VNP46A2 - Hours of Light Capture

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graceb
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VNP46A2 - Hours of Light Capture

by graceb » Wed Nov 02, 2022 1:38 pm America/New_York

Hello,

I have a question regarding which night the data are referring to for a specific Julian calendar day. For example, is the data captured for January 19th, 2012 (Julian day 19 of the year 2012) referring to the night lights captured between 00:00 ~ 06:00 of the 19th? Or, is it referring to the night of the 19th to the morning of the 20th which would be 20:00 of the 19th to 06:00 of the 20th? My assumption here is that the tile in this example is not near high latitudes where night light hours seasonally vary.

Thanks.

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LAADS_UserServices_M
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Re: VNP46A2 - Hours of Light Capture

by LAADS_UserServices_M » Wed Nov 02, 2022 4:34 pm America/New_York

Dear Grace:
The data acquisition time is stored in VNP46A1 layer UTC_Time. More info can be found from Black Marble users guide
https://viirsland.gsfc.nasa.gov/PDF/BlackMarbleUserGuide_v1.2_20220916.pdf

Section 3.2 for filenames.
Table 3 for layers
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graceb
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Re: VNP46A2 - Hours of Light Capture

by graceb » Wed Nov 02, 2022 5:31 pm America/New_York

Thanks for your response. Could you just confirm my understanding of the UTC layer?

For example, the tile h08v05 of Julian day 20 of the year 2012 covers an area of the American midwest. The average UTC time for the tile is around ~8 which means that it was around 02:00 hours central time. Another tile on the same day, h27v08, covers an area around Indonesia and the average UTC time is around 19:00 hours. Does this mean that the satellite is capturing lights at 01:00 hours of the morning of January 21st in Indonesia?

Hopefully, my example was clear. Let me know if this is the correct way to understand the output of the layer.

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Re: VNP46A2 - Hours of Light Capture

by LAADS_UserServices_M » Fri Nov 04, 2022 7:35 am America/New_York

Yes, you need to convert the UTC time to the local time.
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