We review development of the TIMS beginning in the early part of the decade and up to preliminary results of work in progress. We describe a geostationary application (geoCARB) at near PDR maturity for mapping CO2, CH4 and CO column mixing ratios on continental scale areas (e.g., Australia and East Asia) several times per day on contiguous samples with spacing the order 3 km at the sub satellite point. Measurements per footprint are expected to be acquired with median mission SNRs >> 300, 300 and 240 in the traditional spectral regions (e.g., OCO and TANSO-FTS on GOSAT) for CO2, namely the O2 A-band, and the weak and strong bands of CO2 near 1.61 and 2.06 microns; and >> 200 in a region near 2.32 microns for CO and CH4.The resolving powers are 15000, 15000, 11000 and 11000 in the 4 regions, respectively. Given this performance the median mission retrieval for CO2, CH4 and CO column mixing ratio is expected to be considerably better than 0.7, 1.0 and 10.0%, respectively. These measurements over several years would provide a break through reduction in the uncertainty for the sources of CO2 and CH4 within the large geostationary field of regard of the geoCARB, and the CO measurement would assist in source attribution.
Spectrometers, in which a grating is coupled with a two dimensional detector array to provide high resolution spectra
without the need for spectral scan mechanisms can be designed in compact, rugged, configurations, making them well
suited for spaceborne spectral mapping applications.
We are pursuing the use of this technology for spaceborne tropospheric air quality monitoring, targeting high spectral
resolution solar reflective and thermal emission spectroscopy in the wavelength range 2 to 5 μm. In this region key tropospheric
pollutant and greenhouse gases such as O3, CO, CO2, CH4, HCHO, and H2O, have strong spectral features.
The relatively short wavelengths allow for the use of well-developed detector technology and passive cooling. With sufficient
resolving power, sensitivity, and judicious combination of spectra, good information on tropospheric vertical distributions,
including boundary layer data, can be obtained.
This paper describes the performance characteristics of a laboratory prototype of such a spectrometer, focused on the
measurement of CO spectra in the range 4.56 to 4.73 μm. The design uses a cooled grating and optical train, coupled
with a cooled 1024 x 1024 pixel HgCdTe array. It achieves a spectral resolution of ~0.32 cm-1 and NESR of 5.8x10-9
w/cm2/sr/cm-1. Both laboratory absorption spectra and zenith-looking air emission spectra of CO are presented. The
spectrometer is the pre-cursor to a combined 4.6/2.33 μm instrument being developed under NASA funding and designed
to demonstrate the unique vertical information capability of such a combination for tropospheric CO measurement.
We give a brief discussion of a spaceborne concept focused on this technique.
We are currently developing grating mapping spectrometers (GMS) with very high spectral resolution, very low noise, and very wide field of view. These also would be very compact facilitating deployment in either a leo or geo application. The measurement set could be very comprehensive, addressing air quality, climate change and meteorology, or subsets of these. For this presentation we'll focus on potential applications of these GMS for air quality measurements of the species ozone O3, formaldehyde HCHO and carbon monoxide CO. We will discuss these applications at various levels of complexity and the commensurate value for application to understanding and forecasting air quality. At lowest complexity we would utilize a single GMS operating in the solar reflective infrared region for column measurements of O3 and HCHO. A more complex approach would utilize a second and/or third GMS for thermal emissive O3 measurements that provide improved vertical resolution, and for CO profile. Our major emphasis is the lowest tropospheric air layer 0-2 km. For realistic models of these GMS we'll present retrieval performance as predicted by a linear error analysis. In a polar leo orbit the most complex approach could provide twice daily global mapping with some footprints as small as 1.6 km at nadir. We'll present results from an in house lab demonstration GMS. This demo is a predecessor to an advanced design that we are currently developing with support of the NASA ESTO Instrument Incubator Program (IIP).
We present test data for a solid ZnSe air gapped etalon with free spectral range 3 cm-1 and finesse >70 (i.e., spectral resolution <0.043 cm-1). We present an instrument concept, the Tropopsheric Ozone Sounding (TOS) Dual Etalon Cross Tilt Order Sorting Spectrometer (DECTOSS), that would use an etalon like this to acquire nadir data at resolution <0.06 cm-1 and signal to noise the order 1000 on a range from 1036 to 1071 cm-1 in footprints with crosstrack dimension selectable (e.g., the order tens to hundreds of km), and with along track dimension the order 17 km. Instrument accommodation is the order 25 kg, 110 W and 1 mbps. We present linear error analysis for retrieval of tropospheric ozone from the data acquired by the TOS-DECTOSS. Indication is that more than 2.5 vertical layers of information on tropospheric information are retrievable. An example of the deployment of the TOS-DECTOSS would be as an instrument of opportunity (IOO) add on to the US National Polar-orbiting Operational Environmental Satellite System (NPOESS). The huge advantage of the TOS-DECTOSS as compared with UV techniques for tropospheric ozone measurement is that it the can be used both day and night, the latter is not possible in the UV. The considerable advantage in signal to noise compared with a Fourier Transform Spectrometer (FTS) for tropospheric ozone measurement, on considering that for a given footprint the DECTOSS and FTS integration times are comparable, is that the DECTOSS noise per spectral sample is dominated by statistical fluctuations of signal photons that are passed through its narrow 0.06 cm-1 bandpass, while for a similar FTS spectral sample the noise is due to fluctuations of the signal photons through the FTS bandpass of tens of cm-1. The TOS-DECTOSS signal to noise advantage on the FTS is also enhanced in that the spectral sample density of the TOS-DECTOSS data is more than one hundred times larger than for the FTS.
The Dual Etalon Cross Tilt Order Sorted Spectrometer (DECTOSS) uses relatively inexpensive off the shelf components in a small and simple package to provide ultra high spectral resolution over a limited spectral range. For example, the modest first try laboratory test setup DECTOSS we describe in this presentation achieves resolving power ~ 105 on a spectral range of about 1 nm centered near 760 nm. This ultra high spectral resolution facilitates some important atmospheric remote sensing applications including profiling cirrus and/or aerosol above bright reflective surfaces in the O2 A-band and the column measurements of CO and CO2 utilizing solar reflectance spectra. We show details of the how the use of ultra high spectral resolution in the O2 A-band improves the profiling of cirrus and aerosol. The DECTOSS utilizes a Narrow Band Spectral Filter (NBSF), a Low Resolution Etalon (LRE) and a High Resolution Etalon (HRE). Light passing through these elements is focused on to a 2 Dimensional Array Detector (2DAD). Off the shelf, solid etalons with airgap or solid spacer gap are used in this application. In its simplest application this setup utilizes a spatially uniform extended source so that spatial and spectral structure are not confused. In this presentation we'll show 2D spectral data obtained in a desktop test configuration, and in the first try laboratory test setup. These were obtained by illuminating a Lambertian screen with (1) monochromatic light, and (2) with atmospheric absorption spectra in the oxygen (O2) A-band. Extracting the 1D spectra from these data is a work in progress and we show preliminary results compared with (1) solar absorption data obtained with a large Echelle grating spectrometer, and (2) theoretical spectra. We point out areas for improvement in our laboratory test setup, and general improvements in spectral range and sensitivity that are planned for our next generation field test setup.
The Waves middle class Explorer mission (WE) is proposed to observe and quantify the effects of small-scale internal Gravity Waves (GW) in the Earth's atmosphere from source regions in the troposphere and lower stratosphere to the mesosphere, lower thermosphere, and ionosphere (MLTI) where the GW have their most dramatic effects. These are now understood to be a key element in defining large-scale circulation, thermal and constituent structures, and variability of the stratosphere and MLTI. The WE instrumentation consists of 5 nadir and limb viewing sensors of the wave perturbed emission structure due to GW throughout the source and affected regions. The WE PI is Prof. G.R. Swenson. This paper addresses the measurement strategy and implementation for two of these instruments, the Source Wave And Propagation Imager (SWAPI), and the Hydroxyl Airglow Wave Imager (HAWI). The SWAPI uses multi-spectral sublimb imaging measurements in the CO2 (nu) 3 band near 4210 nm to identify GW sources, and their propagation through the stratosphere. Its measurement strategy is driven by data, particularly sublimb images in the CO2 (nu) 3 band that were obtained by instrumentation deployed on the Midcourse Space Experiment (MSX) satellite, and by the WE team member's data analysis and models. Similarly team member's ground based observational experience and data analysis drives the HAWI measurement strategy.
Passive radiative cooling is desirable for space borne detectors because it is generally cheaper, less massive and power consumptive than cooling by a mechanical refrigerator or expendable cryogens. Our interest is space borne nadir imaging the OH airglow in Q-branch features of the 9->6 band at approximately 1382.3 nm, and the 2->0 band at approximately 1434.4 nm with sufficient signal to noise to quantitatively retrieve wave structure. Low noise 256 X 256- 40 micrometer pitch HgCdTe detector arrays are available for our application. E.g., the Rockwell Science Center standard 2.5 micrometers PACE product bonded on to the PICNIC read out MUL satisfies our high sensitive and low read noise requirements, but would require a mechanical refrigerator or expendable cryogen to cool sufficiently to satisfy our dark current requirement. To demonstrate an option that would provide our required performance at viable passive radiative cooling temperature, we have procured examples of the more recent RSC double layer planar heterogenous HgCdTe 2D arrays with shorter wavelength cutoff and produced by molecular beam epitaxy on a CdZnTe substrate, and bonded to the PICNIC MUL. Here we describe our test procedures and results that these at relatively warm temperature, the order 160 to 170K, satisfy the requirements for our OH airglow wave imaging application. We describe an instrument model and observational operations to observe the OH airglow wave structure with signal to noise > 100.
For IR detectors that require cooling to temperatures lower than viable by passive radiative cooling, the mechanical refrigerator is an attractive alternative to expendable cryogen. It provides dramatic reduction in mass, and increased lifetime. For very low noise detectors, there may be some concern that mechanical cooler operation could provide an additional significant detector noise source. Here at LMAATC we have developed a mini-cooler for space borne application, a Stirling compressor driving a pulsetube, and have conducted test to determine if it would induce significant additional noise no cooling a low noise Mie HgCdTe 2D detector array with 3800 nm cutoff. We set up to cool the detector with our mini-cooler, and measure the noise with the cooler running, and with it turned off. We found that cooler operation increased noise barely perceptibly over the cooler off case. We will present implications for our planned space borne instrument, the Source Wave and Propagation Imager. It is an imaging spectrometer that will obtain measurements just below the limb in the 4180 to 4250 nm region of the CO2 band. Tropospheric production of atmospheric internal gravity waves, and their subsequent propagation through stratospheric will be retrieved from these data.
The objective of this effort is to improve x-ray absorption and light production while maintaining high spatial resolution in x-ray imaging phosphor screens. Our current target is to improve screen absorption efficiency and screen brightness by factors of 2 or greater over existing screens that have 10-1p/mm resolution. In this program, commercial phosphor screens are combined with highly absorbing, high-resolution scintillating fiber-optic (SFO) face plates to provide a hybrid sensor that exhibits superior spatial resolution, x-ray absorption, and brightness values over the phosphor material alone. These characteristics of hybrid scintillators can be adjusted to meet specific x-ray imaging requirements over a wide range of x-ray energy. This paper discusses the design, fabrication, and testing of a new series of hybrid scintillators.
A Loral 1024 X 1024 CCD array with 15-micron pixels has been incorporated as the focal plane detector in a new imaging spectrometer for auroral research. The large format low-noise CCD provides excellent dynamic range and signal to noise characteristics with image integration times on the order of 60 seconds using f/1.4 camera optics. Further signal enhancement is achieved through on-CCD pixel binning. In the nominal binned mode the instrument wavelength resolution varies from 15 to 30 angstrom across the 5000 to 8600 angstrom spectral range. Images are acquired and stored digitally on a Macintosh computer. This instrument was operated at a field site in Godhavn, Greenland during the past two winters (1993, 1994) to measure the altitude distribution of the various spectral emissions within auroral arcs. The height resolution on an auroral feature 300 km distant is approximately 1 km. Examples of these measurements are presented here in snapshot and summary image formats illustrating the wealth of quantitative information provided by this new imaging spectrometer.
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