Canada | Seeing the Light

It has been three years since the devastating Fort McMurray Fire – the costliest natural disaster in Canada’s recorded history. The city is still rebuilding. And, I’m sure the people of Alberta don’t want to be reminded of it. I hate to be the bearer of bad news but, here we go again! This time, it is the town of High Level, Alberta that has been evacuated. This is one of four “out of control” fires currently burning in Alberta:

Map of known fires from Alberta Wildfire (24 May 2019)

The red flame icons represent “out of control” fires, while the green ones are “under control”. This map was produced by Alberta Wildfire on 24 May 2019. The northernmost red flame represents the fire near High Level. According to Alberta Wildfire, the southern two fires started on 18 May 2019, while the other two started on 11-12 May 2019. But, as you shall see, they became out of control on 18 May.

This being Alberta, satellites are often the first source for spotting fires in these remote areas. Plus, Alberta is in a unique position: it is far enough east to be within the view of GOES-16 and far enough west to be within view of GOES-17. It is also far enough north to get several overpasses from VIIRS on both Suomi-NPP and NOAA-20 each day. So, what do these satellites have to say about these fires? Let’s take a look.

We’ll begin with GOES-16, which is now operational as “GOES-East” and has been since December 2017. Since April 2019, the GOES-16 ABI has been running in Mode 6 operations. This means full disk images every 10 minutes. Here is the GeoColor loop (link goes to PDF file) from GOES-16, starting at 1700 UTC on 18 May 2019 and running until sunset (~0400 UTC 19 May 2019):

For the uninitiated, GeoColor is a blend of true color imagery during the day with a low cloud/fog detection product at night. True color is particularly useful for detecting smoke, as we have seen before. So, did you see it in the above loop?

I’ll admit, the clouds make it difficult to see, but there are two smoke plumes visible in that video. (It helps to view the video in full screen mode.) The smoke plume for the fire near High Level shows up first (around 1940 UTC), then the smoke from the pair of fires northeast of Lesser Slave Lake appears around 2150 UTC and become obvious around 0000 UTC on 19 May.

Here is the Fire Temperature RGB from GOES-16 ABI over the same time period:

This product is sensitive to hot spots – not smoke – but it still has an issue with clouds, which block the signal. Notice that the northernmost fire near High Level first appears in the Fire Temperature RGB around 1840 UTC – an hour before the smoke is really evident. The first fire northeast of Lesser Slave Lake is visible starting at 2200 UTC (10 min. after the smoke is visible), and the second one first appears at 2320 UTC (about 20 min. after its smoke plume appears). The fourth fire (the westernmost of all of them, and the one in the middle north-to-south) first appears at 2100 UTC, although it is in and out of the clouds, and its smoke plume is never very visible.

Compare that with GOES-17 ABI, which is now operational as “GOES-West” (since February 2019), and is also running Mode 6 operations:

These videos cover the same time period as the GOES-16 versions.

Moving clockwise starting at the fire near High Level, the first sighting of smoke according to GOES-17 is around 1900 UTC, 2200 UTC (first Lesser Slave Lake fire), 0000 UTC (second Lesser Slave Lake fire), and 2310 UTC (westernmost fire). This is 40 min. earlier, similar to, similar to, and not easily comparable to GOES-16. (Since GOES-16 never got a clear look at the smoke from the westernmost fire, it’s difficult to compare times against GOES-17, which was able to detect smoke from that fire. Also, I say a 10 minute difference is “similar to”, since it is a judgement call as to which image an analyst would confidently say they first saw the smoke or the hot spot.)

Comparing the hot spots in the same clockwise fashion, the time of first detection was 1820 UTC, 2140 UTC, 2320 UTC and 2050 UTC. This is 20 min. earlier, 20 min. earlier, similar to, and similar to GOES-16. If you don’t believe me, compare them frame by frame in this video:

GOES-16 is on the right and GOES-17 is on the left. When you can directly compare them like this, a few things jump out. 1) The fire near High Level appears to grow more in GOES-17 than it does in GOES-16. 2) The two fires near Lesser Slave Lake appear so close together in GOES-17 that they are difficult to distinguish as separate fires, although they are clearly separate fires according to GOES-16. 3) The westernmost fire is the most difficult to see due to the presence of clouds.

Here, it is important to note a few things. A) Alberta is generally closer to GOES-17 than it is to GOES-16. B) The position the clouds and the motion of the fire relative to the lines of sight of the satellites lead to significant differences between GOES-16 and GOES-17 in this case. C) The plural of “anecdote” is not “data”. The exact circumstances that apply here are not going to apply in the future but the concepts still will.

The fire near High Level moves perpendicular to the GOES-17 line of sight, and moves parallel to the GOES-16 line of sight (away from the satellite on a curved surface to boot!) with slightly more coarse resolution in GOES-16. That explains the differences in apparent motion between the two views. The differences in viewing angle also explain why the fires near Lesser Slave Lake appear separate (GOES-16) or together (GOES-17). And, clouds blocking the line of sight explain why the fire near High Level appears later in GOES-16, while GOES-17 had a clear view of the fire 20 minutes earlier. The differences in perspective offered by the two satellites also make it tough to tell which fire is furthest west (the one I’ve been calling “westernmost”).

Both geostationary satellites are viewing Alberta at a high angle. What about VIIRS on Suomi-NPP and NOAA-20, which flies more-or-less directly overhead? Since these satellites are polar-orbiting, they can’t provide 10 min. imagery of the fires. But, they do combine to provide ~50 min. imagery of the fires for a few orbits in the early afternoon and early morning hours, and they have much better spatial resolution.

The first VIIRS overpass was from NOAA-20 at around 1920 UTC, and here’s what it saw (according to the Fire Temperature RGB):

NOAA-20 VIIRS Fire Temperature RGB (1920 UTC, 18 May 2019)

This image is about an hour after GOES-17 first spotted the fire near High Level, which is the most obvious fire in the scene. In fact, the westernmost fire is hidden under the clouds and the fires near Lesser Slave Lake haven’t started yet. The closest time matched images with both GOES are provided below.

GOES-17 (left) and GOES-16 (right) Fire Temperature RGB (1920 UTC 18 May 2019)

Approximately 50 min. later, Suomi-NPP passed overhead and provided this view:

S-NPP VIIRS Fire Temperature RGB (2011 UTC, 18 May 2019) — S-NPP VIIRS Fire Temperature RGB (20:11 UTC, 18 May 2019)

And the GOES view from approximately the same time:

GOES-17 (left) and GOES-16 (right) Fire Temperature RGB (2010 UTC 18 May 2019)

Here the fire near High Level is quite a bit more intense, as you could see if you were to toggle back and forth between the two VIIRS images. This is a definite sign that the fire is a dangerous one! Still no sighting of the Lesser Slave Lake fires, which didn’t get going for another 110 minutes or so, according to both GOES. There was another NOAA-20 overpass at 2101 UTC, then a Suomi-NPP overpass that caught the western portion of the scene at 2151 UTC. Combine all four overpasses in an animation, and you get this:

Animation of VIIRS Fire Temperature RGB images (18 May 2019)

Remember to click on the image to get the animation to play. If you look closely, you can see both of the fires northeast of Lesser Slave Lake in the NOAA-20 image from 2101 UTC – a full 50 minutes earlier than either GOES was able to detect them. But, they are just single pixels at this point, making them likely too small for ABI to detect them (particularly at such a high viewing angle).

As for the smoke, it’s not any easier to detect with VIIRS given all the clouds overhead (click to play):

Animation of VIIRS True Color images (18 May 2019)

You might have noticed in the above animation that Lake Athabasca is still covered in ice, which is not apparent in the Fire Temperature RGB. That means fire season in Alberta started before all the ice had melted!

Over the next few days, clouds weren’t as much of a problem so the hot spots and smoke plumes were easily visible from all four satellites. Here are direct comparisons between GOES-16, GOES-17 and the two VIIRS for 19 May 2019:

NOAA-20 VIIRS Fire Temperature RGB (1901 UTC, 19 May 2019)

GOES-17 (left) and GOES-16 (right) Fire Temperature RGB (1900 UTC, 19 May 2019)

S-NPP VIIRS Fire Temperature RGB (1959 UTC, 19 May 2019)

GOES-17 (left) and GOES-16 (right) Fire Temperature RGB (2000 UTC, 19 May 2019)

We can even do better than that and really zoom in:

Comparison between GOES-17 ABI, NOAA-20 VIIRS, and GOES-16 ABI Fire Temperature RGB images (1900 UTC, 19 May 2019) zoomed in at 400%

Comparison between GOES-17 ABI, S-NPP VIIRS, and GOES-16 ABI Fire Temperature RGB images (2000 UTC, 19 May 2019) zoomed in at 400%

Here, the previous images have been cropped to the fire near High Level, and zoomed in 400%. The difference in resolution between VIIRS and ABI at this latitude is obvious.

Here is a four day loop of True Color RGB images from both VIIRS (click to play):

Animation of VIIRS True Color images (18-21 May 2019)

And, click on these links for similar loops of GeoColor from GOES-16, GOES-17 and a side-by-side comparison of the two. (The videos are too large to embed here.)

And, finally, here is a four day loop of Fire Temperature RGB images from both VIIRS (click to play):

Animation of VIIRS Fire Temperature RGB images (18-21 May 2019)

And links to MP4 video files from GOES-16, GOES-17 and a side-by-side comparison of the two.

It’s not everyday that one comes across something that is truly surprising. But, here’s something I recently came across that surprised me: a website on ghosts, angels and demons with useful scientific information. Of relevance here is the section on lens flare and ghosting. Although, maybe it shouldn’t be surprising. If you’re looking for “real” ghosts, you have to be able to spot the “fake” ones.

Simply put, lens ghosting (or optical ghosting) is a consequence of the fact that no camera lens in existence perfectly transmits 100% of the light incident upon it. Some of the light is reflected from the back of lens to the front, and then back again, as in the first diagram on this website. When the source of this light is bright enough, the component of this light that bounces around due to internal reflections within the lens may be as bright or brighter than the rest of the incoming light and will show up on the film (for you old fogies) or recorded by the array of detector elements that convert light into an electric signal (pretty much any camera purchased after 2004). That leads to the phenomena known as “flaring” and “ghosting”.

We’ve all seen pictures or movies that contain these artifacts. Here’s an example of flaring. Here’s an example of ghosting. And here’s both in the same image:

Photo credit: Nasim Mansurov (photographylife.com)

Professional photographers use flaring and ghosting to their advantage. Amateurs wonder why it ruined their picture.

In the particular case of “ghosts”, the light you see often takes on the shape of the aperture, which gives you polygonal or circular shapes like these:

Examples of lens ghosts. Pictures courtesy Angels&Ghosts.com.

I hate to be a stickler but those are pentagons, not hexagons. (Keep on your toes!) Flaring and ghosting is so prevalent in cameras of all kinds that animated movies replicate it in order to look “more real.” And, they are two examples of the many artifacts produced by cameras. (Take a look at the differences between CCD and CMOS detectors, as an example of others.)

Why bring this up on a blog about a weather satellite? Because the VIIRS Day/Night Band is, in a manner of speaking, just a really high-powered CCD camera. It, too, is subject to ghosts. (More so than other VIIRS bands because of its high sensitivity to low levels of light.)

Before we get to that, see if you notice anything unusual about this Day/Night Band image:

VIIRS Day/Night Band image (00:42 UTC 9 February 2015).

Those with photographic memories will recognize this image from an earlier post about the N-ICE field campaign in 2015 (which I hid in one of the animations). See that row of 6 bright lights north of Svalbard? Those aren’t boats and they’re not optical ghosts – they are 6 images of the same satellite (using the more liberal definition of satellite: 2a).

Don’t believe me? Here’s the explanation: VIIRS is on a satellite that orbits the Earth at about 835 km. That means two things: 1) there are plenty of satellites (or bits of space junk) that orbit at lower altitudes; and 2) every time a satellite crosses over to the nighttime side of the terminator, there is a period of time that the object is still illuminated by the sun before it passes behind the Earth’s shadow. And, there’s a third thing to consider: lower orbiting objects travel faster than higher orbiting objects. If one of these lower orbiting satellites should pass through the field-of-view of VIIRS while it is still illuminated by the sun, it can reflect light back to VIIRS, where the Day/Night Band can detect it. It’s a form of glint, like sunglint or moonglint. If it moves only slightly faster than VIIRS, it will be in the field-of-view for multiple scans, like in the image above.

It happened again in the same area 4 days later, only with 5 bright spots this time:

VIIRS Day/Night Band image (06:10 UTC 13 February 2015).

With all the striping that is present in the above image, you can clearly see the outline of each VIIRS scan. Note the relative position of the bright light in each scan in which it is imaged. See how it moves in the along-track dimension from one edge of the scan to the other? (The along-track dimension is basically perpendicular to the scan lines.)

Here are the two previous images zoomed in at 400%:

VIIRS Day/Night Band image (06:10 UTC 13 February 2016) zoomed in at 400%.

If this “satellite” reflects a high amount of light back to VIIRS, it can cause optical ghosts like in this image:

VIIRS Day/Night Band image (11:50 UTC 1 March 2014).

The ghosting is obvious. The “satellite” is less obvious, but you should be able to see the six smaller dots indicating its location. Eagle-eyed observers may click on it to see the full resolution image and note the two partial dots at either end of the row, indicating where this “satellite” was only partially within the VIIRS field-of-view. Even when the “satellite” was not in the field-of-view of VIIRS, it still caused ghosts – just like how the sun doesn’t have to be in a camera’s field-of-view to cause flares and ghosts.

The yellow line demarcates where the solar zenith angle is 108° on the Earth’s surface and the green line demarcates the lunar zenith angle of 108°. The yellow line is the limit of astronomical twilight. (Astronomical twilight exists to the right of that line.) Even though the surface is dark where this ghosting occurs (astronomical night), satellites are still illuminated by the sun (and moon) in this region. In fact, my back-of-the-envelope calculation indicates that VIIRS (at ~835 km) doesn’t pass into the Earth’s shadow until the sub-satellite point reaches a solar zenith angle of ~118°. (As an aside, the International Space Station is much lower [~400 km], so it is illuminated only to a solar zenith angle of ~110°.)

Here is the above image zoomed in at 200%:

Now that you’ve passed the crash course, see if you can earn your PhD. How many ghosts you can find in this image from last month? Make sure you click on it to see it in full resolution:

VIIRS Day/Night Band image (11:50 UTC 4 May 2016).

Where is the “satellite” in this case? What is the “real” image? And what are the “ghosts”? Are they even ghosts? As shown on the Angels & Ghosts website, objects that are out of focus are not necessarily ghosts – either “real” ghosts or “fake” ones. VIIRS is focused on the Earth’s surface (835 km away), so if another satellite were orbiting the Earth just a few kilometers lower in altitude, it would definitely appear out of focus and it would have a very similar speed to VIIRS, so it could be causing ghosts in the Day/Night Band for a long time, as you see here.

Here are all the ghosts that I found:

Close ups of the ghosts from 11:50 UTC 4 May 2016 (kept at native resolution).

But, is that what we’re seeing? Are we seeing one satellite? Or is it a clutter of space junk? Did VIIRS just come close to a collision with something (because we’re seeing nearby out-of-focus objects)? Or are they optical ghosts from an object well below VIIRS, so we don’t have to worry about it? Maybe it’s a UFO! What about that!?

For once, I don’t have all the answers. But, the truth is out there! (Cue music…)

UPDATE (6/24/2016): Thanks to Dan L. for pointing out an instance of the high-resolution Landsat-8 Operational Land Imager quite clearly spotting the lower-orbiting International Space Station. With a different instrument scan strategy, it produces a different kind of artifact: tracking the ISS motion from one band to the next!

Seeing the Light

VIIRS in the Arctic

Tag Archives: Canada

A four-birds-eye-view of fires in Alberta

Optical Ghosts