Optical Imagery is Eclipsed!

Solar eclipse across the USA captured by Suomi NPP VIIRS satellite on 21st August. Image courtesy of NASA/ NASA’s Earth Observatory.

Last week’s eclipse gave an excellent demonstration of the sun’s role in optical remote sensing. The image to the left was acquired on the 21st August by the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the NOAA/NASA Suomi NPP satellite, and the moon’s shadow can be clearly seen in the centre of the image.

Optical remote sensing images are the type most familiar to people as they use the visible spectrum and essentially show the world in a similar way to how the human eye sees it. The system works by a sensor aboard the satellite detecting sunlight reflected off the land or water – this process of light being scattered back towards the sensor by an object is known as reflectance.

Optical instruments collect data across a variety of spectral wavebands including those beyond human vision. However, the most common form of optical image is what is known as a pseudo true-colour composite which combines the red, green and blue wavelengths to produce an image which effectively matches human vision; i.e., in these images vegetation tends to be green, water blue and buildings grey. These are also referred to as RGB images.

These images are often enhanced by adjustments to the colour pallets of each of the individual wavelengths that allow the colours to stand out more, so the vegetation is greener and the ocean bluer than in the original data captured by the satellite. The VIIRS image above is an enhanced pseudo true-colour composite and the difference between the land and the ocean is clearly visible as are the white clouds.

As we noted above, optical remote sensing works by taking the sunlight reflected from the land and water. Therefore during the eclipse the moon’s shadow means no sunlight reaches the Earth beneath, causing the circle of no reflectance (black) in the centre of the USA. This is also the reason why no optical imagery is produced at night.

This also explains why the nemesis of optical imagery is clouds! In cloudy conditions, the sunlight is reflected back to the sensor by the clouds and does not reach the land or water. In this case the satellite images simply show swirls of white!

Mosaic composite image of solar eclipse over the USA on the 21st August 2017 acquired by MODIS. .Image courtesy of NASA Earth Observatory images by Joshua Stevens and Jesse Allen, using MODIS data from the Land Atmosphere Near real-time Capability for EOS (LANCE) and EOSDIS/Rapid Response

A second eclipse image was produced from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor aboard the Terra satellite. Shown on the left this is a mosaic image from the 21st August, where:

  • The right third of the image shows the eastern United States at about 12:10 p.m. Eastern Time, before the eclipse had begun.
  • The middle part was captured at about 12:50 p.m. Central Time during the eclipse.
  • The left third of the image was collected at about 12:30 p.m. Pacific Time, after the eclipse had ended.

Again, the moon’s shadow is obvious from the black area on the image.

Hopefully, this gives you a bit of an insight into how optical imagery works and why you can’t get optical images at night, under cloudy conditions or during an eclipse!

Algae Starting To Bloom

Algal Blooms in Lake Erie, around Monroe, acquired by Sentinel-2 on 3rd August 2017. Data Courtesy of ESA/Copernicus.

Algae have been making the headlines in the last few weeks, which is definitely a rarely used phrase!

Firstly, the Lake Erie freshwater algal bloom has begun in the western end of the lake near Toledo. This is something that is becoming an almost annual event and last year it interrupted the water supply for a few days for around 400,000 residents in the local area.

An algae bloom refers to a high concentration of micro algae, known as phytoplankton, in a body of water. Blooms can grow quickly in nutrient rich waters and potentially have toxic effects. Although a lot of algae is harmless, the toxic varieties can cause rashes, nausea or skin irritation if you were to swim in it, it can also contaminate drinking water and can enter the food chain through shellfish as they filter large quantities of water.

Lake Erie is fourth largest of the great lakes on the US/Canadian border by surface area, measuring around 25,700 square km, although it’s also the shallowest and at 484 cubic km has the smallest water volume. Due to its southern position it is the warmest of the great lakes, something which may be factor in creation of nutrient rich waters. The National Oceanic and Atmospheric Administration produce both an annual forecast and a twice weekly Harmful Algal Bloom Bulletin during the bloom season which lasts until late September. The forecast reflects the expected biomass of the bloom, but not its toxicity, and this year’s forecast was 7.5 on a scale to 10, the largest recent blooms in 2011 and 2015 both hit the top of the scale. Interestingly, this year NOAA will start incorporating Sentinel-3 data into the programme.

Western end of Lake Erie acquired by Sentinel-2 on 3rd August 2017. Data

Despite the phytoplankton within algae blooms being only 1,000th of a millimetre in size, the large numbers enable them to be seen from space. The image to the left is a Sentinel-2 image, acquired on the 3rd August, of the western side of the lake where you can see the green swirls of the algal bloom, although there are also interesting aircraft contrails visible in the image. The image at the start of the top of the blog is zoomed in to the city of Monroe and the Detroit River flow into the lake and the algal bloom is more prominent.

Landsat 8 acquired this image of the northwest coast of Norway on the 23rd July 2017,. Image courtesy of NASA/NASA Earth Observatory.

It’s not just Lake Erie where algal blooms have been spotted recently:

  • The Chautauqua Lake and Findley Lake, which are both just south of Lake Erie, have reported algal blooms this month.
  • NASA’s Landsat 8 satellite captured the image on the right, a bloom off the northwest coast of Norway on the 23rd July. It is noted that blooms at this latitude are in part due to the sunlight of long summer days.
  • The MODIS instrument onboard NASA’s Aqua satellite acquired the stunning image below of the Caspian Sea on the 3rd August.

Image of the Caspian Sea, acquired on 3rd August 2017, by MODIS on NASA’s Aqua satellite. Image Courtesy of NASA/NASA Earth Observatory.

Finally as reported by the BBC, an article in Nature this week proposes that it was a takeover by ocean algae 650 million years ago which essentially kick started life on Earth as we know it.

So remember, they may be small, but algae can pack a punch!

If no-one is there when an iceberg is born, does anyone see it?

Larsen C ice Shelf including A68 iceberg. Image acquired by MODIS Aqua satellite on 12th July 2017. Image courtesy of NASA.

The titular paraphrasing of the famous falling tree in the forest riddle was well and truly answered this week, and shows just how far satellite remote sensing has come in recent years.

Last week sometime between Monday 10th July and Wednesday 12th July 2017, a huge iceberg was created by splitting off the Larsen C Ice Shelf in Antarctica. It is one of the biggest icebergs every recorded according to scientists from Project MIDAS, a UK-based Antarctic research project, who estimate its area of be 5,800 sq km and to have a weight of more a trillion tonnes. It has reduced the Larsen C ice Shelf by more than twelve percent.

The iceberg has been named A68, which is a pretty boring name for such a huge iceberg. However, icebergs are named by the US National Ice Centre and the letter comes from where the iceberg was originally sited – in this case the A represents area zero degrees to ninety degrees west covering the Bellingshausen and Weddell Seas. The number is simply the order that they are discovered, which I assume means there have been 67 previous icebergs!

After satisfying my curiosity on the iceberg names, the other element that caught our interest was the host of Earth observation satellites that captured images of either the creation, or the newly birthed, iceberg. The ones we’ve spotted so far, although there may be others, are:

  • ESA’s Sentinel-1 has been monitoring the area for the last year as an iceberg splitting from Larsen C was expected. Sentinel-1’s SAR imagery has been crucial to this monitoring as the winter clouds and polar darkness would have made optical imagery difficult to regularly collect.
  • Whilst Sentinel-1 was monitoring the area, it was actually NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) instrument onboard the Aqua satellite which confirmed the ‘birth’ on the 12th July with a false colour image at 1 km spatial resolution using band 31 which measures infrared signals. This image is at the top of the blog and the dark blue shows where the surface is warmest and lighter blue indicates a cooler surface. The new iceberg can be seen in the centre of the image.
  • Longwave infrared imagery was also captured by the NOAA/NASA Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite on July 13th.
  • Similarly, NASA also reported that Landsat 8 captured a false-colour image from its Thermal Infrared Sensor on the 12th July showing the relative warmth or coolness of the Larsen C ice shelf – with the area around the new iceberg being the warmest giving an indication of the energy involved in its creation.
  • Finally, Sentinel-3A has also got in on the thermal infrared measurement using the bands of its Sea and Land Surface Temperature Radiometer (SLSTR).
  • ESA’s Cryosat has been used to calculate the size of iceberg by using its Synthetic Aperture Interferometric Radar Altimeter (SIRAL) which measured height of the iceberg out of the water. Using this data, it has been estimated that the iceberg contains around 1.155 cubic km of ice.
  • The only optical imagery we’ve seen so far is from the DEMIOS1 satellite which is owned by Deimos Imaging, an UrtheCast company. This is from the 14th July and revealed that the giant iceberg was already breaking up into smaller pieces.

It’s clear this is a huge iceberg, so huge in fact that most news agencies don’t think that readers can comprehend its vastness, and to help they give a comparison. Some of the ones I came across to explain its vastness were:

  • Size of the US State of Delaware
  • Twice the size of Luxembourg
  • Four times the size of greater London
  • Quarter of the size of Wales – UK people will know that Wales is almost an unofficial unit of size measurement in this country!
  • Has the volume of Lake Michigan
  • Has the twice the volume of Lake Erie
  • Has the volume of the 463 million Olympic-sized swimming pools; and
  • My favourite compares its size to the A68 road in the UK, which runs from Darlington to Edinburgh.

This event shows how satellites are monitoring the planet, and the different ways we can see the world changing.

Locusts & Monkeys

Soil moisture data from the SMOS satellite and the MODIS instrument acquired between July and October 2016 were used by isardSAT and CIRAD to create this map showing areas with favourable locust swarming conditions (in red) during the November 2016 outbreak. Data courtesy of ESA. Copyright : CIRAD, SMELLS consortium.

Spatial resolution is a key characteristic in remote sensing, as we’ve previously discussed. Often the view is that you need an object to be significantly larger than the resolution to be able to see it on an image. However, this is not always the case as often satellites can identify indicators of objects that are much smaller.

We’ve previously written about satellites identifying phytoplankton in algal blooms, and recently two interesting reports have described how satellites are being used to determine the presence of locusts and monkeys!

Locusts

Desert locusts are a type of grasshopper, and whilst individually they are harmless as a swarm they can cause huge damage to populations in their paths. Between 2003 and 2005 a swarm in West Africa affected eight million people, with reported losses of 100% for cereals, 90% for legumes and 85% for pasture.

Swarms occur when certain conditions are present; namely a drought, followed by rain and vegetation growth. ESA and the UN Food and Agriculture Organization (FAO) have being working together to determine if data from the Soil Moisture and Ocean Salinity (SMOS) satellite can be used to forecast these conditions. SMOS carries a Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) instrument – a 2D interferometric L-band radiometer with 69 antenna receivers distributed on a Y-shaped deployable antenna array. It observes the ‘brightness temperature’ of the Earth, which indicates the radiation emitted from planet’s surface. It has a temporal resolution of three days and a spatial resolution of around 50 km.

By combining the SMOS soil moisture observations with data from NASA’s MODIS instrument, the team were able to downscale SMOS to 1km spatial resolution and then use this data to create maps. This approach then predicted favourable locust swarming conditions approximately 70 days ahead of the November 2016 outbreak in Mauritania, giving the potential for an early warning system.

This is interesting for us as we’re currently using soil moisture data in a project to provide an early warning system for droughts and floods.

Monkeys

Earlier this month the paper, ‘Connecting Earth Observation to High-Throughput Biodiversity Data’, was published in the journal Nature Ecology and Evolution. It describes the work of scientists from the Universities of Leicester and East Anglia who have used satellite data to help identify monkey populations that have declined through hunting.

The team have used a variety of technologies and techniques to pull together indicators of monkey distribution, including:

  • Earth observation data to map roads and human settlements.
  • Automated recordings of animal sounds to determine what species are in the area.
  • Mosquitos have been caught and analysed to determine what they have been feeding on.

Combining these various datasets provides a huge amount of information, and can be used to identify areas where monkey populations are vulnerable.

These projects demonstrate an interesting capability of satellites, which is not always recognised and understood. By using satellites to monitor certain aspects of the planet, the data can be used to infer things happening on a much smaller scale than individual pixels.

Monitoring Fires From Space

Monitoring fires from space has significant advantages when compared to on-ground activity. Not only are wider areas easier to monitor, but there are obvious safety benefits too. The different ways this can be done have been highlighted through a number of reports over the last few weeks.

VIIRS Image from 25 April 2017, of the Yucatán Peninsula showing where thermal bands have picked-up increased temperatures. Data Courtesy of NASA, NASA image by Jeff Schmaltz, LANCE/EOSDIS Rapid Response.

Firstly, NASA have released images from different instruments, on different satellites, that illustrate two ways of how satellites can monitor fires.

Acquired on the 25 April 2017, an image from the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite showed widespread fire activity across the Yucatán Peninsula in South America. The image to the right is a natural colour image and each of the red dots represents a point where the instrument’s thermal band detected temperatures higher than normal.

False colour image of the West Mims fire on Florida/Georgia boundary acquired by MODIS on 02 May 2017. Data courtesy of NASA. NASA image by Jeff Schmaltz, LANCE/EOSDIS Rapid Response.

Compare this to a wildfire on Florida-Georgia border acquired from NASA’s Aqua satellite on the 02 May 2017 using the Moderate Resolution Imaging Spectroradiometer (MODIS). On the natural colour image the fires could only be seen as smoke plumes, but on the left is the false colour image which combines infrared, near-infrared and green wavelengths. The burnt areas can be clearly seen in brown, whilst the fire itself is shown as orange.

This week it was reported that the Punjab Remote Sensing Centre in India, has been combining remote sensing, geographical information systems and Global Positioning System (GPS) data to identify the burning of crop stubble in fields; it appears that the MODIS fire products are part of contributing the satellite data. During April, 788 illegal field fires were identified through this technique and with the GPS data the authorities have been able to identify, and fine, 226 farmers for undertaking this practice.

Imaged by Sentinel-2, burnt areas, shown in shades of red and purple, in the Marantaceae forests in the north of the Republic of Congo.
Data courtesy of Copernicus/ESA. Contains modified Copernicus Sentinel data (2016), processed by ESA.

Finally, a report at the end of April from the European Space Agency described how images from Sentinel-1 and Senintel-2 have been combined to assess the amount of forest that was burnt last year in the Republic of Congo in Africa – the majority of which was in Marantaceae forests. As this area has frequent cloud cover, the optical images from Sentinel-2 were combined with the Synthetic Aperture Radar (SAR) images from Sentinel-1 that are unaffected by the weather to offer an enhanced solution.

Sentinel-1 and Sentinel-2 data detect and monitor forest fires at a finer temporal and spatial resolution than previously possible, namely 10 days and 10 m, although the temporal resolution will increase to 5 days later this year when Sentinel-2B becomes fully operational.  Through this work, it was estimated that 36 000 hectares of forest were burnt in 2016.

Given the danger presented by forest fires and wildfires, greater monitoring from space should improve fire identification and emergency responses which should potentially help save lives. This is another example of the societal benefit of satellite remote sensing.

Earth observation satellites in space in 2016

Blue Marble image of the Earth taken by the crew of Apollo 17 on Dec. 7 1972. Image Credit: NASA

Blue Marble image of the Earth taken by the crew of Apollo 17 on Dec. 7 1972.
Image Credit: NASA

Earth Observation (EO) satellites account for just over one quarter of all the operational satellites currently orbiting the Earth. As noted last week there are 1 419 operational satellites, and 374 of these have a main purpose of either EO or Earth Science.

What do Earth observation satellites do?
According to the information within the Union of Concerned Scientists database, the main purpose of the current operational EO satellites are:

  • Optical imaging for 165 satellites
  • Radar imaging for 34 satellites
  • Infrared imaging for 7 satellites
  • Meteorology for 37 satellites
  • Earth Science for 53 satellites
  • Electronic Intelligence for 47 satellites
  • 6 satellites with other purposes; and
  • 25 satellites simply list EO as their purpose

Who Controls Earth observation satellites?
There are 34 countries listed as being the main controllers of EO satellites, although there are also a number of joint and multinational satellites – such as those controlled by the European Space Agency (ESA). The USA is the leading country, singularly controlling one third of all EO satellites – plus they are joint controllers in others. Of course, the data from some of these satellites are widely shared across the world, such as Landsat, MODIS and SMAP (Soil Moisture Active Passive) missions.

The USA is followed by China with about 20%, and Japan and Russia come next with around 5% each. The UK is only listed as controller on 4 satellites all related to the DMC constellation, although we are also involved in the ESA satellites.

Who uses the EO satellites?
Of the 374 operational EO satellites, the main users are:

  • Government users with 164 satellites (44%)
  • Military users with 112 satellites (30%)
  • Commercial users with 80 satellites (21%)
  • Civil users with 18 satellites (5%)

It should be noted that some of these satellites do have multiple users.

Height and Orbits of Earth observation satellites
In terms of operational EO satellite altitudes:

  • 88% are in a Low Earth Orbit, which generally refers to altitudes of between 160 and 2 000 kilometres (99 and 1 200 miles)
  • 10% are in a geostationary circular orbit at around 35 5000 kilometres (22 200 miles)
  • The remaining 2% are described as having an elliptical orbit.

In terms of the types of orbits:

  • 218 are in a sun-synchronous orbit
  • 84 in non-polar inclined orbit
  • 16 in a polar orbit
  • 17 in other orbits including elliptical, equatorial and molniya orbit; and finally
  • 39 do not have an orbit recorded.

What next?

Our first blog of 2016 noted that this was going to be an exciting year for EO, and it is proving to be the case. We’ve already seen the launches of Sentinel-1B, Sentinel-3A, Jason-3, GaoFen3 carrying a SAR instrument and further CubeSat’s as part of Planet’s Flock imaging constellation.

The rest of the year looks equally exciting with planned launches for Sentinel-2B, Japan’s Himawari 9, India’s INsat-3DR, DigitalGlobe’s Worldview 4 and NOAA’s Geostationary Operational Environmental Satellite R-Series Program (GOES-R). We can’t wait to see all of this data in action!

Four Fantastic Forestry Applications of Remote Sensing

Landsat Images of the south-east area of Bolivia around Santa Cruz de la Sierra 27 years apart showing the changes in land use. Data courtesy of USGS/NASA.

Landsat Images of the south-east area of Bolivia around Santa Cruz de la Sierra 27 years apart showing the changes in land use. Data courtesy of USGS/NASA.

Monitoring forest biomass is essential for understanding the global carbon cycle because:

  • Forests account for around 45 % of terrestrial carbon, and deforestation accounts for 10% of greenhouse gas emissions
  • Deforestation and forest degradation release approximately identical amounts of greenhouse gases as all the world’s road traffic
  • Forests sequester significant amounts of carbon every year

The United Nations (UN) intergovernmental Reducing Emissions from Deforestation and forest Degradation in developing countries (REDD+) programme, was secured in 2013 during the 19th Conference of the Parties to the UN Framework Convention on Climate Change. It requires countries to map and monitor deforestation and forest degradation, together with developing a system of sustainable forest management. Remote sensing can play a great role in helping to deliver these requirements, and below are three fantastic remote sensing initiatives in this area.

Firstly, the Real Time System for Detection of Deforestation (DETER) gives monthly alerts on potential areas of deforestation within Amazon rainforests. It uses data from MODIS, at 250 m pixel resolution, within a semi-automated classification technique. A computer model detects changes in land use and cover such as forest clearing that are then validated by interpreters. It has been valuable helping Brazil to reduce deforestation rates by around 80% over the last decade; however, it takes two weeks to produce the output of this computer model.

Zoomed in Landsat Images of the south-east area of Bolivia around Santa Cruz de la Sierra 27 years apart showing the changes in land use. Data courtesy of USGS/NASA.

Zoomed in Landsat Images of the south-east area of Bolivia around Santa Cruz de la Sierra 27 years apart showing the changes in land use. Data courtesy of USGS/NASA.

A similar initiative is FORest Monitoring for Action (FORMA), which also use MODIS data. FORMA is fully automated computer model which combines vegetation reflectance data from MODIS, active fires from NASA’s Fire Information for Resource Management and rainfall figures, to identify potential forest clearing. Like DETER it produces alerts twice a month, although it works on tropical humid forests worldwide.

A third initiative aims to provide faster alerts for deforestation using the research by Hansen et al, published in 2013. The researchers used successive passes of the current Landsat satellites to monitor land cover, and when gaps appear between these passes it is flagged. These will be displayed on an online map, and the alerts will be available through the Word Resources Institute’s Global Forest Watch website, starting in March 2016. With the 30 m resolution of Landsat, smaller scale changes in land use can be detected than is possible for sensors such as MODIS. Whilst this is hoped to help monitor deforestation, it doesn’t actually determine it, as they could be other reasons for the tree loss and further investigation will be required. Being an optical mission, Landsat has problems seeing both through clouds and beneath the forestry canopy, and so it’s use will be limited in areas such as tropical rain forests.

Finally, one way of combat the weather and satellite canopy issue is to use radar to assess forests, and the current AfriSAR project in Gabon is doing just that – although it’s with flights and Unmanned Aerial Vehicles (UAV) rather than satellites. It began in the 2015 with overflights during the dry season, and the recent flights in February 2016 captured the rainy season. This joint ESA, Gabonese Space Agency and Gabon Agency of National Parks initiative aims of the project is to determine the amount of biomass and carbon stored in forests, by using the unique sensitivity of P-band SAR, the lowest radar frequency used in remote sensing at 432–438 MHz. NASA joined the recent February missions adding its Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) and the Land, Vegetation and Ice Sensor (LVIS) instrument, which are prototypes of sensors to be used on future NASA missions. Overall, this is giving a unique dataset on the tropical forests.

These are just four example projects of how remote sensing can contribute towards towards understanding what is happening in the world’s forests.

Reprocessing Data Challenges of Producing A Time Series

August 2009 Monthly Chlorophyll-a Composite; data courtesy of the ESA Ocean Colour Climate Change Initiative project

August 2009 Monthly Chlorophyll-a Composite; data courtesy of the ESA Ocean Colour Climate Change Initiative project

Being able to look back at how our planet has evolved over time, is one of the greatest assets of satellite remote sensing. With Landsat, you have a forty year archive to examine changes in land use and land cover. For in situ (ground based) monitoring, this is something that’s only available for a few locations, and you’ll only have data for the location you’re measuring. Landsat’s continuous archive is an amazing resource, and it is hoped that the European Union’s Copernicus programme will develop another comprehensive archive. So with all of this data, producing a time series analysis is easy isn’t it?

Well, it’s not quite that simple. There are the basic issues of different missions having different sensors, and so you need to know whether you’re comparing like with like. Although data continuity has been a strong element of Landsat, the sensors on Landsat 8 are very different to those on Landsat 1. Couple this with various positional, projection and datum corrections, and you have lots of things to think about to produce an accurate time series. However, once you’ve sorted all of these out and you’ve got your data downloaded, then everything is great isn’t it?

Well, not necessarily; you’ve still got to consider data archive reprocessing. The Space Agencies, who maintain this data, regularly reprocess satellite datasets. This means that the data you downloaded two years ago, isn’t necessarily the same data that could be downloaded today.

We faced this issue recently as NASA completed the reprocessing of the MODIS Aqua data, which began in 2014. The data from the MODIS Aqua satellite has been reprocessed seven times, whilst its twin, Terra, has been reprocessed three times.

Reprocessing the data can include changes to some, or all, of the following:

  • Update of the instrument calibration, to take account of current knowledge about sensor degradation and radiometric performance.
  • Appyling new knowledge, in terms of atmospheric correction and/or derived product algorithms.
  • Changes to parallel datasets that are used as inputs to the processing; for example, the meteorological conditions are used to aid the atmospheric correction.

Occasionally, they also change the output file format the data is provided in; and this is what has caught us out. The MODIS output file format has changed from HDF4 to NetCDF4 with the reason being that NetCDF is a more efficient, sustainable, extendable and interoperable data file format. A change we’ve known about for a long time, as it resulted from community input, but until you get the new files you can’t check and update your software.

We tend to use a lot of Open Source software, enabling our clients to carry on working with remote sensing products without having to invest in expensive software. The challenge is that it takes software provider time to catch up with the format changes. Hence, the software is unable to load the new files or the data is incorrectly read e.g., comes in upside down. Sometimes large changes, mean you may have to alter your approach and/or software.

Reprocessing is important, as it improves the overall quality of the data, but you do need to keep on top what is happening with the data to ensure that you are comparing like with like when you analyse a time series.

Ocean Colour Cubes

August 2009 Monthly Chlorophyll-a Composite; data courtesy of the ESA Ocean Colour Climate Change Initiative project

August 2009 Monthly Chlorophyll-a Composite; data courtesy of the ESA Ocean Colour Climate Change Initiative project

It’s an exciting time to be in ocean colour! A couple of weeks ago we highlighted the new US partnership using ocean colour as an early warning system for harmful freshwater algae blooms, and last week a new ocean colour CubeSat development was announced.

Ocean colour is something very close to our heart; it was the basis of Sam’s PhD and a field of research she is highly active in today. When Sam began studying her PhD, Coastal Zone Color Scanner (CZCS) was the main source of satellite ocean colour data, until it was superseded by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) that became the focus of her role at Plymouth Marine Laboratory.

Currently, there are a number ocean colour instruments in orbit:

  • NASA’s twin MODIS instruments on the Terra and Aqua satellites
  • NOAA’s Visible Infrared Imager Radiometer Suite (VIIRS)
  • China’s Medium Resolution Spectral Imager (MERSI), Chinese Ocean Colour and Temperature Scanner (COCTS) and Coastal Zone Imager (CZI) onboard several satellites
  • South Korea’s Geostationary Ocean Color Imager (GOCI)
  • India’s Ocean Colour Monitor on-board Oceansat-2

Despite having these instruments in orbit, there is very limited global ocean colour data available for research applications. This is because the Chinese data is not easily accessible outside China, Oceansat-2 data isn’t of sufficient quality for climate research and GOCI is a geostationary satellite so the data is only for a limited geographical area focussed on South Korea. With MODIS, the Terra satellite has limited ocean colour applications due to issues with its mirror and hence calibration; and recently the calibration on Aqua has also become unstable due to its age. Therefore, the ocean colour community is just left with VIIRS; and the data from this instrument has only been recently proved.

With limited good quality ocean colour data, there is significant concern over the potential loss of continuity in this valuable dataset. The next planned instrument to provide a global dataset will be OLCI onboard ESA’s Sentinel 3A, due to be launched in November 2015; with everyone having their fingers crossed that MODIS will hang on until then.

Launching a satellite takes time and money, and satellites carrying ocean colour sensors have generally been big, for example, Sentinel 3A weighs 1250 kg and MODIS 228.7 kg. This is why the project was announced last week to build two Ocean Colour CubeSats is so exciting; they are planned to weigh only 4 kg which reduces both the expense and the launch lead time.

The project, called SOCON (Sustained Ocean Observation from Nanosatellites), will see Clyde Space, from Glasgow in the UK, will build an initial two prototype SeaHawk CubeSats with HawkEye Ocean Colour Sensors, with a ground resolution of between 75 m and 150 m per pixel to be launched in early 2017. The project consortium includes the University of North Carolina, NASA’s Goddard Space Flight Centre, Hawk Institute for Space Sciences and Cloudland Instruments. The eventual aim is to have constellations of CubeSats providing a global view of both ocean and inland waters.

There are a number of other planned ocean colour satellite launches in the next ten years including following on missions such as Oceansat-3, two missions from China, GOCI 2, and a second VIIRS mission.

With new missions, new data applications and miniaturised technology, we could be entering a purple patch for ocean colour data – although purple in ocean colour usually represents a Chlorophyll-a concentration of around 0.01 mg/m3 on the standard SeaWiFS colour palette as shown on the image at the top of the page.

We’re truly excited and looking forward to research, products and services this golden age may offer.

Goodbye HICO, Hello PACE – Ocean Colour’s Satellite Symmetry

HICO™ Data, image of Hong Kong from the Oregon State University HICO Sample Image Gallery, provided by the Naval Research Laboratory

HICO™ Data, image of Hong Kong from the Oregon State University HICO Sample Image Gallery, provided by the Naval Research Laboratory

Ocean colour is the acorn from which Pixalytics eventually grew, and so we were delighted to see last week’s NASA announcement that one of their next generation ocean colour satellites is now more secure with a scheduled launched for 2022.

Unsurprisingly the term ocean colour refers to the study of the colour of the ocean, although in reality it’s a name that includes a suite of different products, with the central one for the open oceans being the concentration of phytoplankton. Ocean colour is determined by the how much of the sun’s energy the ocean scatters and absorbs, which in turn is dependent on the water itself alongside substances within the water that include phytoplankton and suspended sediments together with dissolves substances and chemicals. Phytoplankton can be used a barometer of the health of the oceans; in that phytoplankton are found where nutrient levels are high and oceans with low nutrients have little phytoplankton. Sam’s PhD involved the measurement of suspended sediment coming out of the Humber estuary back in 1995, and it’s remained an active field of her research for the last 20 years.

Satellite ocean colour remote sensing began with the launch of NASA’s Coastal Zone Colour Scanner (CZCS) on the 24th October 1978. It had six spectral bands, four of which were devoted to ocean colour, and a spatial resolution of around 800m. Despite only having an anticipated lifespan of one year, it operated until the 22nd June 1986 and has been used as a key dataset ever since. Sadly, CZCS’s demise marked the start of a decade gap in NASA’s ocean colour data archive.

Although there were some intermediate ocean colour missions, it was the launch of the Sea-viewing Wide Field-of-view (SeaWiFS) satellite that brought the next significant archive of ocean colour data. SeaWiFS had 8 spectral bands optimized for ocean colour and operated at a 1 km spatial resolution. One of Sam’s first jobs was developing a SeaWiFS data processor, and the satellite collected data until the end of its mission in December 2010.

Currently, global ocean colour data primarily comes from either NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) on-board the twin Aqua and Terra satellites, or the Visible Infrared Imaging Radiometer Suite (VIIRS) which is on a joint NOAA / NASA satellite called Suomi NPP. MODIS has 36 spectral bands and spatial resolution ranging from 250 to 1000 m; whilst VIIRS has twenty two spectral bands and a resolution of 375 to 750 m.

Until recently, there was also the ONR / NRL / NASA Hyperspectral Imager for the Coastal Ocean (HICO) mission on-board the International Space Station. It collected selected coastal region data with a spectral resolution range of 380 to 960nm and 90m spatial resolution. It was designed to collect only one scene per orbit and has acquired over 10,000 such scenes since its launch. However, unfortunately it suffered during a solar storm in September 2014. Its retirement was officially announced a few days ago with the confirmation that it wasn’t possible to repair the damage.

In the same week we wave goodbye to HICO, NASA announced the 2022 launch of the Pre-Aerosol and ocean Ecosystem (PACE) mission in a form of ocean colour symmetry. PACE is part of the next generation of ocean colour satellites, and it’s intended to have an ocean ecosystem spectrometer/radiometer called built by NASA’s Goddard Space Flight Centre and will measure spectral wavebands from ultraviolet to near infrared. It will also have an aerosol/cloud polarimeter to help improve our understanding of the flow, and role, of aerosols in the environment.

PACE will be preceded by several other missions with an ocean colour focus including the European Sentinel-3 mission within the next year; it will have an Ocean and Land Colour Instrument with 21 spectral bands and 300 m spatial resolution, and will be building on Envisat’s Medium Resolution Imaging Spectrometer (MERIS) instrument. Sentinel-3 will also carry a Sea and Land Surface Temperature Radiometer and a polarimeter for mapping aerosols and clouds. It should help to significantly improve the quality of the ocean colour data by supporting the improvement of atmospheric correction.

Knowledge the global phytoplankton biomass is critical to understanding the health of the oceans, which in turn impacts on the planet’s carbon cycle and in turn affects the evolution of our planet’s climate. A continuous ocean colour time series data is critical to this, and so we are already looking forward to the data from Sentinel-3 and PACE.