Jeffrey S. Myers

Airborne Scanning Spectrometer for Remote Sensing of Cloud, Aerosol, Water Vapor, and Surface Properties

An airborne scanning spectrometer was developed for measuring reflected solar and emitted thermal radiation in 50 narrowband channels between 0.55 and 14.2mm. The instrument provides multispectral images of outgoing radiation for purposes of developing and validating algorithms for the remote sensing of cloud, aerosol, water vapor, and surface properties from space. The spectrometer scans a swath width of 37 km, perpendicular to the aircraft flight track, with a 2.5-mrad instantaneous field of view. Images are thereby produced with a spatial resolution of 50 m at nadir from a nominal aircraft altitude of 20 km. Nineteen of the spectral bands correspond closely to comparable bands on the Moderate Resolution Imaging Spectroradiometer ( MODIS ) , a facility in- strument being developed for the Earth Observing System to be launched in the late 1990s. This paper describes the optical, mechanical, electrical, and data acquisition system design of the MODIS Airborne Simulator and presents some early results obtained from measurements acquired aboard the National Aeronautics and Space Administration ER-2 aircraft that illustrate the performance and quality of the data produced by this instrument.

Vicarious calibration of the moderate-resolution imaging spectroradiometer airborne simulator thermal-infrared channels.

We made an experimental vicarious calibration of the Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator (MAS) thermal infrared (TIR) channel data acquired in the field campaign near Mono Lake, Calif. on 10 March 1998 to demonstrate the advantage of using high-elevation sites in dry atmospheric conditions for vicarious calibration. With three lake-surface sites and one snow-field site, we estimated the MAS noise-equivalent temperature difference as 0.7-1.0 degrees C for bands 30-32 in the 3.68-4.13-microm region and 0.1-0.5 degrees C for bands 42, 45, 46, and 48 in the 8-13.5-microm region. This study shows that the MAS calibration error is within +/-0.4 degrees C in the split-window channels (at 11 and 12 microm) and larger in other TIR channels based on the MAS data over Mono Lake and in situ measurement data over the snow-field site.

Blackbody emissivity considerations for radiometric calibration of the MODIS Airborne Simulator (MAS) thermal channels

The impact of non-unit calibration blackbody emissivity on MODIS airborne simulator (MAS) absolute thermal calibration accuracy is investigated. Estimates of blackbody effective emissivity were produced for MAS infrared channels using laboratory observations of a thermally controlled external source in a stable ambient environment. Results are consistent for spectrally close atmospheric window channels. SWIR channels show an effective emissivity of about 0.98; LWIR channels show an effective emissivity of about 0.94. Using non-unit blackbody effective emissivity reduces MAS warm scene brightness temperatures by about 1 degree Celsius and increases cold scene brightness temperatures by more than 5 degrees Celsius as compared to those inferred from assuming a unit emissivity blackbody. To test the MAS non- unit effective emissivity calibration, MAS and high- resolution interferometer sounder (HIS) LWIR data from a January 1995 ER-2 flight over the Gulf of Mexico were compared. Results show that including MAS blackbody effective emissivity decreases LWIR absolute calibration biases between the instruments to less than 0.5 degrees Celsius for all scene temperatures, and removes scene temperature dependence from the bias.

MODIS Airborne Simulator radiometric calibration

Over the past few years, the MODIS airborne simulator (MAS) has been providing imagery for EOS scientific algorithm development. Primarily flown aboard NASAs ER-2 aircraft, the MAS provides high spatial resolution (50 m at nadir) in 50 spectral channels from 0.55 to 14.2 micrometer, overlapping many MODIS and ASTER channels. This paper focuses on calibration of the short-wave (0.55 - 2.38 micrometer) channels, both radiometric and spectral, and calibration of the integrating sources. Also discussed is the dependence of the short-wave calibration on instrument temperature, showing significant reduction in the thermal sensitivity after recent instrument enhancements and upgrades. The procedures for intercomparison of MAS and AVIRIS (airborne visible/infrared imaging spectrometer) data are also discussed. Some limited comparisons for flights over Alaska (June 1995) are presented, although this analysis is in its initial stages and quantitative results are preliminary.

The NASA enhanced MODIS airborne simulator

The new NASA Enhanced MODIS Airborne Simulator (eMAS) is based on the legacy MAS system, which has been used extensively in support of the NASA Earth Observing System program since 1995. eMAS consists of two separate instruments designed to fly together on the NASA ER-2 and Global Hawk high altitude aircraft. The eMAS-IR instrument is an upgraded version of the legacy MAS line-scanning spectrometer, with 38 spectral bands in the wavelength range from 0.47 to 14.1 μm. The original LN2-cooled MAS MWIR and LWIR spectrometers are replaced with a single vacuum-sealed, Stirling-cooled assembly, having a single MWIR and twelve LWIR bands. This spectrometer module contains a cold optical bench where both dispersive optics and detector arrays are maintained at cryogenic temperatures to reduce infrared background noise, and ensure spectral stability during high altitude airborne operations. The EMAS-HS instrument is a stand-alone push-broom imaging spectrometer, with 202 contiguous spectral bands in the wavelength range from 0.38 to 2.40 μm. It consists of two Offner spectrometers, mated to a 4-mirror anastigmatic telescope. The system has a single slit, and uses a dichroic beam-splitter to divide the incoming energy between VNIR and SWIR focal plane arrays. It will be synchronized and bore-sighted with the IR line-scanner, and includes an active source for monitoring calibration stability. eMAS is intended to support future satellite missions including the Hyperspectral Infrared Imager ( HyspIRI,) the National Polar-orbiting Operational Environmental Satellite System (NPOESS) Preparatory Project (NPP,) and the follow-on Joint Polar Satellite System (JPSS.)

Accuracy assessment of MODIS Airborne Simulator (MAS): 1996 to 1998

In an effort to provide accuracy assessment of the visible and shortwave infrared (SWIR) channels of MODIS Airborne Sensor (MAS), the results of three inflight calibration/validation experiments from 1996-1998 are presented. Both MAS and Airborne Visible InfraRed Imaging Spectrometer (AVIRIS) were flown together on NASAs ER-2 over two bright targets (San Diego 1996 and Lunar Lake 1997). Additionally MAS acquired another bright target, Ivanpah Playa, in 1998. AVIRIS data were spectrally and spatially convolved to MAS specifications for cross comparisons. Predicted at sensor radiances using MODTRAN 3.5 constrained with in situ data are then compared to MAS and convolved AVIRIS radiances for absolute radiometric calibration.