Christopher C. Moeller

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.

Assessing MODIS LWIR band calibration accuracy

Five cases using NASA ER-2 aircraft based SHIS and MAS radiances have been used to assess the L1B radiometric performance of Terra and Aqua MODIS Collection 5 radiances for LWIR bands 31-36. The composite results of these cases show that the split window bands 31 (11 μm) and 32 (12 μm) have performed well within the 0.5% radiometric specification over their lifetime. This is in agreement with results from other ground based and satellite based comparisons that are discussed in the paper. However, the LWIR CO2-sensitive bands 34-36 appear to be performing outside of their 1% accuracy specification, especially for Terra MODIS. This is also observed in global Aqua AIRS-MODIS comparisons. Possible causes for this behavior are under investigation, with the most likely contributors being spectral characterization error, OOB influences due to spectral filter leaks, or possibly scan mirror characterization. It seems that an optical leak from Terra MODIS band 31 into bands 32-36 is probably not a significant contributor to the large residuals of bands 34-36, owing to an effective radiometric correction. Calibration coefficient error is probably only a small contributor since, after adjustments in 2002, the on-orbit calibration now closely follows that of the pre-launch calibration.

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.

Radiometric evaluation of MODIS emissive bands through comparison to ER-2-based MAS data

The calibration accuracy of the Moderate resolution Imaging Spectro-radiometer (MODIS) on Terra near its one year anniversary of first light has been assessed using ER-2 aircraft underflights during the Terra eXperiment (TX-2001) in the spring, 2001. The ER-2, equipped with the MAS and SHIS instruments, underflew Terra several times viewing clear sky earth scenes of the Gulf of Mexico. MAS and SHIS form a powerful tandem, combining high spatial resolution imaging with high spectral resolution sampling in the midwave to longwave infrared region. The assessment is based on co-located MODIS and MAS fields of view with matching viewing geometry and removes spatial and spectral dependencies. The MAS L1B calibration accuracy is improved by transferring the SHIS calibration accuracy (conservatively 0.5 K) to MAS. The early results of two days from TX-2001 indicate that MODIS bands are performing well, but not optimally. The MODIS MWIR window bands appear to be close to the 0.75 - 1% radiometric accuracy specification for the uniform warm, low reflectance scenes assessed, perhaps suggesting that known electronic crosstalk in MODIS SWIR and MWIR bands is small for such scenes. MODIS LWIR window bands show residuals of about 0.5 K to 0.6 K, larger than the 0.5% radiometric accuracy specification. However with the 0.5 K (window bands) to 1 K (atmospheric bands) uncertainties associated with the current assessment, it is not possible to definitively state whether these MODIS bands are or are not within specification. MODIS LWIR atmospheric CO2 bands appear to perform near the 1% accuracy specification with the exception of bands 35 and 36, the upper tropospheric CO2 bands at 13.9micrometers and 14.1micrometers . Different MODIS viewing geometry on the two days seems to suggest that scan mirror reflectance dependence on mirror angles may be influencing the MODIS L1B calibration for some bands, most notably the 8.6micrometers and LWIR CO2 bands; however this assessment is dependent upon the accuracy of the spectral correction (a function of atmospheric conditions), which will be further investigated in coming months. It was surprising to find large MODIS residuals for several bands when the mirror angle to the earth scene closely matched that of when MODIS views its onboard blackbody.

Comparison of two methodologies for calibrating satellite instruments in the visible and near-infrared

Traditionally, satellite instruments that measure Earth-reflected solar radiation in the visible and near infrared wavelength regions have been calibrated for radiance responsivity in a two-step method. In the first step, the relative spectral response (RSR) of the instrument is determined using a nearly monochromatic light source such as a lamp-illuminated monochromator. These sources do not typically fill the field of view of the instrument nor act as calibrated sources of light. Consequently, they only provide a relative (not absolute) spectral response for the instrument. In the second step, the instrument views a calibrated source of broadband light, such as a lamp-illuminated integrating sphere. The RSR and the spheres absolute spectral radiance are combined to determine the absolute spectral radiance responsivity (ASR) of the instrument. More recently, a full-aperture absolute calibration approach using widely tunable monochromatic lasers has been developed. Using these sources, the ASR of an instrument can be determined in a single step on a wavelength-by-wavelength basis. From these monochromatic ASRs, the responses of the instrument bands to broadband radiance sources can be calculated directly, eliminating the need for calibrated broadband light sources such as lamp-illuminated integrating spheres. In this work, the traditional broadband source-based calibration of the Suomi National Preparatory Project Visible Infrared Imaging Radiometer Suite sensor is compared with the laser-based calibration of the sensor. Finally, the impact of the new full-aperture laser-based calibration approach on the on-orbit performance of the sensor is considered.

Assessment of Aqua MODIS and AIRS TIR band L1B radiances using ER-2-based observations during TX-2002

During the Terra-Aqua experiment -- 2002 (TX-2002), a NASA ER-2 was used to underfly the EOS Aqua satellite over the Gulf of Mexico for the purpose of gaining insight on the accuracy of MODIS and AIRS thermal infrared (TIR) band radiances. The ER-2 payload included the MODIS Airborne Simulator (MAS) and the Scanning High resolution Interferometer Sounder (SHIS); these instruments have flown previously on the ER-2 for assessing Terra MODIS TIR band radiances. On November 21, 2002, the ER-2 flew directly under the Aqua satellite, with MODIS and AIRS, as it swept over a clear sky region of the Gulf of Mexico. The MAS and SHIS observations were used to simulate the MODIS thermal IR band radiances for the warm (~ 295 K) Gulf of Mexico scene. The results of comparing the simulated MODIS radiances with the MODIS observations show Aqua MODIS TIR bands are performing well. The residuals (MAS - MODIS) in most bands are within or very near specification. The split window 11 and 12 μm band residuals are small and very close to one another at -0.15°C and -0.13°C, respectively. The comparisons suggest that MODIS LWIR CO2 sensitive bands 35 (13.9 μm) and 36 (14.2μm) may be calibrated slightly warm by about 1°C. Early direct comparisons between MODIS and AIRS on Aqua also suggest that the MODIS bands 35 and 36 may be calibrated slightly warm.

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.)

Improvements to Terra MODIS L1B, L2, and L3 science products through using crosstalk corrected L1B radiances.

Observations in the Terra MODIS PVLWIR bands 27 – 30 are known to be influenced by electronic crosstalk from those bands as senders and into those same bands as receivers. The magnitude of this crosstalk affecting L1B radiances has been steadily increasing throughout the mission lifetime, and has resulted in several detectors within these bands to be unusable for making L2 and L3 science products. In recent years, the crosstalk contamination has been recognized as compromising the climate quality status of several MODIS L2 and L3 science products that depend on the PVLWIR bands. In response, the MODIS Characterization Support Team (MCST) has undertaken an effort to generate a crosstalk correction algorithm in the operational L1B radiance algorithm. The correction algorithm has been tested and established and crosstalk corrected L1B radiances have been tested in several Terra MODIS L2 science product algorithms, including MOD35 (Cloud Mask), MOD06 (Cloud Fraction, Cloud Particle Phase, Cloud Top Properties), and MOD07 (Water Vapor Profiles). Comparisons of Terra MODIS to Aqua MODIS and Terra MODIS to MetOp-A IASI show that long-term trends in Collection 6 L1B radiances and the associated L2 and L3 science products are greatly improved by the crosstalk correction. The crosstalk correction is slated for implementation into Collect 6.1 of MODIS processing.

Spectral characterization of MODIS Airborne Simulator (MAS) LWIR bands and application to MODIS science data cloud products

Several MODIS cloud product algorithms are being developed at the University of Wisconsin for the generation of day-1 products after the launch of MODIS. MODIS Airborne Simulator (MAS) radiometric data collected form NASAs ER-2 platform is being used to simulate MODIS spectral bands for testing and refinement of the cloud product algorithms. Spectral characterization is an important component of the MAS calibration. MAS LWIR bands are spectrally characterized in ambient conditions using a monochromator and are corrected for source spectral shape and atmospheric attenuation. An atmospheric correction based on LBLRTM forward model transmittances demonstrates that strong spectral absorption features, such as Q-branch CO2 absorption near 13.9 micrometers , are effectively removed from the spectral measurements with the aid of a small spectral position correction. Comparisons of MAS in-flight data to well- calibrated HIS instrument data indicate that MAS LWIR spectral calibration drift over time is less than 5 percent of FWHM. The MODIS CO2 cloud top height retrieval shows small dependence on the spectral characterization, with retrieved cloud top height changing by less than 0.5 km in response to a 5 percent spectral position change. This is within the tolerance of other error sources in the cloud top properties algorithm.

MODIS cross-talk effects and areas of potential performance differences for Terra from Aqua characteristics.

Optical and electronic cross-talk effects are present in the Terra MODIS sensor. Those effects are reviewed and the physical (engineering) characteristics that give rise to the effects are described when they are known. The potential for performance degradation also is assessed for each effect. The long-term consequence of these effects is to give rise to Terra MODIS to Aqua MODIS performance trend differences and researchers are cautioned to use care in interpretation of these trend differences as potential diurnal environmental effects.