There are several issues concerning the spectral response functions for the MODIS instruments, including:
The following is a simple analysis showing the detector specific spectral response functions for each of the visible bands convolved with a hyper-spectral water-leaving radiance typical of clear, open ocean water.
| Band 8 - 412nm (View Graph) | Band 9 - 443nm (View Graph) | |||
|---|---|---|---|---|
| Detector | Aqua Lw412nm (mW/m2/um/sr) |
Terra Lw412nm (mW/m2/um/sr) |
Aqua Lw 443nm (mW/m2/um/sr) |
Terra Lw 443nm (mW/m2/um/sr) |
| 1 | 1.53831 | 1.56724 | 1.38255 | 1.38223 |
| 2 | 1.53831 | 1.56621 | 1.38255 | 1.38237 |
| 3 | 1.53834 | 1.56578 | 1.38242 | 1.38241 |
| 4 | 1.53842 | 1.56551 | 1.38239 | 1.38240 |
| 5 | 1.53850 | 1.56505 | 1.38236 | 1.38243 |
| 6 | 1.53871 | 1.56486 | 1.38234 | 1.38249 |
| 7 | 1.53901 | 1.56414 | 1.38232 | 1.38254 |
| 8 | 1.53930 | 1.56341 | 1.38232 | 1.38262 |
| 9 | 1.53994 | 1.56308 | 1.38232 | 1.38275 |
| 10 | 1.53998 | 1.56308 | 1.38249 | 1.38275 |
| AVG | 1.53888 | 1.56484 | 1.38240 | 1.38250 |
| % Diff | 100*(T-A/A) | 1.68663 | 100*(T-A/A) | 0.00698490 |
| Band 10 - 488nm (View Graph) | Band 11 - 531nm (View Graph) | |||
|---|---|---|---|---|
| Detector | Aqua Lw 488nm (mW/m2/um/sr) |
Terra Lw 488nm (mW/m2/um/sr) |
Aqua Lw 531nm (mW/m2/um/sr) |
Terra Lw 531nm (mW/m2/um/sr) |
| 1 | 1.03870 | 1.04801 | 0.399058 | 0.400915 |
| 2 | 1.03870 | 1.04796 | 0.399058 | 0.400915 |
| 3 | 1.03870 | 1.04790 | 0.399058 | 0.400927 |
| 4 | 1.03868 | 1.04798 | 0.399090 | 0.400930 |
| 5 | 1.03869 | 1.04795 | 0.399110 | 0.400930 |
| 6 | 1.03872 | 1.04797 | 0.399124 | 0.400926 |
| 7 | 1.03874 | 1.04791 | 0.399128 | 0.400902 |
| 8 | 1.03877 | 1.04790 | 0.399136 | 0.400603 |
| 9 | 1.03882 | 1.04790 | 0.399107 | 0.400603 |
| 10 | 1.03889 | 1.04790 | 0.399107 | 0.400603 |
| AVG | 1.03874 | 1.04794 | 0.399098 | 0.400825 |
| % Diff | 100*(T-A/A) | 0.885213 | 100*(T-A/A) | 0.432887 |
| Band 12 - 551nm (View Graph) | Band 13 - 667nm (View Graph) | |||
|---|---|---|---|---|
| Detector | Aqua Lw 551nm (mW/m2/um/sr) |
Terra Lw 551nm (mW/m2/um/sr) |
Aqua Lw 667nm (mW/m2/um/sr) |
Terra Lw 667nm (mW/m2/um/sr) |
| 1 | 0.314212 | 0.316653 | 0.0209927 | 0.0209237 |
| 2 | 0.314212 | 0.316646 | 0.0209927 | 0.0209271 |
| 3 | 0.314220 | 0.316653 | 0.0209922 | 0.0209275 |
| 4 | 0.314228 | 0.316668 | 0.0209941 | 0.0209261 |
| 5 | 0.314223 | 0.316657 | 0.0209943 | 0.0209266 |
| 6 | 0.314226 | 0.316650 | 0.0209943 | 0.0209274 |
| 7 | 0.314232 | 0.316650 | 0.0209934 | 0.0209249 |
| 8 | 0.314225 | 0.316651 | 0.0209944 | 0.0209235 |
| 9 | 0.314213 | 0.316617 | 0.0209950 | 0.0209232 |
| 10 | 0.314257 | 0.316617 | 0.0209942 | 0.0209232 |
| AVG | 0.314225 | 0.316646 | 0.0209937 | 0.0209253 |
| % Diff | 100*(T-A/A) | 0.770560 | 100*(T-A/A) | -0.325838 |
| Band 14 - 678nm (View Graph) | ||
|---|---|---|
| Detector | Aqua Lw 678nm (mW/m2/um/sr) |
Terra Lw 678nm (mW/m2/um/sr) |
| 1 | 0.0184740 | 0.0184604 |
| 2 | 0.0184740 | 0.0184601 |
| 3 | 0.0184737 | 0.0184603 |
| 4 | 0.0184738 | 0.0184604 |
| 5 | 0.0184737 | 0.0184610 |
| 6 | 0.0184737 | 0.0184613 |
| 7 | 0.0184741 | 0.0184604 |
| 8 | 0.0184741 | 0.0184611 |
| 9 | 0.0184737 | 0.0184628 |
| 10 | 0.0184727 | 0.0184628 |
| AVG | 0.0184738 | 0.0184610 |
| % Diff | 100*(T-A/A) | -0.0688040 |
The differences between the detectors are insignificant. There appears to be no need for detector-specific spectral response functions. A single, average sepectral response function will suffice. In fact, all bands have several detectors with identical spectral response functions. The reason for this is discussed below under "Measurement Issue".
The above, detector-specific analysis shows that the differences between sensors are slightly greater than the inter-detector differences, but it may be that this is solely due to the OOB component, which is often quite similar between the two sensors (see the associated figures). The OOB component does differ, but may be more of a magnitude or measurement issue than any real difference.
The MODIS RSRs were measured by SBRS with a Spectral Measurement Assembly (SpMA). In the SpMA, a monochromator is used to produce light of a single wavelength at an exit slit. The slit was measured by MODIS, and the output wavelengths of the SpMA were varied. Unfortunately, for one output wavelength of the SpMA, there is a small spectral variation along the slit, e.g. the upper region of the slit has a slightly different wavelength than the center. The MODIS detectors measured different parts of the slit, which resulted in shifts of the center wavelengths of the measured RSRs of the detectors relative to each other. SBRS determined that the central area of the slit contains the correct information, and shifted the RSRs of the outer detectors such that they matched the center wavelength of the central detectors. (In some cases, some of the outer detectors did not produce valid measurements, and their RSRs were copied from nearby detectors.) Therefore, the corrected RSRs of all detectors have the same center wavelength. The amount of the shift is less than 0.1% (e.g. at 500nm, a shift of less than 0.5nm). The magnitude and direction of the shift is consistent with an independent characterization of the SpMA (without the use of MODIS). However, it is unclear whether using the corrected RSRs achieves superior results than simply using a common RSR for all detectors of a band.
The detector-averaged center wavelength (in-band) for the Terra and Aqua ocean bands are:
| Band | Aqua(nm) | Terra(nm) | Difference(nm) |
|---|---|---|---|
| 8 | 412.463 | 411.896 | 0.567 |
| 9 | 442.220 | 442.114 | 0.106 |
| 10 | 487.428 | 486.995 | 0.433 |
| 11 | 530.110 | 529.722 | 0.388 |
| 12 | 547.198 | 546.954 | 0.244 |
| 13 | 665.997 | 665.757 | 0.240 |
| 14 | 677.580 | 676.973 | 0.607 |
| 15 | 746.831 | 746.612 | 0.219 |
| 16 | 866.877 | 866.348 | 0.529 |
The difference between the center wavelengths for Aqua and Terra are within the magnitude of the detector specific adjustments made to compensate for the SpMA instrument artifact. It is unclear as to whether or not the differences between Aqua and Terra are real or an artifact of the measurement, or a combination.
There has been some concern raised regarding the OOB component of the spectral response functions for both MODIS instruments. MCST does not have confidence in the OOB component for the spectral response functions, and in fact do not use them. Experience with SeaWiFS has shown that the out-of-band component can be important, especially for ocean processing, so it was decided to use the OOB component for MODIS processing as well in order to maintain consistency.
It has been suggested that the OOB for MODIS is too high. The effect of the OOB component is evident in the bandpass averaged water absorption coefficients, especially in the blue bands (412 and 443nm). This topic was addressed on the Ocean Color Forum
If the OOB component for MODIS is in error, there will be an effect on the ocean color processing. This effect will vary depending upon the source function to which the spectral response function is applied (e.g. Rayleigh, water absorption, etc.).
