MODIS Aqua Chl-a Product

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febryanto25
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MODIS Aqua Chl-a Product

by febryanto25 » Sun Oct 24, 2021 11:38 am America/New_York

As far as I know, the Aqua MODIS satellite observes the equator in the afternoon at about 1.30 PM, which means the Aqua MODIS satellite will only have one measurement over the equator region at 1.30 PM, and so for other regions (please correct me if I am wrong). My question is, how are the Level-3 daily chlorophyll-a products derived from the Aqua MODIS satellite represent the daily chlorophyll-a since the satellite itself only observes a certain region at one time?

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gnwiii
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Re: MODIS Aqua Chl-a Product

by gnwiii » Mon Oct 25, 2021 8:21 am America/New_York

The MODIS instrument is constantly scanning. Aqua's orbit goes over the poles and takes about 99 minutes: https://en.wikipedia.org/wiki/Aqua_(satellite), so over 24 hours you get 14.5 orbits. Generally you can download 5 minutes of data as a "granual", see: https://modis-images.gsfc.nasa.gov/MOD05_L2/grids.html for an animation.

febryanto25
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Re: MODIS Aqua Chl-a Product

by febryanto25 » Mon Oct 25, 2021 9:55 am America/New_York

Sorry, I still don't get the point.
Firstly, the orbital period is 99 minutes means it takes 99 minutes for the Aqua satellite to move from the south pole to the north pole. Am I correct?
Secondly, you mentioned over 24 hours we get 14.5 orbits. In my imagination, the Aqua satellite provides 14.5 orbits, which provide data at the different areas around the world. My question is, how many times per day does the Aqua satellite monitor the same area?
Last, as the animation shown from the link you provided, 'each MOD05_L2 product file covers a five-minute time interval (based on the start time of each MODIS Level-1B granule)'. Could you please explain more detail about this?

Looking forward to hearing from you soon.
Thank you.

gnwiii
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Re: MODIS Aqua Chl-a Product

by gnwiii » Mon Oct 25, 2021 5:47 pm America/New_York

febryanto25 wrote: Mon Oct 25, 2021 9:55 am America/New_York Firstly, the orbital period is 99 minutes means it takes 99 minutes for the Aqua satellite to move from the south pole to the north pole. Am I correct?
The orbital period is time for a complete orbit, so south pole to north pole and back around to south pole.
febryanto25 wrote: Mon Oct 25, 2021 9:55 am America/New_York Secondly, you mentioned over 24 hours we get 14.5 orbits. In my imagination, the Aqua satellite provides 14.5 orbits, which provide data at the different areas around the world. My question is, how many times per day does the Aqua satellite monitor the same area?
At the equator, less than once a day. Close to a pole, 14.5 times a day. The area imaged in each track can be visualized as series of rubber bands stretched around a ball and crossing at the "poles". The bands will overlap at the crossing points, but unless you can find a really wide rubber band and a small ball, there will be gaps at the equator. Another consideration for chlorophyll is that you only get data for daylight hours (there are other geophysical quantities that can be measured at night).

febryanto25
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Re: MODIS Aqua Chl-a Product

by febryanto25 » Tue Oct 26, 2021 12:28 am America/New_York

If there is only one observation in the Equator, let say in Indonesia, then how is it possible for the MODIS daily chl-a products represent the real daily chl-a? Technically, how is the process for generating the MODIS daily chl-a products?

As the animation shown from the link you provided, 'each MOD05_L2 product file covers a five-minute time interval (based on the start time of each MODIS Level-1B granule)'. Could you please explain more detail about this?

Looking forward to hearing from you soon.
Thank you.

gnwiii
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Joined: Fri Jan 29, 2021 5:51 pm America/New_York
Answers: 2

Re: MODIS Aqua Chl-a Product

by gnwiii » Tue Oct 26, 2021 6:32 am America/New_York

Daily products just combine the available granuals, so can't capture the details when there are rapid changes in chlorophyll level. Other products (PAR) that have daily cycles, aren't usable on time scales less than 30 days, but daily products are still generated and then used to produce monthly products. Having daily data provides an opportunity to use 30-day averages for periods other than months.

febryanto25
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Re: MODIS Aqua Chl-a Product

by febryanto25 » Tue Oct 26, 2021 10:49 am America/New_York

In short, the daily chl-a product from Aqua MODIS in the equator region represents one observation?

Please correct me if I am wrong, 'each MOD05_L2 product file covers a five-minute time interval (based on the start time of each MODIS Level-1B granule)' means the MOD05_L2 product has 5 minute time interval of chl-a product on global scale?

Looking forward to hearing from you soon.
Thank you.

gnwiii
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Joined: Fri Jan 29, 2021 5:51 pm America/New_York
Answers: 2

Re: MODIS Aqua Chl-a Product

by gnwiii » Tue Oct 26, 2021 4:08 pm America/New_York

Not sure what you mean by
febryanto25 wrote: Tue Oct 26, 2021 10:49 am America/New_York 'each MOD05_L2 product file covers a five-minute time interval (based on the start time of each MODIS Level-1B granule)' means the MOD05_L2 product has 5 minute time interval of chl-a product on global scale?
The MODIS sensor scans across the track, so each 5-minute granual provides data for a the scans that were made during the interval covered by the granual. The data can be viewed as an array whose coordinates are along-track scan line and cross track pixel, so you often see references to pixel and line coordinates.

https://appliedsciences.nasa.gov/sites/ ... sion_2.pdf has an introduction to remote sensing that may be helpful. It is important when working with
these data to have an understanding of the underlying processes as there are often practical considerations that can affect the suitability of the data for a given application.

febryanto25
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Re: MODIS Aqua Chl-a Product

by febryanto25 » Thu Oct 28, 2021 11:55 am America/New_York

You mentioned earlier the Aqua satellite monitors the same area less than once a day at the equator while close to a pole, 14.5 times a day. In my understanding, based on your explanation, the Aqua satellite scan once within 24 hours over the equator area, which means one observation of chl-a represents daily chl-a (regarding the level 3 chl-a product). Am I correct?

I hope you get what I mean.
Anyway, I appreciate the file you attached.


Looking forward to hearing from you soon.
Thank you.

OB.DAAC - amscott
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Re: MODIS Aqua Chl-a Product

by OB.DAAC - amscott » Thu Oct 28, 2021 3:23 pm America/New_York

Perhaps we don't understand your goal--Are you trying to understand how the instrument collects data on a daily basis, or are you trying to decide when you visit a data repository, like OBDAAC, how to find the data you want, or are you trying to understand the data groupings we offer at OBDAAC, such as 'daily', 'monthly', 'yearly', '8-day', etc or other goal?
--
Alicia

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