Pretty Parallax Planes

While scanning the European Space Agency’s (ESA) Sentinel-2 satellite images for signs of the Sanchi oil slick, I came across an unusual sight of what appeared to be three, brightly-colored aircraft flying in tight formation. I’m not enough of a GIS rookie to be fooled into thinking China’s latest stealth jets were malfunctioning, what I was observing was a single aircraft’s image split into three spectral bands of red, green, and blue.

This flight was snapped by Sentinel-2 on its way to Tokyo (flight data from Flightradar24.com).

To explain why this happens, we need to take a look at the source of these images: Sentinel-2’s MultiSpectral Instrument (MSI) sensor. This can be thought of as a very advanced camera that can see beyond the usual visual spectrum and into the near-infrared (great for monitoring vegetation) and shortwave infrared. Instead of just one sensor in a camera, the MSI sensor has 12 in a row. For a more technical explanation, take a look at ESA’s guide on the MSI sensor here. Imagine a push-broom with 12, wide bristles and you’ll have an idea of how these sensors sweep across the Earth as the satellite flies overhead. Each sensor splits the image into 10 different spectral bands using a stripe filter which means not only is each band detected at a slightly different angle, they are also detected at slightly different times. What this means for an image like the one above, a “true color” composite made up of the MSI’s red, green, and blue bands, is that when the bands are combined, an assumption has to be made about how far away the object is to correct for the parallax and “focus” the image on the target — and for earth-observation systems like Sentinel, the target is the surface of the earth. An element of parallax is factored in when we combine the bands in the same way that our brains adjust for the parallax of the different angles our eyeballs are seeing. This is called orthorectification. For an example of this, hold your finger halfway between this screen and your face and focus on these words. As well as being a bit blurry, you should be seeing more than one finger. In the same way, the RGB bands are combined with the focus on the surface of the Earth so an aircraft at a higher altitude splits into three images, one for each band. Since this Airbus A321 was cruising at an altitude of about 33,000 feet, the aircraft’s position was projected onto the Earth’s surface resulting in three different images, one for each of the bands.

The time difference between when each band is detected also adds to the offset. This isn’t noticeable for stationary or slow-moving objects but an aircraft is moving fast enough to see a difference. In the image we found, the aircraft’s speed, about 550kts (according to Flightradar24.com), is probably the biggest cause of the shift between images but if you look closely at the contrails, you can see some sideways drift between the first and last image of the plane. The image below, from just off the east coast of Bulgaria, better highlights the two effects of the forward motion of the aircraft and the sideways shift due to parallax.

Example of parallax off the east coast of Bulgaria.

If we really wanted to fix the aircraft’s image, we would need to adjust for the parallax at that distance as well as the delay between each band’s detection (to account for the aircraft’s speed). The result would be that the aircraft would now be one, complete image but everything else would be a multicolor mess.

For more info on this effect, check out this post by Tyler Erickson, or some direct information from the European Space Agency (skip to chapter 2.5).

U.S. Secretary of State John Kerry looks on as Paul Woods, SkyTruth CTO, demonstrates the Global Fishing Watch interface. Credit: Franz Mahr, Oceana

Read our Annual Report for an Overview of Our Environmental Impact

Together with partners from around the world, SkyTruth uses the view from space to motivate people to protect the environment. SkyTruth is committed to transparency in all things. In the spirit of that, we wanted to share our annual report with you which covers the impact we’ve been able to have as a watchdog, innovator, and motivator for environmental good.

Global Flaring Map Reset

The wasteful practice of flaring off natural gas from oil and gas fields is again making news, coinciding with a new release of SkyTruth’s Global Flaring Map that visualizes gas flaring activity around the globe. This map relies on the Nightfire data provided by NOAA’s Earth Observation Group, which has written extensively about their work detecting and characterizing sub-pixel hot sources using multispectral data collected globally, each night, by the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi-NPP satellite. Read about the algorithm that creates Nightfire data here and methods for estimating flared gas volumes here.

SkyTruth’s enhanced map has these added features:

  • NOAA has published two additional years of flaring data, allowing our map to extend back to March 2012.
  • A location search box lets you go directly to a city, state, country, landmark, etc.
  • Date range selection helps you limit the visualization to the time-frame of interest.
  • You can identify your rectangular Area of Interest and download flaring data within that AOI (works best in Chrome browsers).
  • We’ve caught up with NOAA’s daily download after adjusting to recent changes in their web security.


About our Global Flaring Map

Please read about some of the uses for this map and how SkyTruth processes NOAA’s data in this original post describing our map. If you don’t see a flaring detection you expected to see, consider the caveats:  some flares don’t burn hot enough to be included in our dataset, they may not have been burning when the satellite passed overhead, the flare may not be frequent enough to make it past the 3 detection threshold, heavy clouds may have obscured the flare from the sensor, etc.

If you find this map useful, drop us an email at info@skytruth.org to let us know.

Why Flaring is In the News Again

In November 2016 the Interior Department announced a new Methane and Waste Prevention Rule to reduce wasteful flaring and leaks of natural gas from oil and gas operations on public and Indian lands. Although Congress tried repealing the rule after the 2016 elections, that effort failed to advance out of the Senate after a May 2017 vote.

Despite the Senate’s action to keep the methane rule, the Environmental Protection Agency just announced (as of 6/15/2017) they would suspend implementation of the rule for 90 days — an action leading environmental groups claim is unlawful.

Transshipment in the Fishing Industry Getting a Critical Look

Our collaboration with Global Fishing Watch on the problem of transshipment at sea in the fishing industry is at the forefront of a growing movement to take a critical look at this practice, which is increasingly regarded as a key driver of overfishing, and an enabler of illegal fishing and other fisheries crime including crew enslavement. Our work is funded by the Walton Family Foundation and being led by Bay-area skytruthers Aaron Roan and Nate Miller.

Some hot-off-the-presses resources on this issue:

A new Walton Family Foundation blog post on our work — How Big Data is Helping in Battle Against Illegal Fishing: Satellite Monitoring Tracks ‘Pervasive Problem’ of Global Transshipments

Just-published research concluding transshipment at sea should be banned to curb illegal fishing — Potential Ecological and Social Benefits of a Moratorium on Transshipment on the High Seas

SkyTruth collaboration with DigitalGlobe to target transshipment with high-resolution satellite imagery — Satellites Leave No Place to Hide for Rogue Thai Fishing Fleet

Worldview-3 satellite image of likely transshipment courtesy DigitalGlobe.

Oceana report — No More Hiding at Sea: Transshipping Exposed

SkyTruth + Global Fishing Watch report, map and dataset showing 5,000 likely transshipment events over four years, detected using vessel tracking data — The Global View of Transshipment: Preliminary Findings

 

SkyTruth CTO: Paul Woods

When Paul Woods moved to Shepherdstown, West Virginia, SkyTruth’s home base, he was looking to get away from the Washington, D.C. area where we had been consulting in the tech industry during the dot com boom. His goal had been to find a slower pace and a more soul-satisfying lifestyle than the world of maximizing profit margins through software development. Now, he’s setting off to help save the oceans by revolutionizing the way the fishing industry works.

As the Chief Technology Officer at SkyTruth, Paul was instrumental in bringing Global Fishing Watch into being. [You can read about that here] Now, the platform we developed for identifying and tracking every commercial fishing vessel on the oceans is spinning off into an independent non-profit organization with Paul at the helm. As the interim CEO of Global Fishing Watch, Paul will be guiding the new organization through the transition. While we’re still keeping him in the fold, we thought it was a good time to sit down for a brief reflection on his path, his time at SkyTruth and a look into what’s next.

It’s a small town, so I guess when you landed in Shepherdstown in 2001, it was only a matter of time before you and SkyTruth found each other. How did you get involved?

It’s true just about everybody in Shepherdstown knows SkyTruth. When I met John (SkyTruth President, John Amos), I was working with another company, but I did a few side projects for SkyTruth. I also joined the board as technology advisor. Then, as the other work was winding down and I was looking for the next thing, I realized I just got a lot more out of the SkyTruth stuff than I did out of creating products to maximize clicks or streamline business processes.

In 2010, when the Deepwater Horizon spill happened, I helped John set up a rapid response website. Of course the whole oil spill incident opened an opportunity for growth at SkyTruth, and I realized I could apply my skills in the stuff I really like doing directly to issues that made a real difference in the world. That kind of direct application to saving the environment is so much more satisfying than just writing a check or writing a letter to your congressman.

By 2013, I came on full time, and one of the first projects we did was SkyTruth Alerts, which is still in use today.

I’m sure it’s rewarding to see Global Fishing Watch mature into its own organization. Do you have any reflections to share as you look back at your time at SkyTruth?

Over the years I’ve been working on many different projects at SkyTruth that have been deeply rewarding to me. Now that one of those projects has gotten big enough that it requires all of my time and attention to keep it running, which is enormously exciting.

What are your hopes for the future:

Clearly my immediate hopes and dreams are focused on the continued success and growth of Global Fishing Watch. I hope to see Global Fishing Watch arrive at a long term sustainable model that will propel its growth beyond me and be wildly successful at making fishing sustainable and helping save the oceans.

Personally, I guess I’m always looking for the next thing. I’m a start-up guy. That’s what I do. It’s what I like to do, so I guess my hope is that there will be another Global Fishing Watch around the corner a few years from now —another project with the same great impact and the same great opportunity to make the world a better place, and I’ll get to be involved in it. There’s a good chance that project is in its infancy right now at SkyTruth.

If you could see any place in the world from space, where would it be?

Anyplace? Well, we have recently detected new planets only four-and-a-half light years away, and at least one of them potentially has liquid water on it. The surface of Proxima Centauri B. That’s my first answer.

Great answer. What about here on Earth. If you could aim the SkyTruth “eye” where would you aim?

What would be really fantastic to see from space would be the bottom of the ocean, the sea floor. Unfortunately we can’t do that right now, but I think that would be the place I’d want to see.