Adjusting Aqua MODIS TEB nonlinear calibration coefficients using iterative solution

Transcription

1 Adjusting Aqua MODIS TEB nonlinear calibration coefficients using iterative solution Aisheng Wu a, Zhipeng (Ben) Wang a, Yonghong Li a, Sriharsha Madhavan b, Brian N. Wenny a, Na Chen a, and Xiaoxiong (Jack) Xiong c a Sigma Space Corp., 4801 Forbes Blvd, Lanham, MD b Sciences and Systems Applications, Inc., Greenbelt Rd., Lanham, MD c Sciences and Exploration Directorate, NASA/GSFC, Greenbelt, MD ABSTRACT Radiometric calibration is important for continuity and reliability of any optical sensor data. The Moderate Resolution Imaging Spectroradiometer (MODIS) onboard NASA EOS (Earth Observing System) Aqua satellite has been nominally operating since its launch on May 4, The MODIS thermal emissive bands (TEB) are calibrated using a quadratic calibration algorithm and the dominant gain term is determined every scan by reference to a temperature-controlled blackbody (BB) with known emissivity. On a quarterly basis, a BB warm-up and cool-down (WUCD) process is scheduled to provide measurements to determine the offset and nonlinear coefficients used in the TEB calibration algorithm. For Aqua MODIS, the offset and nonlinear terms are based on the results from prelaunch thermal vacuum tests. However, on-orbit trending results show that they have small but noticeable drifts. To maintain data quality and consistency, an iterative approach is applied to adjust the prelaunch based nonlinear terms, which are currently used to produce Aqua MODIS Collection-6 L1B. This paper provides details on how to use an iterative solution to determine these calibration coefficients based on BB WUCD measurements. Validation is performed using simultaneous nadir overpasses (SNO) of Aqua MODIS and the Infrared Atmospheric Sounding Interferometer (IASI) onboard the Metop-A satellite and near surface temperature measurements at Dome C on the Antarctic Plateau. Keywords: MODIS, calibration, blackbody, nonlinearity 1. INTRODUCTION MODIS is a key instrument on-board both NASA EOS Terra and Aqua satellites launched in December 1999 and May 2002, respectively 1-2. The Terra and Aqua MODIS are nearly identical Earth Observing Missions and Sensors: Development, Implementation, and Characterization III, edited by Xiaoxiong Xiong, Haruhisa Shimoda, Proc. of SPIE Vol. 9264, 92640R 2014 SPIE CCC code: X/14/$18 doi: / Proc. of SPIE Vol R-1

2 in their instrument design. The two satellites fly in sun-synchronous polar orbits at a nominal altitude of 705 km, with an equatorial crossing time of 10:30 am for Terra and 1:30 pm for Aqua. MODIS has 16 TEB with a wavelength range from 3.5 to 14.4 μm and 10 detectors in each band. All TEB detectors are located on two separate focal plane assemblies: short-wave infrared or mid-wave infrared (SWIR/MWIR) and long-wave infrared (LWIR). The two focal plane assemblies are normally controlled at 83 K using a passive radiative cooler. MODIS uses a double-sided scanning mirror that scans over an angular range of ±55 o off nadir for earth scene data collection, covering a cross-track swath of 2330 km. The nadir spatial resolution for all TEB detectors is 1 km. The calibration of the MODIS TEB detectors is conducted using an onboard BB on a scan-byscan basis to adjust the dominant gain coefficients for each band, detector, and mirror side 3. This is achieved by reference to the BB radiance calculated using the Planck equation with known emissivity and the BB temperature determined by taking measurements from 12 embedded thermistors. On a quarterly basis, a scheduled blackbody WUCD cycle is conducted during which temperature varies between 270 and 315K. This allows determination and tracking of the stability of the offset and nonlinear coefficients used in the TEB calibration. For Aqua MODIS, the TEB at-launch offset and nonlinear coefficients were derived from the prelaunch calibration results 4. Since MODIS prelaunch tests were conducted in better controlled BB temperature conditions and over a much wider dynamic range than on-orbit BB WUCD activities, the prelaunch calibration coefficients are more accurate than those derived on orbit. However, onorbit trending results indicate that there is a need to adjust the offset and nonlinear coefficients and a direct implementation of these coefficients derived from BB WUCD activities would cause a discontinuity in the L1B data, particularly for scene temperatures beyond the BB temperature range. In this study, an iterative approach is used to adjust the prelaunch based coefficients based on BB WUCD measurements. This provides the ability to maintain data quality and consistency and at the same time to make necessary on-orbit adjustment of the coefficients. Section 2 describes the methodology of using an iterative solution to determine the adjusted prelaunch coefficients. Section 3 shows the impact of the adjusted prelaunch coefficients on L1B based on simulated detector responses. Validation is conducted using observations from Aqua MODIS and Metop-A IASI SNO and the near surface temperature measurements at Dome C on the Antarctic Plateau, Antarctica. 2. METHODOLOGY The TEB on-orbit calibration is based on a quadratic equation that converts the detector response to radiance when viewing a temperature controlled BB 3-4 : 2 ΔL BB = a 0 + b 1 dn BB + a 2 dn BB (1) Proc. of SPIE Vol R-2

3 where ΔLBB is the radiance at the BB view subtracted by the background impact, dnbb is the detector raw count at the BB view subtracted by that of its space view. The dominant linear term, b1, is computed on a scan-by-scan basis and offset and nonlinear terms, a0, and a2, are derived from prelaunch calibration tests or on-orbit scheduled BB WUCD activities. The term ΔLBB is calculated by ΔL BB = RVS BB ε BB L(T BB ) + (RVS SV RVS BB )L(T SM ) + RVS BB (1 ε BB )ε cav L(T cav ) (2) where RVSBB and RVSSV are the response versus scan angle (RVS), εbb and εcav are the BB and instrument cavity emissivity. L is the radiance computed at the respective temperatures (TBB, TSM and Tcav) using the Planck equation. Based on Equations (1) and (2), the linear term b1 is determined by b 1 = ΔL 2 BB a 0 a 2 dn BB dn BB (3) The Earth view (EV) radiance can be determined by applying the calibration coefficients, a0, b1 and a2 to the detector response to the EV L EV = 1 {(a RVS 0 + b 1 dn EV + a 2 dn 2 EV ) (RVS SV RVS EV )L sm } (4) EV The nominal TEB calibration is performed with the BB temperature controlled at 285 K. MODIS also performs quarterly BB WUCD activities, during which the BB temperatures vary between 270 and 315 K. This allows determination of the terms a0 and a2 and their trending results are used to evaluate the stability of each detector. Figure 1 shows the on-orbit trends of b1 and a2 for band 30. Trending results show that although the dominant term b1 is very stable through the mission, there is a gradual upward trend in a2, indicating the nonlinearity of the detector responses changes on orbit. However, a direct use of the a0 and a2 values determined from quarterly BB WUCD activities would cause a discontinuity in the L1B data because there are systematic differences between the on-orbit and prelaunch a0 or a2. Figure 2 shows a discontinuity in brightness temperature (BT) at the typical radiance for all TEB bands by replacing the prelaunch a0 and a2 coefficients with two sets of on-orbit coefficients obtained in 2002 and 2010, respectively. There are no results shown here for bands 20, 31 and 32 since the on-orbit based calibration coefficients are used for these bands. The reason that the prelaunch a0 and a2 were used is because they were more accurately estimated and better represented the nonlinear behavior of the detectors during the initial stage of the Aqua mission. Figure 3 compares the ratio, dn/l, over the whole range of detector response (0 4095) for bands 20 and 34 obtained from prelaunch tests with those from on-orbit BB WUCD measurements on day 203 of It is shown that the prelaunch tests cover a much wider dynamic range, corresponding to Proc. of SPIE Vol R-3

4 a BB temperature range between 170 and 340 K, compared with a typical BB WUCD temperature range between 270 and 315 K. Since the nonlinearity of the detector responses changes on orbit, it is necessary to adjust a0 or a2 accordingly to maintain a consistent quality of the calibrated L1B data. As discussed previously, a direct use of the a0 and a2 values determined from BB WUCD activities would cause a discontinuity in the L1B data. Thus, an iterative (i = 0, 1, 2, 3..) solution is used to make finite adjustment of a2 by adding a small term Δa2 i and such an adjustment is based on BB WUCD measurements in order to catch the on-orbit changes in detector nonlinearity [BT EV (a 2 + a 2 i ) BT EV (a 2 )] [BT EV WUCD BT EV WUCD_ini ] δt EV (5) { a i 2 = dt(dn EV a 2 0 = dl dt(dn EV dl 2 dn BB dn EV ) [BT EV WUCD BT EV WUCD ini ] 2 dn BB dn EV ) [BT EV(a 2 + a 2 i 1 ) BT EV (a 2 )] (i = 1, 2.. ) (6) where BTEV is the brightness temperature at the typical radiance, BT with superscripts WUCD and WUCD_ini are the BT derived using a2 from current and initial (year 2002) BB WUCD events, respectively, dl/dt is a function of spectral wavelength and temperature determined by the Planck equation. Value of δtev is a predetermined temperature difference tolerance with which the iteration converges. The calculation of Δa2 i at each iteration step is based on the combination of equations (1) to (3). Currently, there is no adjustment of a0, which is still based on the values provided from prelaunch tests. Aqua Cool-down Al Trending (Collection6; SBRS Order) band 30 Aqua Cool -down A2 Trending (Collection6; SHRS Order) 4x10 band Jr." 2x10 -B g x o... I Epoch x Epoch CIO CM ChS Chi CM CM Ch? CM CMO Figure 1. Trends of Aqua C6 band 30 b1 (left, marked with A1) and a2 (right, marked with A2) determined from quarterly BB WUCD data. Detectors are marked with different colors and two mirror sides are marked with different symbols. Proc. of SPIE Vol R-4

5 Figure 2. BT differences at the typical temperature between using prelaunch and on-orbit a0/a2 for all TEB detectors in 2002 and Figure 3. dn versus L from prelaunch tests and on-orbit BB WUCD data on day 203 of 2004 for bands 20 and RESULTS 3.1 Impact of the adjusted prelaunch a2 on L1B The adjusted prelaunch a2 coefficients are determined based on the calculation of BT at the typical radiance, Ltyp, as shown in the previous section. It is important to examine the impact of the new value of a2 over the effective calibration range between 0.3Ltyp and 0.9Lmax (Lmax is the maximum radiance). Figure 4 shows the simulated BT differences as a function of temperature for bands 20 and 30 between using the adjusted and unadjusted prelaunch a2 in three recent years (2009, 2012 and 2013) when the impacts are noticeably larger than early years. We only provide Proc. of SPIE Vol R-5

6 results of the two bands since they have the largest on-orbit change in detector response nonlinearity, as shown in Figure 1 for band 30. Over the effective calibration range, the impact due to the use of the adjusted prelaunch a2 is within ±0 K. bond band o CO 0.0. ~ ' band bond 30 Y, CO ',..i Y I- CO S bond band I- C a Figure 4. BT differences between using prelaunch and on-orbit a0/a2 as function of BT for bands 29 and 30 in 2009, 2012 and Different lines represent detectors Validation A comparison is made using hyperspectral measurements by IASI onboard the polar-orbiting satellite, MetOp-A, launched in Its infrared spectra spans 645 to 2760 cm-1 (3.62 to 15.5 μm) with 8461 spectral samples and the spectra have a sampling interval of 5 cm-1. IASI is an Proc. of SPIE Vol R-6

7 1 across track scanning system with scan range of ±48, centered at the nadir direction. A nominal scan line covers 30 scan positions towards the Earth. At each scan position, the effective field of view consists of a 2 x 2 matrix of instantaneous fields of view (IFOV), which corresponds to a ground resolution of 12 km at nadir at a satellite altitude of 819 km. Similar to MODIS, IASI uses an internal blackbody and deep space view for its calibration. Details on the calibration and performance can be found in recent studies 6-7. Comparison was conducted using measurements from SNO between the Metop-A satellite and Aqua satellites. SNO times and locations were determined based on satellite orbital two-line element sets and the SNO time difference was limited to within 30 seconds. Each IASI instantaneous field of view of 12 km is co-located with multiple MODIS pixels. The corresponding IASI simulated MODIS radiances are derived by convolutions of IASI hyperspectral data and MODIS relative spectral response functions. Finally, the radiances are converted into the BT values using the Planck function. (MODIS -IASI) Time Series B29 (MODIS -IASI) Time Series B x 0.5 x ó w co R 0 w epoch day since epoch day since Figure 5. Trends of BT differences between Aqua and IASI for bands 29 and 30 from C5 and C6. Lines represent a linear fit using the data stating from day 208 of 2009 (epoch 2765) when the first iteration adjusted a2 LUT was implemented. Figure 5 shows the time series of BT differences between MODIS and IASI for bands 29 and 30 with data starting from the beginning of 2007 when IASI data became available. The Collection 5 (C5) results are derived with the unadjusted prelaunch a2 coefficients and the Collection 6 (C6) results are from the adjusted coefficients 8-9. It should be noted that SNO between Aqua and MetOp-A orbits appear at approximately ±75 latitude. Thus the BT range is mostly from 240 to 275 K for band 29 and 220 to 260 K for band 30. Results from C5 show a noticeable upward trend, indicating an increased BT difference over time. By comparison, results from C6 are generally flat. The reason that results from C5 and C6 during the first two years (2007 and 2008) are very close is because the first L1B look-up-table (LUT) with the adjusted prelaunch a2 coefficients starts from the middle of Based on the results shown in Figure 4, the Proc. of SPIE Vol R-7

8 simulation with the adjusted coefficients produces slightly lower BT by ~ K over the SNO temperature ranges. This is consistent with the trending results shown in Figure 5 where BT from C6 is lower than those from C5 by about 0.1 to K in the SNO results after These results indicate that the MODIS L1B data generated using the adjusted prelaunch a2 coefficients as used in C6 not only become more consistent than those from the unadjusted coefficients, they agree better with IASI measurements. (MODIS -IASI) Scatter Plot B29 (MODIS -IASI) Scatter Plot B3O -t o zoo Simulated t o zoo Simulated Figure 6. Trends of BT differences between Aqua C5/C6 and Terra C6 for bands 29, 30 and 35 using IASI. The BT differences between MODIS and IASI are further plotted versus BT (Figure 6). Results from C6 are lower than C5 and in better agreement with IASI for the both bands. For band 29, the magnitude of the BT differences decreases with increasing temperature. This is expected since the high end of the SNO temperatures is around 280 K, which is close to the Aqua nominal BB temperature of 285 K, where the calibration is dominant by the linear term b1. For band 30, the scene temperatures are much lower than those observed by band 29 but the trend shows an increase in BT differences with temperature. This is believed to be caused by some unaccounted effects in calibration such as electronic leak from other bands, which has been observed in the case of Terra MODIS bands 27 to 30. We also track the BT difference, ΔT, using the near surface temperature measurements obtained from Concordia Station at a location called Dome C on the Antarctic Plateau, Antarctica 10. Since Dome C is one of the coldest places on Earth, the trending result is useful to evaluate calibration performance at extreme low temperatures. Figure 7 shows the band 29 BT difference trends from C5 and C6. It should be noted that the differences between BT and station s near surface temperature measurements are large (up to 7 K in summer) but on average and over the long term, the differences should be consistent and stable. Results from band 29 are more useful than Proc. of SPIE Vol R-8

9 those from band 30, which has a larger atmospheric impact. The trends indicate that both C5 and C6 results are stable over the mission. In order to find out the existing small difference between C5 and C6 in the Dome C result, the ΔT difference between C5 and C6 is calculated, as shown in Figure 8. This result indicates that C6 with the adjusted a2 coefficients produces slightly lower BT than C5 by up to 0 K at extreme low temperatures, starting from the middle of This is consistent with the difference between C5 and C6 found both in the simulated results and SNO measurements. A LAL 4,44 4A, CE, Figure 7. Trends of BT subtracted by AWS-T, ΔT, using Aqua C5/C6 for band 29 over Dome C. Each data point is obtained by taking a monthly average over all available data. Figure 8. Trends of ΔT difference between C5 and C6 for band 29 over Dome C. Proc. of SPIE Vol R-9

10 4. SUMMARY MODIS is a key instrument onboard both EOS Terra and Aqua satellites launched in December 1999 and May 2002, respectively. It has 16 TEB bands with a wavelength range between 3.5 and 14.4 μm and 10 detectors in each band. TEB on-orbit calibration is performed on a scan-by-scan basis to determine the dominant gain response through observations of a temperature controlled on-board BB. On a quarterly basis, the offset and nonlinear coefficients are derived using measurements from scheduled BB WUCD cycles. Trending results show that the on-orbit offset and nonlinear coefficients of a few TEB have a small but noticeable drift. To maintain data quality and consistency, an iterative method is applied to adjust the prelaunch based nonlinear terms, which are currently used to produce Aqua MODIS Collection-6 L1B. This paper provides details on how to use an iterative solution to determine these calibration coefficients. Validation is performed using observations from Aqua MODIS and Metop-A IASI SNO and the near surface temperature measurements obtained from the Dome C AWS on the Antarctic Plateau, Antarctica. Results show that C6 with the adjusted coefficients produces BT lower than C5 by about 0.1 to K in both SNO and Dome C trends after 2009 and C6 shows a more consistent trend in comparison with IASI. REFERENCES [1] W.L. Barnes and V.V. Salomonson, MODIS: A global image spectroradiometer for the Earth Observing System, Critical Reviews of Optical Science and Technology, CR47, , (1993). [2] C.L. Parkinson, Summarizing the first ten years of NASA s Aqua mission, IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 6(3), , (2013). [3] Xiong, X., K. Chiang, J. Esposito, B. Guenther, and W. Barnes, "MODIS On-Orbit Calibration and Characterization", Metrologia, vol. 40, issue 1, pp , (2003). [4] X. Xiong, B. Wenny, A. Wu, W.L. Barnes, and V. Salomonson, Aqua MODIS Thermal Emissive Bands On-orbit Calibration, Characterization, and Performance, IEEE Trans. Geosci. Remote Sensing, 47(3), , (2009). [5] Li, Y., A. Wu, and X. Xiong, "Evaluating calibration of MODIS thermal emissive bands using infrared atmospheric sounding interferometer measurements ", Proc. SPIE 8724: Ocean Sensing and Monitoring V 87240X, (2013). [6] Elliott D.A., Aumann H.H., and Broberg S.E., Comparison of AIRS and IASI co-located radiances for cold scenes, Proc. SPIE, vol.7807, 78070J, doi: / (2010). Proc. of SPIE Vol R-10

11 [7] Wang L., Wu X., Goldberg M., Cao C., Li Y., Sohn S., Comparison of AIRS and IASI radiances using GOES imagers as transfer radiometers towards climate data records, Journal of Applied Meteorology and Climatology, vol.49, no.3, pp (2010). [8] Toller, G., X. Xiong, J. Sun, B. N. Wenny, X. Geng, J. Kuyper, A. Angal, H. Chen, S. Madhavan, and A. Wu, "Terra and Aqua Moderate-resolution Imaging Spectroradiometer Collection 6 Level 1B Algorithm", Journal of Applied Remote Sensing, vol. 7, issue 1, (2013). [9] Wenny, B. N., A. Wu, S. Madhavan, Z. Wang, Y. Li, N. Chen, V. Chiang, and X. Xiong, "MODIS TEB calibration approach in collection 6", Proc. SPIE 8533, Sensors, Systems, and Next-Generation Satellites XVI, 85331M, (2012). [10] Wenny, B., and X. Xiong, "Using a Cold Earth Surface Target to Characterize Long-term Stability of the MODIS Thermal Emissive Bands", IEEE Geosci. Remote Sens. Let., vol. 5, issue 2, pp , (2008). Proc. of SPIE Vol R-11