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.)
KEYWORDS: Calibration, Digital signal processing, Analog electronics, MODIS, Sensors, Modulation transfer functions, Black bodies, Head, Video, Spectrometers
NASA has built two airborne multi-spectral sensors to simulate space-borne instruments recently launched on the EOS (Earth Observing System) Terra satellite. The MODIS Airborne Simulator (MAS) and the MODIS/ASTER Simulator (MASTER) were designed to provide initial data sets to EOS investigators for algorithm development. MAS and MASTER are currently conducting calibration and validation under-flights for the MODIS and ASTER orbital instruments. These imaging spectrometers produce 50 spectral channels of 16-bit co-registered imagery data, from the blue wavelengths out though the thermal IR bands. Both systems share a common digitizer design developed originally for MAS. Greater accuracy and flexibility is achieved with high precision digital signal processors (DSPs) and field programmable gate arrays controlling the zero restoration, gain and antialiasing oversampling. Digitization rates of up to 100K samples per second per channel allow five-times oversampling at 6.25 scans per second and single sampling at 25 scans per second, resulting in aggregate data rates up to 2 Megabytes per second to disk. Both systems were designed for possible unattended operation on a NASA-ER2, but also support a realtime operator display for interactive mission evaluation on DOE’s B200 and NASA’s DC8. System design, characterization and performance will be covered by this paper.
KEYWORDS: Black bodies, Calibration, Long wavelength infrared, Temperature metrology, MODIS, Short wave infrared radiation, Carbon dioxide, Absorption, Clouds, Spectral resolution
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.
Over the past few years, the MODIS airborne simulator (MAS) has been providing imagery for EOS scientific algorithm development. Primarily flown aboard NASA's 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.
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