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.

It’s World Space Week!!

world-space-week-logoDid you know it’s World Space Week? It occurs between the 4th and 10th October each year, because:

  • On 4th October 1957 the first human-made Earth satellite, Sputnik 1, was launched; and
  • On 10th October 1967: The Treaty on Principles Governing the Activities of States in the Exploration and Peaceful Uses of Outer Space, including the Moon and Other Celestial Bodies was signed – see previous blog for more details.

This annual international celebration aims to inspire everyone about space, encourage young people to get involved in science, technology, engineering and maths and to demonstrate the benefits, and use, of space technology. The first World Space Week occurred in 2000, and each year has a specific theme.

2016 World Space Week
We’re really excited this year as the theme is ‘Remote Sensing: Enabling our Future’. It’s celebrating Earth Observation (EO), and highlighting the variety of EO missions in space and the applications which use their data.

There are over 1,000 events taking place all over the world to celebrate remote sensing, and they are all listed on the World Space Week website. It seems as though Brazil is holding the most events this year, a whopping 159! Have a look through and see if there is anything you’d like to go to. If not, create your own event –

  • Spend a night looking at the stars.
  • Use Google Earth to look at your local area from space.
  • Get some friends together and watch classic space films.
  • Build your own spacecraft – Both ESA and SSTL have cut out models you can use.

Competition!!

Competition Image courtesy of ESA.

Competition Image courtesy of ESA.

Here at Pixalytics, we couldn’t let the Remote Sensing theme go by without getting involved. So we’ve decided to run our first ever Twitter competition!! The prize is a copy of our book ‘Practical Handbook of Remote Sensing’, which guides complete beginners through the process of finding, downloading, analysing and applying remote sensing data. We’ll post the book, free of charge, anywhere in the world!

The competition has now closed. Thanks to everyone who entered.

The location was Angkor Wat in Cambodia, read more about the site our next blog.

Celebrating World Space Week!

Did you know this is World Space Week? In 1999, the United Nations declared that World Space Week would occur between the 4th and 10th October each year. It chose these two dates because:

  • On 4th October 1957 the first human-made Earth satellite, Sputnik 1, was launched; and
  • On 10th October 1967 The Treaty on Principles Governing the Activities of States in the Exploration and Peaceful Uses of Outer Space, including the Moon and Other Celestial Bodies was signed – which was discussed in last week’s blog.

This annual international celebration supports events in countries around the world to educate people about space, encourage everyone to benefit from the space industry and inspire young people to get involved in science, technology, engineering and maths.

Space: Guiding Your Way is the theme for 2014 and focuses on all aspects of satellite navigation, from the GPS in your smartphone, though road navigation, shipping and disaster recovery. According to the World Space Week website there are over 700 events in over 60 countries taking place during this week’s celebration: everywhere from Afghanistan to Venezuela has an event, supported by a number of global events. The events vary from educational presentations, conferences and demonstrations through to water rocket competitions, training like an astronaut or even having coffee with an astronomer. The UK Space Agency has a ‘Tweet the Expert 2014’ event running between 2pm and 3pm each day this week.

Rumple Quarry in Plymbridge Woods

Rumple Quarry in Plymbridge Woods

Here at Pixalytics, we didn’t want World Space Week to go by without getting involved, and so we’ve taken part in the EarthCache Virtual 5K. Although, don’t let the word virtual fool you as there has been running! EarthCache aims to teach people about the world by highlighting interesting geologic or geographic phenomenon or features you can visit, and there are almost eighteen thousand such sites worldwide. To participate in the Virtual 5K run, you have to run at least 5K starting or ending at a registered EarthCache site.

Sam Lavender at the start of her 5K run

Sam Lavender at the start of her 5K run

Our closest EarthCache site is a quarry site in Plymbridge Woods, pictured above. Records indicate it has been worked as far back as 1683, and slate from here was reportedly used in the building of Devonport Dockyard. In addition to the quarry site, there is also a viaduct and ruins of a waterwheel and quarry workers cottages. To the right is the picture of Sam at the start of her 5K run.

Are you going to join in World Space Week? Have a look at the website and see if you can find an activity, or join us in the virtual run. Whatever you decide to do, remember you’re celebrating space and your part in this industry.