The Ozone Monitoring Instrument (OMI) is an ultravioletvisible imaging spectrograph that uses two-dimensional CCD detectors to register both the spectrum and the swath perpendicular to the flight direction. This allows having a 114 degrees wide swath combined with an unprecedented small ground pixel (nominally 13 x 24 km2), which in turn enables global daily ground coverage with high spatial resolution. The OMI instrument is part of NASA’s EOSAURA satellite, which will be launched in the second half of 2004. The on-ground calibration of the instrument was performed in 2002. This paper presents and discusses results for a number of selected topics from the on-ground calibration: the radiometric calibration, the spectral calibration and spectral slit function calibration. A new method for accurately calibrating spectral slit functions, based on an echelle grating optical stimulus, is discussed. The in-flight calibration and trend monitoring approach and facilities are discussed.
A Space-based Calibration Transfer Spectroradiometer (SCATS) is combined with a ground calibration
spectral albedo radiometric standard which consists of an opaque quartz glass Mie scattering diffuser (MSD) which
has very good Lambertian scattering properties in both reflectance and transmittance modes. This system provides
the capability for determining long term changes in the spectral albedo calibrations which operate in the solar
reflective wavelength region. The spectral albedo calibration would be traceable to the SIRCUS and STARR NIST
calibration facilities. The on-orbit radiometric standard is the Sun. The NIST traceable ground spectral albedo
calibration is invariant between the ground and on-orbit over the instrument lifetime due to the use of a field of view
defining mechanical baffle to differentiate between radiance and irradiance.
Emerging instrumental requirements for remotely sensing tropospheric trace species have led to a rethinking by some of
the paradigm for Système International d'Unités (SI) traceability of the spectral irradiance and radiance radiometric
calibrations to spectral albedo (sr-1) which is not a SI unit. In the solar reflective wavelength region the spectral albedo
calibrations are tied often to either the spectral albedo of a solar diffuser or the Moon.
This new type of Mie scattering diffuser (MSD) is capable of withstanding high temperatures, and is more Lambertian
than SpectralonTM. It has the potential of covering the entire solar reflective wavelength region. Laboratory
measurements have shown that the specular reflectance component is negligible, and indicate that internal absorption by
multiple scattering is small. This MSD, a true volume diffuser, exhibits a high degree of radiometric stability which
suggests that measurements at the National Institute of Standards and Technology (NIST) could provide a spectral
albedo standard. Measurements are currently in progress of its radiometric stability under a simulated space environment
of high energy ionizing and ultraviolet (UV) solar radiation for its eventual use in space as a solar diffuser.
The Passive A-Band Wind Sounder (PAWS) was funded through NASA's Instrument
Incubator Program (IIP) to determine the feasibility of measuring tropospheric wind speed profiles
from Doppler shifts in absorption O2 A-band. It is being pursued as a low-cost and low-risk alternative
capable of providing better wind data than is currently available. The instrument concept is adapted
from the Wind Imaging Interferometer (WINDII) sensor on the Upper Atmosphere Research Satellite.
The operational concept for PAWS is to view an atmospheric limb over an altitude range from the
surface to 20 km with a Doppler interferometer in a sun-synchronous low-earth orbit. Two orthogonal
views of the same sampling volume will be used to resolve horizontal winds from measured line-of-sight
winds.
A breadboard instrument was developed to demonstrate the measurement approach and to
optimize the design parameters for the subsequent engineering unit and future flight sensor. The
breadboard instrument consists of a telescope, collimator, filter assembly, and Michelson
interferometer. The instrument design is guided by a retrieval model, which helps to optimize key
parameters, spectral filter and optical path difference in particular.
Ghosting between detector and window surfaces is a common source of stray light in packaged detectors. Simple
formulas are presented for predicting window ghost characteristics. Stray light paths in silicon detectors, which are
semi-transparent to near-infrared light, can involve non-optical structures deep within the device. Methods for
identifying subsurface stray light paths are presented.
The Ozone Monitoring Instrument is an UV-Visible imaging spectrograph using two-dimensional CCD detectors to register both the spectrum and the swath perpendicular to the flight direction. This allows having a wide swath (114 degrees) combined with a small ground pixel (nominally 13 x 24 km2). The instrument is planned for launch on NASA’s EOS-AURA satellite in January 2004. The on-ground calibration measurement campaign of the instrument was performed May-October 2002, data is still being analyzed to produce the calibration key data set. The paper highlights selected topics from the calibration campaign, the radiometric calibration, spectral calibration including a new method to accurately calibrate the spectral slitfunction and results from the zenith sky measurements and gas cell measurements that were performed with the instrument.
The background and an approach for deriving tropospheric ozone, water vapor and aerosols from direct sun observations, zenith sky radiances and the ratio of radiances at large zenith (near horizon) to the corresponding zenith sky radiances is described. Surface based remote sensing measurements of zenith sky and large zenith angle atmospheric radiance measurements have been made in the wavelength range of 305 nm to 923 nm near local noon from a mountain site at an altitude of 2.6 km in the vicinity of Boulder, Colorado. The wavelength dependence of the limb/zenith radiance ratios indicate significant limb darkening in the ultraviolet that changes to limb brightening with increasing wavelengths that is a maximum in the blue. Model calculations of a molecular atmosphere with absorption by ozone for wavelengths between 300 and 400 nm indicates that this ratio is sensitive to tropospheric ozone. Results from measurements and model calculations of the sensitivity of these ratios to tropospheric ozone are presented.
Radiometric calibration of large aperture space-borne remote sensing instruments designed to measure atmospheric radiances in the 250 to 400 nm wavelength range is difficult. Historically the spectral radiance calibrations of these instruments have been derived from aperture radiances of integrating spheres illuminated internally by quartz-tungsten-halogen (QTH) lamps. Typical aperture radiances increase by a factor of 400 from 250 to 400 nm and by an additional factor of 10 from 400 to 900 nm. The characteristics of the aperture radiances of 51 cm diameter Spectralon sphere illuminated by an external xenon arc and by internal QTH lamps have been measured. The aperture radiance of the sphere illuminated externally by the xenon arc is 15 times larger at 250 nm than the radiance from internal QTH lamp illumination. The radiometric stability and the aperture uniformity at 290 nm from the two types of illumination are comparable. These measurements have been made with a calibration transfer standard spectroradiometer using 14 narrow ion-assisted deposition filters covering the wavelength region from 250 to 920 nm. The calibration scale of the transfer radiometer is tied to a NIST 1000 W FEL lamp spectral irradiance standard.
Side by side comparisons of Langley type Dobson AD double wavelength pair direct sun observations between SBUV/2, SSBUV flight models and the NOAA world primary standard Dobson spectrophotometer 83 and 61 show that the SBUV/2 type instruments yield column ozone amounts that are 2% higher than the NOAA Dobson spectrometers. Similar results have been obtained with a radiometrically stable multi-filter spectroradiometer (MFS) under less than ideal conditions. A new approach based on a modeled table look-up method for using zenith sky radiances to derive total column ozone from zenith clear sky conditions has been tested using SBUV/2, SSBUV flight models and the MFS equipped with narrow band interference filters at the Dobson AD wavelength pairs. These clear zenith sky observations yield total column ozone that is in good agreement with ozone from the NOAA Dobson spectrophotometer direct sun observations. New model calculations on the relationship between satellite nadir radiances and surface based zenith clear sky radiances suggest that the combination of a very radiometrically stable MFS combined with a compact high performance double monochromator could be used to derive a common radiometric scale among satellite borne ozone monitoring instruments using the solar backscatter ultraviolet technique, and to be able to determine the drift in the radiometric calibration of ozone remote sensing instruments in space.
In the early 1990s a series of surface-based direct sun and zenith sky measurements of total column ozone were made with SBUV/2 flight models and the SSBUV Space Shuttle instrument in Boulder, Colorado which were compared with NOAA Dobson Instrument direct sun observations and TOMS instrument overpass observations of column ozone. These early measurements led to the investigation of the accuracy of derived total column ozone amounts and aerosol optical depths from zenith sky observations. Following the development and availability of radiometrically stable IAD narrow band interference filter and nitrided silicon photodiodes a simple compact multifilter spectroradiometer was developed which can be used as a calibration transfer standard spectroradiometer (CTSS) or as a surface based instrument remote sensing instruments for measurements of total column ozone and aerosol optical depths. The total column ozone derived from zenith sky observations agrees with Dobson direct sun AD double wavelength pair measurements and with TOMS overpass ozone amounts within uncertainties of about 1%. When used as a calibration transfer standard spectroradiometer the multifilter spectroradiometer appears to be capable of establishing instrument radiometric calibration uncertainties of the order of 1% or less relative to national standards laboratory radiometric standards.
Comparisons of a new class of ultra stable ion-assisted- deposition (IAD) narrow band interference filters fabricated from thin films of refractory metal oxides and SiO2 have produced 'hard' filters which are radiometrically stable under conditions of extreme environmental stress such as high temperature, humidity, and space. The new IAD interference filter technology can be used to derive detector-based radiometric scales with significantly smaller uncertainties than a source-based radiometric calibration scale. For the ultraviolet, 250 to 400 nm, a calibration transfer spectroradiometer consisting of a small tandem Ebert-Fastie double monochromator and a multifilter spectroradiometer consisting of a series of narrow band IAD filters combined with a NIST quantum efficiency silicon photodiode for the ultraviolet should be capable of defining a detector-based radiometric calibration scale with uncertainties less than 1.0%. The double monochromator and the filter calibration transfer standard spectroradiometer have been used to investigate the radiometric uncertainty and repeatability of three radiance sources; a FEL lamp-diffuser, a xenon arc- diffuser, and the aperture of an internally illuminated sphere.
KEYWORDS: Transmittance, Optical filters, Interference filters, Humidity, Temperature metrology, Linear filtering, Signal to noise ratio, Ionizing radiation, Sensors, Ultraviolet radiation
A series of in-band and out-of-band transmittance measurements of filters covering the wavelength range from 317 to 1019 nm and a linear variable filter for the 300 to 400 nm range have been made. The bandpass filters and the linear variable were fabricated using the ion-assisted-deposition or similar processes. The radiometric stability of the central wavelength, bandpass (FWHM), and peak transmittances were measured before and after exposures to combined high temperature and humidity, a thermal vacuum cycle, an ionizing particle radiation environment, flight on the Space Shuttle and at two temperature ranges. Representative radiative signal-to-noise ratios are given for solar irradiance observations with a silicon photodiode detector.
New developments in interference filter technology utilizing low thermal expansion coefficient materials which are deposited free of voids using variants of ion-assisted- deposition techniques have made possible the development of a new class of low-cost, lightweight remote sensing instruments. These instruments can easily have a throughput two orders of magnitude larger than similar dispersive monochromators in addition to having a stray light rejection somewhere between that of a single and a double monochromator for a bandpass of the order of one nanometer. Results from environmental testing, measurements of stability in space, and unique problems associated with spectral radiance calibrations with these interference filter instruments are described.
A new instrument has been developed to measure spacecraft attitude which utilizes ultraviolet radiation scattered in the Earth's limb. The sensor consists of a very stable UV bandpass filter with a center wavelength at 355 nm, imaging optics, and a linear diode array detector. The radiance of the limb at this wavelength is dominated by Rayleigh scattering and typically decreases by 15% per kilometer above 20 km. The theoretical resolution at the limb of this device is 0.39 km per pixel for a nominal orbital altitude of 306 km (approximately equals 0.012 degree(s)) and represents a significant improvement over typical infrared-based attitude sensors which have an accuracy of approximately equals 0.1 degree(s). This system was integrated with the Shuttle Solar Backscatter Ultraviolet experiment and flown on STS-72 in January of 1996. The calibration and optical characterization of the device will be presented. Results from the first flight of this instrument, showing an agreement with available shuttle pointing data of +/- 0.05 degree(s), will also be discussed.
In recent years, the consistency of ultraviolet solar spectral irradiance measurements has improved to the extent that broad band averages of solar irradiances agree to within a few percent over ranges of a few hundred nanometers. Over spectral ranges from tens to tenths of nanometers significantly larger disagreements in spectral irradiances are observed. Some well known factors which contribute to measurement uncertainties over smaller wavelength intervals are wavelength calibration errors, uncertainties in radiometric standards especially in the transition region from one standard to another, and differences and uncertainties in slit scattering function between instruments. Extensive pre-launch radiometric calibration of SBUV-2 ozone monitoring instruments in air and vacuum have indicated new sources of radiometric calibration uncertainties. These are effects of Woods anomalies in grating efficiency and wavelength dependent changes (sometimes with significant structure) in the reflectance of MgF2 overcoated aluminum surfaces which seems to be associated with the absorption and desorption of water vapor by the MgF2 thin film. These effects combine to produce wavelength dependent radiometric calibration differences between air and vacuum conditions of as much as 10 percent or more. These results indicate that one should not assume that an accurate radiometric calibration in air is valid in space for the wavelength region of 200 - 400 nm.
Beginning in the mid 1960's large aperture scanning radiometers have been used in space to determine spectroradiometric properties of earth scenes in the red and near infrared regions. Panel diffusers as calibration sources for these radiometers were abandoned in favor of internally illuminated integrating spheres because of problems of illuminating the panel diffuser uniformly.1 Since 1970 spectroradiometric instruments used in space for remote sensing of the atmosphere in the ultraviolet for the determination of stratospheric ozone and total column amounts have used spectral radiance calibrations derived from calibrated panel diffusers illuminated by NIST standards of spectral irradiance. An advantage of the panel diffuser technique is simplicity of the experimental set up. Stratospheric ozone profiles and total column amounts are derived from ratios of atmospheric radiances to corresponding solar irradiances incident at the top of the atmosphere in the wavelength region of 250 - 340 nm. An inherent problem associated with measurements for the remote sensing of stratospheric ozone which is not shared with remote sensing measurements of earth scenes at longer wavelengths of the solar scattering and reflective region is the extremely large dynamic range of atmospheric radiances and the steep gradients of radiance with wavelength. For a typical wavelength scan the spectral radiance changes by about lO and the average signal level of a spectral scan can shift by another factor of 25 or more due to solar zenith angle changes between the subsolar point and the solar zenith angle limit of useful scan information which is within a couple of degrees of the terminator. The derivation of spectral radiance calibrations using either the sphere or panel diffuser techniques for ultraviolet remote sensing instruments are single point calibrations at each wavelength. A subsequent linearity calibration of the detector and electronics is made in non dispersed or white light over the entire dynamic range of the instrument of more than six decades. Consequently the derived radiometric calibration constants consist of a radiometric sensitivity term and a signal dependent linearity correction. An initial comparison of spectral radiance calibrations of SBUV-2 instruments using spherical integrator and panel diffuser techniques has been given by Heath et al.2. Subsequent work by Heath et al. describes the results from comparisons of four spectral radiance calibrations derived using panel diffuser techniques with five spectral radiance calibrations derived using spherical integrator techniques for three different SBUV-2 instruments. The comparability of the sphere and panel diffuser spectral radiance calibration techniques is assesed by comparing derived average BRDF values of panel diffusers based upon the sphere technique with laboratory measurements of BRDF of the panel diffusers. The sphere radiances determined relative to NIST standards of spectral irradiance are compared with measurements of sphere radiance relative to a NIST high temperature blackbody source. This work describes the evaluation of the consistency of spectral radiance calibration scales established using panel diffuser and internally illuminated spherical integrator techniques for the SSBUV and SBUV-2 Flight Model 5 instruments using zenith sky radiance measurements as a function of solar zenith angle (UMKEHR) which coincident in space and time. Also described are the spectral radiance calibrations of the Global Ozone Monitoring Experiment (GOME) with the NASA sphere which has been used to intercalibrate SBUV-2 and SSBUV instruments. These spectral radiance calibration constants are compared with those derived using a Spectralon panel diffuser whose BRDF was measured at NASA Goddard Space Flight Center by J. Butler.
Spectral radiance calibrations have been made for several SBUV/2 instruments using techniques based upon an internally illuminated spherical integrator and diffuse reflectance panels with BRDF measurements from NIST. Both techniques are referenced to NIST standards of spectral irradiance which are used to derive the spectral irradiance calibrations of the instruments. The spectral radiance of the aperture of the internally illuminated spherical integrator also has been calibrated by NIST against a high temperature blackbody. The consistency of the spectral radiance calibrations and the problems specific to each of the techniques are described.
A comparison has been made of spectral radiance calibrations of SBUV-2 instruments using two techniques. In one, a source of spectral radiance is obtained by illuminating a spectralon panel diffuser whose BRDF has been measured by NIST with a 1000 W FEL quartz-tungsten- halogen standard of spectral irradiance from NIST. In the other, the spectral irradiance of the aperture of an internally illuminated spherical integrator is measured relative the same NIST standard of spectral irradiance used with the panel diffuser. The spectral radiance of the sphere is calculated for a source of irradiance which is uniform and radiates as a Lambertian source over the area of the aperture. The agreement between the average of four spectral radiance calibrations of SBUV-2 instruments obtained with the spectralon panel diffuser technique with those obtained from five spectral radiance calibrations with the internally illuminated spherical integrator technique is better than one percent in the wavelength range of 250 - 340 nm. Repeated use of two spectralon panel diffusers over a two year period indicates a stability of diffuser BRDF of better than one percent.
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