasses, BK7 and LaK9, and their radiation-resistant counterparts. The strongest effect was observed for the radiation-hard lanthanum crown LaK9G15: At a 0.65-Mrad dose the index change was approximately 3 x 10(-5). Radiation-hard glasses are used in optical systems operating in radiation environments because they prevent spectral transmission degradation in the visible. However, such glasses are not protected against radiation-induced refractive-index perturbations, and a diffraction-limited optical system based on such glasses may fail owing to radiation-induced aberrations.

Calibration Methodology for the Airborne Dispersive Pushbroom Imaging Spectrometer (APEX)

APEX is a dispersive pushbroom imaging spectrometer operating in the spectral range between 380 - 2500 nm. The spectral resolution will be better than 10 nm in the SWIR and < 5 nm in the VNIR range of the solar reflected range of the spectrum. The total FOV will be ± 14 deg, recording 1000 pixels across track with about 300 spectral bands simultaneously. A large variety of characterization measurements will be performed in the scope of the APEX project, e.g., on-board characterization, frequent laboratory characterization, and vicarious calibration. The retrieved calibration parameters will allow a data calibration in the APEX Processing and Archiving Facility (PAF). The data calibration includes the calculation of the required, time-dependent calibration coefficients from the calibration parameters and, subsequently, the radiometric, spectral and geometric calibration of the raw data. Because of the heterogeneity of the characterization measurements, the optimal calibration for each data set is achieved using a special assimilation algorithm. In the paper the different facilities allowing characterization measurements, the PAF and the new data assimilation scheme are outlined.

Status of the airborne dispersive pushbroom imaging spectrometer APEX (Airborne Prism Experiment)

Over the past few years, a joint Swiss/Belgian initiative resulted in a project to build a new generation airborne imaging spectrometer, namely APEX (Airborne Prism Experiment) under the ESA funding scheme named PRODEX. APEX is designed to be a dispersive pushbroom imaging spectrometer operating in the solar reflected wavelength range between 400 and 2500 nm. The spectral resolution is designed to be better than 10 nm in the SWIR and 5 nm in VIS/NIR range of the spectrum. The total FOV is on the order of /spl plusmn/14/spl deg/, recording 1000 pixels across track, and max. 300 spectral bands simultaneously. The final radiance data products are well characterized and calibrated to be traceable to absolute standards. APEX is subdivided into an industrial team responsible for the optical instrument, the calibration home base, and the detectors, and a science and operational team, responsible for the processing and archiving of the imaging spectrometer data, as well as its operation. APEX is in its design phase with partial breadboarding activities and will be operationally available to the user community in the year 2005.

Signal-to-noise ratio of pseudo-random noise continuous wave backscatter lidar with analog detection

Backscatter lidars are useful tools for range determination in various applications, particularly if they provide compact and robust set-ups with suitable light sources, efficient sensors and adequate signal processing. A promising perspective to achieve such requirements is offered by the Pseudo-Random Noise continuous wave (PRN cw) lidar technique, employing cw laser diodes as transmitter. Here the Signal-to-Noise ratio (SNR) of such lidar will be investigated in detail for the following cases: power (amplitude) modulation of the transmitted laser beam and analogue detection by an avalanche photodiode, surface detection. The SNR is calculated numerically, allowing selection of the factors limiting the lidar detection under various environmental conditions. A set of various measurements, obtained with a PRN cw lidar, employing a diode laser in the near IR spectral range and an avalanche photodiode will be presented. The results from the analytical and numerical study are compared with the experimental results. Various applications for the PRN cw lidar will be examined and critical factors influencing its detection performances discussed. Finally an outlook is provided for possible applications of such a lidar in specific atmospheric and surface measurements.

Dynamic holography for the space qualification of large reflectors

The next generation of infrared - sub mm space telescopes requires reflectors with large dimensions, high quality and, according to weight issues, are based on composite or new materials technology. The challenging tasks of on-ground testing are to achieve the required accuracy in the measurement of these reflectors shape and antenna structures and to verify their performance under simulated space conditions (vacuum, low-high temperatures). A holographic camera for the verification and validation of this type of reflector in a space environment is presented. A diffuser is implemented to measure the deformations of reflective surfaces in a more flexible way. The system has been made compatible with the vacuum conditions. Some elements of the holographic camera (camera lenses, CCD, crystal, optical fibre) have been adapted and tested under vacuum. The metrological certification of the whole system is realised by the measurement of a parabolic CFRP reflector with a 1.1 meter diameter. The results are compared to the one achieved with a high spatial resolution IR interferometer on the same reflector in laboratory conditions and under thermal vacuum conditions. This later test consists in measuring the deformations of the reflector between an initial state at a selected temperature and a final state at another temperature. The comparison between the high spatial resolution IR interferometer and this dynamic holographic method showed very good qualitative and quantitative agreement between the techniques, thus verifying the potential of this new Holographic approach.

Space radiation sensitivity of glasses: first results toward a comprehensive dose coefficients database

Radiation sensitivity of glass is a general concern for the designer of space optical instruments. It has been proposed that the effect of radiation can be described within the dose coefficient approximation. In the paper we discuss the effect of gamma and proton radiation on the transmission and the refractive index of a number of commercial optical glasses. This experimental study is intended for establishment of a data-base, which will be useful to predict the effect of space radiation on optical systems.

APEX-Airborne Prism Experiment: The Realization Phase of an Airborne Hyperspectral Imager

APEX (Airborne Prism EXperiment) is a project of the European Space Agency ESA focusing on high accuracy simulation, calibration and validation for spaceborne remote sensing instruments. The instrumentation comprises a hyperspectral imager for various standard airborne platforms, a fixed installed calibration home base and a complete facility for data processing and archiving. The pushbroom-type instrument accommodates two spectrometer channels covering a spectral range from 0.38 up to 2.5 micron. The spatial/spectral resolution amounts to 1000 samples at 28-degree field of view with 312 spectral bands. The overall instrument design and its built-in characterization unit will allow excellent performance stability under various flight conditions. The presentation will focus on the design, development and realization phases of the instrument and discuss various highlights of technical achievements, as there are the infrared HgCdTe detector with extended array format for the short wave infrared channel, the thermal/mechanical stabilization of the spectrometer and the realization of the infrastructure for high accuracy characterization and calibration of the instrument.

Experimental investigations of the potential of commercial laser sources for spaceborne imaging laser sensors

We present results from experimental investigations on different laser sources for 3D imaging lidars. The lasers investigated include microchip lasers and fiber lasers and the results are compared to results obtained with diode lasers. The potential of the laser candidates for spaceborne 3D imaging sensors based on the pulsed time-of-flight principle is analyzed. Analysis include limitations in radar key parameters such as peak power, pulse width, beam divergence, parameter stability, efficiency, and impact on sensor mass and volume. After trade-off and selection of suitable laser sources, breadboard models have been realized and tested.

Canister-free videogrammetry system for thermal vacuum and space applications

The development of a canister-free videogrammetry system is presented. Applications in view, are coordinate measurements during thermal vacuum test and on-baord space flight metrology of mechanical structures, reflectors and antennas. The paper presents the breadboard system architecture. Two breadboards have been developed. One is based on a space-qualified micro-imager camera. Lenses and flashers are all commercial components and have been made vacuum compatible. Results of accuracy (typically 50ppm) and resolution (typically 25 ppm) tests, in ambient and in vacuum are also presented.