Tag Archives: Arctic

Optical Ghosts

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)

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.

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)

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)

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 (00:42 UTC 9 February 2015)

VIIRS Day/Night Band image (00:42 UTC 9 February 2015) zoomed in at 400%.

VIIRS Day/Night Band image (06:10 UTC 13 February 2016)

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)

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%:

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

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

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

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!

The nice (and dedicated) people of N-ICE

Imagine this scenario: you’re stuck on a boat in the Arctic Ocean in the middle of the night. The winds are howling, the air is frigid, and the boat you’re in is completely encased in ice. Step off the boat and your face is constantly sand-blasted by tiny ice particles. Blink at the wrong time and your eyes freeze shut. The ice may crack under your feet (or between you and the boat)  – without notice – leaving icy water between you and the only warm place for hundreds of kilometers. Have to swim for it? Look out for jellyfish. Decide to stay on your crumbling patch of ice? I hear polar bears can get pretty hungry. Death awaits every misstep and every wrong turn. Cowering in the boat? Internet access is limited, there are no re-runs of Friends to keep you entertained, and the shuffleboard court is outside. (Actually, it’s worse than that: there is no shuffleboard court!)

Now imagine this: you actually wanted to be there!

Most people would say, “That’s crazy! I would never do that!” But, for the scientists and crew aboard the research vessel (RV) Lance, it is a unique opportunity to further our understanding of the Arctic and its role in the Earth’s climate system.

You see, we are nearing the mid-point of the N-ICE 2015 field experiment, which is taking place from 1 January to 1 August 2015. The idea behind the experiment is to take a boat, freeze it in the Arctic ice sheet, and constantly monitor the environment around the boat for about six months. A group of scientists work in six-week shifts where they monitor everything from the weather to local biology. Of course, the primary objective is to see what happens with the ice itself.

One of our very own researchers at CIRA (and one of the world’s leading experts on snow) was on board during the first leg of the experiment.  So, what is a snow expert doing on a ship whose primary purpose is to study ice?

Here’s the lowdown. There are two types of ice that concern Arctic researchers: “young” and “multi-year”. As the name implies, multi-year ice is ice that survives the summer and lasts for more than one year. Young ice does not reach its first birthday – it melts over the summer. Arctic researchers have been finding out that, not only is the Arctic ice sheet shrinking, it’s lost most of its multi-year ice, which is being replaced by young ice.

Multi-year ice is thicker, more resilient and tends to be brighter (more reflective), while young ice is thinner, darker (less reflective of sunlight), and less resilient. The less sunlight that is reflected, the more sunlight is absorbed into the system and this leads to warming, which melts more ice (and is a positive feedback). The less ice there is, the more open ocean there is, and open water is a lot less reflective than ice, which leads to more absorption of sunlight, more warming, more melting, etc.

The thinner “young ice” breaks up more easily due to wind and waves. This creates more leads of open water. The water, being much warmer than the air above it, pumps heat and moisture into the atmosphere, creating more clouds and snow – just like lake-effect or sea-effect snow. And, while most people have a hard time believing it, snow is a good insulator. Snow on top of the ice will create a blanket that protects the ice from the really cold air above. This reduces the rate at which the ice thickens up, keeping the ice thinner, and we have another positive feedback.

That’s just one of the things being studied on the 2015 Norwegian Young Sea Ice Cruise. Of course, I wouldn’t be mentioning any of this unless VIIRS could provide information to help out with the mission.

Go back to the N-ICE 2015 website. Notice the sliding bar/calendar on the bottom of the map. You can use that to follow the progress of the ship. Or, you can use the VIIRS Day/Night Band.

At the time of this writing, the Lance is docked in Longyearbyen, the largest town on the island of Spitsbergen in Norway. (Spitsbergen is part of the Svalbard archipelago, which has a direct connection to VIIRS. Svalbard has a receiving station used by NOAA that collects and distributes data from nearly all of their polar-orbiting satellites.) Longyearbyen is where the RV Lance and Norwegian icebreaker KV Svalbard departed for the Arctic back in mid-January. KV Svalbard escorted the RV Lance into the ice sheet, then returned to Longyearbyen while the Lance froze itself into the ice. See if you can see that in this loop of VIIRS Day/Night Band images from 12 – 17 January 2015:

Animation of VIIRS Day/Night Band images from 12-17 January 2015

Animation of VIIRS Day/Night Band images from 12-17 January 2015. These images cover the area of the N-ICE field experiment, north of Svalbard.

Notice how the one bright light follows a lead in the ice until it stops. Then the light appears to split in two, with one light source heading back the way it came and the other stuck in the ice. That is the start of N-ICE 2015!  The KV Svalbard did its duty. If you look closely, there are also some other boats hanging out in the open water near the edge of the ice sheet during this time.

If you suspect there are jumps in the images you’re right. VIIRS passes over this area every day 6-8 times between 00 and 12 UTC, with no overpasses for the next 12 hours.

Toward the end of January you can see how the RV Lance drifted to the west along with the ice:

Animation of VIIRS Day/Night Band images from 23-30 January 2015

Animation of VIIRS Day/Night Band images from 23-30 January 2015. These images cover the area of the N-ICE field experiment, north of Svalbard.

This was all according to plan. But, then, in February, the winds shifted and helped the ice spit the boat back out towards the open water:

Animation of VIIRS Day/Night Band images from 8-15 February 2015

Animation of VIIRS Day/Night Band images from 8-15 February 2015. These images show the area of the N-ICE field experiment, north of Svalbard.

After this, the RV Lance needed help from the KV Svalbard to be repositioned in the ice sheet near where it started a month earlier. Otherwise, all the instruments they placed in the ice would no longer be in the ice – they’d be at the bottom of the ocean as the ice sheet broke up all around them.

If you want to know why the ship seems to disappear and reappear every day, you can thank the sun. You see, the first few weeks of the experiment took place during the long polar night. But, by mid-February, twilight began to encroach on the domain during the afternoons. This was enough light to drown out the light from the ship. (Sunrise occurred in early March.)

Another thing to notice with these last two animations: the cloud streets that form over the open water near Svalbard. The direction these cloud streets move gives a pretty good indicator of where the ice is going to go, since both the clouds and icebergs are being pushed and pulled by the same wind.

It’s fascinating to watch the movement of the ice over the first 6 weeks of the field experiment. To save on file size and downloading time, the animation below only uses one image per day (between 10 and 11 UTC). Here’s 6 weeks of images in 5 seconds:

Animation of VIIRS Day/Night Band images from 11 January to 28 February 2015

Animation of VIIRS Day/Night Band images from 11 January to 28 February 2015. These images show the area of the N-ICE field experiment, north of Svalbard.

And you probably thought of sea ice as being relatively static.

Once again, we lose sight of the RV Lance because of afternoon twilight in mid-February, so we can’t see it or the KV Svalbard after that. And note that there’s a lot less open water near Svalbard by the end of the period.

What if we didn’t have the Day/Night Band? You wouldn’t be able to see the ships at all, that’s for sure! Plus, this area was under darkness (no direct sunlight) for this six week period, so none of the other visible wavelength channels will work.  That leaves us with the infrared (IR), which looks like this:

Animation of VIIRS IR (M-15) images from 11 January to 28 February 2015

Animation of VIIRS IR (M-15) images from 11 January to 28 February 2015. These images cover the area of the N-ICE field experiment, north of Svalbard.

Note that clouds appear to have a greater impact on the detection of ice (and distinction between ice and clouds) in the IR. When it’s relatively cloud-free, there is enough of a temperature contrast between the open water and ice to see the icebergs but, pretty much any cloud will obscure the ice. So, why doesn’t the Day/Night Band have this problem?

That has to do with the optical properties of clouds at visible and IR wavelengths. Most of these clouds are optically thick in the IR and optically thin in the visible. The Day/Night Band can see through these clouds (most of them, anyway) while channels like M-15 (10.7 µm) shown here, can’t. We’ve seen more extreme examples of this before.

In the rapidly changing Arctic, it is nice to know that there are a few dedicated individuals who risk frostbite, hypothermia and polar bears to provide valuable information on how the ice impacts the environment both locally and globally. Me: I’ll just stick to analyzing satellite data from my nice, comfortable office, thank you.

By the way, the N-ICE field experiment has it’s own blog, and pictures and other snippets of information about the people and progress of the mission are regularly posted to Instagram, Facebook and Twitter.