LDOPE KNOWNISSUE

Opening Date: 2020-10-13 Last update: 2021-09-29 Status: Note

On DOY 2020.279 (10.05.20) MODIS Terra experienced a PWA failure that affected the PWA-41 (supersets 80 and 81), bringing down the total number of MODIS PWA to 14 with only 28 supersets. Given this, there is an increased chance of buffers being filled and data overwritten, resulting in unscheduled and unrecoverable data loss. So, FOT and MODIOT have decided to change the day/night coverage ratio from current 50/50 to 44.2/55.8 to minimize the overflow of onboard buffers. This ratio change occurred on October 7th (2020.281) at 20:00z. Due to this change there is an observed reduction in the overall Terra daytime coverage, specifically at the end of orbits in the northern latitudes. Below is a global browse image of DOY 2020.280, prior to the implementation of the ratio change. The second image on the right shows DOY 2020.282, where the ratio change has been implemented for the full day, and has impacted retrieval over high latitude. In this second image you can observe the reduced daytime coverage over Greenland and portions of Northern Russia as well as North America and Alaska. For comparison, the same DOY 282 from 2019 is shown in the third image, which verifies the difference is not due to seasonal affect.

MOD09, DAILY LAND SURFACE REFLECTANCE BANDS 1,4,3
day A2020280 (2020-10-06), 006
Global Browse image.

MOD09, DAILY LAND SURFACE REFLECTANCE BANDS 1,4,3
day A2020282 (2020-10-08), 006
Global Browse image.

MOD09, DAILY LAND SURFACE REFLECTANCE BANDS 1,4,3
day A2019282 (2019-10-08), 006
Global Browse image.

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UPDATE 11.18.20
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MODIOT switched to a Short Term Schedule (STS) for Terra on 11/8/2020 (2020-313), allowing for more flexibility for scheduling alternating orbits, enabling greater Day Rate collection over the poles. This will be an improvement over the 44.2/55.8 ratio that was being followed before, for day/night coverage, immediately following the Terra PWA failure. The global browse image from 11.15.20 (2020.320) below shows the impacts of these changes. Coverage of greater polar areas every alternating orbit results in some increased gaps over the northern high latitude regions.

MOD09, DAILY LAND SURFACE REFLECTANCE BANDS 1,4,3
day A2020320 (2020-11-15), 006
Global Browse image.

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UPDATE 05.04.21
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On May 2, 2021, MODIOT updated the Terra formatter scheduling again, from the STS (short-term schedule) method that had gone into effect on November 8, 2020. The updated STS uses the solar angle variation throughout the year to adjust the alternating orbits to focus more on the North Pole during October - March and on South Pole during April - September. This method adds slightly more coverage over the pole where it has less day rate data under the STS that went into effect on November 8, by removing excess coverage over the other pole where there is greater day rate coverage. For more details on this new STS please refer to this presentation made by MODIOT on this new STS . The effect of the updated STS that went in effect on May 2, over the poles, can be reviewed from this document and from the two images shown below. The change to the new STS on May 02, shows slightly more coverage over the southern polar region.

MOD09, DAILY LAND SURFACE REFLECTANCE BANDS 1,4,3
day A2021121 (2021-05-01), 006
Global Browse image.

MOD09, DAILY LAND SURFACE REFLECTANCE BANDS 1,4,3
day A2020122 (2020-05-02), 006
Global Browse image.

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UPDATE 07.06.21
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On June 29, 2021 (DOY 180), Terra experienced another PWA failure (PWA-47), affecting the supersets of 92/93 that further impacted the MODIS buffer. Following this latest Terra PWA failure on June 29, 2021, Terra day rate coverage was temporarily brought down from 44% prior this current anomaly (loss of 5:56 minutes daytime data per orbit) to 37.6% after the anomaly (loss of 11:56 minutes daytime data per orbit), with all day time data loss happening over the poles. The effect of the updated STS went into operation post PWA anomaly, on July 2nd (2021.183). Below are global browse images of reflectance data showing examples of before and after this change. The before image is from June 28th (2021.179) and the after image is from July 3rd (2021.184).

MOD09, DAILY LAND SURFACE REFLECTANCE BANDS 1,4,3
day A2020179 (2021-06-28), 006
Global Browse image.

MOD09, DAILY LAND SURFACE REFLECTANCE BANDS 1,4,3
day A2021184 (2021-07-03), 006
Global Browse image.

But, given the larger loss of daytime science data with this 37.6% day rate coverage, FOT/IOT decided to revert back to the dynamic STS that was in effect prior to this latest PWA anomaly, since May 2021, even at the risk of potential data loss, in case of occasional onboard buffer overflow. This earlier dynamic STS is in effect since July 4, 2021 (day 185) onward. Please note that this earlier STS ensures a day/night coverage ratio of ~44%/56% but does run the risk of occasional data loss due to onboard buffer overflow.

MOD09, DAILY LAND SURFACE REFLECTANCE BANDS 1,4,3
day A2021185 (2021-07-04), 006
Global Browse image.

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UPDATE 09.29.21
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FOT conducted a successful MODIS Terra SSR reset on 09.22.21 and has been able to restore all the PWAs. For more details regarding this please refer here. Starting on 2021269-00:00 (09.26.21), the Terra day/night collection coverage has been restored to the earlier 50/50 ratio following the successful SSR reset. Below are example global browse images, which compare DOY 2021.268 (09.25.21) against the newly implemented 50/50 collection ratio on DOY 2021.269 (09.26.21). Slightly fuller coverage is observed over the Southern pole on DOY 2021.269 (09.26.21), highlighted by the red box.

MOD09, DAILY LAND SURFACE REFLECTANCE BANDS 1,4,3
day A2021268 (2021-09-25), 006
Global Browse image.

MOD09, DAILY LAND SURFACE REFLECTANCE BANDS 1,4,3
day A2021269 (2021-09-26), 006
Global Browse image.

Occurrence: TERRA C6 / C61 L1B and all Downstream products DOY 2020281 and forward.

PGE:

Case Name: PM_MOD_20280