New Scientist - USA (2019-10-05)

36 | New Scientist | 5 October 2019

of the forest possible. “We’ve designed the

instrument, its lasers, detectors, and other

technology, to get through dense tropical

forests,” says Ralph Dubayah at the

University of Maryland, who is principal

investigator on GEDI.

GEDI’s lidar uses near infrared light, which

is reflected off leaves so that the canopy looks

brighter than it does in the visible spectrum.

Nevertheless, it has some limitations. For a

start, because it is hitching a ride with the

ISS, over its two-year mission, it will sample

only a fraction of Earth’s surface and won’t

collect data north of about 52 degrees latitude,

therefore missing most of the boreal forest.

In addition, because lidar uses light in the near

infrared part of the spectrum, it can’t penetrate

clouds. So GEDI can’t do this all alone. That is

where other systems that use radar come in.

Radar systems send out microwave

radiation, which passes through cloud and

scatters when it hits a solid object. The sensor

detects this backscatter and the patterns it

produces can be analysed to form pictures of

the landscape. Microwaves range from about

1 millimetre to 1 metre in length, and the

wavelength a particular radar uses dictates

what it can see. Shorter wavelengths detect

Christiana Figueres will speak about the next steps on climate

change on the main stage at New Scientist Live on 10 October

newscientistlive.com

SOURCE: doi.org/cp29

Plants

450

gigatonnes of carbon

70

Bacteria Fungi 12 Archaea 7

Viruses0.2

Protists 4 Animals 2

The stuff of life

Plants account for by far the largest proportion

of biomass on Earth, and most of that living

matter is in trees

smaller objects. Longer ones can penetrate

the forest canopy to bounce off larger objects

below, such as tree trunks and branches.

Too long, however, and the wavelength gets

scattered in Earth’s ionosphere – a layer of

the atmosphere extending from about 60 to

1000 kilometres up that contains a lot of ions

and free electrons.

What is needed is a “Goldilocks” wavelength,

not too long and not too short. The one that

is just right, at 70 centimetres, is known as

the P-band. Unfortunately, that wavelength

couldn’t be used for many years. Satellites

carrying P-band radars were banned because

they would interfere with Earth-based

operations that use the same wavelength,

including military defence systems. For

example, missions including the German

Earth observation satellite TanDEM-X operate

at much shorter wavelengths, so they aren’t

very sensitive to biomass.

Radar unlocked

However, in 2004, the blanket ban on using

P-band radar in orbit was lifted. Since then,

Shaun Quegan at the University of Sheffield,

UK, and his colleagues have been developing

a system based on the Goldilocks wavelength

to measure biomass from space. In 2013,

the European Space Agency finally selected

this system – the Biomass satellite – for

implementation. It is set to launch in 2022.

The mission will systematically map forests

where information is most urgently needed,

including all the world’s tropical forest and

most subtropical and boreal forest. Remaining

defence embargoes will prevent Biomass from

surveying the 22 per cent of Earth’s boreal

forests that are in Canada and Alaska, however.

During the initial 14-month phase of its

five-year mission, the sensor will operate in a

tomography mode. Like a CAT scanner imaging

the human body in multiple slices, Biomass

will build up a three-dimensional picture of

the forest. Because it uses an imaging system

at a long wavelength, instead of the discrete

sampling system of GEDI, it will create a

different picture: a continuous map of the

woody parts of the forest structure that would

otherwise be obstructed by the canopy. “At

these wavelengths, the leafy canopy becomes

transparent and we can see right down to the

ground,” says Quegan. Thereafter, the satellite

will switch to using interferometry – extracting

information from interference patterns – and

visit the same locations every seven months.

This will allow scientists to estimate canopy

biomass and height on a more frequent

Orbital tech will

reveal the secrets

of the forest

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