OLCI mapping offset

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andrew.meredith
Posts: 33
Joined: Tue Dec 31, 2013 10:29 am America/New_York

OLCI mapping offset

by andrew.meredith » Thu Jun 08, 2017 3:34 pm America/New_York

I'm seeing offsets in the mapping of OLCI data processed with l2gen from L1B source data. The offset amount varies by day and with elevation but appears to repeat based on the Sentinel-3 27-day orbit cycle.

The first attached image shows the L1B and L2 files overlaid with a vector polygon. The approximate scene center lon/lat is 119 00' 55"W/38 00' 32"N (Mono Lake, CA). The source L1B file was S3A_OL_1_EFR____20170525T175211_20170525T175511_20170525T194025_0179_018_098_2340_SVL_O_NR_002.SEN3.

Based on a SNAP forum suggestion (http://forum.step.esa.int/t/updated-olci-test-data-set/639), I forced SNAP Sentinel-3 OLCI reader to use per-pixel gecoding. If using the SNAP default (tie-point geocoding) to display the L1B file, i getting a mapping offset as noted in the second image.

Thanks,
Andrew


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OB.DAAC - SeanBailey
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OLCI mapping offset

by OB.DAAC - SeanBailey » Fri Jun 09, 2017 9:52 am America/New_York

Andrew,

The l2gen file reader for OLCI is not setting the terrain_corrected field to true in the internal data record...so...terrain correction is being applied to the terrain corrected geolocation.  After modifying the reader to set the bit, the geolocation is happy.  This fix will be included in the next release.  If you want to rebuild l2gen to include the fix before then, add:   

l1file->terrain_corrected = 1;

to the openl1_olci function in the $OCSSWROOT/build/src/l2gen/l1_olci.c file

Sean

andrew.meredith
Posts: 33
Joined: Tue Dec 31, 2013 10:29 am America/New_York

OLCI mapping offset

by andrew.meredith » Tue Jun 13, 2017 2:12 pm America/New_York

Thanks Sean. I was able to implement the fix and the first test looks good!

Andrew

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