MODIS Data Processing Program

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zhooukee
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MODIS Data Processing Program

by zhooukee » Mon Apr 15, 2024 11:24 pm America/New_York

In the user guide for MOD09A1, I learned that the products of the MOD_PR09A.exe program are used to generate eight-day composite reflectance data from daily reflectance data. I would like to ask what "Observations with the highest coverage" in the text corresponds to in terms of observational coverage. What does observational coverage mean, and how is it calculated? Additionally, is this program publicly available?

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LP DAAC - lien
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Re: MODIS Data Processing Program

by LP DAAC - lien » Tue Apr 16, 2024 2:43 pm America/New_York

Hi @zhooukee
It is a priority to use the observation that makes up the largest percentage of a given Sinusoidal gridded MODIS pixel. Each MODIS tile is geographically fixed. So, by that I mean any given pixel from any gridded Sinusoidal product from any day or temporal range will have the exact upper left identifying coordinate. However, these are originally produced from the L1B MOD/MYD02 radiance swath products and these pixels are not gridded and are not constant. For example, a 1-kilometer pixel at nadir is 1-kilometer square at the edge of the swath a 1-k pixel is 4.8-k cross track and 2-k along track.
Primarily this applies to pixels that are off nadir. Which the satellites that the MODIS sensors are on board have a 16-day revisit cycle and this is an 8-day product so there will certainly be pixels off nadir. Hence the User Guide says: "The criteria for selection include observational coverage and view angle, and whether the observation is flagged as cloudy, clear, containing high aerosol or low aerosol, or in cloud shadow."

MODAPS in the past had a portal users could register for to download the processing programs. I do not know if that still exists. If it does you will need to contact them: https://modaps.modaps.eosdis.nasa.gov/services/production/pge_versions.html
Thanks,
Brett

zhooukee
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Re: MODIS Data Processing Program

by zhooukee » Tue Apr 16, 2024 10:01 pm America/New_York

Dear Brett,

Thank you very much for your response.Through your response and reading the source code, I learned that the eight-day synthesis is performed on a per-pixel basis. The observation coverage corresponds to the values in the "obscov_500m" layer in MOD09GA. Initially, the observation data within the eight days are grouped by orbit number to obtain the maximum obscov for each orbit. Then, a scoring system is used to compare the optimality of pixels within multiple orbits. However, I still haven't found information on how to retrieve the satellite image orbit numbers. Could it be related to one of the parameters in MOD09GA data such as "orbit_pnt", "granule_pnt", or "iobs_res"?
Thanks,
Zhou
Last edited by zhooukee on Wed Apr 17, 2024 9:13 am America/New_York, edited 7 times in total.

zhooukee
Posts: 11
Joined: Tue Dec 26, 2023 8:38 am America/New_York
Answers: 0

Re: MODIS Data Processing Program

by zhooukee » Wed Apr 17, 2024 10:26 pm America/New_York

Dear Brett,
Through reading the source code, I learned that the eight-day synthesis is performed on a per-pixel basis. The observation coverage corresponds to the values in the "obscov_500m" layer in MOD09GA. Initially, the observation data within the eight days are grouped by orbit number to obtain the maximum obscov for each orbit. Then, a scoring system is used to compare the optimality of pixels within multiple orbits. However, I still haven't found information on how to retrieve the satellite image orbit numbers. Could it be related to one of the parameters in MOD09GA data such as "orbit_pnt", "granule_pnt", or "iobs_res"?
Thanks,
Zhou

LP DAAC - lien
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Re: MODIS Data Processing Program

by LP DAAC - lien » Fri Apr 26, 2024 3:28 pm America/New_York

Hi,
There really is no way that we know to definitively ascertain which of the 8 daily MOD09 granules was used in the specific 8 day granule that they coincide with. You can try and deduce it from the different ancillary pointer files but there really is no guarantee that will be accurate.
Thanks,
Brett

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