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Terra and Aqua MODIS On-orbit Spectral Characterization for Reflective Solar Bands

Terra and Aqua MODIS On-orbit Spectral Characterization for Reflective Solar Bands. Taeyoung (Jason) Choi a , Xiaxiong (Jack) Xiong b , Zhipeng Wang a , Daniel Link a a Sigma Space Corporation, 4801 Forbes Boulevard, Lanham, MD 20706 USA

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Terra and Aqua MODIS On-orbit Spectral Characterization for Reflective Solar Bands

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  1. Terra and Aqua MODIS On-orbit Spectral Characterization for Reflective Solar Bands Taeyoung (Jason) Choia, Xiaxiong (Jack) Xiongb, Zhipeng Wanga, Daniel Linka aSigma Space Corporation, 4801 Forbes Boulevard, Lanham, MD 20706 USA bSciences and Exploration Directorate, NASA Goddard Space Flight Center

  2. Outline • Introduction • MODIS • Spectro-Radiometirc Calibration Assembly (SRCA) • Methodology • On-orbit SRCA spectral calibration • Results • On-orbit events • Center Wavelength (CW) and Band Width (BW) trending for the nominal bands • Band 2 results • SRCA configuration changes and its solution • Short Wave IR (SWIR) band results • Summary

  3. Introduction • Terra and Aqua MOIDS have been successfully operated over 13+ and 11+ years. • MODIS has 36 spectral bands (490 detectors) • 20 reflective solar bands (RSB) • bands 1-19, 26 covering 0.41-2.2 µm • 16 thermal emissive bands (TEB) • bands 20-25 and 27-36 covering 3.5-14.4 µm • 3 spatial resolutions at nadir • 250 m (bands 1-2), 500 m (bands 3-7), 1 km (bands 8-36) • Global temporal coverage within 2 days • 40 data products for land, oceans and atmospheric studies.

  4. Introduction • MODIS on-board calibrators • SDSM, SD, SRCA, BB and SV Figure 1. MODIS on-board calibrators.

  5. Introduction • SpectroRadiometric Calibration Assembly (SRCA) • Radiometric mode: • Long-term DN trending and mirror side ratios  inputs to RSB calibration. • Spatial mode: • Band-to-Band Registration (BBR) in along-scan and along-track directions, Modulation Transfer Function (MTF) • Spectral mode: • Relative Spectral Response (RSR). • Band width, and center wavelength

  6. Methodology • Two lamp states, 30W and 10W, are used. • Different filter combinations are pre-programmed. • The two slits and a grating are in place. • The two SiPDs are functional. • Capable of wavelength self-calibration • The embedded didymium filter. • From grating equations. • The full spectral mode takes four Macros on-orbit in space-dark. SiPD (Silicon photo detector) Figure 2. Simplified SRCA schematic.

  7. Results • On-orbit events • After the 2nd 10W lamp failure, the 30W lamp configuration was replaced by a 20W configuration after • February 18, 2006 for Terra MODIS • June 28, 2005 for Aqua MODIS • Terra and Aqua MODIS CW/BW results remained steady around this lamp configuration changes. Table 1. Number of on-orbit spectral calibrations for Terra and Aqua MODIS SRCA. *Constant current mode started on 2005-178 for Terra and 2005-283 for Aqua.

  8. Results • CW and BW trending for the nominal bands ±4 ±5.6 48nm 46nm Figure 4. Terra & Aqua CW & BW trending results for the nominal bands.

  9. Results: Band 2 RSR • A measured spectral response (MSR) from SRCA is the convolution of the slit function and the actual RSR. • The pre-launch slit function can be recovered through the de-convolution but… • The original band 2 slit function can not be used • Because of the on-orbit modification of the calibration sequence. • Moved orbit #2 with 30W &order ‘-2’  orbit #1 with 10W & order ‘-1’.

  10. Results: Band 2 RSR • Instead of using the pre-launch RSR, the first on-orbit calibration RSR is used to get the slit function. • The first on-orbit slit function can be used to track the on-orbit RSR changes. • The band 2 CW and BW variations are well within the specifications. • CW within ±2.2 nm and BW within ±4.3 nm

  11. Results: Band 2 RSR Figure 5. Terra & Aqua band 2 RSR trending results. Table 3. The band 2 BW shifts of Terra and Aqua Table 2. The band 2 CW shifts of Terra and Aqua • Spec. within ±2.2 nm Spec. within ±4.3 nm

  12. Results: SWIR bands • The two SiPDs that are used to normalize effects • spectrally shaped light source, grating efficiency, and SRCA system transmittance. • The cutoff frequency of the SiPD is at 1.1µm. • Historically, SWIR bands are not included in the SRCA results. • B5 (1.2 µm), B6 (1.7 µm), B7 (2.1µm), and B26 (1.4 µm). • However, this cutoff wavelength does not cause evident uncertainty in the spectral calibration for these SWIR bands. • Hence, a normal methodology can be applied to SWIR bands.

  13. Results: SWIR band Figure 6. Terra pre-launch and on-orbit yearly averaged RSR in SWIR bands.

  14. Results: SWIR bands Figure 7. Aqua pre-launch and on-orbit yearly averaged RSR in SWIR bands.

  15. Results: SWIR bands Table 4. Terra on-orbit yearly averaged SWIR band CW and BW changes from pre-launch. Table 5. Aqua on-orbit yearly averaged SWIR band CW and BW changes from pre-launch.

  16. Summary • Band 2 can be measured and monitored using the first on-orbit RSR as a reference. • Showing very stable CW and BW trending results within 1 nm (well below the spec.). • The mission-lifetime changes for several SWIR bands are larger than other RSB bands, • but are still within specification with large margin. • Terra and Aqua MODIS continue to provide quality data to the scientific community. • In all the RSB bands, without significant spectral changes.

  17. Back up slides

  18. On-orbit SRCA Spectral Calibration Methodology • The SRCA spectral mode tracks band center wavelength shifts and monitoring band spectral response changes for bands (<1µm). • The embedded didymium filter is functioned as wavelength calibrator so that the SRCA has wavelength self-calibrated capability.

  19. On-orbit SRCA Spectral Calibration Methodology • The monochromatic beam exiting from the main slit λ is related to the grating angle θM by the grating equation • where A=4.24µm is the grating spacing, m is the order of diffraction, the half angle β and grating motor offset angle θoff • For wavelength self-calibration, β and θoff are calculated • from an off-axis calibration SiPD and a didymium filter • where Δ=1.319º is the angular displacement between the main slit and the didymium slit.

  20. On-orbit SRCA Spectral Calibration Methodology • Wavelength self-calibration utilizes the sharp peaks of a didymium filter with known center wavelengths. • Calibration SiPD signal is normalized by reference SiPD signal to eliminate spectral modification of the light source (< 1µm). • The grating angle is converted to wavelength when the three normalized signal peaks best match three didymium peaks. Figure 3. SiPD signal normalization

  21. SRCA Functional Configuration

  22. Specifications

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