24. February 2016

Radar over the rain­for­est

The Do 228 af­ter a mea­sure­ment flight
Image 1/3, Credit: DLR (CC-BY 3.0).

The Do 228 after a measurement flight

The twin en­gined tur­bo­prop Do 228-212 air­craft is well suit­ed to the in­stal­la­tion of spe­cial cam­era and radar sys­tems be­cause of its rect­an­gu­lar cab­in and large open­ings in the floor.
Radar im­age ac­quired by the F-SAR sys­tem over the Pon­gara Na­tion­al Park south of Li­bre­ville, Gabon
Image 2/3, Credit: DLR (CC-BY 3.0).

Radar image acquired by the F-SAR system over the Pongara National Park south of Libreville, Gabon

In Pon­gara Na­tion­al Park, there are still large ar­eas of pris­tine man­grove for­est, while the in­land bor­ders merge in­to dense rain­for­est. The colour scheme shows dif­fer­ent types of re­flec­tion of the radar sig­nals. For sig­nals in the L band (23 cen­time­tre wave­length) main­ly veg­e­ta­tion-free sur­faces (for ex­am­ple, wa­ter or open fields) ap­pear blue in the im­age. A dom­i­nant dou­ble re­flec­tion (for ex­am­ple, from the ver­ti­cal man­grove trees fol­lowed by the wa­ter sur­face) ap­pears red. A pre­dom­i­nant­ly om­ni­di­rec­tion­al re­flec­tion, es­pe­cial­ly from the vol­ume of the rain­for­est tree­tops ap­pears green.
Radar re­flec­tor
Image 3/3, Credit: DLR (CC-BY 3.0).

Radar reflector

The re­flec­tor, with its three mu­tu­al­ly per­pen­dic­u­lar sur­faces, col­lects all of the in­ci­dent en­er­gy and sends it back di­rect­ly in the di­rec­tion from which it came – anal­o­gous to a cat’s eye road re­flec­tor in the vis­i­ble wave­length range. The to­tal en­er­gy ap­pears to come di­rect­ly from the in­ner cor­ner of the re­flec­tor. The po­si­tion of this rear cor­ner was mea­sured very pre­cise­ly us­ing GPS, so it can be de­ter­mined whether the radar da­ta is ge­o­met­ri­cal­ly pre­cise to with­in a few cen­time­tres. Pos­si­ble prob­lems in the hard­ware can be de­tect­ed, for ex­am­ple, by an ap­par­ent change in the po­si­tion of the re­flec­tor. The strength and phase re­la­tion­ships of the in­ci­dent hor­i­zon­tal­ly or ver­ti­cal­ly po­larised waves are eval­u­at­ed with the aim of en­sur­ing that the F-SAR hard­ware is func­tion­ing cor­rect­ly in the air­craft.

Dense rainforest, a twin-engine turbo-prop aircraft and advanced radar technology – researchers from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) are performing numerous measurement flights from 1 to 29 February 2016 over the central African country of Gabon to determine the state of the rainforest. The data obtained will help to improve climate models and better understand global warming. The campaign is being carried out in cooperation with the European Space Agency (ESA), the French National Aerospace Research Center (Office National d'Etudes et Recherches Aérospatiales; Of­fice Na­tion­al d'Etudes et de Recherch­es Aérospa­tiales (ON­ERA)), the Gabonese Agency for the Study of and Observation from Space (Agence d'Études et d'Observation Spatiales; AGEOS) and US Space Agency NASA. ONERA already performed measurement flights in July 2015, during the dry season, while DLR and NASA will research the rainforest with three aircraft during the rainy season. The results will serve as reference data for the ESA satellite mission 'Biomass', planned for launch in 2020, and the German Tandem-L mission proposal.

Biomass – investigating critical quantities

Earth's climate system is highly complex and is dependent on many factors. A high degree of uncertainty exists for land use change emis­sions, and hence for the full carbon cycle budget (according to the In­ter­gov­ern­men­tal Pan­el for Cli­mate Change, IPCC).

Approximately one third of Earth's land surface is covered by forest. Since half of the total carbon in forest ecosystems is found in biomass, any changes (for example, by windthrow or forest clearing) have a particularly strong impact. Therefore, a global inventory of forests and any changes they undergo is particularly critical to more accurately determining how much carbon is bound.

With their large mass of vegetation, rainforests play an important role. The problem is that the extent of the existing rainforests and their changes, due to deforestation or construction, has not been observed or quantified up to now. It is also not clear how much carbon is actually bound and released again within a given period. A better understanding of the carbon cycle would improve the determination of the carbon budget immensely. This will ultimately lead to more sustainable countermeasures against global warming.

At total of 88 percent of Gabon is rainforest, making it ideal for the measurement campaign. Researchers from the DLR Microwaves and Radar Institute will measure the above ground, terrestrially bound biomass of the lavish rainforest vegetation in the jungles of the central African west coast. To that end, the scientists have equipped the DLR Dornier DO 228 research aircraft with modern L- and P-band radar technology.

Mission images in our Flickr gallery

Looking through the treetops

Conventional radar technology can only record the superficial structure of a rainforest area. The DLR twin satellite duo TanDEM-X, for example, measures the scattering of the radar signal at the treetops, but cannot penetrate through them. With the help of F-SAR (Synthetic Aperture Radar) airborne radar system developed by DLR, it is possible to measure in several frequency ranges simultaneously. The new radar technology is based on polarimetric SAR Interferometry (Pol-In­SAR). This makes it possible to measure various parameters over wide ranges. "Thanks to the use of very long wave L- and P band frequencies, we are able to see through to the forest floor and can capture the entire area between the ground and the treetops," explains Irena Hajnsek, Manager of the AfriS­AR project at DLR. The height of the forest is calculated and a vertical 3D profile is generated from the data collected. "With these parameters, we can deduce how much carbon is actually bound in the rainforest biomass," Hajnsek adds. The evaluated data will then be used to significantly improve existing climate models. Cleared and deforested areas can be made clearly visible as a by-product of the mapping process. This is very difficult to achieve with optical sensors due to constant cloud cover.

20,000 feet above the forest

The Do 228-212 aircraft is well suited to the installation of special camera and radar systems because of its rectangular cabin and large openings in the floor. Since it does not have a pressurised cabin and the measurements are taken at an altitude of 20,000 feet (approximately 6100 metres), the pilots fly with oxygen masks. "At these altitudes, within a short amount of time, the brain no longer receives enough oxygen. For this reason, wearing a mask is mandatory," emphasises Oliver Brieger, Head of DLR Flight Operations.

One of the major challenges when flying is the weather. Violent thunderstorms in the evening and dense clouds in the morning are not uncommon in the intertropical convergence zone where equatorial Gabon is located. During the 15 measurement flights the aircraft has to be flown very smoothly in order to make the radar measurements and the predetermined course must be followed very precisely – the pilots should not deviate more than two metres from the flight path. This accuracy is required to produce a tomographic data set using the radar data acquired in several parallel flight paths. From this, a 3D model of the forest can further be derived.

  • Fabian Locher
    Ger­man Aerospace Cen­ter (DLR)
    Cor­po­rate Com­mu­ni­ca­tions
    Cor­po­rate Com­mu­ni­ca­tions, Ed­i­tor Aero­nau­tics
    Telephone: +49 2203 601-3959
    Fax: +49 2203 601-3249

  • Prof. Irena Hajnsek
    Ger­man Aerospace Cen­ter (DLR)
    Mi­crowaves and Radar In­sti­tute
    Sci­ence Co­or­di­na­tion
    Telephone: +49 8153 28-2362
    Münchener Straße 20
    82234 Oberpfaffenhofen-Weßling
  • Oliver Brieger
    Head Flight Ex­per­i­ments
    Ger­man Aerospace Cen­ter (DLR)
    Flight Ex­per­i­ments
    Telephone: +49 531 295-2800
    Fax: +49 8153 28-1347
    Lilienthalplatz 7
    38108 Braunschweig
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