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MODIS Aqua R2022 granule start time vs R2018

Posted: Tue Aug 16, 2022 5:31 am America/New_York
by dem1
Hi,

We found that with R2022 the granule start time in the new filenaming convention is not consistent with R2018: with R2018 seconds were always set to 00, but with R2022 there is a mix of 00 and 01, is it normal?
Example:
R2018:
A2003001004500.L2_LAC_OC.nc
A2003001005000.L2_LAC_OC.nc
A2003001005500.L2_LAC_OC.nc
A2003001010000.L2_LAC_OC.nc
A2003001010500.L2_LAC_OC.nc
A2003001011000.L2_LAC_OC.nc
A2003001011500.L2_LAC_OC.nc
A2003001012000.L2_LAC_OC.nc
A2003001012500.L2_LAC_OC.nc
A2003001013000.L2_LAC_OC.nc
R2022:
AQUA_MODIS.20030101T004501.L2.OC.nc
AQUA_MODIS.20030101T005001.L2.OC.nc
AQUA_MODIS.20030101T005501.L2.OC.nc
AQUA_MODIS.20030101T010001.L2.OC.nc
AQUA_MODIS.20030101T010501.L2.OC.nc
AQUA_MODIS.20030101T011000.L2.OC.nc
AQUA_MODIS.20030101T011500.L2.OC.nc
AQUA_MODIS.20030101T012000.L2.OC.nc
AQUA_MODIS.20030101T012501.L2.OC.nc
AQUA_MODIS.20030101T013001.L2.OC.nc
We also found files with seconds different than 00 or 01 and minutes not multiple of 5 with R2022 where they were multiple of 5 and seconds to 00 with R2018, example (files found using the File Search API):
AQUA_MODIS.20020806T151705.L2.OC.nc
AQUA_MODIS.20020824T100534.L2.OC.nc
AQUA_MODIS.20020911T145515.L2.OC.nc
AQUA_MODIS.20020912T215605.L2.OC.nc
AQUA_MODIS.20021014T103306.L2.OC.nc
AQUA_MODIS.20021017T043218.L2.OC.nc
AQUA_MODIS.20021017T190607.L2.OC.nc
AQUA_MODIS.20021017T223149.L2.OC.nc
AQUA_MODIS.20030512T004157.L2.OC.nc
AQUA_MODIS.20031108T193616.L2.OC.nc
AQUA_MODIS.20041212T031059.L2.OC.nc
AQUA_MODIS.20050416T025349.L2.OC.nc <== this one seems to be A2005106025000.L2_LAC_OC.nc with R2018
AQUA_MODIS.20050513T155505.L2.OC.nc
AQUA_MODIS.20050526T163757.L2.OC.nc
AQUA_MODIS.20050710T175247.L2.OC.nc
AQUA_MODIS.20060705T072233.L2.OC.nc
AQUA_MODIS.20060705T085546.L2.OC.nc
AQUA_MODIS.20060727T093502.L2.OC.nc
AQUA_MODIS.20060727T111002.L2.OC.nc
AQUA_MODIS.20060727T111502.L2.OC.nc
AQUA_MODIS.20060727T112002.L2.OC.nc
AQUA_MODIS.20060727T112502.L2.OC.nc
...
AQUA_MODIS.20210511T120509.L2.OC.nc <== this one (at least) seems not available in the L2 browser
...
AQUA_MODIS.20211001T113505.L2.OC.nc
...
Thanks,
Julien

Re: MODIS Aqua R2022 granule start time vs R2018

Posted: Tue Aug 16, 2022 8:07 am America/New_York
by OB.DAAC - SeanBailey
Julian,

Yes, this is normal (and has been the case for SST L2s since 2019). We decided to stop lying to you :D
The start time in the L2 files is derived from the first valid scan line in the scene. Quite often that is indeed on the 5 minute boundary of the arbitrary chunking that is done to the MODIS data, however, the MODIS scan rate isn't an exact multiple of seconds, so sometimes the time bleeds into the next second. Other times, for one reason or another, the sensor doesn't have any valid data for a fair bit of time into the 5 minute chunk, so the L2 filename time will have a value that isn't so close to the 5 minute boundary as the L1 filename time would have you believe it was.

As for the one you noted appears missing from the L1/2 browser, it's there...but it's a significantly smaller file than the 5 minute variety and only the northern portion of the scene has valid data:

https://oceancolor.gsfc.nasa.gov/cgi/browse.pl?file=AQUA_MODIS.20210511T120509.L2.OC.nc

Sean

Re: MODIS Aqua R2022 granule start time vs R2018

Posted: Thu Oct 06, 2022 8:45 am America/New_York
by dem1
Hi,

Sorry to bother you again about products start time, but I found these cases which seem suspect to me:

AQUA_MODIS.20220606T185503.L2.OC.nc which is ~3 seconds after its
time_coverage_start = "2022-06-06T18:55:00.545Z"
and also the corresponding NRT products was AQUA_MODIS.20220606T185500.L2.OC.NRT

but this other case is reversed:
AQUA_MODIS.20210511T120509.L2.OC.nc which is ~4 minutes before its
time_coverage_start = "2021-05-11T12:08:58.004Z"

Julien

Re: MODIS Aqua R2022 granule start time vs R2018

Posted: Thu Oct 06, 2022 9:52 am America/New_York
by OB ODPS - towens
The L2 file names are not intended to be an exact representation of the start_time of the data.
That is why there are metadata in the file.

The L2 file names are derived from the reported L1A start times.
The actual L2 retrievals are independent of the L1A reported start time.

There are times when not all of the scans are vaild.

This L1A is short with only 1980 scans and the valid data for L2 start later.

AQUA_MODIS.20210511T120509.L2.OC
:time_coverage_start = "2021-05-11T12:08:58.004Z" ;
:time_coverage_end = "2021-05-11T12:10:00.043Z" ;

Parent:
A2021131120500.L1A_LAC
start_time: 2021-05-11 12:05:09.040
stop_time: 2021-05-11 12:10:00.043
scan_lines: 1980


This was the same case in the R2018 :

A2021131120500.L2_LAC_OC.nc
:time_coverage_start = "2021-05-11T12:08:58.004Z" ;
:time_coverage_end = "2021-05-11T12:10:00.043Z" ;

Use the metadata - not the file names.

Tommy

Re: MODIS Aqua R2022 granule start time vs R2018

Posted: Thu Oct 06, 2022 10:03 am America/New_York
by dem1
Thanks Tommy this is clear now.