Time_of_observation in C3M data

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polunma
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Time_of_observation in C3M data

by polunma » Wed May 20, 2020 1:51 pm America/New_York

I am working on some analysis of the C3M data and have a question about the variable “Time_of_observation”. The unit is day, but I am not sure how to interpret. For example, for the file with the date 20080701, the first value for Time_of_observation is 2454648.500080335. What does it mean? The file that has the date of 20080701 has the first Time_of_observation of about 2454648.5 and the last being 2454649.5. Is it UTC time? This might sound naive but I thought aqua passes the equator around 1:30pm local time and near the poles it is within an hour or so from 1:30pm. If so why the Time_of_observation differs so much within a day?

Thanks!

Po-Lun

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ASDC - mcook
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Re: Time_of_observation in C3M data

by ASDC - mcook » Thu May 21, 2020 4:35 pm America/New_York

Thank you for your question. A Subject Matter Expert has been notified and will answer your question shortly. Please stand by!

wfmiller
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Re: Time_of_observation in C3M data

by wfmiller » Tue May 26, 2020 3:48 pm America/New_York

The Time of Observation in the C3M and SSF product is a continuous count of time in whole and fractional days elapsed at the Greenwich meridian since noon on January 1, 4714 BCE. So, the .500080335 portion would be slightly after midnight on July 1, 2008.

The GMT is determined by first computing the number of seconds in the day since midnight:
if JFract > 0.5,then Seconds = 86400.0 * (JFract-0.5)
if JFract <= 0.5, then Seconds = 86400.0 * (JFract+0.5)
Then compute HH, MM, and SS where:
HH = Int(Seconds/3600)
MM = Int(Seconds-(HH*3600.0)/60)
SS = Seconds-(HH*60.0 + MM)*60.0

In your note, the fraction is 0.500080335. Since it is greater the 0.5, we subtract 0.5 to get 0.000080335. Multiply it by 86400 and get 6.93792 seconds.
Therefore HH = 0, MM = 0, SS = 6.93792 or 00:00:06.93792.

The nonfraction part of the time is related to the day, since the day on the CCCM is related to noon, 2454648 runs from noon on June 30, 2008 until just before noon on July 1, 2008. After noon, the Julian day becomes 2454649 or July 1, 2008.

If you subtract the first time of observation from the last, it will be approximately 1 meaning the file contains a days worth of data.

Since this is a day file, it will have 14 equatorial crossings at 1:30 Local Time and cross a pole approximately 50 minutes before or after each one.

The SSF and CCCM documentation is helpful in explaining variables on the CCCM. The core of the product is an SSF. Deviations from the SSF and additional variables are explained in the c3m document.
https://ceres.larc.nasa.gov/documents/c ... G_R2V1.pdf
https://asdc.larc.nasa.gov/documents/ce ... _B1_v2.pdf

polunma
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Re: Time_of_observation in C3M data

by polunma » Tue May 26, 2020 3:56 pm America/New_York

Thank you so much!!

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