Highly accurate FTIR observations from the Scanning HIS aircraft instrument Henry E. Revercomb a , David C. Tobin a , Robert O. Knuteson a , Fred A. Best a , William L. Smith b , Paul van Delst a , Daniel D. LaPorte a , Scott D. Ellington a , Mark W.Werner a , Ralph G. Dedecker a , Ray K. Garcia a , Nick N. Ciganovich a , H.Benjamin Howell a , Erik Olson a , Steven Dutcher a , and Joe K. Taylor a a University of Wisconsin-Madison, Space Science and Engineering Center 1225 West Dayton Street, Madison Wisconsin, 53706 b Hampton University, Hampton, VA ABSTRACT Development in the mid 80s of the High-resolution Interferometer Sounder (HIS) instrument for the high altitude NASA ER2 aircraft demonstrated the capability for advanced atmospheric temperature and water vapor sounding and set the stage for new satellite instruments that are now becoming a reality [AIRS(2002), CrIS(2006), IASI(2006), GIFTS(200?), HES(2013)]. Follow-on developments at the University of Wisconsin that employ Fourier Transform Infrared (FTIR) for Earth observations include the ground-based Atmospheric Emitted Radiance Interferometer (AERI) and the new Scanning HIS aircraft instrument. The Scanning HIS is a smaller version of the original HIS that uses cross-track scanning to enhance spatial coverage. Scanning HIS and its close cousin, the NPOESS Airborne Sounder Testbed (NAST), are being used for satellite instrument validation and for atmospheric research. A novel detector configuration on Scanning HIS allows the incorporation of a single focal plane and cooler with three or four spectral bands that view the same spot on the ground. The calibration accuracy of the S-HIS and results from recent field campaigns are presented, including validation comparisons with the NASA EOS infrared observations (AIRS and MODIS). Aircraft comparisons of this type provide a mechanism for periodically testing the absolute calibration of spacecraft instruments with instrumentation for which the calibration can be carefully maintained on the ground. This capability is especially valuable for assuring the long-term consistency and accuracy of climate observations, including those from the NASA EOS spacecrafts (Terra, Aqua and Aura) and the new complement of NPOESS operational instruments. It is expected that aircraft flights of the S-HIS and the NAST will be used to check the long-term stability of AIRS and the NPOESS operational follow-on sounder, the Cross-track Infrared Sounder (CrIS), over the life of the mission. Key Words: Interferometer, Spectroscopy, Remote Sensing, Calibration, Radiance 1. INTRODUCTION Our involvement in making spectral radiance observations of the atmosphere with high absolute accuracy for using calibrated FTIR is summarized in Table 1. It began with the UW High-resolution Interferometer Sounder (HIS) program that extended over more than two decades, beginning with simulation studies in the late ‘70s. The UW effort includes the design of new satellite instruments and retrieval techniques, the development of new aircraft and ground-based measurement capability, as well as the conduct of field campaigns and the associated science. The satellite instrument work has laid the foundation for future operational sounding instruments from both polar orbit (CrIS for NPOESS is an outgrowth of the ITS Phase A study performed for EUMETSAT in 1990-91) and from geosynchronous orbit (the Geosynchronous Imaging Fourier Transform Spectrometer, GIFTS, as a precursor to the GOES-R Hyperspectral Environmental Suite). HIS aircraft data has been used to demonstrate improved vertical resolution and accuracy of temperature and water vapor retrievals from higher spectral resolution. Also, the high radiometric and spectral calibration accuracy of HIS data has enabled many improvements in clear sky forward modeling. The HIS also led to the ground-based Atmospheric Emitted Radiance Interferometer (AERI), developed at the UW-SSEC for the DOE Atmospheric Radiation Measurement (ARM) program. The AERIs are capable of continuous operation in the field and provide accurately calibrated spectra directly over the internet. They are used both for atmospheric forward model work and for remote sensing of the boundary layer. Multispectral and Hyperspectral Remote Sensing Instruments and Applications II, edited by Allen M. Larar, Makoto Suzuki, Qingxi Tong, Proc. of SPIE Vol. 5655 (SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.579018 41
Transcript
Highly accurate FTIR observations from the Scanning HIS aircraft instrument
Henry E. Revercomba, David C. Tobina, Robert O. Knutesona, Fred A. Besta, William L. Smithb, Paul van Delsta, Daniel D. LaPortea, Scott D. Ellingtona,
Mark W.Wernera, Ralph G. Dedeckera, Ray K. Garciaa, Nick N. Ciganovicha, H.Benjamin Howella, Erik Olsona, Steven Dutchera, and Joe K. Taylora
aUniversity of Wisconsin-Madison, Space Science and Engineering Center
1225 West Dayton Street, Madison Wisconsin, 53706 bHampton University, Hampton, VA
ABSTRACT Development in the mid 80s of the High-resolution Interferometer Sounder (HIS) instrument for the high altitude NASA ER2 aircraft demonstrated the capability for advanced atmospheric temperature and water vapor sounding and set the stage for new satellite instruments that are now becoming a reality [AIRS(2002), CrIS(2006), IASI(2006), GIFTS(200?), HES(2013)]. Follow-on developments at the University of Wisconsin that employ Fourier Transform Infrared (FTIR) for Earth observations include the ground-based Atmospheric Emitted Radiance Interferometer (AERI) and the new Scanning HIS aircraft instrument.
The Scanning HIS is a smaller version of the original HIS that uses cross-track scanning to enhance spatial coverage. Scanning HIS and its close cousin, the NPOESS Airborne Sounder Testbed (NAST), are being used for satellite instrument validation and for atmospheric research. A novel detector configuration on Scanning HIS allows the incorporation of a single focal plane and cooler with three or four spectral bands that view the same spot on the ground. The calibration accuracy of the S-HIS and results from recent field campaigns are presented, including validation comparisons with the NASA EOS infrared observations (AIRS and MODIS).
Aircraft comparisons of this type provide a mechanism for periodically testing the absolute calibration of spacecraft instruments with instrumentation for which the calibration can be carefully maintained on the ground. This capability is especially valuable for assuring the long-term consistency and accuracy of climate observations, including those from the NASA EOS spacecrafts (Terra, Aqua and Aura) and the new complement of NPOESS operational instruments. It is expected that aircraft flights of the S-HIS and the NAST will be used to check the long-term stability of AIRS and the NPOESS operational follow-on sounder, the Cross-track Infrared Sounder (CrIS), over the life of the mission.
1. INTRODUCTION Our involvement in making spectral radiance observations of the atmosphere with high absolute accuracy for using calibrated FTIR is summarized in Table 1. It began with the UW High-resolution Interferometer Sounder (HIS) program that extended over more than two decades, beginning with simulation studies in the late ‘70s. The UW effort includes the design of new satellite instruments and retrieval techniques, the development of new aircraft and ground-based measurement capability, as well as the conduct of field campaigns and the associated science. The satellite instrument work has laid the foundation for future operational sounding instruments from both polar orbit (CrIS for NPOESS is an outgrowth of the ITS Phase A study performed for EUMETSAT in 1990-91) and from geosynchronous orbit (the Geosynchronous Imaging Fourier Transform Spectrometer, GIFTS, as a precursor to the GOES-R Hyperspectral Environmental Suite).
HIS aircraft data has been used to demonstrate improved vertical resolution and accuracy of temperature and water vapor retrievals from higher spectral resolution. Also, the high radiometric and spectral calibration accuracy of HIS data has enabled many improvements in clear sky forward modeling. The HIS also led to the ground-based Atmospheric Emitted Radiance Interferometer (AERI), developed at the UW-SSEC for the DOE Atmospheric Radiation Measurement (ARM) program. The AERIs are capable of continuous operation in the field and provide accurately calibrated spectra directly over the internet. They are used both for atmospheric forward model work and for remote sensing of the boundary layer.
Multispectral and Hyperspectral Remote Sensing Instruments and Applications II,edited by Allen M. Larar, Makoto Suzuki, Qingxi Tong, Proc. of SPIE Vol. 5655(SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.579018
* Teaming Participants included: (1) Bomem,Inc, for all FTS subsystems; (2) SBRC for HIS, GHIS, GAP, ITS; (3) ITT for GHIS, Small FTS; (4) Denver University for HIS, and (5) Lincoln Labs, who were primary for NAST, (6) NASA LaRC, Utah State SDL and many others for GIFTS.
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2. THE SCANNING HIGH-RESOLUTION INTERFEROMETER SOUNDER (S-HIS) The Scanning HIS (S-HIS) is an advanced version of the HIS ER-2 instrument 1, 2, developed between 1996 and 1998 with the combined support of the US DOE, NASA, and the NPOESS Integrated Program Office. It has flown in many field campaigns beginning in 1998, has proven to be very effective and dependable, and has been integrated on the NASA DC8, the NASA ER2 and the Proteus aircraft from Scaled Composites. We also expect S-HIS to be integrated on the WB57 this year. The instrument as installed in the NASA DC8 is shown in Figure 1.
Figure 1. Scanning HIS installed in the bay of the NASA DC8 on which it was the first deployed.
The compact Sensor Module, consisting of an afocal fore optics (reducing angles by a factor of 2.5), dynamically aligned interferometer, aft-optics consisting of a 45-degree flat mirror with central-obscuration hole for the converged beam from a spherical converging mirror, detector dewar with diamond window, mechanical Stirling cooler, and preamplifier/ADC electronics is shown in Figure 2.
The continuous spectral coverage from 3.3 to 16.7 µm at 0.5 cm-1 resolution is illustrated in Figure 3 by a sample spectrum from the SAFARI-2000 mission in South Africa. This coverage is divided into three bands with separate detectors (two photoconductive HgCdTe and one InSb) to achieve the required noise performance. The bands use a common field stop to ensure accurate spatial co-alignment and spectral calibration. The longwave band provides the primary information for temperature sounding, cloud phase, and particle size. The midwave band provides the primary water vapor sounding information and further cloud property information. The shortwave band provides information on cloud reflectance and augments sounding information.
Longwave
Midwave
CO2CO
N2O
H2O
H20
N2O/CH4
CO2
O3
150
Rad
ianc
e (m
W/m
2 sr
cm
-1)
-2
8
N2O
Shortwave
1800 2600
0
Wavenumber (cm-1)
1800500
Figure 3. Sample Scanning HIS spectrum from NASA SAFARI-2000 mission. There is overlap between the longwave and the midwave band that is useful for diagnostics (about 1025-1200 cm-1).
The basic spectral and geometrical sampling characteristics are summarized in Table 2. The optical design is very efficient, providing useful signal-to-noise performance from a single 0.5 second dwell time. This allows imaging with 2-3 km resolution to be accomplished by cross-track scanning. Onboard reference blackbodies are viewed as part of each cross-track scan, providing updated calibration information every 20-30 seconds.
The S-HIS employs a customized commercial interferometer (DA5 from Bomem, Inc, Quebec, Canada), with dynamically aligned plane mirrors. The moving Michelson mirror is voice coil driven and its support mechanism was designed and built at UW-SSEC to make use of a linear bearing approach to minimize tilts. The spectral characteristics of the measurements are very well known and stable because of the use of a HeNe laser to control optical delay sampling. A 1/4-wave quadrature system is used to assure that no samples are dropped or miscounted and the laser is also used to maintain alignment. Any residual tilt misalignments are measured as a diagnostic and as the basis for corrections as needed.
The fundamental measurement consists of one numerically filtered interferogram from each of the three spectral bands collected every 0.5 second. These interferograms are stored on a microprocessor-controlled hard drive flown in a pressurized container. The hard drive capacity is 12 Gbytes, sufficient for 5-6 normal data flights. On the ground, data is downloaded over an internet connection and processed to calibrated radiances in the field. This allows initial conclusions to be made in a timely way for evaluating the success of experiment objectives as the field campaign progresses. Detailed instrument health and performance data is also processed each day to assure that a healthy instrument is ready for the next flight.
The rapid sampling frequency of the S-HIS allows cross-track imaging at 2 km resolution with a swath width on the ground of 30-40 km. An example of how this capability can be used to build up an image of a larger area by flying a mapping pattern with the ER-2 is shown in Figure 4.
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Table 2. Scanning HIS Characteristics Characteristic Scanning HIS Interferometer: Type Dynamically Aligned plane
mirror-[Custom Bomem DA 5] Laser Metrology System & Mirror Drive Feedback
HeNe laser; white light start up; continuous fringe counting (¼-wave quadrature)-[Bomem]
Michelson Mirror Assembly
Linear bearing with voice coil [UW-SSEC];
Beamsplitter / Compensator
KBr; wedged; Antimony tri-sulphide and Ge coating
Spectral Resolution 0.5 cm-1 (unapodized) Maximum OPD* ±1.037 cm OPD scan speed 4 cm/s Interferogram time 0.5 sec Beam Diameter 4.5 cm Beam angular FOV ±20 mrad Mirror tilt monitoring 0-2 KHz Spectral Coverage: LW Band (HgCdTe) 600-1100 cm-1 MW Band (HgCdTe) 1000-1825 cm-1 SW Band (InSb) 1825-3000 cm-1 Detector / Coolers: Geometry Sandwich focal plane: 2mm
InSb over 1mm MW & LW Cooler Single, 0.6 W Litton Stirling Temperature 78 K Spatial Sampling: Angular FOV 100 mrad Footprint Diameter 2 km from 20 km Cross-track samples Programmable
(15 earth, 10 cal common) Scan time (12.5 sec) Aircraft ER2 Centerline; DC8; Proteus Mass 70 Kg Power 225W *Optical Path Difference (unapodized resolution = 1/ (2 * maxOPD)
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Figure 4. Scanning HIS image of the Okavanga Delta from the ER-2 during SAFARI-2000 compared to a 0.25 km MODIS image (0.65 µm). The S-HIS image is for the average of 980-985 cm-1, an example of a clean window region selection as illustrated by the water (lower) and barren land (upper) example spectra shown. Images can be made from any of the nearly 5000 spectral samples of the S-HIS. For clouds, linear combinations of clean window channels are used to investigate the spatial distribution of cloud properties.
2.1 Scanning HIS Calibration Achieving high absolute accuracy has been a major objective of the HIS program at UW-SSEC. As a result, our aircraft and ground-based observations have contributed significantly to improved atmospheric spectroscopy3-7. Early experience with the High-resolution Interferometer Sounder (HIS) aircraft instrument demonstrated that Fourier Transform Spectrometers (FTS) are especially well-suited to absolute emission measurements of broad spectral bands at high resolution6, 8-11. Radiometric calibration of the HIS (and its successors, including AERI, S-HIS, and NAST) is accomplished with the same basic technique used in low resolution radiometry12. Periodic viewing of two high emissivity, uniform temperature blackbody references provides the responsivity and offset parameters needed to convert measured spectra to radiances. One property of interferometric measurements which is important for accurate calibration is the phase. The proper handling of the phase for warm FTS instruments was developed at UW-SSEC 9.
Two bodies were sufficient for the HIS because its detectors (arsenic doped silicon) were very linear. We have also demonstrated with AERI that two bodies are sufficient even when using Photoconductive HgCdTe detectors that are non-linear. This depends on characterizing the non-linearity with a physical model that allows the non-linearity to be corrected before performing the calibration13 and has been verified using a third blackbody at LN2 and other intermediate temperatures. The same approach has been applied to the S-HIS and the NAST-I14. The blackbody reference sources for the AERI, S-HIS, and NAST-I are high emissivity cavities (about 0.999 known to <0.001) carefully designed, fabricated, and characterized at UW-SSEC15. The resulting absolute radiometric accuracy of these instruments is better than 0.5 K for downlooking and better than 0.5 % of ambient radiance for uplooking. The reproducibility is better than 0.2 K and 0.2% respectively. These are conservative estimates of the uncertainty, with the absolute accuracy representing a not to exceed value. An RSS of the error contributors indicates expected uncertainties that are about half of these values, and ground tests with a third blackbody confirm this tighter expectation.
The spectral calibration of these instruments is accurate (better than 1 part in 106) and stable (better than 0.3 parts in 106), based on the use of an HeNe reference laser. The interferometer approach, coupled with laser triggered sampling, yields an instrument for which accurate central wavelengths and spectral weightings are mathematically defined from a few design parameters or a single adjustable parameter16. The importance of the extremely accurate wavelength calibration possible with FTS increases with increasing spectral resolution, because the large slopes on the sides of lines can create large effective radiance errors for very small wavelength errors.
A practical demonstration of radiometric and spectral integrity is shown in Figure 5 that compares example S-HIS and AERI downwelling spectra. The AERI absolute calibration has received a good deal of scrutiny, because of its use for improving atmospheric spectroscopy by the DOE Atmospheric Radiation Measurement (ARM) program17. Calibrated AERI spectra on a standardized spectral scale are derived automatically on site and provided directly to the ARM science center via the internet for use by the science team. An AERI comparison with a NIST maintained blackbody during the Miami IR Workshop in 1998 showed agreement to < 0.05°C at 60°C, < 0.04°C at 30°C, and <0.02°C at 20°C.
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Figure 5. Examples of S-HIS uplooking spectra overlayed on AERI spectra. On the left is a blowup of a portion of the 15 micron CO2 band from the S-HIS longwave band; on the right is a shortwave band comparison from the same time period.
The Proteus implementation also provides a zenith view to augment calibration information and to observe upper level water vapor, as illustrated in Figure 6. Note the accurate zero determined from the warm and intermediate temperature blackbody spectra shown, even for the non-linear longwave and midwave bands.
Figure 6. Comparison of zenith (lower) and nadir (upper) atmospheric radiance spectra from the Proteus at 14 km over the DOE Atmospheric Radiation Measurement (ARM) program Southern Great Plains site on 16 November 2002. The smooth Planck function spectra are those of the reference blackbodies used for calibration.
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The estimated S-HIS calibration uncertainties based on a formal error analysis are shown as a function of scene brightness temperatures in Figure 7. Band-to-band overlap agreement is also used to verify calibration and non-linearity correction. An example of the excellent overlap of the S-HIS bands is shown in Figure 8.
Figure 7. Three-sigma calibration accuracy estimates for Scanning HIS with two different ambient blackbody temperatures. Conditions apply to AIRS validation flights on 21 November 2002 on the ER2 over the Gulf of Mexico (left) and 16 November 2002 on Proteus over the DOE Atmospheric Radiation Measurement (ARM) site in Oklahoma. TABB and THBB are the ambient and hot blackbody temperatures.
Figure 8. S-HIS brightness temperature spectrum from the Proteus at 14 km over the DOE Atmospheric Radiation Measurement (ARM) program Southern Great Plains site on 16 November 2002. The lower spectra overlay the long- and mid-wave spectral overlap regions (left); the mid- and short-wave overlap (right).
2.2 Scanning HIS Noise Performance The noise performance for individual S-HIS 0.5 second spectra is shown in Figure 9. These noise estimates were derived from calibration blackbody views collected in-flight on 01 April 2001 during the Texas 2001 field experiment. Included in the figure is the total noise (1 standard deviation) and the two basic components: (1) random noise, uncorrelated with wavenumber from the detector and electronics, and (2) spectrally correlated noise due to vibration-
induced wavefront tilts for the ambient blackbody view. These noise levels are sufficiently low to allow temperature and water vapor profiling, cloud properties, and surface properties to be derived from each individual field of view. However, the size of the vibration-induced correlated noise varies with aircraft platform and for some, correction techniques are needed to remove this noise for single-FOV application. It is also possible to remove random noise as necessary by taking advantage of Principal Component Analysis18.
Figure 9. Scanning HIS noise performance on the NASA ER2, April 2001. The curve with the highest levels is the total noise (random plus tilt-induced correlated) and the lowest is the noise correlated in wavenumber.
3. VALIDATION OF AIRS WITH SCANNING HIS The Atmospheric Infrared Sounder (AIRS) on the NASA Aqua spacecraft launched on 20 May 2002 is a cryogenic cross-dispersed grating spectrometer19. It employs 7 different orders of dispersion to map the spectrum onto 12 detector modules with a total of 15 linear arrays. A single spatial footprint is detected at a time, with the field being focused on the grating to minimize the impact of non-uniform cloudy scenes on spectral calibration.
There are significant differences in spatial and spectral sampling for the S-HIS and the AIRS that need to be accounted for in making accurate comparisons. The technique selected for doing this is to make use of calculations that account for the actual spectral and spatial characteristics of each instrument. The calculated spectra allow the observation-minus-calculation residual for each instrument to be compared, avoiding the first-order effects of these differences. To improve this comparison even further, the residuals are each convolved with the Instrument Line Shape (ILS) of the other. This is equivalent to eliminating grating contributions from optical path differences larger than measured by S-HIS and weakly apodizing S-HIS to match the effect of the AIRS ILS.
The result of applying this technique for data from a flight over the Gulf of Mexico on 21 November 2002 as part of the Terra Aqua Experiment (TX 2002) is shown in Figures 10 and 11. The scene uniformity for this case is excellent, with the peak-to-peak brightness temperature differences in the 12 micron MODIS channel over the 8 chosen AIRS footprints was about 0.5 K. To minimize the effects of random noise on this comparison, the spectra from each instrument have been noise filtered using principle component techniques.
The final comparison between S-HIS and AIRS radiances as given in Tobin, et al.20 is repeated here for convenience in Figure 12. The difference between these residual differences and those of Figures 10 and 11 are that channels with significant contribution from above the aircraft flight level have been objectively eliminated. The excellence of the agreement is demonstrated by the histograms for each module, as discussed in the caption. Note that while the small residual differences are not just for window regions, but extend deeply into the 15-micron carbon dioxide band and also into the 6.3-micron water vapor band.
4. ASSESSMENT OF MODIS CALIBRATION FROM AIRS Having validated AIRS with S-HIS, it is useful to consider the implications for other infrared instruments. Since the AIRS provides reasonably high spectral resolution, it can be used to simulate the radiance of lower resolution instruments, such as the EOS MODIS imager, by convolving the AIRS spectrum with the normalized MODIS spectral resolution function for each of the 15 MODIS IR bands. Note that there are some gaps in the AIRS spectrum that create significant convolution errors, unless they are accounted for.
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Figure 10. The top panel shows the S-HIS and AIRS mean brightness temperature spectra for this longwave region. The middle panel shows the observed-minus-calculated brightness temperature residual for each. For the CO2 region 650-700 cm-1 AIRS spectra and residuals are larger than for S-HIS and the opposite is true for the O3 region 1000-1100 cm-1. Finally, the AIRS minus S-HIS residual difference is shown in the bottom panel.
Figure 11. Same as Figure 10 for the midwave spectral region.
wavenumberwavenumber
wavenumberwavenumber
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Figure 12. Summary of final AIRS to Scanning HIS comparison. Channels with significant sensitivity above the ER2 are eliminated. The different detector modules of the AIRS instrument (separate linear detector arrays) are labeled in the difference spectrum on top, and on the corresponding histogram of differences. Note the excellent agreement! The mean agreement over most modules is order 0.1 °C or less (M-04b is the one exception with a mean of 0.26 °C). Also, the standard deviations are with one exception less than 0.2°C.
The results of comparing co-located AIRS and MODIS for a large selection of global samples, chosen from reasonably uniform AIRS footprints is shown in Figure 13. Note that the numbers of comparisons range from a few 100,000 for window channels and the ozone channel to almost 2 million for the most opaque channel 36. The distributions of AIRS-MODIS differences shown in blue for each band are the final results with corrections for convolution errors determined from a standard atmosphere. The blue bars give an estimate of the convolution errors and are centered at the mean AIRS-MODIS difference. Given the excellent agreement between AIRS and S-HIS, it is expected that the largest part of these differences are issues with MODIS. The most significant issues occur for the opaque 15-micron CO2 bands (34-36) and for water vapor (27, 28).
5. SUMMARY These results of comparing AIRS in orbit on the Aqua platform to aircraft observations from S-HIS illustrate the substantial advantages of high spectral resolution observations for accurate calibration applications. Although they demonstrate considerable progress in achieving high calibration accuracy, we can and should do even better for climate. A simple mission aimed at providing the ultimate IR reference for climate should be a priority as soon as possible.
ACKNOWLEDGEMENTS We gratefully acknowledge the support of the Integrated Project Office (IPO), contract 50-SPNA-1-00039 and of NASA contract NAS5-31375 for support of S-HIS instrument refinement, field deployment, and data analysis for this work. Integration of S-HIS to the Proteus and early development was supported by the DOE ARM Program.
(AIR
Sob
s-A
IRS
calc
)-(S
HIS
obs-
SH
ISca
lc)
(K)
(AIR
Sob
s-A
IRS
calc
)-(S
HIS
obs-
SH
ISca
lc)
(K)
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Figure 13. Distributions of brightness-temperature differences between AIRS and MODIS for a large number of samples (N) taken from scenes with reasonably small spatial non-uniformity. An spectral shift in the MODIS spectral response functions could account for most of these differences. It has been demonstrated that a single spectral shift would make the differences small for a wide range of scene brightness temperature conditions.
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Red=without accounting for convolution errorBlue=accounting for convolution error with mean correction from standard atmospheres
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20. Tobin, David C., Henry E. Revercomb, Chris Moeller, Robert O. Knuteson, Fred A. Best, William L. Smith, Paul van Delst, Daniel D. LaPorte, Scott D. Ellington, Mark W.Werner, Ralph G. Dedecker, Ray K. Garcia, Nick N. Ciganovich, H. Benjamin Howell, Steven Dutcher, Joe K. Taylor, "Validation of Atmospheric InfraRed Sounder (AIRS) Spectral Radiances with the Scanning High-resolution interferometer Sounder (S-HIS) aircraft instrument", in Remote Sensing Europe 2004, Klaus P. Schäfer, Adolfo Comerón, eds., Proc. SPIE 5571 (2004).
