Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


Experimental radar provides 3-D forest view


An advanced radar technique to image forests in three dimensions has undergone an ESA-backed test campaign in Indonesia. A future space-based version could measure global biomass to sharpen the accuracy of climate change models.

The campaign, called the Second Indonesian Airborne Radar Experiment (INDREX-II), involved flying a test instrument called the Experimental Synthetic Aperture Radar (E-SAR), built by the German Aerospace Centre (DLR), in a Dornier-228 aircraft over eight test sites around Kalimantan on the island of Borneo.

The sites varied in character from pristine rainforest to coastal mangroves and oil palm and rubber plantations. They were also measured in detail on the ground to provide ’ground truth’ for the radar results, around 200 Gigabytes of raw data having been gathered during three weeks of flights. "We had already carried out tests in European forests," explained INDREX-II team member Dirk Hoekman of the University of Wageningen. "We were able to extract the difference between the tree canopy and the forest floor – and from knowing tree height, we can use specially-developed algorithms to estimate forest biomass with a reasonable degree of certainty. "What we needed to know was if the same was true of much denser tropical forests. So with ESA’s support and the co-operation of the Indonesian Ministry of Forestry we carried out this aerial survey over test sites that were also measured from the ground, in order to gather a sizeable tropical radar database. There is still a lot of analysis to be done, but early findings look promising."

Hoekman added that the instrument also showed high sensitivity to flooded, burnt and logged forest areas, while Irena Hansek of DLR recounted that individual tall trees standing above the surrounding canopy could be detected. An improved global picture of forests – which cover about 27% of the Earth’s total land surface – would have multiple uses. Woodland is an economic resource that is also central to the environment: as a haven for biodiversity, preventing soil erosion and flooding and influencing local climate. Forests are also the most significant onshore stores of carbon, helping to absorb excess carbon dioxide that would otherwise increase global warming. So the Kyoto Protocol, about to come into force, allows nations with forested areas to set them against carbon emission. Accurate quantification of total forest biomass would provide verification for Kyoto and also shrink current uncertainties within climate change models.

The campaign took place during last November and December. Details of INDREX-II were presented during a workshop in ESA’s European Space Research Institute (ESRIN). Some 135 researchers from 27 countries attended the POLINSAR 2005 event, named for a relatively new radar technology called synthetic aperture radar (SAR) polarimetric interferometry (Pol-InSAR). Participants in the weeklong workshop heard that other potential applications of the technology included monitoring of urban areas, ice fields and agriculture.

What is Pol-InSAR?

Radar has come a long way since its initial development for range and direction finding in the run-up to World War Two. Today satellites such as ESA’s Envisat routinely scan the Earth with radar signals, building up highly detailed surface images - even through clouds and local darkness – out of reflected radar backscatter. In addition to simply imaging a site, two radar images of the same location acquired from close to the same position in space can be combined together using a technique called SAR interferometry (InSAR). Working on the same basis as stereoscopic vision, a dual image InSAR dual-image combinations throw up colourful interference pattern ’fringes’ that resemble contour lines on the map.

These fringes contain topographic height information that can be used to generate highly accurate digital elevation models (DEMs). InSAR can also be applied to detecting millimetre-scale ground movement taking place between acquisitions, such as ground subsidence or tectonic motion. Extra information can also be gathered on a site based on the fact that radar signals can have different polarisations; just as visible light does – as demonstrated by the high amount of glare screened out with polarized sunglasses.

Envisat’s Advanced Synthetic Aperture Radar (ASAR) is a dual-polarisation instrument, meaning that it can transmit and receive in either Horizontal (H) or Vertical (V) polarisations. In Alternating Polarisation (AP) mode it transmits in the same way, but can receive back radar pulses in both H and V polarisations. Combinations of HH/VV HH/HV or VV/VH polarisation pairs are possible. Depending on the physical and chemical properties of the surface, the responses from these different combinations of polarisation pairs can vary considerably, for example enabling the differentiation of separate crop species in the same field, or telling sea ice from open water.

The new generation of airborne radar sensors go one better than ASAR because they include full quad-polarisation capability: this means their signal can be retrieved simultaneously in H and V polarisations, so that by sending alternating H and V pulses a full spread of combinations can be obtained, generating more information-rich imagery.

These two techniques of InSAR and polarimetry have both been around a long time. Pol-InSAR is the combination of these two processes – the performance of interferometry from multi-polarised radar images. "Using Pol-InSAR brings us an extra dimension in the imaging of forests," said Scott Hensley of California’s Jet Propulsion Laboratory (JPL). "The interferometry part gives us the height data, while the polarisation part provides information on depth and also the orientation of objects relative to the instrument. This can then be processed to form a complete three-dimensional picture."

In the case of forests, Pol-InSAR typically relies on longer L- or P- band wavelengths upon in preference to the shorter C-band wavelengths used by the radar satellites currently in orbit - these code letters being a legacy from radar’s wartime origins. Envisat’s C-band radar has a wavelength of 5.8 centimetres, which is too short to pierce through a dense forest canopy. Instead Pol-InSAR measurements are made using L- and P-band wavelengths, which range between around 15 to 100 cms – closer to the dimensions of the trees researchers are interested in. The E-SAR sensor used during INDREX-II has multi-polarisation X-, C- L- and P-band wavelengths available. Hensley’s team has been surveying tropical forests in Costa Rica with the AIRSAR airborne instrument, similarly with multi-polarisation C-, L- and P-band wavelengths.

Putting Pol-InSAR into space?

Pol-InSAR is a relatively young field, albeit one that has come on greatly since the first POLINSAR workshop at ESRIN back in January 2003. That meeting concluded with recommendations that Pol-InSAR be tested on further forest types, which led to the INDREX-II campaign. This time discussions were centred on the prospect of the current generation of airborne sensors eventually making it into orbit aboard to a Pol-InSAR-capable satellite. There are several new radar missions due to enter service - including Germany’s TerraSAR-X, Japan’s Advanced Land Observing Satellite (ALOS), the Italian-French CosmoSkyMed and Canada’s Radarsat-2 – but while they offer researchers an increased range of radar wavelengths and polarization combinations, none are optimised for Pol-InSAR.

A major hindrance is that a single satellite can take time to revisit the same area – 35 days in the case of Envisat. In just a few days between images, vegetation growth in forested areas can be so great to prevent InSAR coherence: the two scenes have changed too much for interferometry to work. One simple but costly solution that was raised was to launch constellations of satellites to minimise revisit times. Another suggestion was to forego satellites in favour of high-altitude unmanned aerial vehicle (UAV) platforms over areas of continuing interest.

The next POLINSAR meeting is due to take place in January 2007.

Mariangela D’Acunto | alfa
Further information:

More articles from Ecology, The Environment and Conservation:

nachricht Invasive Insects Cost the World Billions Per Year
04.10.2016 | University of Adelaide

nachricht Malaysia's unique freshwater mussels in danger
27.09.2016 | The University of Nottingham Malaysia Campus

All articles from Ecology, The Environment and Conservation >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: New 3-D wiring technique brings scalable quantum computers closer to reality

Researchers from the Institute for Quantum Computing (IQC) at the University of Waterloo led the development of a new extensible wiring technique capable of controlling superconducting quantum bits, representing a significant step towards to the realization of a scalable quantum computer.

"The quantum socket is a wiring method that uses three-dimensional wires based on spring-loaded pins to address individual qubits," said Jeremy Béjanin, a PhD...

Im Focus: Scientists develop a semiconductor nanocomposite material that moves in response to light

In a paper in Scientific Reports, a research team at Worcester Polytechnic Institute describes a novel light-activated phenomenon that could become the basis for applications as diverse as microscopic robotic grippers and more efficient solar cells.

A research team at Worcester Polytechnic Institute (WPI) has developed a revolutionary, light-activated semiconductor nanocomposite material that can be used...

Im Focus: Diamonds aren't forever: Sandia, Harvard team create first quantum computer bridge

By forcefully embedding two silicon atoms in a diamond matrix, Sandia researchers have demonstrated for the first time on a single chip all the components needed to create a quantum bridge to link quantum computers together.

"People have already built small quantum computers," says Sandia researcher Ryan Camacho. "Maybe the first useful one won't be a single giant quantum computer...

Im Focus: New Products - Highlights of COMPAMED 2016

COMPAMED has become the leading international marketplace for suppliers of medical manufacturing. The trade fair, which takes place every November and is co-located to MEDICA in Dusseldorf, has been steadily growing over the past years and shows that medical technology remains a rapidly growing market.

In 2016, the joint pavilion by the IVAM Microtechnology Network, the Product Market “High-tech for Medical Devices”, will be located in Hall 8a again and will...

Im Focus: Ultra-thin ferroelectric material for next-generation electronics

'Ferroelectric' materials can switch between different states of electrical polarization in response to an external electric field. This flexibility means they show promise for many applications, for example in electronic devices and computer memory. Current ferroelectric materials are highly valued for their thermal and chemical stability and rapid electro-mechanical responses, but creating a material that is scalable down to the tiny sizes needed for technologies like silicon-based semiconductors (Si-based CMOS) has proven challenging.

Now, Hiroshi Funakubo and co-workers at the Tokyo Institute of Technology, in collaboration with researchers across Japan, have conducted experiments to...

All Focus news of the innovation-report >>>



Event News

#IC2S2: When Social Science meets Computer Science - GESIS will host the IC2S2 conference 2017

14.10.2016 | Event News

Agricultural Trade Developments and Potentials in Central Asia and the South Caucasus

14.10.2016 | Event News

World Health Summit – Day Three: A Call to Action

12.10.2016 | Event News

Latest News

Resolving the mystery of preeclampsia

21.10.2016 | Health and Medicine

Stanford researchers create new special-purpose computer that may someday save us billions

21.10.2016 | Information Technology

From ancient fossils to future cars

21.10.2016 | Materials Sciences

More VideoLinks >>>