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.

Uncovering Secrets with Remote Sensing

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

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

Recent significant discoveries in Cambodia and Jordan have highlighted the potential offered by remote sensing and satellite imagery to help uncover secrets on Earth – a field known as satellite archaeology.

Cambodia
Helicopter mounted Lidar was used to reveal multiple cities beneath the forest floor near the ancient temples of Angkor Wat in Cambodia. Lidar, which stands for Light Detection and Ranging, is an active optical remote sensing technique that uses a laser scanner to map the Earth’s topography by emitting a laser pulse and then receiving the backscattered signal. In Cambodia, a topographic Lidar with a near infrared laser was used by Australian archaeologist Dr Damian Evans to survey beneath the forest vegetation.

The conurbations discovered, surrounding the stone temple Preah Khan Kompong Svay, are believed to be between 900 to 1 400 years old. Analysis of the survey has shown a large number of homes packed together liked terraced houses, together with structures for managing water and geometric patterns formed from earth embankments – which could be gardens.

At 734 square miles, the 2015 survey is also thought to be the most extensive of its type ever undertaken. Dr Evans work is due to be published in the Journal of Archaeological Science.

Jordan
Archaeologists using high resolution satellite imagery, drones surveys and imagery within Google Earth have discovered a huge structure buried in the sand less than a kilometre south of the city of Petra. The two high resolution satellites used were Worldview-1 and Worldview-2, operated by DigitalGlobe. Worldview-1 was launched in September 2007 and has a half-metre panchromatic resolution; Worldview-2, launched two years later, offers similar panchromatic resolution and 1.85m multispectral resolution.

The outline of the structure measures 56m x 49m, and there is a smaller platform contained inside the larger one. Nearby pottery finds suggest the platform is 2 150 years old, and it is thought that it had a ceremonial purpose. The research undertaken by Sarah Parcak and Christopher Tuttle was published in the May 2016 edition of the Bulletin of the American Schools of Oriental Research.

Benefits of Remote Sensing & Satellites
Angkor Wat and Petra are both World Heritage sites, and the benefits of using remote sensing and satellite technology to undertake archaeological investigations are evident in the statement from Christopher Tuttle who noted that they did not intend to excavate their Petra discovery as ‘The moment you uncover something, it starts to disintegrate.’

Satellite technology allows investigations to take place without disturbing a piece of soil or grain of sand, which is a huge benefit in terms of time, cost and preservation with archaeology. These two discoveries also demonstrate that the world still has secrets to reveal. As Sarah Parcak herself said in 2013, “We’ve only discovered a fraction of one percent of archaeological sites all over the world.”

Who knows what remote sensing and satellite imagery will uncover in the future?