Performance Verification of the Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) On-board Blackbody Calibration System Fred A. Besta, Henry E. Revercomba, David C. Tobina, Robert O. Knutesona, Joseph K. Taylora, Donald J. Thielmana, Douglas P. Adlera, Mark W. Wernera, Scott D. Ellingtona, John D. Elwellb, Deron K. Scottb, Gregory W. Cantwellb, Gail E. Binghamb, and William L. Smithc a University of Wisconsin-Madison, Madison, WI b Utah State University, Logan, UT c Hampton University, Hampton, VA ABSTRACT The NASA New Millennium Program’s Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) instrument was designed to provide enormous advances in water vapor, wind, temperature, and trace gas profiling from geostationary orbit. The top-level instrument calibration requirement is to measure brightness temperature to better than 1 K (3 sigma) over a broad range of atmospheric brightness temperatures, with a reproducibility of ±0.2 K. For the onboard calibration approach used by GIFTS that employs two internal blackbody sources (290 K and 255 K) plus a space view sequenced at regular programmable intervals, this instrument level requirement places tight requirements on the blackbody temperature uncertainty (0.1 K) and emissivity uncertainty (0.001). The blackbody references are cavities that follow the UW Atmospheric Emitted Radiance Interferometer (AERI) design, scaled to the GIFTS beam size. The engineering model blackbody system was completed and fully calibrated at the University of Wisconsin and delivered for integration into the GIFTS Engineering Development Unit (EDU) at the Utah State Space Dynamics Laboratory. This paper presents a detailed description of the methodology used to establish the required temperature and emissivity performance, with emphasis on the traceability to NIST standards. In addition, blackbody temperature data are presented from the GIFTS EDU thermal vacuum tests that indicate excellent temperature stability. The delivered on-board blackbody calibration system exceeds performance goals – the cavity spectral emissivity is better than 0.998 with an absolute uncertainty of less than 0.001, and the absolute blackbody temperature uncertainty is better than 0.06 K. Keywords: Imaging, FTS (Fourier Transform Spectrometer), Radiometric, Calibration, Blackbody, Remote Sensing 1. INTRODUCTION The Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) instrument requires highly accurate radiometric calibration in order to provide its enormous advances in water vapor, wind, temperature, and trace gas profiling from geostationary orbit1-3. The unique GIFTS radiometric calibration scheme uses two internal blackbody sources located behind the instrument telescope, combined with a space view, to provide end-to-end instrument calibration accuracy better than 1 K (3-sigma)4-9. The reproducibility is better than ±0.2 K. The calibration scheme builds on well-established general techniques for calibrating interferometer instruments10-12. There is a significant advantage to the internal blackbody approach used by GIFTS, in large part because it is practical to achieve a high emissivity with a small size (1 inch aperture) blackbody. Because the blackbodies are small and internally located, they also provide significant immunity from solar forcing. The blackbodies are periodically viewed via a flip-in mirror that is located near the field stop behind the instrument telescope (Figure 1). The blackbodies, which are independently controlled to different temperatures, are located on a translating table that positions either the ambient (255 K) or hot (290 K) blackbody into the calibration position - filling the interferometer field of view with the appropriate hot or ambient view. An instrumentlevel GIFTS calibration model involving blackbody temperature and temperature uncertainty, blackbody emissivity and emissivity uncertainty, telescope and flat mirror element temperatures, as well as structural temperatures has been developed and used to assess and budget the allowable blackbody parameter uncertainties needed to meet the overall Multispectral, Hyperspectral, and Ultraspectral Remote Sensing Technology, Techniques, and Applications edited by William L. Smith Sr., Allen M. Larar, Tadao Aoki, Ram Rattan, Proc. of SPIE Vol. 6405, 64050I, (2006) 0277-786X/06/$15 · doi: 10.1117/12.698021 Proc. of SPIE Vol. 6405 64050I-1 GIFTS calibration requirement. The error budget allocation for the blackbody subsystem is 0.5 K (3 sigma)6. The contributions to this error include the blackbody measurement uncertainty ( 0.1 K), and emissivity uncertainty (0.002). There is also a requirement that the surrounding environmental temperature that can reflect back out of the blackbody must be known to within 5 K. c6OK OMVVifl rifler aria ueteciDr MriBy LWIR PlItsr id Dactor Any IFC Flip-in Mirror (240K) Figure 1. The GIFTS EDU optical layout shows the location of the Flip-in mirror (left panel). Both blackbodies are mounted on a linear slide mechanism that positions the desired source in the field of view of the interferometer, via the activated Flip-in mirror (right panel). Table 1 summarizes the blackbody subsystem specifications that flow down from the GIFTS top-level instrument calibration requirement. The right-hand column lists the current best estimate for each parameter at the time of delivery of the engineering model system. Table 1. Blackbody subsystem specifications with as-delivered estimates of performance. nrium wlmlm-(a •€Tht1:Iizttffittfl Ambient Blaekbody Nominal Set Point 255 K Hot Blackbody Nominal Set Point 290 K 290 K <0.1 K (3 sigma) <0.056 K (3 sigma) Ambient Blsckbody Eminninity >0.996 >0.999 Hot Blackbody Eminsinity >0.996 >0.999 Eminninity Uncertainty <0.002 (3 sigma) <0.00072 (3 sigma) Wanelangth 690 - 2,300 cm-i 680 - 2,300 cm-i 2.54cm 2.54cm Temperature Measurement Uncertainty Source Aperture Source POV (full angle) Mann (two blackbodien plan controller) Power average/man Ennulopa 255 K > 10 > 10 <2 4 kg <2 1 kg <2 2i5.2 W <2 2/5.2 W <8n8a15.Scm <SasxtsAcm Section 2 of this paper gives an overview of the blackbody subsystem that includes two blackbodies and a controller that provides temperature measurement and control. Section 3 describes the temperature calibration approach that starts with the calibration of the controller electronics on-board reference resistors, and then proceeds to the end-to-end temperature calibration that uses a temperature standard that is tightly coupled to the cavity along with the calibrated controller electronics. Section 4 describes the blackbody cavity painting procedure and the determination of emissivity. Section 5 presents data collected from the GIFTS EDU testing that demonstrates extremely high temperature stability. 2. SYSTEM OVERVIEW The GIFTS engineering model on-board blackbody calibration system consists of two blackbody sources and a controller that provides independent temperature measurement and control for each blackbody. The two blackbodies are identical Proc. of SPIE Vol. 6405 64050I-2 except for the thermistors used for temperature measurement that are tuned for the appropriate operating range (hot or ambient). The translating slide on which the blackbodies are mounted is attached directly to instrument optics bench. The controller is contained on one (12” x 6”) electronics card that is located in the Sensor Module electronics box. U 1 IU 1_d 1:11 / HH U The blackbody design, illustrated in Figure 2, is scaled from the University of Wisconsin (UW) developed Atmospheric Emitted Radiance Interferometer (AERI) blackbody, scaled to a 1” aperture diameter to accommodate the GIFTS optical design13,14. The blackbody cavities are machined from aluminum and painted with Aeroglaze Z306 diffuse black paint. Each blackbody has four thermistors used for temperature measurement, each mounted in a customized housing that threads into the aluminum cavity wall6. Two of these thermistors are mounted near the apex of the cone and two mounted diametrically opposite near the junction of cylinder and cone. Figure 2. The GIFTS EDU blackbody is shown in the photo on the left without the enclosure. The cut-away figure on the right illustrates the key features of blackbody, including the thermistor locations and painted cavity. The blackbody controller functional block diagram is presented in Figure 3. The controller provides independent temperature measurement and control for each blackbody. A key feature of the controller is the self-calibration scheme that employs the use of on-board reference resistors. These highly stable resistors are read each time the blackbody thermistor resistances are read, thus significantly reducing measurement uncertainty – measurement accuracy becomes largely independent of offset and gain drift of the electronics. The calibration of these on-board reference resistors is performed on the ground as part of the temperature calibration procedure described in the next section. SOL Bus ABa Healer Figure 3. The blackbody controller provides independent temperature measurement and control for both blackbodies. At each data collection cycle the on-board reference resistors are measured along with the blackbody thermistor resistances – providing self-calibration. 3.0 TEMPERATURE CALIBRATION The blackbody temperature measurement scheme is depicted in Figure 4. Many aspects of this NIST traceable calibration approach were pioneered for the UW developed AERI ground based instrument and Scanning Highresolution Interferometer Sounder aircraft instrument15-17. Counts associated with the resistance of each thermistor Proc. of SPIE Vol. 6405 64050I-3 temperature sensor and each on-board reference resistor are read by the blackbody controller. The thermistor counts are converted to calibrated resistance using the counts returned from the on-board reference resistors and the Resistance Self-calibration Constants that are determined once on the ground using a set of Calibration Resistors (read in place of the blackbody thermistors). The calibrated resistances from each thermistor are then converted to calibrated temperature by using the Thermistor Calibration Coefficients. These coefficients are determined during end-to-end temperature calibration tests, using the calibrated blackbody controller and a temperature calibration standard that is tightly coupled to the blackbody cavity. Ground Processing Computer Blackbody Li Blackbody Thermistors Resistance Self Calibration Controller ____________ ____________ ________________ em?erat Ito' esistance Thermistor Counts Reference Resistor Counts Calibration Resistors - -, - Resistance SelfCalibration Constants Used one time on the ground to calibrate on-board reference resistors Calibrated Temperature an nr Figure 4. Block diagram illustrating the steps to get calibrated temperature from the thermistor resistance count data that is read through the blackbody controller. The key calibration steps involve determining: a) the Resistance Self-calibration constants, and b) the Thermistor Calibration constants. Both steps employ the use of NIST traceable transfer standards. To determine the Resistance Self-calibration Constants, 20 highly stable calibration resistors were used over the four measurement ranges of each blackbody. The calibration resistors were measured using a NIST traceable Agilent 7458A DVM. The 3 worst-case equivalent temperature error associated with the calibration resistors is 0.2 K. The Resistance Self-calibration Constants include one unique value for each thermistor channel (associated with the bridge resistance), and a unique value associated with each of the two on-board reference resistors used for the self-calibration. To cover all four measurement ranges for each blackbody there are a total of five on-board reference resistors associated with each blackbody (two reference resistors are used for each range). Fig. 5 shows the extremely high stability of the blackbody controller, where over a period of 60 days following calibration and prior to delivery, the equivalent temperature error is shown to be significantly less than 1 mK. The plot represents the resistance measurement error of the blackbody controller while periodically reading precise and highly stable calibration resistors inserted in each of the eight thermistor channels, expressed in equivalent thermistor temperature. The series plotted on the top is room temperature and is read on the right hand axis. Other testing demonstrated that errors due to the expected power supply voltage variations are < 0.2 mK, and errors due to the expected electronics board temperature variation are less than 0.5 mK. +1 mK 0.9 0.7 0.6 05 t N OmKj to5 I 104 Ooloii'o ro.i -0-C -0.7 BCE 11 -0-i -09 -1 mK Dot. Siamp_File No. 4-26-05 535 Figure 5. The stability of the blackbody controller resistance measurement capability over a period of 60 days, after calibration and prior to delivery is shown to be far better than 1 mK. Proc. of SPIE Vol. 6405 64050I-4 The end-to-end temperature calibration scheme is illustrated in Figure 6. Custom temperature calibration probes (one for each blackbody), were tightly coupled thermally to each blackbody cavity. These probes were made at Thermometrics to UW specifications using their SP-60 thermistors. The calibration probe thermistor resistances were read using a Hart Scientific 2563 Standard Thermistor Module. Each combined probe and read-out module was calibrated as a unit at Hart Scientific, to a 3 absolute accuracy of 5 mK, using NIST traceable standards. Custom made Thennonetrlcs SP-60 calibration standards Figure 6. Blackbody thermistor calibration scheme functional block diagram. For the engineering model system, each blackbody was calibrated at 5 temperatures within the measurement range associated with the nominal temperature set-point (there are four measurement ranges for each blackbody). At each blackbody calibration temperature, the cavity thermistor resistances were read using the calibrated blackbody controller, at the same time the cavity temperature was being read by the calibration probe. For each blackbody thermistor, the resistance and temperature data collected at the five calibration points were used in a least squares fit to the standard three coefficient Steinhart and Hart thermistor relationship. Fit residuals were very small, never exceeded 0.5 mK. Table 2 summarizes the temperature calibration uncertainty budget. The 3 uncertainty is 0.056 K which significantly exceeds the requirement of 0.1 K. Further discussion of the breakdown of the error contributors shown in the table can be found in the original paper describing the GIFTS blackbody system[6]. Table 2. Blackbody temperature uncertainty budget summary. Temperature Uncertainty Temperature Calibration Standard (Thermometrics SPeC Probe with Hart Scientific 2560 Thernnistor Module) •fl.iu' a:.i.a( :k'-lW 0.005 0.056 Blackbody Readout Electronics Uncertainty Readout Electronics Uncertainty (at deilvy) I 0.005 0.005 Blackbody Thennistor Temperature Transfer Uncertainty Gradient Between Temperature Standard and Cavity Thermistors Calibration Flttinq Equation Residual Error 0.010 0.001 0.010 Cavity Temperature Uniformity Uncertainty 0.025 0.008 0.018 Cavity to Thermistor Gradient Uncerlalnty (nnoMs,naenqee.dgsssve Thermistor Wire Heat Leak Temperature Bias Unceflaint Paint Gradient (hthheh fee Sen S nthn* fIBS Tne Bd ntehfn fne Qecnfl 0.032 Long-term Stability 0.030 0.012 Biackbody Thermistor (asndthi*as'ns'g nwCj Blackbody Controller Readout Electronic. 0.032 Effective Radiometric Temperature Weighting Factor Uncertainty Monte Carlo Rev Trace Model Uncertainty In Determining Tilt SI 535 en)eSedgfaasiff I 0.030 0.030 nety aSe,med to be the fell nal,a relcilatnd rot the effect in the tense case thnrmalennirelmtntindmakintcnns&vdtivethennalcoljplintnnsjmptinra) Proc. of SPIE Vol. 6405 64050I-5 0.056 4.0 CAVITY PAINTING AND EMISSIVITY DETERMINATION The blackbody cavity surfaces are painted with Aeroglaze Z306 diffuse high emissivity black paint. Previous work at the UW15 has shown that a dry thickness of 0.003 inch is desired for obtaining the maximum expected emissivity of this paint. This thickness will also keep undesirable temperature gradients through the thickness of the paint to a reasonably low level for on-orbit conditions. The blackbody cavities were spray painted at the UW using well-established procedures developed for the AERI and S-HIS instruments. Forty-eight, one inch diameter witness samples were painted at the same time as the cavities. Four witness samples were held in each of 12 fixtures that were designed to simulate the painting of the cavity cone (see right photo in Figure 7). These witness samples were used to determine dry paint thickness, dry paint density, paint emissivity, and paint adhesion integrity. The paint density measurements, along with mass measurements made of the cavities both before and after painting, provided a means to infer that the desired paint thickness of 3 mil was applied to the cavities. Direct thickness measurements would have damaged the paint surface. Figure 7 presents the average of six GIFTS blackbody witness sample emissivity measurements made at Labsphere. For comparison, emissivity data are plotted from NIST measurements of a sample of Aeroglaze Z306 (painted at NIST for an unrelated program). The GIFTS blackbody witness samples measured at Labsphere compare favorable with the NIST measurements, and generally fall well within the 3 uncertainty quoted by NIST for their measurements. GIFTS EM Blackbody Paint Witness Sample Emissivity - Meaured by Labsphere 0980 0.975 0.970 0.965 0.955 0.950 Fixture holds four 1" dia. samples simulating cone geometry 0.945 0.940 500 2,000 Wavenumber lcmhI Figure 7. The GIFTS Engineering Model paint emissivity measurements made by Labsphere (average of six 1” diameter witness samples). Samples were painted at the same time as the blackbody cavities in fixtures that simulated the cone geometry (right photo). In order to determine the cavity emissivity the Monte Carlo ray trace computer program STEEP3 was used18. Inputs to this model are paint emissivity, paint diffusity, and cavity geometry6,19, as illustrated in the left-hand diagram of Figure8. The right-hand plot in Figure 8 provides the output of the ray-trace model. This plot shows the wavelength dependence of cavity emissivity vs paint emissivity, which can be parameterized in terms of the Cavity Factor (Cf), defined in the figure. The plot on the left of Figure 9 shows a quadratic fit of the wavelength dependence of the Cavity Factor. This relationship was then folded together with the paint emissivity data to provide the cavity emissivity vs wavenumber plot shown on the right of Figure 9. The cavity emissivity is greater than 0.998 for all wavenumbers of interest, significantly exceeding the requirement of 0.996. Table 3 presents a summary of the emissivity uncertainty budget, which remains the same as that presented in the original paper describing the GIFTS Blackbody system6. The 3 cavity emissivity uncertainty is 0.00072 which significantly exceeds the requirement of 0.002. Further discussion of the breakdown of the error contributors shown in the table can be found in the original paper describing the GIFTS blackbody system. Proc. of SPIE Vol. 6405 64050I-6 Paint Emissivity 4 Cavity Geometry 0.999 STEEP3 Cavity Emissivity Monte Carlo Ray Trace Model 0.998 0.94 0.96 0.98 Paint Emissivity Figure 8. The GIFTS blackbody cavity emissivity was calculated using the Monte Carlo ray trace model STEEP3. Inputs to the model are cavity geometry, intrinsic paint emissivity, and paint diffusity. The plot on the right shows the calculated cavity emissivity at 5, 10, and 15, parameterized in terms of the cavity factor (equation in plot). GIFTS Blackbody Cavity Factor () Dependence on Wavelength GIFTS Blackbody Cavity Emissivity 000.00 it —93.00 90.00 o sw A1 - -3.900 80.00 —0.0600 70.00 I -I 50.00 —Fit • Data E o 99OO 3 so.oo Cf 40.00 -J 30.00 '0 0 05 20 O.SS9CO 0.99090 Wavelength [ii] I I 500 0000 I 500 5.0. 2000 2500 b•s 4s-L Figure 9. The GIFTS blackbody cavity emissivity dependence on wavelength shown in Figure 7 was fit using a quadratic (left plot). This relationship was then folded with the paint emissivity data using the equation presented in the right panel to yield cavity emissivity vs wavenumber (right plot). Table 3. Blackbody emissivity uncertainty budget summary. Paint Witness Sample Measurement 0.4% Ep [11 bEp=0.0038 (1/t)*aEp 0.00010 Paint Application Variation 1.0% Ep [21 bEp=0.0094 (1fl)*AEp 0.00024 Long-term Paint Stability 2.0% Ep 131 6Ep=O.0188 (lIf)*AEp 0.00048 30% f 141 At=1 1.7 (1 Ep)ftA2*aJ 0.00046 Cavity Factor RSS Notes: f=(1-Ep(1-Ec) f=Cav.ty Factor Ep=Emissivity of Paint Ec=Emissivity of cavity [1] Factor of 1.5 times NISr Staled Accuracy for 2 sigma (2] Worst case difference belween 1 and 3 coats [3] 2 x above [4] Accounts of Cavity Model Uncertainty * NIST Stated accuracy is 4% of Reflectivity (2 sigma) Proc. of SPIE Vol. 6405 64050I-7 0.00072 5.0 GIFTS EDU TEST RESULTS DEMONSTRATING TEMPERATURE STABILITY Blackbody temperature data collected during the GIFS Engineering Development Unit testing at the Utah State Space Dynamics Laboratory earlier this year, demonstrates very high temperature stability. Figure 10 shows that both long and short-term stability are excellent for the four HBB thermistors. The upper plot presents temperature data from the four HBB thermistors over a period of about a month, from 8/25 through 9/26 of 2006. The changing gradients within the HBB shown in this plot are due to the different temperature environments that surround the blackbody for different test conditions. The lower plot (from 9/6) shows a stability of better than 2 mK over a period of 80 minutes. The lower two thermistors (A and B) are located at the cavity apex and are expected to read close to the same value, which they do (within 2 mK). The upper two thermistors (C and D) are located closer to the cavity heater at the junction of the cavity cone/cylinder junction and are thus running warmer. The 11 mK difference in their readings is most likely due to the asymmetric external environment of the blackbody. The temperature measurements observed are consistent with blackbody thermal model predictions. GIFTS LDU HBB Blackbody Thermistor Temperature 286.60 288.59 Ii 286.58 -ft-i—fl 286.57 U 286.56 + hbbA hbbB 286.55 + hbbC + hbbD I 286.54 II 286.53 1$ 0.10 K 286.52 288.51 286.50 8/18 8/23 8/28 9/2 9/12 Date (mm/ddJ 9/17 9/7 9/22 9/27 10/2 GIFTS EDLJ HBB Blackbody Thermistor Temperatures (9106/06) 256.60 760,55 256. 56 0S—W Q +hbbA 0.046K - - X0600 256.54 + + _____ 0.002 256.52 206.50 50:57:36 19:12:00 59:26:24 10:40:46 19:55:52 0.10 K 240 K Kt 20:06:36 20:24:00 20:36:24 Tim. (hh:mm:..] Figure 10. The Hot Blackbody thermistor temperature stability, long-term (upper plot) and short-term (lower plot), from data obtained during GIFTS EDU testing, shows excellent performance. Proc. of SPIE Vol. 6405 64050I-8 6.0 SUMMARY A blackbody calibration system suitable for spaceflight has been developed to meet the demanding requirements of the GIFTS instrument. The system builds on the strong heritage of the ground and aircraft based FTS instruments developed at the University of Wisconsin. The engineering model version of this system was fully calibrated and shown to exceed required temperature and emissivity accuracies. The engineering model system was delivered to the USU Space Dynamics Laboratory and integrated into the GIFTS Engineering Development Unit, which underwent a full complement of thermal vacuum testing. During this testing the GIFTS on-board blackbody calibration system exceeded all performance expectations. ACKNOWLEDGEMENTS The GIFTS Blackbody development effort at the University of Wisconsin was conducted under a subcontract (C922185) from the Utah State University, Space Dynamics Laboratory. The parent NASA contract number to SDL is NAS100071. REFERENCES 1. Smith, W.L., F. Harrison, D. Hinton, J. Miller, M. Bythe, D. Zhou, H. Revercomb, F. Best, H. Huang, R. Knuteson, D. Tobin, C. Velden, G. Bingham, R. Huppi, A. Thurgood, L. Zollinger, R. Epslin, and R. 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