Differences Between Optical & Radar Satellite Data

Ankgor Wat, Cambodia. Sentinel-2A image courtesy of ESA.

Ankgor Wat, Cambodia. Sentinel-2A image courtesy of ESA.

The two main types of satellite data are optical and radar used in remote sensing. We’re going to take a closer look at each type using the Ankgor Wat site in Cambodia, which was the location of the competition we ran on last week’s blog as part of World Space Week. We had lots of entries, and thanks to everyone who took part!

Constructed in the 12th Century, Ankgor Wat is a temple complex and the largest religious monument in the world. It lies 5.5 kilometres north of the modern town of Siem Reap and is popular with the remote sensing community due to its distinctive features. The site is surrounded by a 190m-wide moat, forming a 1.5km by 1.3km border around the temples and forested areas.

Optical Image
The picture at the top, which was used for the competition, is an optical image taken by a Multi-Spectral Imager (MSI) carried aboard ESA’s Sentinel-2A satellite. Optical data includes the visible wavebands and therefore can produce images, like this one, which is similar to how the human eye sees the world.

The green square in the centre of the image is the moat surrounding the temple complex; on the east side is Ta Kou Entrance, and the west side is the sandstone causeway which leads to the Angkor Wat gateway. The temples can be clearly seen in the centre of the moat, together with some of the paths through the forest within the complex.

To the south-east are the outskirts of Siem Reap, and the square moat of Angkor Thom can be seen just above the site. To the right are large forested areas and to the left are a variety of fields.
In addition to the three visible bands at 10 m resolution, Sentinel-2A also has:

  • A near-infrared band at 10 m resolution,
  • Six shortwave-infrared bands at 20 m resolution, and
  • Three atmospheric correction bands at 60 m resolution.

Radar Image
As a comparison we’ve produced this image from the twin Sentinel-1 satellites using the C-Band Synthetic Aperture Radar (SAR) instrument they carry aboard. This has a spatial resolution of 20 m, and so we’ve not zoomed as much as with the optical data; in addition, radar data is noisy which can be distracting.

Angkor Wat, Cambodia. SAR image from Sentinel-1 courtesy of ESA.

Angkor Wat, Cambodia. SAR image from Sentinel-1 courtesy of ESA.

The biggest advantage of radar data over optical data is that it is not affected by weather conditions and can see through clouds, and to some degree vegetation. This coloured Sentinel-1 SAR image is produced by showing the two polarisations (VV and VH i.e. vertical polarisation send for the radar signal and vertical or horizontal receive) alongside a ratio of them as red, green and blue.

Angkor Wat is shown just below centre, with its wide moat, and other archaeological structures surrounding it to the west, north and east. The variety of different landscape features around Angkor Wat show up more clearly in this image. The light pink to the south is the Cambodian city of Siem Reap with roads appearing as lines and an airport visible below the West Baray reservoir, which also dates from the Khmer civilization. The flatter ground that includes fields are purple, and the land with significant tree cover is shown as pale green.

Conclusion
The different types of satellite data have different uses, and different drawbacks. Optical imagery is great if you want to see the world as the human eye does, but radar imagery offers better options when the site can be cloudy and where you want an emphasis on the roughness of the surfaces.

The Road To Success….

Danube river crossing The Great Romanian Flood Plain. Image acquired by Sentinel-2A on the 3rd December 2015. Data courtesy of ESA.

Danube river crossing The Great Romanian Flood Plain. Image acquired by Sentinel-2A on the 3rd December 2015. Data courtesy of ESA.

‘On the road, you will face many stumbling blocks, twists, and turns… You may never know how far the road will take you.’ **

In my case, the road brought me to Plymouth, a city on the south coast of Devon, England, a magical place with great history and outstanding views.

What I am doing here? Well, I am pursuing my dream of becoming a GIS and Remote Sensing Specialist by doing an internship through the Erasmus + programme at a local company called Pixalytics. My mentor is Dr. Samantha Lavender, is a great professional with vast experience in this field, She is also the Chairman of the British Association of Remote Sensing Companies and former Chairman of the Remote Sensing & Photogrammetry Society. For me, this is about more than just getting a grade, earning credit, or making money; this is an opportunity to learn, ask questions, and impress with my eagerness.

Finding this internship was easy for me. With a short search on Google I found this Pixalytics blog, where a previous student here had posted her impressions and thoughts on the company. I immediately said “This worth trying!” In the next moment I opened my email started writing, I sent wrote emails to multiple addresses, to make sure my message reached the target. After just two days, I received an answer from Mr. Andrew Lavender and it was positive!

I was very happy and because I knew the departure papers would take over a month to be completed, I immediately started doing them. All of this happened at the end of September. After my papers were done, I bought my flight ticket to Luton Airport, then a bus to London and then onto Plymouth. I arrived on December 5th and so, like the previous student, here I am posting my own impressions and thoughts on the Pixalytics blog page.

My first day at Pixalytics started pretty badly, I got lost and arrived a little late. I now remind myself each morning to turn left, not right, when I get off the bus. I got a short introduction to the building where the company is located, and my office for the next three months, which by the way looks very good. The office has a professional, but relaxed, atmosphere and I soon started working, one of my first tasks being the downloading of Sentinel-2A data, which proved a very difficult one due to slow data speeds and functionality of the ESA Data Hub.

Over the next three months, I am expecting to assist Pixalytics in developing their agritech products, explore the potential of Sentinel-2A data and I will be doing my own research into Urban Sprawl in Romania. I am hoping to have the opportunity to present my research at a conference during my placement.

It has been over a week now since I came to Plymouth and I feel great, working at Pixalytics is a great opportunity for my career and I will take full advantage of this. I strongly recommend all students who want to burst their work experience and who want to see what it is like to be in a professional business environment, to search for Erasmus+ placement offers as I did. You will not regret it!

Blog written by Catalin Cimpianu

** Quote is by Tony Hassini, from ‘The Road To Success’

Sentinel-2A Data Released Into The Wild

False Colour Image of Qingdao, China, acquired by Sentinel-2A on the 21st August 2015. Data courtesy of ESA.

False Colour Image of Qingdao, China, acquired by Sentinel-2A on the 21st August 2015. Data courtesy of ESA.

Sentinel -2A is already producing some fantastic images, and last week ESA announced the availability of Sentinel-2A orthorectified products in the Sentinel Data Hub. This will enable Sentinel-2 data to be accessed more widely, although as we found out this week there are still a few teething problems to sort out.

At the top of the blog is a stunning image of the Chinese city of Qingdao, in the eastern Shangdong province. The false colour image shows the city of Qingdao and the surrounding area with the centre dominated by Jiaozhou Bay, which is natural inlet to the Yellow Sea. The bay is 32 km long and 27 km wide, and generally has a depth of around ten to fifteen metres; although there are deeper dredged channels to allow larger ships to enter the local ports. The bay itself has decreased by around 35% since 1928, due to urban and industrial growth in the area.

Jiaozhou Bay Bridge a sub-set of a false colour image of Qingdao, China, acquired by Sentinel-2A on the 21st August 2015. Data courtesy of ESA.

Jiaozhou Bay Bridge a sub-set of a false colour image of Qingdao, China, acquired by Sentinel-2A on the 21st August 2015. Data courtesy of ESA.

There is a tenuous linguistic link between Plymouth, where Pixalytics is based, and Qingdao. Plymouth is branded as Britain’s Ocean City and Qingdao is home to the Ocean University of China. Qingdao does however, have a much greater claim to fame. It is home to the World’s Longest Bridge. The Jiaozhou Bay Bridge is 42 km long and transects the bay. It is clearly visible on the satellite image, although you might not be able to see it on the thumbnail image at the top of the blog. Therefore, if you look at the subset to the right, you should be able to see bridge clearly and boats on the bay.

Now Sentinel-2A data has been released into the Sentinel Data Hub, images like this are waiting for everyone in the world to discover. We’ve been testing Sentinel-2A data for a few months already, as were part of the community who gave feedback to ESA on the quality of the data. Sentinel-2A carries a Multispectral Imager (MSI) that has 13 spectral bands with 4 visible and near infra-red spectral bands with a spatial resolution of 10 m, 6 short wave infrared spectral bands with a spatial resolution of 20 m and 3 atmospheric correction bands with a spatial resolution of 60 m. When the identical Sentinel-2B is launched in late 2016, the pair will offer a revisit time of only 5 days.

The data from Sentinel-2A forms part of the Copernicus program and is freely available to use, as such it is bound to be very popular. So popular in fact, we found it difficult to get on the Data Hub this week, with slow data speeds and a few elements of the functionality not working efficiently. Although, we’re sure that these will be resolved quickly. Also, there are user guides and tutorials available on the website to help people use the data hub.

The Sentinel-2A data release, following on from the microwave data from Sentinel-1, is a watershed moment for Earth Observation companies, given their spatial resolution, revisit time and free availability, they offer a unique opportunity to develop satellite data services. We’re intending to use this data, are you?

Sentinel-2A dips its toe into the water

Detailed image of algal bloom in the Baltic Sea acquired by Sentinel-2A on 7 August 2015. Data courtesy of Copernicus Sentinel data (2015)/ESA.

Detailed image of algal bloom in the Baltic Sea acquired by Sentinel-2A on 7 August 2015. Data courtesy of Copernicus Sentinel data (2015)/ESA.

With spectacular images of an algal bloom in the Baltic Sea, ESA’s Sentinel-2A has announced its arrival to the ocean colour community. As we highlighted an earlier blog, Sentinel-2A was launched in June predominately as a land monitoring mission. However, given it offers higher resolution data than other current marine focussed missions; it was always expected to dip it’s toe into ocean colour. And what a toe it has dipped!

The images show a huge bloom of cyanobacteria in the Baltic Sea, with the blue-green swirls of eddies and currents. The image at the top of the blog shows the detail of the surface floating bloom caught in the currents, and there is a ship making its way through the bloom with its wake producing a straight black line as deeper waters are brought to the surface.

Algal bloom in the Baltic Sea acquired by Sentinel-2A on 7 August 2015. Data courtesy of Copernicus Sentinel data (2015)/ESA.

Algal bloom in the Baltic Sea acquired by Sentinel-2A on 7 August 2015. Data courtesy of Copernicus Sentinel data (2015)/ESA.

To the right is a wider view of the bloom within the Baltic Sea. The images were acquired on the 7th August using the Multispectral Imager, which has 13 spectral bands and the visible, which were used here, have a spatial resolution of 10 m.

The Baltic Sea has long suffered from poor water quality and in 1974 it became the first entire sea to be subject to measures to prevent pollution, with the signing of the Helsinki Convention on the Protection of the Marine Environment of the Baltic Sea Area. Originally signed by the Baltic coastal countries, a revised version was signed by the majority of European countries in 1992. This convention came into force into force on the 17th January 2000 and is overseen by the Helsinki Commission – Baltic Marine Environment Protection Commission – also known as HELCOM. The convention aims to protect the Baltic Sea area from harmful substances from land based sources, ships, incineration, dumping and from the exploitation of the seabed.

Despite the international agreements, the ecosystems of the Baltic Sea are still threatened by overfishing, marine and chemical pollution. However, the twin threats that cause the area to suffer from algal blooms are warm temperatures and excessive levels of nutrients, such as phosphorus and nitrogen. This not only contributes towards the algal blooms, but the Baltic Sea is also home to seven of the world’s ten largest marine dead zones due to the low levels of oxygen in the water, which prevent marine life from thriving.

These images certainly whet the appetite of marine remote sensors, who also have Sentinel-3 to look forward to later this year. That mission will focus on sea-surface topography, sea surface temperature and ocean colour, and is due to the launched in the last few months of 2015. It’s an exciting time to be monitoring and researching the world’s oceans!

Sentinel-2A Ready To Start Its Watch

Integration of the Vega VV05, carrying Sentinel-2A, in the launcher assembly area at Europe's Spaceport in Kourou, French Guiana, on 11 June 2015. Image courtesy of ESA–M. Pedoussaut, 2015

Integration of the Vega VV05, carrying Sentinel-2A, in the launcher assembly area at Europe’s Spaceport in Kourou, French Guiana, on 11 June 2015.
Image courtesy of ESA–M. Pedoussaut, 2015

Sentinel-2A is due to be launched next Tuesday, 23rd June, from French Guiana. It’s the second satellite in the joint European Union and European Space Agency Copernicus programme, following Sentinel-1A’s launch in April 2014. Sentinel-2A carries a Multispectral Imager (MSI) that has 13 spectral bands:

  • 4 visible and near infra red spectral bands with a spatial resolution of 10 m
  • 6 short wave infrared spectral bands with a spatial resolution of 20 m
  • 3 atmospheric correction bands with a spatial resolution of 60 m

It’s advantages over the US Geological Survey Landsat-8 mission includes the higher spatial resolution, and that Sentinel-2A is the first in a pair of satellites that will operate in tandem; Sentinel-2B is due to be launched next year. The key advantage of having an identical paired satellite constellation is that they can map the Earth much faster. On its own Sentinel-2A will return to the same point above the Earth, referred to as the revisit time, every 10 days; whereas it’s currently 16 days for Landsat-8. However, when Sentinel-2B is added the revisit time will halve to only 5 days at the equator. This improvement is hugely significant for the development of time critical applications. Also, there are plans to work Landsat-8 and Sentinel-2 together to provide an even higher repeat coverage of around twice a week

When both Sentinel-2 satellites are operational, they will acquire over 1 Tb of data every single day, and currently this data has ESA Sentinel-2:

  • Land Use & Land Cover (LULC) Monitoring – Providing data on how land on the planet is used, and helping to monitor how this changes over time. For example, monitoring deforestation, desertification, reforestation, drying up of wetlands, urban creep and flood mapping amongst others. The European Commission leads the way in this type of monitoring with the CORINE Land Cover Project, which has produced European wide maps for 1990, 2000, 2006 and 2012 that classify 44 different types of land; available through the Copernicus Land Service.
  • Plant Health – Providing information on vegetation and growth such as leaf water content, which will be particularly helpful for farmers in determining when, and how much, to water crops to improve yields. Also wider uses such as the leaf area index (LAI), which is one of the Essential Climate Variables used by the United Nations to monitor climate change.
  • Inland and Coastal Water Management – Providing higher resolution ocean colour data than available from ocean colour missions, such as MODIS and VIIRS, that supports the monitoring of water quality. Using products such as Chlorophyll-a to help the identification, and mapping, of harmful phytoplankton algal blooms, and turbidity to measure water clarity.
  • Disaster Mapping – Supporting a variety of disaster situations through the Copernicus Emergency Management Service.

The Copernicus satellite programme offers an exciting new data source and is made available free of charge to users, making this a critical resource for everyone working in the Earth observation industry. Every company needs to look at what new products and services they could develop from Copernicus data, or how they can make existing processes more efficiently and effectively. If you don’t, you can guarantee someone else will. How are you going to the use Sentinel-2A data?