Under the first phase of the Centre for Earth Observation Instrumentation (CEOI), a breadboard demonstrator of a novel UV/VIS spectrometer has been developed. Using designs from Surrey Satellite Technology Ltd (SSTL) the demonstrator has been constructed and tested at the University of Leicester’s Space Research Centre. This spectrometer provides an exceptionally compact instrument for differential optical absorption spectroscopy (DOAS) applications from LEO, GEO, HAP or ground-based platforms.
Measurement of atmo spheric compounds with climate change or air quality implications is a key driver for the ground and space-based Earth Observation communities. Techniques using UV/VIS spectroscopy such as DOAS provide measurements of ozone profiles, aerosol optical depth, certain Volatile Organic Compounds, halogenated species, and key air quality parameters including tropospheric nitrogen dioxide. Compact instruments providing the necessary optical performance and spectral resolution are therefore a key enabling technology.
The Compact Air Quality Spectrometer (CompAQS) features a concentric arrangement of a spherical meniscus lens, a concave spherical mirror and a suitable curved diffraction grating. This compact design provides efficiency and performance benefits over traditional concepts, improving the precision and spatial resolution available from space borne instruments with limited weight and size budgets. The breadboard spectrometer currently operating at the University of Leicester offers high throughput with a spectral range from 310 to 450 nm at 0.5nm(UV) to 1.0nm (visible) resolution, suitable for DOAS applications. The concentric design is capable of handling high relative apertures, owing to spherical aberration and coma being near zero at all surfaces. The design also provides correction for transverse chromatic aberration and distortion, in addition to correcting for the distortion called ‘smile’ – the curvature of the slit image formed at each wavelength. These properties render this design capable of superior spectral and spatial performance with size and weight budgets significantly lower than standard configurations.
In this presentation, the design of the spectrometer is detailed, with results from instrument characterisations undertaken at the University of Leicester, including demonstrations of DOAS fits for key air quality species.
The CHRIS instrument was launched on ESA’s PROBA platform in October 2001, and is providing hyperspectral images of selected ground areas at 17m ground sampling distance, in the spectral range 415nm to 1050nm. Platform agility allows image sets to be taken at multiple view angles in each overpass. The design of the instrument is briefly outlined, including design of optics, structures, detection and in-flight calibration system. Lessons learnt from construction and operation of the experimental system, and possible design directions for future hyperspectral systems, are discussed.
Sound policymaking requires high confidence in climate predictions verified against decadal change observations with robustly known accuracy. Yet, our ability to monitor and predict the future of the climate is inadequate as we currently do not possess sufficient accuracy in our observing capability to confidently observe the small but critical climate change signals that are expected to occur over decadal time scales. These signals are fundamental to assessing the accuracy of climate change projections made by models and for the unambiguous attribution of climate change.
This paper describes a Compact High Resolution Imaging Spectrometer (CHRIS) developed at Sira. The imaging spectrometer is flying on PROBA, a small agile satellite, which was launch in 2001. This paper provides details of the instrument design and performance. The main purpose of the instrument is to provide images of land areas, particularly the measurement of the Bi-directional Reflectance Distribution Function (BRDF) properties for selected targets on the Earth surface using multi-angle observations. The platform provides pointing in both across-track and along-track directions, for target acquisition, BRDF and aerosol measurements and slow pitch during imaging in order to increase the integration time of the instrument. This increase in integration time enhances the target radiometric resolution. The spectral range covered by the instrument extends from 400nm to 1050nm. The platform orbits the Earth with an apogee of 673km and a perigee of 560km. The spatial sampling interval at apogee is approximately 17m. In this mode it is possible to readout 19 spectral bands. The location and width of the spectral bands are programmable. Selectable on-chip integration can increase the number of bands to 63 for a spatial sampling interval of 34m. The swath width imaged is 13km at perigee.
This paper describes the initial flight experience of the Compact High Resolution Imaging Spectrometer (CHRIS) developed at Sira Electro-Optics Ltd. The imaging spectrometer is flying on PROBA, a small agile satellite, which was launched in October 2001. The main purpose of the instrument is to provide images of land areas. The platform provides pointing in both across-track and along-track directions, for target acquisition and multi-angle observations, particularly for measurement of the Bi-directional Reflectance Distribution Function (BRDF) properties of selected targets. The platform also provides pitch motion compensation during imaging in order to increase the integration time of the instrument, increasing the number of spectral bands that can be read and enhancing radiometric resolution. The instrument covers a spectral range from 400 nm to 1050 nm, at ≤11 nm resolution. The spatial sampling interval at perigee is approximately 17 m. In this mode it is possible to read out 19 spectral bands. The locations and widths of the spectral bands are programmable. Selectable on-chip integration can increase the number of bands to 63 for a spatial sampling interval of 34 m. The swath width imaged is 13 km at perigee.
The CHRIS instrument is a space-based imaging spectrometer that will provide 10 nm spectral resolution over the spectral range from 415 nm to 1050 nm. The nominal spatial sampling interval will be 25 m, however, larger sampling intervals are possible. Band selection, spectral bandwidths and the spatial sampling interval will be programmable. The instrument is planned to be launched on an agile small satellite of the 100 kg class. This satellite will operate in a sun-synchronous, high inclination orbit at approximately 830 km. At this altitude the instrument can provide 19 spectral bands with a spatial sampling interval of 25 m at nadir. The field of view of CHRIS is 18.6 km. Attitude control of the platform will allow access to non-nadir targets, multi-angle observations of selected targets and improved radiometric resolution. This paper describes the optical design of the instrument, including the telescope, spectrometer detector and in-flight calibration hardware, as well as critical alignment procedures, with emphasis on spectrometer assembly and stray light control. Results of performance and calibration measurements are presented.
KEYWORDS: Sensors, Breast, X-rays, Signal to noise ratio, Monte Carlo methods, Digital mammography, Mammography, Charge-coupled devices, Physics, Modulation transfer functions
This paper describes the requirements and constraints involved in the design of a digital scanning mammography system. The SNR model of Muntz is used to assess the influence of detector size and detected scatter-to-primary ratio on the detectability of microcalcifications in a breast phantom, and hence to consider the design of an optimized system. The effects of patient dose, exposure time and various technological constraints, such as x-ray tube power limits, maximum scanning speed and system MTF, are also discussed. It is demonstrated that the combined use of an air gap between the patient and the detector and a wider scanning slot can give the same scatter-to-primary ratio as a significantly smaller slot, while reducing the x- ray tube load. It is also demonstrated that, to some extent, the tube power constraints can be overcome by using higher kVp, less added filtration and a smaller source to patient distance.
The Medium Resolution Imaging Spectrometer (MERIS) is an optical instrument which is intended to be flown on the first European Polar Platform, scheduled for launch in 1997. The instrument has a spectral range of 400 nm to 1050 nm, a swath width of 1500 km, a spectral resolution of 1.25 nm and spatial resolution of 250 m. The instrument has applications in the fields of oceanographic, land and atmospheric research.
The Medium Resolution Imaging Spectrometer (MERIS) is intended to be flown on the First European Polar Platform (PPF) scheduled for launch in 1997. The instrument designed for ocean monitoring, is capable of transmitting 15 spectral bands programmable in width and position across the extended visible domain 400nm - 1050nm. A spatial resolution of 250m is reached on-ground, with a wide field-of-view (FOV) of 82 degrees. To achieve high radiometric performances for the images, an in-flight calibration system is required with data correction on-board in real time. Calibration coefficients will be applied on raw images to reach both a spatial and spectral uniformity of 0.05 percent, and an absolute accuracy of 2 percent.
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