MODIS on-orbit spatial characterization results using ground measurements

MODerate resolution Imaging Spectro-radiometer (MODIS), as part of NASA’s Earth Observe System (EOS) mission, is widely utilized in diversified scientific research areas. Both Terra and Aqua MODIS observe the earth in sun-synchronous orbit at three nadir spatial resolutions. MODIS has thirty-six bands that are located in four Focal Plane Assembles (FPAs) by wavelength: Visible (VIS), Near-Infrared (NIR), Short-and Middle-wavelength IR (SMIR), and Long wavelength IR (LWIR). MODIS Band-to-Band Registration (BBR) was measured pre-launch at the instrument vendor. Mis-registration exists between bands and FPAs. The spatial characterization could change in storage, at launch, and years on-orbit. In this study, a special ground scene with unique features has been selected as our study area to calculate the spatial registration in both along-scan and along-track for bands 2 7 relative to band 1. The results from the earth scene targets have been compared with on-board calibrator, the Spectro-Radiometric Calibration Assembly (SRCA), with good agreement. The measured differences between the SRCA and our ground scene approach are less than 20m on average for VIS/NIR bands both along-scan and along-track. The differences for SMIR bands are 20m along-scan and 0.1 – 0.18 km for along track. The SMIR FPA crosstalk could be a contributor to the difference. For Aqua MODIS instruments, the spatial deviation is very small between the bands located on the same FPA or between VIS and NIR FPAs but is relatively large between warm (VIS and NIR) and cold (SMIR and LWIR) FPAs. The spatial deviation for MODIS/Terra can be ignorable but not for MODIS/Aqua. The results from this study show that the spatial deviation of Aqua MODIS may impact on the science data when multi-band data from both warm and cold FPAs is combined. Keyword: MODIS, spatial characterization, BBR, SRCA, INTRODUCTION MODerate resolution Imaging Spectro-radiometer, one of the sensors for NASA’s EOS mission, is of great importance in diversified scientific research areas, such as land, ocean, and atmosphere. The MODIS on board Terra spacecraft was launched on December 18, 1999 and the MODIS on Aqua spacecraft was launched on May 4, 2002. The Terra spacecraft crosses the equator at 10:30 AM and the Aqua spacecraft crosses the equator at 1:30 PM. Both Terra and Aqua MODIS are operated at a sun-synchronous orbit with the altitude of 705km, observing the earth with thirty-six spectral bands at three different nadir spatial resolutions: 250m, 500m, and l000m [1, . MODIS has been widely used for land, ocean, atmosphere area, devoted in observing the globe climate change, land cover change, detecting the fires, hurricane, dust storm, generating a set of indices, such as NDVI (Normalized Difference Vegetation Index), NDSI (Normalized Difference Snow Index), NDWI (Normalized Difference Water Index) and so on. Because of MODIS wide application areas, the accuracy of measurements is the most critical specification that the scientists and researchers typically pay great attention to. MODIS has a set of On-Board Calibrators (OBCs) designed for instrument on-orbit characterization including solar Earth Observing Systems XI, edited by James J. Butler, Jack Xiong, Proc. of SPIE Vol. 6296, 62961L, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.680787 Proc. of SPIE Vol. 6296 62961L-1 diffuser (SD), solar diffuser stability monitor (SDSM), blackbody (BB), Space-view (SV) and the Spectro-Radiometric Calibration Assembly (SRCA) [3, . The SD and SDSM are used for radiometric calibration of reflective solar bands (RSB) and the blackbody is for thermal emissive bands (TEB). SV provides the back ground signal, a data point with zero radiance. The SRCA, as an independent device within MODIS instrument, can work at three calibration modes: radiometric, spatial, and spectral. It has the capability of tracking the Band-to-Band Registration (BBR) shift during MODIS entire mission. With the operation of MOIDS/Terra for more than 6 years and MODIS/Aqua for about four and half years, the substantial spatial deviation exists between cold and warm FPAs for Aqua MODIS but is negligible for Terra MODIS. In order to validate the spatial characterization results for the SRCA and serves as the complement for real time measurement, we calculated the spatial deviation using ground measurements (GM) at a particular site. Additionally, using GM to track the spatial characterization is helpful for the sensors which have no on-board spatial characterization capability. For example, two targets including berms between agricultural fields at the Maricopa Agricultural Center, Arizona and the Lake Pontchartrain Causeway, Louisiana are applied for spatial characterization of the NASA EO-1 Advanced Land Imager sensor . Since MODIS has daily global coverage, the approximate real time spatial deviation can be calculated if the measurement is qualified. The specific site chosen for MODIS spatial characterization is located at the Libya desert in Africa, where a series of dark circular targets are arranged regularly. The spatial deviation results computed from the ground measurements are consistent with the results derived by the SRCA. The maximum spatial deviation of MODIS/Aqua is up to one pixel (500m spatial resolution) between warm and cold FPAs. In this paper, the instrument background and the SRCA is briefly introduced in section two. In section three, we illustrated the approach used for calculating the spatial deviation both along-track and along-scan using ground targets in details. The spatial deviation impact on current MODIS products is discussed in section four. Our calculation demonstrates that the careful selected ground targets can be utilized to determine band to band registration in both along-scan and along-track. This is important for majority of remote sensing instruments without on-board devices for BBR measurements. Evident band to band mis-registration can be detected and to be considered for correction in the products of data application. INSTRUMENT BACKGROUND MODIS instrument has 36 spectral bands located on four FPAs: Visible (bands 3-4, 8-12), Near Infrared (bands 1-2, 13-19), Short-and Middle-Wavelength Infrared (bands 5-7, 20-26), and Long-Wavelength Infrared (bands 27-36). Figure 1 is the MODIS optical system. The dichroics separate the incoming beam to different FPAs. The four FPAs are carefully aligned prelaunch using high-precision theodolite. In along-scan direction, the electronic phase-delay can be applied to each FPA to adjust the sampling starting time by a step size equivalent to one tenth of 1km Instant Field-Of-View (IFOV) so that the FPA in along-scan direction can be electronically shifted instead of physically shifting the FPAs for registration. The BBR was measured by a ground calibration device called Integration Alignment Calibrator, IAC, at the instrument vendor. At near the same time and test environment, the SRCA was operated in spatial mode. Because the SRCA fills only 1/5 of the MODIS aperture size, the measurement results from IAC and the SRCA have some difference. The difference is served as bias and is assumed unchanged. On-orbit, the SRCA is operated at a designed time period which allows measurement of BBR change from pre-launch to on-orbit and throughout the MODIS life-time. In pre-launch measurement, the SRCA measured Band-to-Band Registration (BBR) in both along-scan and along-track direction. The SRCA has a rectangular reticle covering 5km IFOV along-scan and 12km IOFV along-track to measure BBR along-scan. The illuminated reticle is sharply imaged onto four FPAs and is scanned across all MODIS bands/detectors. Several illumination levels are available so that the best SNR can be obtained from one of the illumination level for each band. The along-track reticle is formed by four small rectangular reticles. When the reticle scans across the detectors, different detectors are illuminated fully or partially . The band centroid position along-track can be determined. Table 1 listed the average BBR shift of band 1-7 for two MODIS instruments in both along-track and along-scan directions. The left side listed the results of Terra MODIS and the results of Aqua MODIS displayed on the Proc. of SPIE Vol. 6296 62961L-2 right side. Table 1. The average BBR shift measured by the SRCA for band 1-7 in both along-track and along-scan directions Terra/MODIS Along-scan(m) Along-track(m) Aqua/MODIS Along-scan(m) Along-track(m) Band 1 0 0 Band 1 0 0 Band 2 20 0 Band 2 -4 0 Band 3 -28 4 Band 3 30 -9 Band 4 -40 16 Band 4 4 -38 Band 5 40 52 Band 5 -246 366 Band 6 46 18 Band 6 -285 382 Band 7 33 86 Band 7 -223 409 Figure 1. MODIS optical system 0 2 4 6 8 10 12 14 16 18 20 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 Pixel number R ef le ct an ce Spatial deviation along-track for MODIS/AQUA Band1 Band4 Band5 Band7 0 2 4 6 8 10 12 14 16 18 20 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 Pixel number R ef le ct an ce Spatial deviation along-scan for MODIS/AQUA Band1 Band4 Band5 Band7 Figure 2 (a) Spatial deviation between VIS and SMIR FPAs along-track for Aqua MODIS (b) Spatial deviation between VIS and SMIR FPAs along-scan for Aqua MODIS OPTICAL SYSTEM