The geoCARB sensor uses a 4-channel slit-scan infrared imaging spectrometer to measure the absorption spectra of
sunlight reflected from the ground in narrow wavelength regions. The instrument, which is to be hosted on a
geostationary communication satellite, is designed to provide continual monitoring of greenhouse gas over continental
scales, several times per day, with a spatial resolution of a few kilometers. The paper discusses the image navigation and
registration (INR) of the geoCARB optical footprints on to the earth’s surface.
The instrument acquires data in a step and stare mode with 4.08 s stare time and 0.34s step time on 1016 footprints
spaced by 2.7 km at nadir in the NS direction along the slit, which is stepped in 3 km EW increments. Knowledge of the
instrument line of sight is obtained through use of a dual-head star tracker system (STS), high-precision optical encoders
for the scan mirrors, a GPS receiver, and a highly stable common optical bench to which the instrument components, the
scan mirror assembly, and the heads of the STS are kinematically mounted.
While attitude disturbances due to jitter and solar array flex affect spatial resolution, we show that the effect on INR is
negligible. GeoCARB performs a star sighting every 30 minutes to compensate for its diurnal alignment variation
relative to the STS, enabling a 1 sigma INR accuracy of 0.38 and 0.51 km at nadir in the NS and EW directions,
respectively. Coastline identification may be used to improve accuracy by 6%, while an additional 20% improvement is
achievable through identification of systematic errors via extensive post-processing. The paper quantifies all error
sources and describes how each of them affects overall INR accuracy.
The geoCARB sensor uses a 4-channel push broom slit-scan infrared imaging grating spectrometer to measure the absorption spectra of sunlight reflected from the ground in narrow wavelength regions. The instrument is designed for flight at geostationary orbit to provide mapping of greenhouse gases over continental scales, several times per day, with a spatial resolution of a few kilometers. The sensor provides multiple daily maps of column-averaged mixing ratios of CO2, CH4, and CO over the regions of interest, which enables flux determination at unprecedented time, space, and accuracy scales. The geoCARB sensor development is based on our experience in successful implementation of advanced space deployed optical instruments for remote sensing. A few recent examples include the Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI) on the geostationary Solar Dynamics Observatory (SDO), the Space Based Infrared System (SBIRS GEO-1) and the Interface Region Imaging Spectrograph (IRIS), along with sensors under development, the Near Infared camera (NIRCam) for James Webb (JWST), and the Global Lightning Mapper (GLM) and Solar UltraViolet Imager (SUVI) for the GOES-R series. The Tropospheric Infrared Mapping Spectrometer (TIMS), developed in part through the NASA Instrument Incubator Program (IIP), provides an important part of the strong technological foundation for geoCARB. The paper discusses subsystem heritage and technology readiness levels for these subsystems. The system level flight technology readiness and methods used to determine this level are presented along with plans to enhance the level.
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).
Precise measurements of CH4 in a column of near surface air, and in partial columns above this, would be very valuable in identifying sources/sinks of atmospheric CH4, and its transport. For this purpose we have proposed a grating mapping spectrometer (GMS) for deployment as an Instrument of Opportunity (IOO) on the NPOESS that acquires data in the 2990 to 3050 cm-1 spectral region. It will provide measurements of CH4 absorption of sunlight in the weaker CH4 features in the region, and of thermal emission in the stronger CH4 features in the region. It is the combination of the two that provides the vertical information. The IOO will acquire spectra on a crosstrack swath centered on nadir, and with 1/2 width of 55 degrees on each side of nadir (about 2800 km full width swath on the ground for a nominal 828 km satellite altitude). This with footprints that are about 3.1 km on a side at nadir. The small footprint facilitates cloud screening, and identification of CH4 source hotspots. A capability to project the slit to nadir along the direction from satellite to sun will be utilized for over the ocean viewing in order to facilitate measurements in solar glitter. It will obtain spectra with resolution n < 0.58 cm-1 and sample spacing < 0.17 cm-1. Based on the spectral characteristics and currently achievable very low-noise we do a linear error analysis (Rodgers, [1]) for the simultaneous retrieval of multi-column CH4, humidity, and surface parameters and 13CH4 total column. We show that useful multi-column CH4 retrievals can be obtained, with good near surface sensitivity in sunlit conditions. We also show the 13CH4 column can be retrieved with precision better than 3%. Retrieval of 13CH4 column in the earth's atmosphere is analogous in difficulty to retrieval of the major CH4 isotope column in the Martian atmosphere by a similar GMS deployed on a Mars orbiter. We show that H2O vertical information can be retrieved from these measurements and discuss the potential for ethane column retrieval.
Measurements of the column CH4, CO and CO2 are high priorities of the NPOESS Pre-Planned Product Improvement (P3I) data sets. Risk reduction for existing NPOESS instruments, including mitigation of daytime CO2 SWIR non-LTE effects, is also a high priority. We have proposed an NPOESS Instrument Of Opportunity (IOO) to address these priorities. It consists of two grating mapping spectrometers (GMSs). One that would acquire measurements with high spectral resolution Δv < 0.13 cm-1 of CH4, CO and H2O absorption lines in reflected sunlight in the VSWIR region 4281 to 4301 cm-1, and another for measurements with Δv < 0.30 cm-1 in the SWIR region 2355 to 2430 cm-1. The IOO will acquire spectra on a crosstrack swath from nadir to 55 degrees (about 1400 km on the ground) on footprints that are about 1.55 and 3.1 km on a side at nadir for the two GMS, respectively. The small footprint facilitates cloud screening, and identification of pollution hotspots. We use linear error analysis (LEA, based on the Rodgers [1] paper) to estimate the proposed IOO's performance. The LEA indicates that the IOO should be able to provide CH4 and CO column retrieval over sunlit land (and from ocean glitter when it is viewed) that satisfies or exceeds NPOESS P3I Environmental Data Records (EDRs) requirements in all aspects except refresh where the IOO would provide every two days vs the once per day requirement. Further, it shows the VSWIR IOO data when used in combination with the NPOESS Cross Track Infrared Sounder (CrIS) [2] data should provide: (a) CO profile data with sensitivity to CO in near surface air that is enhanced compared to that in the current TERRA-MOPITT, ACQUA-AIRS and AURA-TES data sets because these are limited to thermal infrared measurements that lack sensitivity to CO in near surface air layer where there is little contrast between the air temperature and the ground surface temperature, (b) CH4 profile with sensitivity in the near surface air layer that is crucial for identifying CH4 sources/sinks (c) and significant improvement in the CrIS retrieved humidity in the near surface layer of air. We show the SWIR IOO data can be used for CO2 column retrieval with near surface air layer sensitivity in the daytime. And also that in combination with CrIS SWIR data facilitates CO2 SWIR non-LTE mitigation that is required for advanced sounding quality temperature profile (TP) retrieval from CO2 SWIR data in daytime conditions. This provides risk reduction in case of degradation in the CrIS LWIR region data.
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.
In this paper we focus on the status and development of critical detector and cooler technology in support of our instrument concept for a Geosynchronous orbiting Nadir Etalon Sounding Spectrometer (GeoNESS) for temperature, moisture and trace species. The concept is a technology derivative of the Cryogenic Limb Array Etalon Spectrometer (CLAES) which is deployed on the NASA Upper Atmospheric Research Satellite (UARS).
Progress in the ten years since the previous SPIE publication (Kumer, 1981) on the subject of modeling high-altitude non-LTE CO2 4.3 micron phenomena is summarized. Several models have been developed in these years. These models implement improvements in radiation transport, spectral characterization, rate constants, atmospheric models, and numerics. Applications of these models to the three distinct cases of daytime, ambient night time, and aurora are discussed, and compared with applications of the older model. Several new data bases have just become, or will soon become available. The obvious applications of the models to these data will be discussed.
The CLAES measurement concept, instrument design, and performance are presented, and the scientific capabilities and measurement modes are discussed. The CLAES experiment involves remote measurement of earth-limb emission spectra. Characteristic vibration-rotation line spectral radiances are obtained between 3.5 and 13 microns and inverted through an iterative relaxation process to yield pressure, temperature, and species mixing ratio. The UARS limb-viewing instruments, including CLAES, combined with the 57-deg orbit inclination, allow for measurements to 80-deg latitudes. CLAES requires high spectral resolution and high radiometric sensitivity to isolate and accurately measure weak emissions from trace species such as HCl and NO against intense backgrounds from abundant emitters such as CO2, H2O, and O3. Accuracy and precision of retrieved quantities, observational modes, and calibration modes are also discussed.
The CLAES is calibrated with a full-aperture blackbody on the instrument-aperture door. In laboratory calibration, the blackbody is resistively heated. On orbit, the blackbody is intended to be heated by exposure to radiation from the earth while the door is open; calibration data are then taken at several temperatures after closing the door, as the blackbody cools to the temperature of the instrument's cryogenic telescope. An analysis of radiometric calibration-source accuracy is shown, indicating a nominal value of 2.7 percent at 12.63 microns. Preliminary analysis of calibration data indicates a measurement repeatability of about 1.25 percent. Details of the blackbody design, construction, and thermal instrumentation are given.
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