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.

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.

Beijing Is Sinking

Envisat's ASAR sees Tianjin, China's third largest city.  Image courtesy of ESA.

Envisat’s ASAR sees Tianjin, China’s third largest city. Image courtesy of ESA.

Last month a paper was publishing in the Remote Sensing journal demonstrating that the city of Beijing is gradually sinking as a result of subsidence.

The paper ‘Imaging Land Subsidence Induced by Groundwater Extraction in Beijing (China) Using Satellite Radar Interferometry’ by Chen et al described work undertaken by a team of researchers using satellite images from between 2003 and 2011.

The research was undertaken using Interferometric Synthetic Aperture Radar (InSAR), which is a microwave based technique that uses phase measurements from two or more successive satellite SAR images to determine the Earth’s shape and topography and so to measure millimetre-scale changes.

The study used 41 images from Envisat-ASAR acquired from 2003 to 2010 collected in Stripmap mode with VV polarisation, together with 14 images from TerraSAR-X acquired from 2010 to 2011 also in Stripmap mode, but this time using HH polarisation. The images were processed alongside some baseline interferograms and the Shuttle Radar Topography Mission (SRTM) Digital Elevation Model, at 90 m resolution, to produce land subsidence information. The results showed that parts of Beijing, such as the business district of Chaoyang, had sunk by as much as 11 cm within a year. In addition, they estimated that parts of the city had sunk by more than 75cm during the course of the eight year study period.

Land subsidence is often caused by human activities and the researchers identified the main cause to be the extraction of groundwater from beneath the city. Beijing has an enormous appetite for water and is the fifth most water-stressed city in the world according to a 2014 report published by the Nature Conservancy. However, the amount of groundwater extracted was not the only factor with the researchers also seeing relationships with soil type, aquifer type and distance to the pumping wells. In addition, given the number of buildings that have been erected in Beijing in the last fifteen years the additional weight is also likely to be contributing to the problem.

This sinking is not noticeable to the inhabitants of Beijing, but there is concern how this subsidence will impact the transport infrastructure, particularly high speed trains and research is already underway on this area. It is also not just Beijing that’s suffering from subsidence. The research also identified 45 other areas across China as having had or having significant land subsidence. In addition, Mexico City is sinking by between 6 cm and 28 cm per year and the Indonesian capital of Jakarta has dropped by around 8 cm per year; both are due to groundwater extraction. Even London is sinking, although in our case it only 1 – 2 millimetres per year!

Water is a precious resource and one that is necessary for growing populations and economies alike. Research such as this demonstrates why the world needs to get better at water conservation. If we continue as we are, there could be potentially serious consequences for parts of the globe.