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.

Twinkle, Twinkle, Little SAR

Copyright : NASA/JPL Artist's impression of the Seasat Satellite

Copyright : NASA/JPL
Artist’s impression of the Seasat Satellite

Last week ESA released a new synthetic aperture radar (SAR) dataset from NASA’s Seasat mission; nothing unusual in that you might think, except that this data is over 36 years old. As part of its Long Term Data Preservation Programme, ESA has retrieved, consolidated and reprocessed the Seasat data it holds, and made this available to the Earth observation (EO) community.

Seasat was a landmark satellite in EO terms when it was launched on the 27th June 1978. Not only was it the first satellite specifically designed for remote sensing of the oceans, but it was also the first to carry a SAR instrument. Seasat was only in orbit for 106 days as a problem with the electrical system ended the mission just over three months later on 10th October. Although, there is a conspiracy theory that the electrical fault was just a cover story, and the military actually shut down Seasat once they discovered it could detect submerged submarines wakes!

Synthetic aperture radar (SAR) is so called as it uses a small physical antenna to imitate having a large physical antenna; to detect the long wavelengths would require a physical antenna of thousands of metres, while the same result can be achieved with a synthetic antenna of around 10 metres in length. It is an active sensing radar system which works in the microwave part of the electromagnetic spectrum, and uses pulses of radiation to map the surface of the Earth. Pulses are transmitted with wavelengths of between metres and millimetres, some of these pules are absorbed by the surface, whereas others are reflected back and recorded by the SAR. As the satellite moves, the antenna’s position relative to the area that it is mapping changes over time providing multiple observations. This movement crates a large synthetic antenna aperture, because all the recorded reflections of a particular area are processed together as if they were collected by a single large physical antenna, which gives an improved spatial resolution.

SAR is extremely sensitive to small changes in surface roughness, and can provide both day and night imagery as it works independently of visible light, and is generally unaffected by cloud cover. It is used for assessing changes in waves, sea ice features and ocean topography, and recent research is applying it to other fields such as flood mapping. Seasat blazed the trail for SAR instruments, which has since been followed by many other satellites including ESA’s ERS-1 and ERS-2, ENVISAT’s ASAR, RadarSAT, COSMO-SkyMed, TerraSAR-X; and in 2014 both the Japanese ALOS, and ESA’s Sentinel-1, satellites carried SAR instruments.

The potential value residing in Seasat data is demonstrated not only by ESA reprocessing Seasat, but last year NASA also released a reprocessed Seasat dataset. The use of historic data is one of EO most powerful tools, and it is one the remote sensing community needs to exploit more.

Copernicus ready for lift off!

The first satellite of the European Union’s Copernicus project, Sentinel-1A, is due to be launched next Thursday, April 3rd. The project aims to create a constellation of satellites providing a range of Earth Observation data to aid our understanding, and management, of the planet and its resources.

Sentinel-1 is a two-satellite mission, with a second identical satellite, Sentinel-1B, due to be launched in 2016.  The two satellites will orbit the earth 180° apart, allowing the entire globe to be covered every six days, although the Artic will be revisited every day and Europe, Canada and main shipping routes every three days.

The Sentinel-1A satellite weighs 2,300kg and carries a 12m long C-band Synthetic Aperture Radar (SAR) instrument; this is an advanced radar system that transmits microwave radiation that allows it to capture images of the earth twenty four hours a day, in addition it get images through cloud and rain. This is particularly useful when providing imagery for emergency response during extreme weather conditions. The satellite also has a pair of 10m solar wings to provide independent power, the deployment sequence can in be seen in this European Space Agency video.

Over land Sentinel-1 will capture data in an Interferometric Wide swath mode, which means it takes three scans and then combines them into a single image. Each scan has a width of 250km and a spatial resolution of 5m x 20m, which means each pixel on the image represents a 5m x 20m area. It works slightly different over the oceans, operating on a 5m x 5m spatial resolution enabling the direction, wavelength and heights of waves on the open oceans to be determined.

Image of the port of Maracaibo (Venezuela) using ASAR imagery; courtesy of ESA

Image of the port of Maracaibo (Venezuela) using ASAR imagery; courtesy of ESA

This satellite will replace the ASAR (Advanced Synthetic Aperture Radar) C-band instrument that was on-board the Envisat mission which had a resolution of 150m; until contact was lost in April 2012. The image on the right is of Lake Maracibo in Venezuela, and was  acquired in ASAR Image Mode Precision with a spatial resolution of 12.5 m. The varying colour is created by assigning a different RGB (Red, Green, Blue) colour to different acquisition dates (8 Sep 2004 is red, 26 Feb 2004 is green and 17 Jun 2004 is blue) with the brightness being linked to surface texture, so the rougher the surface the brighter the image

Lake Maracaibo itself is also really interesting.  It was formed 36 million years ago and is the largest natural lake in South America; although it has a direct connection to the ocean and so could be called an inland sea. The port of Maracaibo, located on the west side of the strait (large bright area on the image), is the second city of Venezuela and the lake is also a petroleum-producing region supplying two-thirds of the total Venezuelan petroleum output. However, its biggest claim to fame is atmospheric phenomenon of a semi-permanent lightning storm where the Catatumbo river flows into the lake; making it a magnet for stormchasers the world over.