Tuesday, September 25, 2012

GOES Down! GOES Down!

GOES-13, also known as GOES-East went into a state of hardware failure a couple days ago. GOES-14 which has been on standby has come online and taken it's place as GOES-East.

A couple weeks ago I rolled out the replacement satellite/radar pages, touting that visible satellite loops would now remove a healthy chunk of "night-time" images. Following that I realized that a handful of west coast regions for 1km, 2km and 6km resolution were not properly removing night-time images.

This removal of night-time images was accomplished by taking advantage of the fact that when our images get generated, the file size directly correlates to the average brightness of the image. This means that with visible satellite images, there should be a very natural drop-off in file size for night-time images. Within the code that grabs the most recent set of images, I simply added a file size check for visible images and filtered out the images that fell below a certain file size. This effectively removed night-time images from mix without using any information on sunrise, sunset, time of year, or latitude. I was quite happy with that approach. It meant I didn't have to over think the scripting or tailor the code to each region. Basically; less work, more reward.

Getting back to those west coast regions not removing night-time images; It turns out that GOES-West (GOES-15) changes how it operates when taking images of the dark side of the Earth. It attempts to bring in as much light as possible, which gradually increases the brightness of an otherwise dark image. This results in the minimum file size occurring right around sunrise and sunset, with a relative maximum in file size overnight. Here are some highly sophisticated graphs to help further describe this situation.

Images from GOES-13 would have created a File Size vs. Time graph similar to the one above.

GOES-15 on the other hand creates a graph similar to the one below.

Because of this, I can't set a lower threshold for file size to remove night-time images because it would also be removing valid day-time images.

Initially, I was willing to just let it be since the overwhelming majority of the sectors we provide are covered by GOES-East. But now GOES-East is GOES-14 and wouldn't you know it... GOES-14 operates the same way GOES-15 does. Both satellites now variably adjust the amount of light coming in to help grab as much valid data as possible from the visible channel. But this means that when our images are generated... well, hopefully you get the idea by now.

So... for what it's worth, I got 2 weeks out of a pretty neat feature for our visible satellite loops. But at the moment, I'm not interested in getting excessively cute with the script that grabs the most recent set of images for a loop and make it remove night-time images. I will also not write it in some way that involves me editing the code constantly to adjust for variances in time of year and latitude. Waste of time.

When other, larger tasks are taken care of, I may return to working on the scripts for these pages and try to get clever. For now, "Sorry folks. Can't do it."

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