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A proposed sensor will operate in pushbroom mode. The design uses a single area-array MWIR detector with a 1280 x 1024 element format. A swath approaching 10,000 pixels wide is provided by sub-dividing the swath between 8 telescopes that image onto separate sets of strip filters and detector rows. This allows a total swath width approaching 1000km wide to be scanned at 100m ground sample distance (GSD). The relatively small GSD, and use of multiple detector rows for each swath-section and band, will provide a capability for detection of small fires, with radiant powers in the range 0.5 to 1.0 MW. Wide swath will maximize statistical data collected by the sensor, and will also be of potential interest for development of constellation providing global fire monitoring.
The MSI instrument will be formed in two parts: a visible-NIR-SWIR (VNS) system, radiometrically calibrated using a sunilluminated diffuser, and a thermal IR (TIR) system calibrated using cold space and an internal black-body. The VNS system will perform push-broom imaging, using linear array detectors (silicon and InGaAs) and 4 separate lenses. The TIR system will use a microbolometer array detector in a time delay and integration (TDI) mode. Critical issues discussed for the VNS system include detector selection and detailed optical design trade-offs. The latter are related to the desirability of dichroics to achieve a common aperture, which influences the calibration hardware and lens design. The TIR system’s most significant problems relate to control of random noise and bias errors, requiring optimisation of detector operation and calibration procedures.
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.
• a visible-NIR-SWIR (VNS) optical unit radiometrically calibrated using a sun illuminated quasivolume diffuser and shutter system
• a thermal IR (TIR) optical unit radiometrically calibrated using cold space and an internal black-body. This paper, being the first of a sequence of two, will provide an overview of the MSI and enter into more detail the critical performance parameters and detailed design the MSI TIR optical design.
The TIR concept is to provide pushbroom imaging of its 3 bands through spectral separation from a common aperture. The result is an efficient, well controlled optical design without the need for multiple focal plane arrays. The designed focal plane houses an area array detector and will meet a challenging set of requirements, including radiometric resolution, accuracy, distortion and MTF.
TROPOMI is the next step, scheduled for launch in 2015. It combines the broad wavelength range from SCIAMACHY from UV to SWIR and the broad viewing angle push-broom concept from OMI, which makes daily global coverage in combination with good spatial resolution possible. Using spectral bands from 270-500nm (UV-VIS) 675-775nm (NIR) and 2305-2385nm (SWIR) at moderate resolution (0.25 to 0.6nm) TROPOMI will measure O3, NO2, SO2, BrO, HCHO and H2O tropospheric columns from the UV-VIS-NIR wavelength range and CO and CH4 tropospheric columns from the SWIR wavelength range. Cloud information will be derived primarily from the O2A band in the NIR. This will help, together with the aerosol information, in constraining the light path of backscattered solar radiation. Methane (CH4), CO2 and Carbon monoxide (CO) are the key gases of the global carbon cycle. Of these, Methane is by far the least understood in terms of its sources and is most difficult to predict its future trend. Global space observations are needed to inform atmospheric models. The SWIR channel of TROPOMI is designed to achieve the spectral, spatial and SNR resolution required for this task.
TROPOMI will yield an improved accuracy of the tropospheric products compared to the instruments currently in orbit. TROPOMI will take a major step forward in spatial resolution and sensitivity. The nominal observations are at 7 x 7 km2 at nadir and the signal-to-noises are sufficient for trace gas retrieval even at very low albedos (down to 2%). This spatial resolution allows observation of air quality at sub-city level and the high signal-to-noises means that the instrument can perform useful measurements in the darkest conditions.
TROPOMI is currently in its detailed design phase. This paper gives an overview of the challenges and current performances. From unit level engineering models first results are becoming available. Early results are promising and this paper discusses some of these early H/W results.
TROPOMI is the single payload on the Sentinel-5 precursor mission which is a joint initiative of the European Community (EC) and of the European Space Agency (ESA). The 2015 launch intends to bridge the data stream from OMI / SCIAMACHY and the upcoming Sentinel 5 mission. The instrument is funded jointly by the Netherlands Space Office and by ESA. Dutch Space is the instrument prime contractor. SSTL in the UK is developing the SWIR module with a significant contribution from SRON. Dutch Space and TNO are working as an integrated team for the UVN module. KNMI and SRON are responsible for ensuring the scientific capabilities of the instrument.
A review and analysis was undertaken of the Micro-Optical Electro-Mechanical Systems (MOEMS) available in the market with potential application in systems for Earth Observation. A summary of this review will be presented.
Following the review two space-instrument design concepts were selected for more detailed analysis. The first was the use of a MEMS device to remove cloud from Earth images. The concept is potentially of interest for any mission using imaging spectrometers. A spectrometer concept was selected and detailed design aspects and benefits evaluated. The second concept developed uses MEMS devices to control the width of entrance slits of spectrometers, to provide variable spectral resolution.
This paper will present a summary of the results of the study.
Apertures, and complete spectrometer sizes, are reduced if the gratings provide high angular spectral dispersion (radians/nm) – this is a property of “immersed” gratings 1,2. An immersed grating is a reflecting grating formed on a prism of refracting material, in which the incident and diffracted beams are both in the refracting medium. Instruments will typically use immersed gratings in silicon for short-wave IR bands, and silica for near-IR bands. This paper describes example designs for spectrometers using immersed gratings, and early results from a current development of immersed gratings are outlined.
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