15. July 2015

DLR/NASA – 3D radar mea­sure­ments of for­est ar­eas

Traun­stein For­est – F-SAR im­age
Image 1/5, Credit: DLR.

Traunstein Forest – F-SAR image

Po­lari­met­ric radar im­age of the test area near Traun­stein, in south­east­ern Bavaria, ac­quired up by the F-SAR sen­sor in L-band (22 cen­time­tre wave­length) on 30 June 2015. The im­age is shown in false colours; for­est ar­eas ap­pear in green; sur­faces with low veg­e­ta­tion are shown in blue / red.
Ver­ti­cal pro­file of the for­est
Image 2/5, Credit: DLR.

Vertical profile of the forest

The test area near Traun­stein. This im­age shows an ex­am­ple of a ver­ti­cal pro­file of radar backscat­ter in this for­est. The backscat­ter is scaled in shades of green, from dark green (low backscat­ter) to white (high backscat­ter). The sol­id lines rep­re­sent the heights of the for­est floor and crown, de­ter­mined by li­dar mea­sure­ments.
NASA / DLR team pho­to­graph
Image 3/5, Credit: DLR (CC-BY 3.0).

NASA / DLR team photograph

Team pho­to­graph of the NASA and DLR re­searchers dur­ing the joint mea­sure­ment cam­paign for three-di­men­sion­al sur­vey­ing of for­est ar­eas. The DLR Mi­crowaves and Radar In­sti­tute and NASA’s Jet Propul­sion Lab­o­ra­to­ry (JPL) have had a good work­ing re­la­tion­ship for years.
NASA re­search air­craft
Image 4/5, Credit: DLR (CC-BY 3.0).

NASA research aircraft

Sci­en­tists from the Jet Propul­sion Lab­o­ra­to­ry (JPL) used NASA’s Gulf­stream III re­search air­craft.
DLR re­search air­craft
Image 5/5, Credit: DLR (CC-BY 3.0).

DLR research aircraft

Sci­en­tists from the DLR Mi­crowaves and Radar In­sti­tute used the DLR re­search air­craft Dornier Do-228 for the mea­sure­ment flights with the F-SAR sys­tem. The DLR Fa­cil­i­ty Flight Ex­per­i­ments, based in Oberp­faf­fen­hofen and Braun­schweig, is re­spon­si­ble for the op­er­a­tion of high­ly mod­i­fied re­search air­craft.

A tangle of treetops and branches, through which just the occasional clear area provides glimpses of the trunks and roots growing below. Whether it is woodland with German spruces or a tropical rainforest, very few sensors are able to see through this green carpet and clearly visualise the underlying structures. The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) is developing radar technology that, for the first time, will enable a three-dimensional visual representation of forest areas from the roots to the crowns. The radar experts at DLR partnered with the US aerospace agency NASA in a measurement campaign, which has now confirmed the capability of the F-SAR system. The cooperation was aimed at validating the suitability of the radar data to infer a variety of parameters, including a forest’s height or vertical structure.

"The collaboration with NASA allowed us to really test both the sensor technology and the algorithms. We could not have hoped for better results," said Andreas Reigber, project coordinator at the DLR Microwaves and Radar Institute.

Same task, different sensors

To conduct a cross comparison of the data, NASA and DLR each used different sensor systems while imaging the same area. The measurement flights took place over a test region near Traunstein, in southeastern Bavaria. NASA's Jet Propulsion Laboratory deployed its UAVSAR sensor on board a NASA Gulfstream III research aircraft. All the data of the partners from Pasadena was recorded within two days. Roughly a week later, the conditions and weather were sufficiently similar for the DLR team from the Microwaves and Radar Institute to take to the air for its own measurement campaign. Reigber and his colleagues flew over the test area 10 times in quick succession, using the DLR Dornier DO 228 research aircraft, and mapping the forest from a variety of angles. This is the only way in which the individual data acquisitions can be assembled to produce a three-dimensional image later on.

The F-SAR system developed by the DLR Microwaves and Radar Institute is particularly suited for use in forested regions, as it permits simultaneous measurements at several wavelengths. This means that F-SAR only needs one overflight to map different levels of the same terrain. Three wavelengths were used for the spatial survey of Traunstein Forest; the DLR researchers used sensors in C-band and X-band to scan the upper sections of the forest crown, while the L-band sensors penetrated the vegetation to open a window on the forest floor below.

The first evaluations of the measurement campaign have since confirmed F-SAR's accuracy. Moreover, the DLR researchers can use the comparative data provided by NASA to achieve even greater precision in the radar system’s calibration. The scientists can use the variations in the images to understand the capabilities of each sensor to detect certain forest characteristics. The scientists will produce the 'end product' – a three-dimensional image of the forest – once data analysis is complete.

Biomass as a climate factor

Environmental and climate researchers are not alone in wishing they could understand the characteristics of a forest with just one measurement – however complex, impenetrable and expansive it may be. A forest's structure also reveals the biomass it contains. As a natural storage mechanism for carbon, biomass directly influences the greenhouse effect. Therefore, radar tomography opens up entirely new and efficient avenues to identify one fundamental climate factor. The DLR scientists are keen to establish 3D radar mapping as a standard operating procedure – and also to install the technology on satellites. This successful measurement campaign, conducted in cooperation with NASA, has now confirmed that radar technology from Oberpfaffenhofen is on precisely the right track, and that this cooperation has been an important step forward in getting ready for future Earth observation missions.

The DLR Microwaves and Radar Institute is currently preparing for Biomass, a European Space Agency (ESA) satellite-based mission that will conduct radar observations of Earth's surface. The main focus of the mission is to determine the volume of biomass in the tropical rain forests. In addition, the Institute is at the forefront of Tandem-L, a highly innovative satellite mission tasked with using radar to record the dynamic processes unfolding on Earth's surface. Together, these projects promise a level of scientific insight one would not necessarily expect from flying over an area of woodland.

  • Bernadette Jung
    Com­mu­ni­ca­tions Ober­paf­fen­hofen, Weil­heim, Augs­burg
    Ger­man Aerospace Cen­ter (DLR)

    Com­mu­ni­ca­tions and Me­dia Re­la­tions
    Telephone: +49 8153 28-2251
    Fax: +49 8153 28-1243
    Münchener Straße 20
    82234 Weßling
  • Priv.-Doz. Dr.-Ing. Andreas Reigber
    Ger­man Aerospace Cen­ter (DLR)
    Mi­crowaves and Radar In­sti­tute
    Telephone: +49 8153 28-2360
    Münchener Straße 20
    82234 Oberpfaffenhofen-Weßling
  • Katrin Witte
    Ger­man Aerospace Cen­ter (DLR)
    Flight Ex­per­i­ments
    Telephone: +49 8153 28-1357
    Fax: +49 8153 28-1347
    Münchener Straße 20
    82234 Weßling
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