Aqua MODIS LUT version differences from OceanData ancillary versus MYD021KM

[ssecdc]

Hello,

Our team at the University of Wisconsin-Madison generates Aqua MODIS direct broadcast products for downstream users. The processing software depends on LUTs that are staged on the OceanData ancillary website (https://oceandata.sci.gsfc.nasa.gov/directdataaccess/Ancillary/LUTs/modisa/cal/OPER/). Our software routinely downloads the latest LUTs from this URL to use in near-real time processing of direct broadcast data. One of our downstream users noticed that the brightness temperatures, particularly for MODIS Band 27, were significantly different (a mean difference of ~6 K) between our DB L1B product and the global Aqua MYD021KM NRT L1B product of the same observation earlier this month. Upon further investigation, there were major differences in the LUT versions used. For example, as of July 20th, the direct broadcast product is using these LUTs:

:Reflective_LUT_Serial_Number_and_Date_of_Last_Change = "R698 2026:02:04:15:00";
:Emissive_LUT_Serial_Number_and_Date_of_Last_Change = "E697 2026:02:04:15:00";
:QA_LUT_Serial_Number_and_Date_of_Last_Change = "Q697 2026:02:04:15:00";

While the MYD021KM NRT (downloaded from https://nrt3.modaps.eosdis.nasa.gov/archive/allData/61/MYD021KM/) is using:
:Reflective_LUT_Serial_Number_and_Date_of_Last_Change = "R712 2026:07:02:15:00";
:Emissive_LUT_Serial_Number_and_Date_of_Last_Change = "E711 2026:07:02:15:00";
:QA_LUT_Serial_Number_and_Date_of_Last_Change = "Q711 2026:07:02:15:00";

Is there a reason why the OceanColor LUTs track significantly behind the LUT versions used for MYD021KM NRT? Is there a different source of Aqua MODIS LUTs that we should be using to more accurately track the global L1B product? Thank you.

[OB.DAAC - SeanBailey]

The MODIS L1B LUTs maintained by the OBPG differ from the LUTs generated by MCST (as your user noted). Ocean color processing requires an on-orbit calibration accuracy that is not provided by the MCST LUTs. The OBPG versions use the MCST version as a source, but we adjust the coefficients for the ocean color relevant bands - MODIS' band 27 is not one used in ocean color processing, so is not modified by our team. However, since we make corrections on top of the MCST LUTs, our LUTs may lag MCST (and given the challenges recently with this process, that lag can be significant).

The software we distribute will run perfectly fine with the LUTs MCST produces (and MODAPS distributes). Anyone needing to process MODIS L1Bs for non-ocean color needs should use the MCST LUTs.

For Collection 7, we are modifying our calibration efforts to be based on the "official" MODIS L1B files processed by MODAPS. We will no longer provide ocean color specific L1B LUTs for MODIS once we initiate our Collection 7 reprocessing. We do not have plans to provide the Collection 7 L1B code in our SeaDAS distribution, since our L1B source will be MODAPS.

Regards,
Sean