In recent work at the National Institute of Standards and Technology the Atomic Spectroscopy Group has characterized several sources of wavelength standards useful for remote sensing applications. At low resolution, mercury pencil-type lamps are convenient for use either in the laboratory or the field. We recommend wavelengths for this lamp with an uncertainty of +/- 0.001 angstroms in the region 2535 angstroms to 5790 angstroms. We also provide relative irradiances for the most prominent Hg lines, data that can be used to determine instrumental response. For high- resolution applications, we have measured wavelengths and relative intensities emitted by a commercial Pt/Ne hollow cathode lamp. Wavelengths for 5600 lines from 1130 angstroms to 4330 angstroms, some with uncertainties as small as +/- 0.0004 angstroms, are available in a comprehensive atlas of this lamp. Excellent calibration lines can also be obtained from demountable hollow cathode lamps. We review wavelength standards in Fe I and II, Th I and II, Ar II, and Cu II that can be excited in such lamps. Cu II provides wavelengths with uncertainties of less than +/- 0.0004 angstroms from 670 angstroms through the entire ultraviolet region. For calibrations at shorter wavelengths, we have developed accurate standards that can be excited in a sliding-spark discharge with yttrium electrodes.

Polytetrafluoroethylene (PTFE) is widely used in remote sensing applications requiring a diffuse reflectance standard for detector calibration. The bi-directional and directional-hemispherical reflectance properties of both pressed and sintered PTFE were measured at ultraviolet wavelengths to provide information for their use as standards in this spectral range. The reflectance decreases with decreasing wavelength for both geometries, and the ratio between the reflectances for these geometries remains constant for wavelengths from 300 nm to 400 nm.

The National Institute of Standards and Technology (NIST) serves the growing ultraviolet user community by providing calibration services throughout the spectral range from 2 nm to 400 nm. In this paper we describe the far ultraviolet transfer standard detector program, the NASA-supported Spectrometer Calibration Beamline at the Synchrotron Ultraviolet Radiation Facility, SURF III, and the recent upgrade of the SURF electron storage ring. Several types of transfer standard detectors are issued by NIST in the spectral range from 5 nm to 254 nm; Al₂O₃ windowless photoemissive devices, CsTe photoemissive devices with integrated MgF₂ windows, and radiation-hardened, semiconductive Si photodiodes. The Spectrometer Calibration Beamline makes use of the cathode, undispersed synchrotron radiation from SURF III as a standard of spectral irradiance from 2 nm to 400 nm. The upgrade of SURF has greatly improved the accuracy of calibrations based on SURF, as well as extending the useful spectral range to shorter wavelengths. Taken together, the transfer standard detector program and the calibration beamline at SURF III offer a unique calibration resource for scientists and engineers working in the far ultraviolet spectral region.

We have made direct measurements of the internal quantum efficiency and the reflectivity of UV-damped silicon photodiodes in the spectral range of 125 nm to 320 nm. The above quantities, coupled with absolute spectral responsivities, may yield unique information leading to the identification of the mechanisms responsible for the degradation of performance of the silicon photodiodes in the ultraviolet. The measurements were made using synchrotron radiation from the NIST synchrotron ultraviolet radiation facility and an absolute cryogenic radiometer as a primary standard detector.

Techniques, applications, and results will be discussed with respect to the Low Resolution Airglow/Auroral Spectrograph and Special Sensor Ultraviolet Limb Imager instruments aboard current and upcoming satellite remote sensing missions.

A portable high accuracy double spectrometer for ultraviolet (UV) spectral irradiance measurements has been under development at the National Institute of Standards and Technology (NIST) or several years. It has been used in the comparison of NIST spectral irradiance sources: an electron storage ring, 1000 W quartz-halogen lamps, deuterium arc lamps, and a windowless argon miniarc. A UV spectral irradiance intercomparison with the Physikalische Technische Bundesanstalt of Germany has also been carried out with the instrument. This paper will discuss the modular design of this instrument, preliminary uncertainty analysis, results of standard source comparisons and results of solar ultraviolet measurements using the UV spectrometer with the NIST spectral irradiance field calibrator.

Monitoring the aerosol component of the lowest 10 km of the atmosphere at UV wavelengths (300 - 400 nm) is an important part of the High Resolution Fly's Eye astrophysics experiment. Our method of atmospheric monitoring uses a frequency tripled YAG laser and a steering system that can point the beam anywhere in the sky. The same detector that measures scintillation light from high energy cosmic rays also measures light scattered from this laser system over a range of laser energies, geometries, and polarizations. This paper describes the technique, the laser system, and some recent measurements.

The problem of finding the optimum measurement algorithm is formulated for acousto-optical spectrometers (AOS) exhibiting random spectral access. The problem is treated with the example of the measurements of a substance abundance in a two-species mixture. The optimum algorithm depends on the noise spectral distribution and the abundance of the interfered substances. The optimization being a part of the measurement process makes available the adaptation of that process to the analyzed sample. This DOAS technique based on measurements in a few selected spectral points finds an application to UV AOS-based gas analyzers for ambient air monitoring.

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.

The Naval Research Laboratory has built give Special Sensor Ultraviolet Limb Imagers (SSULIs) for the Defense Meteorological Satellite Program. These sensors are designed to measure vertical intensity profiles of the Earth's airglow in the extreme and far ultraviolet (800 to 1700 angstroms). The data from these sensors will be used to infer altitude profiles of ion, electron and neutral density. The first SSULI is scheduled to launch in 2000. An identical copy of the SSULI sensor called LORAAS was launched aboard the ARGOS spacecraft on February 23, 1999. Data from LORAAS will be used to verify the performance of the SSULI sensors, ground analysis software and validate the UV remote sensing technique. Together with the LORAAS instrument the SSULI program will collect data on the composition of the upper atmosphere for a complete solar cycle.

We describe the analysis methods of the stellar occultation measurement technique, and present preliminary results of upper atmospheric measurements with Princeton University's Interstellar Medium Absorption Profile Spectrometer (IMAPS), which was flown on two Shuttle AstroSPAS missions. The primary objective of IMAPS was to study the interstellar medium, but it also made observations of some hot stars as they were occulted by Earth's upper atmosphere. These observations provided the first detailed measurements of upper atmospheric O and N₂ by the stellar occultation method. We also present a conceptual design for a follow-on instrument, dedicated to stellar occultation measurements of the upper atmosphere, the Global Upper Atmospheric Neutral Density from Stellar Occultations investigation.

We have designed a wide-field, objective grating spectrograph based on a Schmidt optical system, similar to one flown in sounding rocket investigations by the Naval Research Laboratory. The instrument covers a field of view 10 degrees in diameter, has a spectral range of 130 - 300 nanometers, and utilizes a solar blind image intensifier tube coupled to a CCD array camera. The instrument could be flown as a Shuttle Hitchhiker payload, in a Spartan mission, or (eventually) as a Space Station attached payload. Another possible application of the instrument would be to observe any UV emission from lightning `sprites', which would be prominent if these extend to high enough altitudes to be above a significant portion of the ozone layer.

The High Resolution Airglow and Aurora Spectrograph flew on sounding rockets in 1990, 1992, and 1994. The instrument obtained over 300 exposures (600 spectra) varying in length from 0.3 seconds to 10 seconds during the three flights. The first two flights observed the UV dayglow above the White Sands Missile Range, Las Cruces, NM. The instrument was flown a third time from the Poker Flat Research Range, Fairbanks, AK where it observed a proton aurora. We will present an overview of the instrument and discuss its calibration and its performance during the three flights.

The High-resolution Ionospheric and Thermospheric Spectrograph (HITS) is a very high resolution (> 0.5 angstroms resolution over the 500 - 1500 angstroms passband) Rowland circle spectrograph that is currently flying on the USAF Advanced Research and Global Observing Satellite (ARGOS, launched 23 February 1999). The ARGOS is in a sun- synchronous, near-polar orbit at 833 km altitude with an ascending node crossing time of 2:30 PM. The instrument is designed to spectrally resolve the 834 angstroms triplet to demonstrate a new technique for remotely sensing the electron density in the F-region ionosphere. In addition, the HITS can spectrally resolve the rotational structure of the N₂ Lyman-Birge-Hopfield bands, which can be used to infer the thermospheric temperature. The HITS can resolve the radiative recombination continuum produced by recombining O⁺ ions and electrons, which can be used to infer the electron temperature. The HITS will also produce a high spectral resolution array of the 500 - 1000 angstroms passband to produce a more accurate identification of some of the previously unresolved features of the dayglow spectrum. The instrument operates as a limb imager with a limb scan occurring every 100 seconds throughout the expected three year mission life. Its field-of-view is 0.06 degree(s) X 4.6 degree(s), which corresponds to 3 km (altitude) X 230 km (along the horizon) at the limb. The instrument's field-of-regard is 17 degree(s) X 4.6 degree(s), which covers the 100 - 750 km altitude range. We will present an overview of the instrument and discuss its calibration and in-flight performance.

The Ionospheric Spectroscopy And Atmospheric Chemistry Experiment is a high resolution mid-ultraviolet Ebert-Fastie spectrograph that is flying on the USAF Advanced Research and Global Observing Satellite (ARGOS, launched 23 February 1999). The instrument is designed to spectrally resolve the rotational structure of the nitric oxide bands, which will be used to infer the temperature in the lower thermosphere (90 - 200 km altitude range). The instrument is operated as a limb imager with a limb scan occurring every 100 seconds throughout the expected three year mission life. The ARGOS is in a sun-synchronous, near-polar orbit at 833 km altitude with an ascending node crossing time of 2:30 PM. We will present an overview of the instrument and discuss its calibration and in-flight performance.

The Global Imaging Monitor of the Ionosphere (GIMI) is one of nine space research and technology instruments aboard the Air Force Space Test Program's Advanced Research and Global Observation Satellite (ARGOS). The ARGOS was launched into a sun-synchronous polar orbit by a Delta II launch vehicle from Vendenberg AFB, CA on the morning of 23 February 1999. At the time of this writing, GIMI had completed preliminary check-outs in orbit, with actual data takes beginning in late May, 1999. The GIMI instrument consists of two far- ultraviolet cameras, using electron-bombarded CCD array detectors, operating in the 75 - 115 nm wavelength range (Camera 1) and the 131 - 160 and 131 - 200 nm wavelength ranges (Camera 2). Both cameras are mounted on a two-axis gimbaled pointing system and simultaneously view the same 9 degree(s)-square field.

A new database has been created that makes a wealth of information on spectra, energy levels, and transition probabilities for atoms and atomic ions easily available via the World Wide Web. These data will be a valuable resource for interpretation of solar and other astrophysical spectra obtained from ground-based and space observations.

In the minds of many, Fourier transform spectrometry is restricted to applications in the infrared. In the ultraviolet, the increasingly severe demands on optical, data acquisition, and motion control systems of the interferometer diminish the effectiveness of the technique. However, with recent advances in ultraviolet optics, data acquisition and sampling techniques for Fourier transform spectrometers, these stringent demands are easier to meet at vacuum ultraviolet wavelengths and significantly reduce the cost of Fourier transform spectrometers. The FT700 spectrometer at NIST can operate at wavelengths as low as 140 nm, limited by the short wavelength cut-off of the calcium fluoride optics. We illustrate the capabilities of the FT700 spectrometer in the ultraviolet with several recent results in atomic emission spectrometry, plasma diagnostics, and refractometry.

A conventional pn diffused junction silicon photodiode originally developed for visible and near infrared light was optimized for ultraviolet wavelength spectrum. Optimization was performed with numerical simulation tools and experimental work. The results of modeling and experiment are compared and discussed. MOS capacitor was integrated on active region of photodiode in order to control the quality of fabricated surface passivation layer. p⁺n junction structure and antireflective coating have been identified as two most important design and processing steps.

Hollow cathode lamps are well known light sources for calibration of ground based and space based instruments. Long life, high spectral purity, adaptability for specialized applications, and survivability in harsh environments are a few of the desirable features offered by hollow cathode lamps. Historically, hollow cathode lamps with platinum, or platinum-chromium cathodes and neon fill gas have been used onboard the Hubble Space Telescope for NASA and the GOME instrument for ESA, as well as ground based experiments. Recent improvements have increased the usefulness of hollow cathode lamps for calibration in the ultraviolet and visible spectrum.