ASL CrIS ν Cal L. Strow UMBC Overview Pre-Launch Spectral Calibration of the CrIS Sensitivity Approach Sensor on NPOESS/NPP Spectra Results Conclusions L. Larrabee Strow, Howard Motteler, and Scott Hannon Physics Department and Joint Center for Earth Systems Technology University of Maryland Baltimore County (UMBC) October 15, 2008 1 / 21
ASL Context of Cross-track Infrared Sensor (CrIS) CrIS ν Cal CrIS is a new infrared sounder for the NASA NPP platform L. Strow and the NPOESS operational system, 1:30 am/pm orbit. UMBC NASA hopes to “bridge” climate measurements between Overview Sensitivity AIRS on EOS/Aqua and CrIS/NPOESS with CrIS on NPP. Approach IASI on EUMETSAT’s METOP platform (since April 2007) is Spectra CrIS’s counterpart in the 9:30 am/pm orbit. Results Instrument specifications driven by operational weather Conclusions forecasting requirements (as they were for AIRS and IASI). However, AIRS performance is “climate-quality”, IASI appears to be the same (we need more time). This work: Assessment of CrIS spectral performance during thermal vacuum testing (Spring 2008), with an eye towards climate quality. 2 / 21
ASL CrIS Instrument CrIS ν Cal Interferometer with 0.8 cm OPD L. Strow UMBC Three focal planes, each with a 3x3 array of detectors Longwave (LW) focal plane Overview 650-1095 cm − 1 Sensitivity OPD = 0.8 cm, ∆ ν = 0.625 cm − 1 Approach Midwave (MW) focal plane Spectra 1210-1750 cm − 1 Results Data collect to 0.4 cm, ∆ ν = 1.25 cm − 1 Conclusions Shortwave (SW) focal plane 2155-2550 cm − 1 Data collect to 0.2 cm, ∆ ν = 2.50 cm − 1 Metrology laser wavelength determined using on-board Neon lamp measurements, sample rate of ∼ 90 minutes, hopefully asynchronously relative to orbital period. NPP Thermal Vacuum (TVAC) spectral allocation requirements are 10 ppm for spectral registration and ∼ 0.6% for Instrument Line Shape (ILS) width. NPOESS spectral calibration requirement is 5 ppm. 3 / 21
ASL Frequency Errors in B(T) Units for CO 2 Forcing CrIS ν Cal Forcings/Responses L. Strow UMBC Forcing (CO 2 growth rate of 2 ppm/year) is ∼ 0.06K/year at 2388 cm − 1 . Overview Temperature signal ∼ 0.01K/year Sensitivity AIRS stability < 0.01K/year (radiometric and frequency) Approach allows CO 2 trends/variability to < 0.5 ppm. Spectra Frequency requirements Results Conclusions CrIS: ν stability of ∼ 1 ppm = 0.015K at 2388 cm − 1 Suggests need ∆ ν errors on CrIS to 1 ppm (0.5 ppm CO 2 ) CrIS ILS width should remain stable. 4 / 21
ASL Pre-Flight Spectral Calibration Details CrIS ν Cal Detailed ILS Shape L. Strow UMBC Performed on bench (not TVAC) with CO 2 laser, so LW only Highly successful, good test of Sensor Data Record (SDR) Overview software. Sensitivity Spectral Calibration and MW/LW ILS Shape (width) Approach Spectra Record gas cell spectra for LW (CO 2 ), MW (CH 4 ), and SW Results (HBr): truth for ILS ν and width Conclusions Collect data at mission nominal temperature (Mn), and PQH/PQL temperatures (relevant to other orbits) that are ∼ ± 28K offset from Mn expected temperature. Data collect includes Neon measurement for each gas Bottom line: TVAC spectral calibration was highly successful! 5 / 21
ASL Approach CrIS ν Cal Four data collects (plus 2-point radiometric cal measurements L. Strow if needed) UMBC Hot blackbody (BB): cell full, cell empty; (FT1, ET1) 1 Overview Cold BB: cell full, cell empty; (FT2, ET2) 2 Sensitivity Gas cell transmittance τ = FT 2 − FT 1 3 Approach ET 2 − ET 1 FT1, etc. are complex count spectra Spectra Results Complex part of τ very small Conclusions Each interferogram is converted into an uncalibrated spectrum, averaged, and transformed to on-axis transmittance spectra. Our apodization correction matrices are interpolated to the present estimate of the metrology laser λ met . The best estimate of λ met minimizes χ 2 between the Obs and Cal τ . (This is a big loop...) We allow the observed transmittances to be scaled and offset in this loop. Generally the scale factor is ∼ 0.98-0.99 and the offset factor is ∼ 0.01-0.02. 6 / 21
ASL Focal Plane Geometry: CrIS CrIS ν Cal y L. Strow C Yellow is a “Corner” FOV UMBC Overview S Green is a “Side” FOV Sensitivity 7 4 1 Approach M Blue is the “Middle” FOV Spectra 8 5 2 x Results Conclusions 9 6 3 Off-axis FOV spectra are shifted by > 500 ppm, etc. UMBC mini-SDR algorithm adjusts these spectra back to effective on-axis measurements. At 1500 cm − 1 , ∆ ν of 500 ppm = 6K in B(T). Frequency errors will be written out using the above layout for FOVs. 7 / 21
ASL Methodology: Freq. Calibration CrIS ν Cal Keep number of fitted parameters as small as possible L. Strow UMBC Start from scratch with gas cell data (similarly start from scratch with in-orbit data) Overview First determine effective λ met for each FOV , assuming Sensitivity perfectly aligned rectlinear focal plane geometry. Approach Spectra Using known value of d ν obs / dr , where r is the radial Results position of the FOV from the interferometer optical axis, Conclusions least-squares fit for the focal plane dx , dy , and for λ met . Fit rigid focal plane position and metrology laser λ with: � dr i × d ( ppm ) � d ν error + d ν met = i dr where � � ( x 2 i + y 2 ( x i + dx ) 2 + ( y i + dy ) 2 ) − dr = i ) and i is the FOV index. Use 9 FOVs to retrieve dx , dy , and d ν met . 8 / 21
ASL Test Nomenclature CrIS ν Cal Test defined by band (LW/MW/SW) and temperature (MN, L. Strow PQL, PQH) UMBC Often use gas name (CO 2 / CH 4 /HBr) instead of band Overview (LW/MW/SW) item Results listed by test sequence as Sensitivity follows: Approach CO 2 , LW at MN 1 Spectra CO 2 , LW at PQL 2 Results CO 2 , LW at PQH 3 Conclusions CH 4 , MW at MN 4 CH 4 , MW at PQL 5 CH 4 , MW at PQH 6 HBr, SW at MN 7 HBr, SW at PQL 8 HBr, SW at PQH 9 If define Neon effective λ with CO 2 , LW at MN, then you have 8 independent measurements of Neon calibration system. But, might need offsets for each band, giving 6 independent measurements. 9 / 21
Raw Magnitude Spectra ASL Hot BB: empty/filled, Cold BB: empty/filled CrIS ν Cal L. Strow UMBC Overview Sensitivity Approach Spectra Results Conclusions 10 / 21
ASL Uncorrected Raw CO 2 Spectrum CrIS ν Cal L. Strow UMBC Overview Sensitivity Approach Spectra Results Conclusions 11 / 21
LW CO 2 FOV8 Obs versus Calc ASL Signal-to-Noise is Outstanding, as is Stability CrIS ν Cal L. Strow UMBC Overview Sensitivity Approach Spectra Results Conclusions 12 / 21
ASL LW-CO 2 Summary CrIS ν Cal L. Strow UMBC Overview Sensitivity Approach Spectra Results Conclusions 13 / 21
ASL MW-CH 4 Summary CrIS ν Cal L. Strow UMBC Overview Sensitivity Approach Spectra Results Conclusions 14 / 21
ASL SW-HBr Summary CrIS ν Cal L. Strow UMBC Overview Sensitivity Approach Spectra Results Conclusions 15 / 21
Focal Plane Appears to Shift Slightly with ASL Temperature CrIS ν Cal Change in effective d ν met errors for LW (CO 2 ) from PQL to PQH L. Strow (in ppm) are: UMBC y Overview Sensitivity Approach 7 4 1 Spectra 3.2 2.7 3.2 Results -1.7 -1.9 -1.3 5 Conclusions 8 2 -5.1 -5.9 -5.2 x 9 6 3 This behavior allows separation of metrology laser wavelength from focal plane alignment. 16 / 21
Observed Focal Plane Positions ASL Assuming rigid movement of each 3x3 focal plane CrIS ν Cal Mission Nominal focal plane position Note: SW derived from average of Band dx (urad) dy (urad) L. Strow PQL and PQH, SW Mn HBr data has UMBC LW 124 -496 liens MW 146 -472 Overview SW 134 -438 Sensitivity Approach But, figure below shows dy changes with temperature Spectra Results Conclusions 17 / 21
Observed (gas cell) versus Computed ν met ASL (All Units are PPM). CrIS ν Cal Test Constant FP Fitted FP Fit d ν met d ν met (max-min) (max-min) Improvement minus bias L. Strow UMBC LW Mn 2.2 2.1 0.1 -3.0 -0.1 LW PQL 7.2 3.5 3.7 -2.4 0.6 Overview LW PQH 5.7 2.7 3.0 -3.7 -0.8 Sensitivity MW Mn 3.0 2.8 0.2 -3.0 -0.1 Approach MW PQL 7.4 2.2 5.1 -2.0 0.9 Spectra MW PQH 5.2 2.6 2.6 -3.0 -0.1 Results SW Mn 17.5 18.9 -1.4 -2.8 0.1 Conclusions SW PQL 5.8 2.2 3.6 -2.4 0.6 SW PQh 3.2 2.2 1.0 -4.2 -1.2 Mean improvement for fitted FP (excluding HBr SW Mn) is 2.4 ppm. Mean d ν met = -2.9 ± 0.7 ppm If use LW (CO 2 ) Mn -3.0 ppm d ν met to calibrate Neon: Neon_cal becomes +18.0 ppm higher than NIST value Expect +14.7 ppm higher due to FOV divergence (taken from ITT) Agreement to within 3.3 ppm is remarkable 18 / 21
ASL Additional Improvements? CrIS ν Cal The CrIS spectral calibration has a 1-sigma std. of 0.7 ppm L. Strow with 2 adjustable parameters (dx, dy) for each operating UMBC temperature. Overview Sensitivity Are additional adjustments warranted? Approach Note that weather centers won’t bookkeep FOV ID. Spectra Answer: Since LW and SW ν calibration errors are Results reasonably correlated ( ∼ 0.8) over FOV #’s between tests, Conclusions small additional changes in FOV geometry could be warranted. 19 / 21
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