Frequently Asked Questions

NASA Earth Observatory image modified by SkyTruth

Frequently Asked Questions

Frequently Asked Questions

NASA Earth Observatory image modified by SkyTruth

Questions about SkyTruth

SkyTruth does not own satellites, and we don’t have the ability to point a satellite at a specific location on a certain day and time. As technology improves, governments and private companies around the world are launching increasing numbers of satellites. SkyTruth relies heavily on government satellites that provide data for free. We also have established partnerships with private companies to use their satellite data for free or at a reduced cost. You can learn more about the technology we use to do our work here and a list of imagery we regularly use here. If you are interested in developing a partnership with SkyTruth to share your data, please contact us.

Yes. As a nonprofit conservation technology organization, all of SkyTruth’s analysis, tools, databases, images, and maps are free. SkyTruth was founded to level the playing field, and provide citizen’s groups, small conservation organizations, researchers, policymakers, journalists and others access to the same sophisticated tools and technology available to extractive industries. You can learn more about our story here.

SkyTruth is a 501(c)(3) nonprofit organization. This means we are approved by the Internal Revenue Service as a tax-exempt charitable organization dedicated to fulfilling our mission to use the view from space to inspire people to protect the environment. As a nonprofit, we receive most of our funding from charitable foundations and individual donors who believe in our mission. We occasionally perform work for other nonprofits and academic partners on a fee-for-service basis, but that’s a very small part of our funding.

Questions about Satellite Imagery

Most of the free imagery that SkyTruth has access to does not provide enough detail to detect individuals or objects as small as cars. Occasionally we use high-resolution commercial imagery that is detailed enough to see cars – or bags of fish being loaded into a cargo ship. This high-resolution imagery is capable of showing very large animals, such as elephants and, when they’re at the surface, whales. But it’s difficult to intentionally target moving objects with high-resolution satellites, and we’ve never been able to see individual humans on an image.

What we can see depends on the resolution of specific satellites and what is visible at the time that satellite is flying over a particular area. Depending on the satellite, the amount of cloud cover can also affect how much we can see. For example, radar satellites can penetrate clouds, but optical/infrared satellites cannot.

You may be wondering about the “satellite” view in tools like Google Maps, which lets you zoom in extremely close in some areas. The images used for Google Maps are a mix of satellite and aerial images. This makes for extremely high-quality imagery, but it usually can’t be used for monitoring environmental incidents that happened very recently. The images are often composites, meaning they are several images stitched together, to show a cloud-free view of the ground, and it can be difficult to know the exact date and time the image was taken.

Questions about SkyTruth Alerts

Anyone can use SkyTruth Alerts for free. It was funded and developed to be used by individuals and organizations working to protect the environment. If you happen to be using SkyTruth Alerts for commercial purposes, we’d really appreciate you making a donation to help us cover costs. 🙂

Anyone can view the SkyTruth Alerts map without creating an account. You can move around the map, look at recent reports, and filter the types of alerts you see by type and date. However, if you want to receive email notifications, save your areas of interest (AOIs) to make monitoring easier, annotate and mark up a map, or create issue maps to share with other people, etc., you will need to sign up for a free account.

A list of available alerts can be found on the SkyTruth Alerts landing page.

There are three steps to signing up to receive email alerts:

  • sign up for an account
  • select your area of interest
  • select which types of alerts you want to be notified about

U.S. users: we have a quick start guide to get alerts for your county that you can use as a starting point.

We’re very interested in adding new types of alert-worthy information to SkyTruth Alerts, in particular state environmental agency notifications and information for other countries. Please email us a link if you know of a source that makes its data publicly available by providing any one of these:

  • A website that allows us to “scrape” the data
  • A spreadsheet we can download
  • An API that allows us public access.

Currently the main source of satellite imagery in Alerts comes from the Sentinel-1 and Sentinel-2 satellites in the European Union’s Copernicus Program. SkyTruth uses Sentinel Hub, a platform for handling and delivering satellite data, to make the imagery available in Alerts. (You can also browse and download satellite imagery directly from the Hub using the EO Browser.)

Imagery from Sentinel Hub is made up of a mosaic of scenes based on zoom level, the size and location of your area of interest (AOI), and the imagery available. If your AOI is small, chances are the image will be from a single scene. However, for larger AOIs the satellite image you see is created from several stitched-together scenes, which may or may not be from the same date. For this reason, we include the date of the imagery on top of each scene so it’s clear what’s being presented.

When you select a map date, the corresponding scene for that date may only represent a small part of your AOI. Sentinel Hub will back-fill the rest of your map view with the most recent imagery available, regardless of cloud cover.

In some cases, especially for very large AOIs, the scenes presented might even change based on zoom level. This is because Sentinel Hub’s image vault may not have the same view available for each zoom level.

We have added several data sets that we find useful in-house to the Layers tab. If you have an idea for other layers that can be helpful to environmental and conservation organizations, email us a link describing the data.

You can also add your own layers and save them for later use. Find out how to do that here.

Yes! More information about how to download alerts and which format area available is on the Download Alerts Data page.

In most cases, data obtained from state agencies includes a unique ID called the Well API number. You can use this number to find out more information, including the name of the well, at agency websites (for example, the Pennsylvania DEP).

To find the Well API number:

  1. Click on an Alert icon to open the report info window.
  2. Click View Full Report at the top of the window.
  3. In the lefthand sidebar, look for the Well API Number. (You may have to scroll to see it.)

Once you know the Well API number, you can use the keyword search to find other Alerts for the same well:

  1. Check the boxes next to the types of alerts you want to see.
  2. Copy and paste (don’t type) the Well API Number into the Keyword Search (Alerts tab).
  3. Click the Search icon.

Questions about Bilge Dumping

Bilge is the oily wastewater that accumulates in the lower hold of a vessel during normal operations. In the early 1970s an international treaty called MARPOL specified how to properly treat this wastewater, but many vessels at sea continue to dump it directly in the ocean in violation of national and international laws. Learn more about this on our bilge page.

SkyTruth uses satellite radar imagery to identify oil slicks. In this type of imagery, bilge dumps typically take the form of long, slender black streaks on the ocean. In some cases, there is also a bright white spot at one end of the streak. Much of the time, this bright white spot is the vessel responsible for the slick. The image below offers a good example:

Our goal is to stop oil pollution at sea

Not all slicks visible on radar imagery are bilge slicks. Heavy rain, slack wind, leaking oil platforms and other phenomena in the ocean can also create dark slicks on imagery, although bilge slicks are very distinctive. Also, bilge dumping is intentional, but there are other reasons that a slick might be visible trailing a vessel: there could be a serious mechanical problem and the vessel might be in distress. In any event, it’s important to be able to detect these pollution events and identify the vessels responsible.

SAR is Synthetic Aperture Radar imagery. A SAR image is a picture of the Earth’s surface made by beaming radar energy down at the planet from a satellite, and measuring the signal that comes back. We’re particularly interested in SAR imagery collected from two orbiting satellites, called Sentinel-1A and Sentinel-1B, operated by the European Space Agency. The radar sensors on the Sentinel-1 satellites operate at a frequency that makes them very sensitive to the roughness of the ocean surface. An oil slick is a smooth patch on the ocean, so it stands out as a dark area in contrast to the usually wind-ruffled surrounding waters that appear gray.

Because radar satellites create their own source of energy to illuminate what is on the Earth’s surface, they can operate day and night. And they beam down a radar signal at a wavelength that cuts through clouds, smoke and haze, creating useful images under conditions that would render optical (visible and near-infrared) imagery useless. This makes radar a valuable tool for continuous ocean monitoring.

You can look at and download Sentinel-1 imagery here. Learn more about Sentinel-1 radar here.

Satellite radar imagery doesn’t have enough detail to identify the vessels, which simply appear as very bright spots. So we use another dataset to make vessel identification possible. Most of the bilge-dumping incidents we’ve seen over the years have been caused by large cargo ships and oil/chemical tankers. Under international maritime safety law (SOLAS), these vessels and many others (passenger ships, research vessels, large fishing vessels) are required to continuously broadcast a radio-frequency identification signal when they’re at sea, using a public, open safety system called the Automatic Identification System (AIS).

AIS is radio gear installed on a ship to broadcast information as frequently as every few seconds, to alert nearby vessels to a ship’s location, course, speed, identity, and other useful information. These broadcasts are public and unencrypted, designed to help ships avoid collision at sea. The broadcasts are also collected by networks of land-based radio receivers (terrestrial AIS) as well as AIS receivers carried on a rapidly growing number of orbiting satellites (satellite AIS). Companies like Spire and Orbcomm and others package the data for sale. You can also see AIS data on public vessel-tracking sites like Marine Traffic and FleetMon, and it is the data at the heart of the Global Fishing Watch service we built with Google and Oceana.

Questions about Flaring

The data for this map was originally made available by NOAA’s Earth Observation Group. As of 15 October 2019, the data is now freely-available from Earth Observation Group (EOG), Payne Inst. for Public Policy, Colorado School of Mines.

The Earth Observation Group have authored the following papers for those interested in the VIIRS instrument and how the flare volume is calculated.

Elvidge, C. D., Zhizhin, M., Hsu, F-C., & Baugh, K. (2013). VIIRS Nightfire: Satellite Pyrometry at Night. Remote Sensing 5(9), 4423-4449.

Elvidge, C. D., Zhizhin, M., Baugh, K. E, Hsu, F-C., & Ghosh, T. (2015). Methods for Global Survey of Natural Gas flaring from Visible Infrared Imaging Radiometer Suite Data. Energies, 9(1), 1-15.

Elvidge, C. D., Bazilian, M. D., Zhizhin, M., Ghosh, T., Baugh, K., & Hsu, F-C (2018). The Potential Role of Natural Gas Flaring in Meeting Greenhouse Gas Mitigation Targets. Energy Strategy Reviews, 20, 156-162.

Here are a few things that we do at SkyTruth to process and present the data on the Flaring Map.

For each daily Nightfire file we process:

  • We eliminate temperatures under 1,773 kelvin (1,500 celsius).
  • For each day, we merge VIIRS Nightfire detections that are within 1500m of each other. This accounts for the 750m at-nadir resolution of the VIIRS instrument, which is the source of Nightfire detections. When two nearby detections are merged, we average their longitude, latitude, Radiant Output and Temperature, and we sum their Radiative Heat and Footprint.

To create the data used in the flaring map:

  • For each new location in the flaring records from step 1, we total the number of flaring records (also from step 1 for prior dates) that are within 925m of the new location in the previous 30 days. 
  • If the count is 3 or more, we create a flaring location for the map. The location is the centroid of the matching flaring from the previous 30 days. The downloaded file contains data (Radiant Output, Temperature, etc.) that is carried over from the step 1 records.

The data are limited in the extreme northern and southern latitudes due to extreme solar illumination and atmospheric conditions.

There was no data available from NOAA between Sept. 29 – Oct. 16, 2013.

In December 2017, the VIIRS instrument began collecting Nightfire data with an additional spectral band. This change resulted in many more high-temperature determinations, which you can see reflected in the timeline as an abrupt increase in apparent flaring activity.

Applications for this map include:

  •  Demonstrating the tremendous amount of natural gas flaring around the world.
  •  Learning if flaring is a chronic problem in your community or places you care about.
  • Tracking active drilling in gas-producing regions where flaring occurs during the drilling and completion of wells.
  • Verifying when petrochemical facilities were flaring in order to aid identifying the source of noxious air emissions polluting fence-line communities.
  • Holding companies accountable for wasting public and private resources through routine flaring.
  • Informing public health research on the impacts of flaring on respiratory health and other disciplines.
  •  Let us know how you could use flaring detections to skytruth an issue in your community.

Verified on the ground by a team we sent out North Dakota’s Bakken Shale and further cross-referenced against aerial and satellite imagery of other flaring hotspots such as Russia, Africa, and the Middle East, this map is updated daily to show the frequency of infrared detections hot enough to be gas flares.

If you don’t see a flaring detection you expected to see, it may be because of the ways that we process and present the flaring data. 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 have obscured the flare from the sensor, etc.

SkyTruth’s dataset is a highly processed version of the EOG Nightfire data made especially for the flaring map. While it does not reflect all of the flaring details present in the Nightfire dataset, it can still be useful for analyzing local trends. To receive a copy of this dataset, email and ask for the most recent flaring map dataset.

See “How does the map data get processed?” above for details on how we create the file.

Here is what you’ll find in the download:

EOG Reference UOM SkyTruth Spreadsheet EOG Description
RHI W/m2 Radiant Output IR-source radiant heat intensity (derived using Nightfire algorithm)
RH MW Radiative Heat IR-source radiant heat (derived using Nightfire algorithm)
Temp_BB EOG: ° Kelvin
SkyTruth: ° Celsius
Temperature IR-source temperature assuming blackbody source (derived using Nightfire algorithm)
Area_BB m2 Footprint Area of IR-source assuming blackbody source (derived using Nightfire algorithm)

Questions about Cerulean

Cerulean automates satellite-based detections of marine oil pollution incidents and the identification of their likely sources, primarily shipping vessels and offshore oil and gas platforms. Oil slick detections by Cerulean enable investigations of potential bilge dumping incidents and chronic pollution from offshore oil and gas operations.

Cerulean uses machine learning and cloud computing to scan all European Space Agency Sentinel-1 satellite data collected over the ocean and inland seas to detect potential oil slicks. There are a few steps to this process:

  1. Whenever new Sentinel-1 imagery becomes available over the ocean, the scene is automatically scanned by our computer vision model, which is trained specifically to detect oil slicks in Sentinel-1 data.
  2. If oil slicks are detected, another set of models tries to identify the potential source. These models use ship locations, from automatic identification system (AIS) data, and offshore oil platform locations to find the potential sources that most closely match the location and timing of the oil slick.
  3. Slick and source information are added to a database, which is then made available in our map-based user interface.

For detailed information, please review our methodology.

Cerulean data is accessible via a publicly available web application providing oil slick detections, potential source information, and satellite imagery. The technology is free to all, giving users easy access to imagery and data needed for investigations, research, and analysis. Cerulean’s ease of use and openness embody our values of transparency and scientific integrity. For API data access, see our API docs.

SkyTruth makes all oil slick detections publicly accessible on the Cerulean platform and API. We provide capacity building support for investigative journalists, enforcement agencies, advocates, researchers, and other users to interpret pollution trends and incorporate Cerulean into their work. SkyTruth does not determine the intentionality or illegality of any oil slick detected by Cerulean, and SkyTruth does not provide direct response services to ongoing oil spills. If you think Cerulean can help your work, get in touch with us at

Cerulean’s geographic coverage focuses on Exclusive Economic Zones of coastal countries due its dependence on free radar satellite imagery from the European Space Agency’s Sentinel-1 mission. Sentinel-1 provides coverage up to a few hundred to a couple thousand kilometers offshore, covering 47% of global shipping lanes and 85% of offshore oil and gas extraction activity. The map below displays the area and frequency of coverage in 2022. Sentinel-1 radar imagery is unaffected by atmospheric conditions, enabling it to make detections even in cloudy weather.

Cerulean does not currently monitor most of the open ocean. We are exploring options to fill in these gaps, including commercial satellite data and non-radar multispectral imagery. Earth-observation systems planned for launch over the next 3-5 years will greatly improve the extent and frequency of our coverage in the ocean. A new upcoming Sentinel-1 satellite (replacing an older spacecraft that was decommissioned in 2021) will double our monitoring frequency, and other public radar satellites are scheduled to enter service in the next several years.

When the Sentinel-1 satellite passes any given area, its imagery typically becomes available to the Cerulean model within 6-12 hours. In a matter of minutes, Cerulean automatically scans new imagery and generates new results.

Cerulean’s AIS data source, which enables the attribution of oil slicks to potential vessel sources, operates with a 72-hour delay. Therefore, live attribution of oil slicks to potential vessel sources is not currently available.

Cerulean is designed to accurately outline the shape of oil slicks against seawater. A high confidence score of a slick detection indicates that the highlighted patch of sea surface displays characteristics consistent with hundreds of confirmed oil slicks on which we have trained the model.

Attributions of slicks to a potential vessel source or piece of fossil fuel infrastructure are more complex. We consider numerous factors, including an analysis of which vessels, platforms, or other infrastructure were in the area when we detected a slick; how closely a moving vessel(s) passed through the slick area; and how closely the movement of the slick’s centerline matches those vessel trajectories. From this data, Cerulean will generate several possible associations between oil slicks and their potential sources. For each associated vessel or infrastructure site, Cerulean provides a confidence score indicating the likelihood of each association. Cerulean’s attributions are only as good as the AIS data supplied to the model; there is a possibility that the actual polluter is a so-called “dark” vessel that was not broadcasting an AIS signal.

It is not possible to definitively identify oil slicks using synthetic aperture radar (SAR) satellite data alone. All Cerulean detected oil slicks should be considered potential oil slicks, not definitive oil slicks. We are still finalizing our slick detection model, so false positives should be expected. Final model accuracy is to be determined.

One important feature of our user interface is the ability for users to filter what they see on the map. For instance, if someone only wants to see slick detections that have a very high level of confidence, we allow users to set that parameter and restrict the visible slicks. If someone wants to see every possible slick, even the ones with a lower probability of being true, users can adjust the display to show all detections.

Radar is a useful means of detecting oil slicks, but it does have limitations. Oil slicks appear as dark patches in radar imagery, but other phenomena can cause similar dark patches, including calm water in low wind conditions, sea ice, and wind shadows around islands and other objects at sea. Cerulean often detects these lookalikes as potential oil slicks. Cerulean is also limited in distinguishing oil slick type: Cerulean is not able to distinguish between mineral oil and vegetable, for example. Because of these uncertainties, Cerulean cannot determine whether an oil slick represents an illegal action, and we recommend that users independently verify all Cerulean’s detections before pursuing legal or other action.

It is not possible to definitively link a potential oil slick to a vessel or stationary source based on satellite imagery, ship location information (AIS), and offshore infrastructure locations alone. All potential oil slick sources identified by Cerulean should be considered potential sources, not definitive sources. Our models narrow the list of potential sources by finding the nearest AIS tracks or stationary sources (such as oil platforms) and providing those to users as a list of candidates. Users can then visualize those AIS tracks or stationary sources on the map and determine which, if any, are the most likely source. In some cases, it is not possible to make a clear determination, either because a ship isn’t broadcasting AIS at all, or because there are too many vessels close together that the source could be any of them.

We can’t measure volume, but we can estimate it using the area covered by each slick. Based on available research, we estimate that oil slicks visible in Sentinel-1 imagery are on average at least 1 micrometer thick, which is about seventy times thinner than a human hair. For every square kilometer of oil we see on the surface, we estimate that represents at least one cubic meter of oil.

For detailed information, please review this oil slick volume methodology here.

Cerulean is the first free and publicly available marine oil pollution monitoring tool, and its primary purpose is to reveal the hidden consequences of global shipping and offshore oil and gas development and to facilitate public action and accountability.