Monitoring Water Quality from Space

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

Two projects using Earth Observation (EO) data to monitor water quality caught our eye recently. As we’re in process of developing two water quality products for our own online portal, we’re interested in what everyone else is doing!

At the end of January UNESCO’s International Hydrological Programme launched a tool to monitor global water quality. The International Initiative on Water Quality (IIWQ) World Water Quality Portal, built by EOMAP, provides:

  • turbidity and sedimentation distribution
  • chlorophyll-a concentration
  • Harmful Algal Blooms indicator
  • organic absorption
  • surface temperature

Based on optical data from Landsat and Sentinel-2 it can provide global surface water mosaics at 90 m spatial resolution, alongside 30 m resolution for seven pilot river basins.  The portal was launched in Paris at the “Water Quality Monitoring using Earth Observation and Satellite-based Information” meeting and was accompanied by an exhibition on “Water Quality from the Space – Mesmerizing Images of Earth Observation”.

The tool, which can be found here, focuses on providing colour visualizations of the data alongside data legends to help make it as easy as possible to use. It is hoped that this will help inform and educate policy makers, water professionals and the wider public about the value of using satellite data from monitoring water resources.

A second interesting project, albeit on a smaller scale, was announced last week which is going to use Sentinel-2 imagery to monitor water quality in Scottish Lochs. Dr Claire Neil, from the University of Stirling, is leading the project and will be working with Scottish Environment Protection Agency. It will use reflectance measures to estimate the chlorophyll-a concentrations to help identify algal blooms and other contaminants in the waters. The project will offer an alternative approach to the current water quality monitoring, which uses sampling close to the water’s edge.

An interesting feature of the project, particularly for us, is the intention to focus on developing this work into an operational capability for SEPA to enable them to improve their approach to assessing water quality.

This transition from a ‘good idea’ into an operational product that will be used, and therefore purchased, by end users is what all EO companies are looking for and we’re not different. Our Pixalytics Portal which we discussed a couple of weeks ago is one of the ways we are trying to move in that direction. We have two water quality monitoring products on it:

  • Open Ocean Water Quality product extracts time-series data from a variety of 4 km resolution satellite datasets from NASA, giving an overview what is happening in the water without the need to download a lot of data.
  • Planning for Coastal Airborne Lidar Surveys product provides an assessment of the penetration depth of a Lidar laser beam, from an airborne survey system, within coastal waters based on the turbidity of the water. This ensures that companies who plan overflights can have confidence in how far their Lidar will see.

We’re just at the starting point in productizing the services we offer, and so it is always good to see how others are approaching the similar problem!

Celebrating Landsat & the Winter Olympics

First Landsat image acquired in 2013 showing area around Fort Collins, Colorado. Data courtesy of NASA/USGS.

The Landsat programme achieved a couple of significant milestones over the last two weeks. Firstly, the 11th February marked the five year anniversary of the launch of Landsat 8 which took place at the Vandenberg Air Force Base, California, in 2013. The image to the right is the first one acquired by Landsat 8 and shows the area around Fort Collins, Colorado with the Horsetooth Reservoir very clear left of centre.

This anniversary is an interesting one because Landsat 8 was only designed for an operational life of five years. Obviously it has already exceeded this and these planned lifespans are very conservative. More often the amount of fuel on board is a more relevant assessment for lifespan and for Landsat 8 the initial assessment was a 10 year lifespan. However, even this tends to be a conservative estimate. As an example, nineteen years ago Landsat 7 was launched with similar planned operational lifespans. It is still working today, although there have been some degradation issues, and IT achieved its own significant milestone on the 1st February when it completed its 100,000th orbit of the Earth.

Landsat 8 is in a sun-synchronous orbit at an altitude of 705 km, circles the Earth every 98.9 minutes and in the last five years has undertaken over 26,500 orbits according to NASA who have produced a short celebratory video.

It has two main instruments, an Operational Land Imager (OLI) and the Thermal Infrared Sensor (TIRS), which together measure eleven different spectral bands. The TIRS has two thermal bands which are used for sensing temperature, whereas the OLI measures nine spectral bands:

  • Three visible light bands that approximate red, green and blue
  • One near infrared band
  • Two shortwave infrared bands
  • Panchromatic band with a higher spatial resolution
  • The two final bands focus on coastal aerosols and cirrus clouds.

With the exception of the highest polar latitudes, Landsat 8 acquires images of the whole Earth every 16 days which has meant it has acquired over 1.1 million images of the Earth that accounts for 16 percent of all the data in the Landsat multi-mission archive.

Landsat 8 image of Pyeongchang, South Korea, which is hosting the 2018 Winter Olympics. Data acquired 11th February 2018. Data courtesy of NASA/USGS.

The image to the left is the Pyeongchang region of South Korea where the Winter Olympics are currently taking place acquired by Landsat on its five year anniversary on the 11th February. Pyeongchang is in the north west of South Korea in the TaeBaek Mountains just over one hundred miles from the capital, Seoul. The left area of the image shows the mountain range where the skiing, biathlon, ski jumping, bobsled, luge and skeleton events take place and to the right is the coastal city of Gangneung, where the ice hockey, curling, speed skating and figure skating are taking place.

With its forty-five year archive, Landsat offers the longest continuous dataset of Earth observations and is critical to researchers and scientists. Landsat 9 is planned to be launched in 2020 and Landsat 10 is already being discussed.

Congratulations to Landsat 7 and 8, and we look forward to many more milestones in the future.

EO Market Is a-Changin’

Artist's rendition of a satellite - paulfleet/123RF Stock Photo

Artist’s rendition of a satellite – paulfleet/123RF Stock Photo

Historically, if you wanted satellite Earth Observation (EO) data your first port of call was usually NASA, or NOAA for meteorological data, and more recently you’d look at the European Union’s Copernicus programme. Data from commercial operators were often only sought if the free-to-access data from these suppliers did not meet your needs.
However, to quote Bob Dylan, The Times They Are a-Changin’. NASA, NOAA and Copernicus are buying, or intending to buy, data from commercial operators.

However, as with many activities there are often precedents. For example, the SeaWiFS mission was built to NASA’s specifications and launched in 1997. It was owned by the commercial organisation Orbital Sciences Corporation and NASA conducted a ‘data-buy’. They’ve moved back in this direction last month as NASA issued a Request for Information for US companies interested in participating in the Earth Observations from Private Sector Small Satellite Constellations Pilot. The aim of this programme is to identify commercial organisations collecting EO data relating to Essential Climate Variables (ECV), and then to evaluate whether this would be a cost effective approach to gathering data rather than, or alongside, launching their own satellites.

To interest NASA the companies need to have a constellation of at least three satellites in a non-geostationary orbits, and the ECV dataset will need to include details of both instrument calibration and processing techniques used. Initially, NASA plans to provide this data to researchers to undertake the evaluation. According to Space News, 11 responses to the request had been received. Discussions will take place with responding companies over the next month and it’s anticipated orders will be placed in March 2018.

NOAA is another US agency looking to the private small satellite sector through their Commercial Weather Data pilot programme. To supplement their own data collections they’ve already purchased GPS radio occupation data and are planning to buy both microwave sounding and radiometry data.

Not everyone is aware that the Copernicus Programme also purchases data from commercial sources as part of its Contributing Missions Programme. Essentially, if data is not available for any reason from the Sentinel satellites, then the equivalent data is sought from one of 30 current contributing missions which include other international partners such as NASA, but also commercial providers.

Whilst part of the drive behind this approach is to ensure data continuity, in the US the backdrop has a more long term concern with President Trump’s intention to move NASA away from EO to focus efforts on deep space exploration. It’s not been fully confirmed yet, but there is due to be a Congress budget discussion later this week and if approved it could mean the loss of the following four NASA missions:

• Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) satellite
• Orbiting Carbon Observatory-3 (OCO-3)
• Climate Absolute Radiance and Refractivity Observatory (CLARREO) Pathfinder
• Deep Space Climate Observatory (DSCOVR)

Whilst buying data from commercial providers may offer opportunities, it also has a number of challenges including how to buy this whilst maintaining their commitment to free-to-access data, and with the shorter lifespans of small satellites the increased pressure on calibration and validation work.

It’s clear that things are evolving in the EO market and the private sector is coming much more to the fore as a primary data supplier to researchers, national and international bodies.

Four Key Earth Observation Trends For 2018

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

This week we’re looking at this year’s key trends in Earth Observation (EO) that you need to know.

Rise of the Data Buckets!
EO data is big! Anyone who has tried to process EO data knows the issues of downloading and storing large files, and as more and more data becomes available these challenges will grow. Amazon recognised this issue and set up Amazon Web Services which automatically downloads all freely available data such as Copernicus and Landsat, offering people who want to process data a platform where they don’t have to download the data – for a price!

The European Commission also picked up on this and awarded four commercial contracts at the end of last year to establish Copernicus Data and Information Access Services (DIAS) which will offer scalable processing platforms for the development of value-added products and services.

The four successful DIAS consortiums are led by Serco Europe, Creotech Instruments, ATOS Integration & Airbus Defence and Space respectively, and a fifth DIAS is planned to be established by EUMETSAT. It’s hoped this will kick-start the greater use and exploitation of Copernicus data.

Continued Growth of Data
There are some exciting EO launches planned this year continuing to increase the amount of data available. Earlier this week China launched the last two satellites of the high resolution optical SuperView constellation. In addition, some of the key larger satellites going into orbit this year include:

  • ESA’s Sentinel-3B and its Aeolus wind mission.
  • NASA’s Gravity Recovery and Climate Experiment Follow-on (GRACE-FO) and the Ice, Cloud and land Elevation Satellite (ICESat-2).
  • Japan’s Advanced Satellite with New system Architecture for Observation (ASNARO 2) which is x-band SAR radar satellite with a 1 m ground resolution.
  • NOAA’s GOES-S is the second of four upgraded weather observatories.

In addition, as we described last week, cubesats will continue to have regular launches. We are still a long way from the high watershed of EO data!

SaaS Will Become The Norm
The rise of the data buckets will encourage the Software-as-a-service (SaaS) approach to EO to become the norm. Companies will develop products and services and offer them to customers on a platform via the internet, rather than the historic bespoke application approach. For companies this will be a more effective way of using their resources and will allow them to better leverage products and services. For the customers, it will enable them greater use EO and geospatial data without the need for expert knowledge.

Pixalytics is due to launch its own Product Portal at the Data.Space 2018 conference at the end of this month.

Artificial Intelligence (AI)
AI is becoming more and more important to EO. Part of this is the natural development of AI, however certain EO tasks are far more suited to AI. For example, change detection, identification of new artefacts in imagery, etc. These aspects have a base image and looking for differences, computers can do this much quicker than any human researcher. Although, it’s also true that humans can see artefacts much more easily than you can program a computer to identify them. Therefore, these AI applications are strongly dependent on training datasets created by humans.

However, things are now moving beyond these simple AI tasks and it’s becoming an integral part of EO products and services. For example, last year Microsoft launched their AI for Earth programme, support by a $50 m investment, which will deploy their cloud computing, AI and other technology to researchers around the world to help develop new solutions for the agriculture, biodiversity, climate change, and water challenges on the planet.

These are a snapshot of our view of the key trends. What do you think? Have we missed anything? Let us know.

Unintended Consequences of Energy Saving

Black Marble 2016: Composite global map created from data acquired by VIIRS in 2016. Image courtesy of NASA/NASA’s Earth Observatory.

Last month a report in Science Advances got a lot of publicity as it described the increase in global light pollution following research using satellite data. Even more interesting was the fact that one of the key drivers, although not the only one, was the switch to LED lights which have mainly being bought in due to their increased energy efficiency.

Recently there has been a lot of night-time imagery released as photographs taken from the International Space Station, and we’ve used them in our blogs. However, night time imagery has also been collected from the uncalibrated Operational Linescan System (OLS) on the Defense Meteorological Satellite Program (DMSP) satellites for a number of years. This was followed by the Suomi National Polar-orbiting Partnership (Suomi NPP) research mission in 2011 that carries the Visible Infrared Imaging Radiometer Suite (VIIRS) which had a planned life expectancy of around five years, however it is still in orbit and continues to collect data. Much more recently, on the 18th November 2017, a second VIIRS instrument was launched aboard the NOAA-20 satellite (previously called JPSS-1).

The role of LED lights in the increase in light pollution was described in detail in the paper ‘Artificially lit surface of Earth at night increasing in radiance and extent’ by Kyba et al which was published on the 22nd November 2017. The paper was based on satellite data collected between 2012 and 2016 from the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi National Polar-orbiting Partnership (Suomi NPP) satellite and one of the key drivers behind the new research is that VIIRS offered the first calibrated and georeferenced night time radiance global dataset. Within the 22 spectral bands the instrument measures is a day/night panchromatic band (DNB). This band has a 750 m spatial resolution and operates on a whiskbroom approach with a swath of approximately 3,000 km which means it provides global coverage twice a day, visiting every location at 1:30 pm and 1:30 am (local time).

The team from the GFZ German Research Centre for Geosciences who did the research concluded that outdoor light pollution has increased by 11% over 5 years. However, for us, the really interesting part was that new LED lights are linked to this increase in light pollution.

Over the last decade within the UK, a lot of local Councils have switched to using LED streetlights mainly due to the energy, and associated cost, savings. However, there was also a message that this would reduce light pollution as they would direct light downwards and reduce nightglow. This is coupled with the fact that businesses and consumers have also been pushed to move towards this type of light for the same reasons. This was brought home to us recently as a firm opposite our home installed new outside LED lights. It has made a significant different to the amount of light in our room and even in the middle of the night it is never completely black.

What the research team found by comparing VIIRS images from 2012 and 2016 was that:

  • The lower cost of LED lights has actually led to more lights going up, mainly on the outskirts of towns and cities. A 2010 paper by Tsao et al published in Physics Today indicated that we tend to purchase as much artificial light as possible for around 0.7% of GDP and so as lighting becomes cheaper, the quantity increases.
  • Flat composite global map created from data acquired by VIIRS in 2016. Image courtesy of NASA/NASA’s Earth Observatory.

    There has been a shift in the spectra of artificial light within cities from the yellow/orange of the old streetlights to the white of LED’s.

  • The majority of countries of the world had seen an increase in light pollution. Although, perhaps surprisingly some of the world’s brightest nations such the US, UK, Germany, Netherlands, Spain and Italy had stayed stable; which may suggest there is a point of saturation of outdoor lighting. The only countries that had less light pollution were areas of conflict or whether there was issue with the data, such as Australia where there were significant wildfires when the first data was collected.

Light pollution has a negative impact on flora and fauna, particularly nocturnal wildlife, and there is increasing evidence that it is also negative for humans. This is an example of why we have to be so careful with the concept of cause and effect. Decisions made for improved energy efficiency look to have had unintended consequences for light pollution.

5 Signs You Work In Earth Observation

Sentinel-2A image of UK south east coastline, acquired on 4th September 2017. Data courtesy of ESA/Copernicus.

Do you recognise yourself in any these five signs? if so, you’re definitely working in the Earth observation industry.

  1. You have a favourite satellite or instrument, or image search tool.
  2. When a satellite image appears on television, you tell everyone in the room which satellite/sensor it came from.
  3. You’ve got an irrational hatred for clouds (unless you’re working on clouds or using radar images).
  4. Anything space related happens and your family asks whether you’re involved with it, and thinks you know everyone who works at NASA or ESA.
  5. Your first reaction to seeing an interesting location isn’t that you should plan to go there. Instead, you wonder whether it would make a good satellite image.

We tick all of these signs at Pixalytics! Last week we suffered from number five when we saw a snippet from the season finale of the UK TV programme ‘Liar’. It wasn’t a programme we’d watched, but as we caught an atmospheric panning shot of the location, and only one thought when through our minds, ‘That would make a great satellite image!’

It was a stunning shot of a marshland with water interwoven between islands. Without knowing anything about the programme, we were expecting it to have been filmed in a far flung Nordic location. Following a bit of impromptu googling we were surprised to discover it was actually Tollesbury on the Essex coast in the UK. It also turns out that we were late to the party on the discovery of the programme and the location.

Sentinel-2A image of Mersea Island and surrounding area, acquired on 4th September 2017. Data courtesy of ESA/Copernicus.

The image on the right shows Mersea Island, which has brown saltmarshes above it within the adjacent inlets of the Blackwater Estuary. To the left of the island is the village of Tollesbury and the Tollesbury marina, which is located within the saltmarshes. This area is the largest of the saltmarshes of Essex, but only the fifth largest of the UK. They play a key role in flood protection and can reduce the height of damaging waves in storm surge conditions by 20%. However, they are disappearing due to sea erosion that’s caused a sixty percent reduction in the last 20 years.

The image itself is a zoomed in pseudo-true-colour composite at 10 m spatial resolution using data acquired by Sentinel-2A on the 4th September 2017 – a surprisingly cloud free day for the UK. The full Sentinel-2 image can be seen at the top of the blog.

As often happens when we look in detail at satellite images, something catches our eye. This time it was the three bluish looking strips just above Mersea island. These are the 82,944 solar panels which make up Langenhoe Solar Farm, and have the capacity to generate 21.15 MW of solar power.

So how many of you recognise our signs of working in Earth observation? Any you think we’ve missed? Get in touch, let us know!

Inspiring the Next Generation of EO Scientists

Artist's rendition of a satellite - 3dsculptor/123RF Stock Photo

Artist’s rendition of a satellite – 3dsculptor/123RF Stock Photo

Last week, whilst Europe’s Earth Observation (EO) community was focussed on the successful launch of Sentinel-5P, over in America Tuesday 10th October was Earth Observation Day!

This annual event is co-ordinated by AmericaView, a non-profit organisation, whose aim to advance the widespread use of remote sensing data and technology through education and outreach, workforce development, applied research, and technology transfer to the public and private sectors.

Earth Observation Day is a Science, Technology, Engineering, and Mathematics (STEM) event celebrating the Landsat mission and its forty-five year archive of imagery. Using satellite imagery provides valuable experience for children in maths and sciences, together with introducing subjects such as land cover, food production, hydrology, habitats, local climate and spatial thinking. The AmericaView website contains a wealth of EO materials available for teachers to use, from fun puzzles and games through to a variety of remote sensing tutorials. Even more impressive is that the event links schools to local scientists in remote sensing and geospatial technologies. These scientists provide support to teachers including giving talks, helping design lessons or being available to answer student’s questions.

This is a fantastic event by AmericaView, supporting by wonderful resources and remote sensing specialists. We first wrote about this three years ago, and thought the UK would benefit from something similar. We still do. The UK Space Agency recently had an opportunity for organisations interested in providing education and outreach activities to support EO, satellite launch programme or the James Webb Space Telescope. It will be interesting to see what the successful candidates come up with.

At Pixalytics we’re passionate about educating and inspiring the next generation of EO scientists. For example, we regularly support the Remote Sensing and Photogrammetry Society’s Wavelength conference for students and early career scientists; and sponsored the Best Early-Career Researcher prize at this year’s GISRUK Conference. We’re also involved with two exciting events at Plymouth’s Marine Biological Association, a Young Marine Biologists (YMB) Summit for 12-18 year olds at the end of this month and their 2018 Postgraduate conference.

Why is this important?
The space industry, and the EO sector, is continuing to grow. According to Euroconsult’s ‘Satellites to Be Built & Launched by 2026 – I know this is another of the expensive reports we highlighted recently – there will be around 3,000 satellites with a mass above 50 kg launched in the next decade – of which around half are anticipated as being used for EO or communication purposes. This almost doubles the number of satellites launched in the last ten years and doesn’t include the increasing number of nano and cubesats going up.

Alongside the number of satellites, technological developments mean that the amount of EO data available is increasing almost exponentially. For example, earlier this month World View successfully completed multi-day flight of its Stratollite™ service, which uses high-altitude balloons coupled with the ability to steer within stratospheric winds. They can carry a variety of sensors, a mega-pixel camera was on the recent flight, offering an alternative vehicle for collecting EO data.

Therefore, we need a future EO workforce who are excited, and inspired, by the possibilities and who will take this data and do fantastic things with it.

To find that workforce we need to shout about our exciting industry and make sure everyone knows about the career opportunities available.

Flip-Sides of Soil Moisture

Soil Moisture changes between 19th and 25th August around Houston, Texas due to rainfall from Hurricane Harvey. Courtesy of NASA Earth Observatory image by Joshua Stevens, using soil moisture data courtesy of JPL and the SMAP science team.

Soil moisture is an interesting measurement as it can be used to monitor two diametrically opposed conditions, namely floods and droughts. This was highlighted last week by maps produced from satellite data for the USA and Italy respectively. These caught our attention because soil moisture gets discussed on a daily basis in the office, due to its involvement in a project we’re working on in Uganda.

Soil moisture can have a variety of meanings depending on the context. For this blog we’re using soil moisture to describe the amount of water held in spaces between the soil in the top few centimetres of the ground. Data is collected by radar satellites which measure microwaves reflected or emitted by the Earth’s surface. The intensity of the signal depends on the amount of water in the soil, enabling a soil moisture content to be calculated.

You can’t have failed to notice the devastating floods that have occurred recently in South Asia – particularly India, Nepal and Bangladesh – and in the USA. The South Asia floods were caused by monsoon rains, whilst the floods in Texas emanated from Hurricane Harvey.

Soil moisture measurements can be used to show the change in soil saturation. NASA Earth Observatory produced the map at the top of the blogs shows the change in soil moisture between the 19th and 25th August around Houston, Texas. The data is based on measurements acquired by the Soil Moisture Active Passive (SMAP) satellite, which uses a radiometer to measure soil moisture in the top 5 centimetres of the ground with a spatial resolution of around 9 km. On the map itself the size of each of the hexagons shows how much the level of soil moisture changed and the colour represents how saturated the soil is.

These readings have identified that soil moisture levels got as high as 60% in the immediate aftermath of the rainfall, partly due to the ferocity of the rain, which prevented the water from seeping down into the soil and so it instead remained at the surface.

Soil moisture in Italy during early August 2017. The data were compiled by ESA’s Soil Moisture CCI project. Data couresy of ESA. Copyright: C3S/ECMWF/TU Wien/VanderSat/EODC/AWST/Soil Moisture CCI

By contrast, Italy has been suffering a summer of drought and hot days. This year parts of the country have not seen rain for months and the temperature has regularly topped one hundred degrees Fahrenheit – Rome, which has seventy percent less rainfall than normal, is planning to reduce water pressure at night for conservation efforts.

This has obviously caused an impact on the ground, and again a soil moisture map has been produced which demonstrates this. This time the data was come from the ESA’s Soil Moisture Climate Change Initiative project using soil moisture data from a variety of satellite instruments. The dataset was developed by the Vienna University of Technology with the Dutch company VanderSat B.V.

The map shows the soil moisture levels in Italy from the early part of last month, with the more red the areas, the lower the soil moisture content.

Soil moisture is a fascinating measurement that can provide insights into ground conditions whether the rain is falling a little or a lot.

It plays an important role in the development of weather patterns and the production of precipitation, and is crucial to understanding both the water and carbon cycles that impact our weather and climate.

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!