Larry Paxton, Andrew Christensen, Daniel Morrison, Brian Wolven, Hyosub Kil, Yongliang Zhang, Bernard Ogorzalek, David Humm, John Goldsten, Robert DeMajistre, Ching-I. Meng
The Global Ultraviolet Imager (GUVI) is an imaging spectrometer on the NASA TIMED spacecraft which was launched on December 7, 2001. This instrument produces a far ultraviolet (FUV) data cube of spatial and spectral information at each step of a scan mirror - that scan mirror covers 140 deg in the cross track direction - a span that includes on limb. GUVI produces simultaneous monochromatic images at five "colors" (121.6 nm, 130.4 nm, 135.6 nm, and in broader bands at 140-150 nm and 165-180 nm) as its field of view is scanned from horizon to horizon. The instrument consists of a scan mirror feeding a parabolic telescope and Rowland circle spectrometer, with a wedge-and-strip detector at the focal plane. We describe the design, and give an overview of the environmental parameters that will be measured. GUVI is a modified version of the Special Sensor Ultraviolet Spectrographic Imager (SSUSI), which was launched on the DMSP Block 5D3 F16 satellite on October 18, 2003 and is slated to fly on DMSP satellites F17 through F20, as well. We present some results the science analysis of the GUVI data to demonstrate its relevance to the space weather community.
The Special Sensor Ultraviolet Spectrographic Imager (SSUSI) is currently slated for launch on the Defense Meteorological Satellite Program (DMSP) F-16 in November 2001. This instrument consists of a scanning imaging spectrograph (SIS) whose field-of-view is scanned from horizon-to-horizon and a nadir-looking photometer system (NPS). It will provide operational information about the state of the atmosphere above 100 km. The unique problems incurred by the observational requirements (e.g. that we be able to make daytime and nighttime observations) and the design trade-offs needed to meet those requirements were strong drivers on calibration requirements. Those design trade-offs and the expectation that the instrument calibration will change appreciably in-flight have led to the requirement to perform a large instrument characterization in-flight using only natural sources. We focus, in this paper, on the flight characterization of the SSUSI instrument. This includes discussions of the stellar calibration approach for radiometric calibration, measurements of internally scattered light, sensitivity to the South Atlantic Anomaly, measurements of changing pulse height distributions, and measuring changing reflectivity of a nadir viewing scan mirror. In addition, the calibration of the NPS system using natural sources is addressed.
Operational sensors are designed and intended to reliably produce the measurements needed to develop high-value key environmental parameters. The Special Sensor Ultraviolet Spectrographic Imager (SSUSI) is slated to fly on the next five Defense Meteorological Satellite Program launches (beginning with the launch of F16 in Fall 2001). SSUSI will routinely produce maps of ionospheric and upper atmospheric composition and image the aurora. In this paper we describe these products and our validation plans and the process through which we can assure our sponsors and data products users of the reliability and accuracy of these products.
The Stellar Absorption and Refraction Sensor (STARS) is a compact, large-aperture instrument that combines a UV-IR imaging spectrograph with a co-aligned visible-light imager to make simultaneous absorptive and refractive stellar occultation measurements. The absorption measurements provided by the spectrograph allow the determination of vertical profiles of atmospheric constituents. The coincident refraction observations made by the image yield high-precision measurements of atmospheric density, pressure, and temperature and provide independent knowledge of both the refracted light path and Rayleigh extinction, which are critical in reducing the uncertainty in the retrieved constituent profiles in the lower atmosphere. STARS employs a two-axis gimbaled telescope to acquire and track the star and a two-axis, high-precision, fast-steering mirror to correct for spacecraft jitter and maintain the star within the spectrograph field of view. The relative star position measured by the imager provides position feedback to the active tracking loop of the fast-steering mirror. With funding from NASA's Instrument Incubator Program, a laboratory facility has been developed to demonstrate the overall instrument performance and, in particular, its capability to acquire and track a setting, refracting, and scintillating star, to compensate for various degrees of platform jitter, and to provide the pointing knowledge required for accurate determination of the atmospheric quantities. The combination of built-in image tracking and motion compensation capabilities, small size, and limited spacecraft resource requirements makes STARS and its tracking mechanism suitable for deployment on existing and future commercial spacecraft platforms for applications that require high-precision pointing. In this paper, we present details of the instrument design and its expected performance based on our laboratory tests.
The Self-Calibrating H2O and O3 Nighttime Environmental Remote Sensor (SCHOONERS) is a compact, integrated UV-IR imaging spectrograph and imager. The instrument has a 25 cm diameter aperture and employs a two- axis gimbaled telescope to provide acquisition and tracking of the star. It also uses a two-axis high-precision vernier mirror to correct for spacecraft jitter and maintain the star within the field-of-view. The imaging spectrograph, covering a spectral range between 300 and 900 nm, measures the varying absorption of starlight as a star sets through the nighttime Earth's atmosphere to determine vertical profiles of atmospheric constituents. The relative star position measured by the co-aligned imager not only provides position feedback to the acting tracking loop of the vernier mirror, but also measures the star refraction angle for determining the atmospheric density and temperature profiles. The SCHOONERS scanning platform and its high- precision tracking mirrors provide 44 microradian azimuth pointing stability and 60 microrad altitude tracking accuracy (3(sigma) ). Its built-in image tracking and motion compensation mechanism, coupled with its small size and limited spacecraft resources required, makes it suitable for deployment on existing and future commercial spacecraft platforms as an instrument-of-opportunity after the year 2002. A laboratory facility has been developed to demonstrate the instrument performance, especially its capability to acquire and track a setting, refracting, and scintillating star, to compensate for various degrees of platform jitter, and to provide the pointing knowledge accuracy required for the determination of atmospheric density and temperature. Hardware includes an accurately moving variable intensity point source to simulate the star and motion stages to generate jitter at the instrument. Software simulates the stellar refraction, attenuation, and scintillation for a full end-to-end test of the instrument.
Larry Paxton, Andrew Christensen, David Humm, Bernard Ogorzalek, C. Pardoe, Daniel Morrison, Michele Weiss, W. Crain, Patricia Lew, Dan Mabry, John Goldsten, Stephen Gary, David Persons, Mark Harold, E. Brian Alvarez, Carl Ercol, Douglas Strickland, Ching-I. Meng
The Global Ultraviolet Imager (GUVI) on the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission will determine the variability in thermospheric composition, and its response to auroral inputs as well as measuring those inputs. GUVI is the result of twenty years of work in designing large field of regard far ultraviolet (110 - 180 nm) imagers for spaceflight. These systems are based on the concept of a horizon-to-horizon 'monochromatic' imager. The field of view of a spectrograph is swept from horizon to horizon using a scan mirror. The spectrograph uses a grating to spectrally disperse the light. A two-dimensional detector is used to record spatial and spectral information simultaneously. Images are obtained at discrete wavelengths without the use of filters; this reduces if not eliminates much of the concern about instrumental bandpasses, out-of-band rejection, and characterization of filter responses. Onboard processing is used to bin the spectral information into 'colors' thereby reducing the overall data rate required. The spectral bandpass is chosen to lie in the far ultraviolet so that the sunlit and dark aurora can be imaged. We review the instrument's as delivered performance and the TIMED science requirements. TIMED will be launched May 18, 2000 and will inaugurate the Solar-Terrestrial Connections program at NASA.
The Global Ultraviolet Imager of the NASA Thermosphere, Ionosphere, and Mesosphere Energetics and Dynamics mission has been calibrated at the Optical Calibration Facility of the Applied Physics Laboratory. This spectrographic imager has a 0.74 degree(s) X 11.6 degree(s) field-of-view, a 140 degree(s) X 11.6 degree(s) field-of-regard and collects data in 176 wavelength bins in the spectral range from 120 - 180 nm. The calibration of this far ultraviolet instrument requires continuously variable wavelengths and angles within a high- vacuum system from the light source to the instrument. An optical calibration facility has been developed providing a bright, uniform, wavelength-selectable, collimated light beam, which is mapped in situ to correct for intensity drifts in the lamp. The facility design and the calibration procedure are discussed.
David Humm, Larry Paxton, Andrew Christensen, Bernard Ogorzalek, C. Pardoe, Ching-I. Meng, Daniel Morrison, Douglas Strickland, J. Evans, Michele Weiss, W. Crain, Patricia Lew, Dan Mabry, John Goldsten, Stephen Gary, Keith Peacock, David Persons, Mark Harold, E. Brian Alvarez, Carl Ercol
The Global UV imager (GUVI) is an imaging spectrometer on the NASA TIMED spacecraft. GUVI produces simultaneous monochromatic images at five 'colors' as its field of view is scanned from horizon to horizon. The instrument consists of a scan mirror feeding a parabolic telescope and Rowland circle spectrometer, with a wedge-and-strip detector at the focal plane. We describe the design, and give an overview of the environmental parameters that will be measured. GUVI is a modified version of the Special Sensor UV Spectrographic Imager (SSUSI), which will fly on the DMSP Block 5D3 satellites S-16 through S-20, We present some results from the optical calibration of the five SSUSI units.
Atmospheric mesoscale (100's of meters to a few kilometers) temperature structure and the structure associated with thin cirrus and aerosol layers in the upper stratosphere and lower mesosphere are difficult to measure by ground and satellite based techniques. We show in this paper that the altitude range between about 10 and 80 km is amenable to satellite sounding techniques in the UV-visible-near infrared bands (approximately 200 to 900 nm). The rapid change in optical depth vs. line-of-sight (LOS) end point along a downward-viewing LOS in the 200 - 350 nm spectral range allows separation of atmospheric regions according to the LOS optical weighting functions. The UV imager weighting functions (200 - 300 nm) in combination with the satellite- sensor zenith angle effect allows sounding in the approximately 40 to 80 km region, while the visible band imagery allows detection and separation of high altitude cloud structure leakage from the UV images of clear-air density structure. The instrument requirements necessary to detect such structure and to discriminate aerosol-induced Mie scatter from Rayleigh scatter components consists of UV to visible band spectral imagers having sufficient spatial, temporal and spectral resolutions. Only moderate spectral resolution imagery in the 200 to 900 nm region over a range of sensor line of sight nadir angles is required to detect clouds and infer cloud types. However, high signal to noise ratios and high spatial resolution are required to characterize the structure power spectral density of clouds and clear-air scatter components. Middle atmosphere structure sounding capability on the mesoscale level allows connection between turbulent-like small scale atmospheric phenomenology and larger scale cloud-related and weather- driven atmospheric variability. We demonstrate the stratosphere-mesosphere sounding concept by applying a low altitude mesoscale stochastic structure (LAMSS) model. This model was derived from the NSS (non-stationary stochastic structure) model which utilizes multi-dimensional Fourier- space descriptions of wavelike, turbulent-like, and deterministic, large scale structure to simulate the effects of atmospheric earthlimb structure. LAMSS specifically address tropospheric background clutter processes such as clear-air wind shears, turbulence, temperature inversions, and cirrus cloud structure. The empirical models are applied to synthesis of visible, UV, and IR clutter backgrounds as measured by passive spectral imaging sensors such as the UVISI (UV, Visible Imagers and Spectral Imagers) sensors on the Mid-course Space Experiment (MSX). This paper analyzes images from MSX-UVISI to obtain cloud and atmospheric density structure characteristics in the 200 - 230 nm UV and 300 - 900 nm visible bands. These data illustrates the feasibility of the UV structure sounding concept by comparison to the synthesized structured backgrounds.
KEYWORDS: Data modeling, Atmospheric modeling, Device simulation, Imaging systems, Rayleigh scattering, Data conversion, Data acquisition, Space operations, Airglow, Aerospace engineering
Numerous instruments for UV-visible optical measurements of terrestrial backgrounds have recently flown or are scheduled for launch in the near future. In order to maximize the scientific return from such flight opportunities, simulations of data acquired by imaging and spectrographic imaging instruments spanning wide wavelength ranges are required to support experiment planning and post-launch data analysis/fusion activities. We are currently developing comprehensive capabilities for modeling these types of remote sensing data suitable for a number of mission-support applications, with specific focus on data acquired by the UVISI instruments on the Midcourse Space Experiment satellite. These capabilities are described in this presentation. The core modeling capabilities reside in a suite of well-tested first principles and empirical modeling codes for atmospheric radiances arising from a variety of physical processes (e.g., photoelectron impact excitation, Rayleigh and aerosol scattering, solar resonance and resonant fluorescence scattering, chemistry). Image generation and LOS spectral radiance evaluation techniques permitting continual change in observer location and viewing geometry without incurring large computational burdens have been set up to ingest the radiance modeling results to create high fidelity synthetic satellite data. Illustrative examples are presented.
The Midcourse Space Experiment Satellite (MSX) has a suite of ultraviolet and visible imaging spectrographs and imagers that cover the wavelength range from 110 to 900 nm. The versatile pointing capability of the satellite allows observations in the earth limb and below the horizon with observations during the day and night. The wavelength resolution (1 - 3 nm) for the spectrographs and high spatial resolution in the filtered imagers allows experiments covering a multitude of background phenomenology issues. Experiments are designed to look at ultraviolet through the visible clutter issues for many different scene conditions in the earth limb and below the horizon. Hyperspectral images of terrain and ocean features for specific locations are in the planning stages specially at specific ground truth locations. Atmospheric emission sources during the day and night in different global locations. Atmospheric emission sources during the day and night in different global locations from the poles to the equator will be observed for both assessment of radiance and clutter issues as well as for input into atmospheric radiance models.
While visible and UV range sensors are frequently designed to see the ground, the short-wavelength region known as 'solar-blind' (below 320 nm wavelength) includes emissions originating at altitudes above 30-140 km. The solar-blind range is being investigated for remote sensing of the composition, dynamics, and energetics of the Earth atmosphere, as well as for use in passive sensor acquisition and tracking systems. An examination is presently conducted of the PSD of mid-UV images from a recently launched satellite.
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