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Earth surveyors in action


 
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Eyes on Earth from Space:  Professor Alfredo Huete, Professor of Plant Functional Biology and Climate Change Cluster at the University of Technology Sydney, takes a closer look at remote sensing and its uses.
Remote sensing is done by cameras that may be in space or balloons, on mountains or towers or airplanes that observe and monitor a certain part of the earth’s surface. The accumulated images and pictures can then be interpreted, often after enhancement and then used for ecological, environmental or management purposes.

 
The Earth as a single integrative ecosystem from space (courtesy of NASA).    

In its early days the equipment was very large and very heavy and big balloons were used to get the systems up into the air. They were often used for military purposes to see into enemy territory. These days we have systems orbiting the planet, numbering several hundred. Some are designed to look very closely at the Earth’s surface, like mapping every household in a particular city. Others can take thermal images to map the temperatures in different parts of the city, monitoring which parts are warmer or cooler. Other camera systems are geared to look at entire regions, such as the state of NSW. The advantage of this is that a picture can be taken every day, accumulating data that would otherwise take years.


 
Below:Fundamentals of the remote sensing process involving energy source, surface detection, sensor systems and derivation of thematic imagery (Image courtsey of the NASA Terra Project).
    

While there are many systems circling the Earth, most are recording very different information and their sensors take pictures over different scales. Some may be designed to capture fine details of a limited area; others may be for larger areas like habitats and ecosystems.
Different systems are available that can record in different parts of the spectrum and can even capture the thermal spectrum in images of the earth, something human eyes can’t do. The sensors can also use the microwave spectrum.


 
Since microwaves are not blocked by clouds they can be highly useful in cloudy regions like rainforests. The microwave wavelengths can also be tuned to pass through different vegetation, ice, or soil layers to various depths. They were used to map the Panama rainforest, one wavelength capturing the canopy and another longer wavelength mapping the soil. There is currently much interest in mapping the moisture content of the soil with microwave imaging sensors.
There has been some concern about potential effects of microwaves on living tissues, however, remote sensing merely records the earth’s natural release of microwave radiation and this long wavelength energy is very weak and relatively benign. In fact, one of the main advantages of these remote sensors is being able to measure and monitor the Earth’s surface without causing damage.

 
Below: Remote sensing is highly useful in detection and mapping of fires along with burned areas and subsequent ecosystem recovery. A 2002 astronaut photograph reveals the orange scars of plant-free sand caused by fire in the Simpson Desert of central Australia (Courtesy of NASA Earth Observatory).    
Plants and trees don’t need to be cut or moved to determine the properties of ecosystems, such as biomass. Impenetrable rainforests, ice sheets, deserts and mountains can now be readily surveyed without the need for expensive infrastructure.
Different science communities are now getting together, like the NASA mission that put into orbit a state of the art sensor, MODIS (moderate resolution imaging spectrometer).

 
Below: The Multi-angle Imaging SpectroRadiometer (MISR). Viewing the sunlit Earth simultaneously at nine widely spaced angles, MISR provides accurate measures of the brightness, contrast, and colour of reflected sunlight. The change in reflection at different view angles affords the means to distinguish different types of atmospheric particles (aerosols), cloud forms, and land surface covers. Combined with stereoscopic techniques, this enables construction of 3-D models and estimation of the total amount of sunlight reflected by Earth's diverse environments (Courtesy NASA).    
This was designed with 36 different spectral channels, ten of which were specifically requested by terrestrial ecologists, farmers and agriculturalists to collect the data they needed to assess and monitor their area of interest.
Another set of bands was specifically put together by the ocean scientists, remote sensing being very useful to monitor the oceans which make up two thirds of the planet. Data can be collected on temperature, sediments, pollution and even phytoplankton.

 
Another big player is the group known as atmosphere scientists whose interests are clouds, pollution, aerosols, dust, carbon monoxide and all the gases in the atmosphere. The fourth group call themselves the cryosphere scientists who monitor the Arctic and Antarctic, the ice and land ice, glaciers. They devote their time to just studying ice and snow, and monitoring their size, appearance and disappearance, and their movements on a weekly basis and from year to year. This is how one can determine that Mt Kilimanjaro has lost 80% of its glacial mass with adverse impacts on the fertile valleys below which depend on melting snow for water.

 
Fires in northern Australia for a single day (June 25, 2012). The 1 km resolution pixels that exceed a temperature threshold are labelled red for suspected presence of fires. From MODIS Rapid Response website: http://earthdata.nasa.gov/data/near-real-time-data/rapid-response    
These four scientific groups work together to determine the best platforms, instruments, spectral channels and wavelengths to encompass their interests.
Clouds and aerosols affect the sensor signals so it is the atmosphere scientists who measure these signals for their interests as well as help the land, ice, ocean scientists to mask the clouds and correct their data for atmosphere influences. In this way, each group’s data help the others, a fascinating way to meet and understand the needs of other stakeholders and scientists.

 
Australia at night    
Many countries are putting up their own satellite systems (Thailand, Indonesia, Vietnam) and governments are quick to listen to scientists to do this.
Australia does not have any satellites but is one of the biggest users of remote sensing, especially to monitor its vast remote areas.
Australia plays an important role in receiving satellite data in our remote corner of the planet, and facilitating the transmission of data back to the satellite owner countries located on the other side of the globe.

 
Australia thereby relies on its good relations with Europe, North America, China and India, having a lot of receiving dishes for these satellites where data can be safely downloaded.

As a terrestrial scientist Alfredo Huete likes to monitor the land, including coastline mangroves, wetlands, grasslands, forests and savannahs. He is fascinated by the dynamics of the week-to-week changes in these different areas, something humans often don’t notice. He can track temperature changes, rainfall patterns and the sun, all of which affect the living things in those areas. The plants, trees and grasses are all indicators of how those environments are changing.
Some satellites have been collecting data for 40 years so they have documented evidence which can answer questions about global warming, ice melting and other changes. One grassland patch had its native plants taken over by invasive species but then two years later this was reversed. In this way the remote sensing is acting like a warning system, issuing alerts when patterns change.
Indeed one planet-wide sensor that focuses on each pixel on an image can send a hotspot alert if the temperature in that pixel exceeds a certain threshold value. People can subscribe to this service, receiving messages on their mobile phones with the coordinates of a potential fire. The 40 years of data are also useful in studying bushfire recovery and regeneration, drawing a big picture.
In terms of our environment this tool may be remote but it is a sensible tool.

 
Below: Sydney - two time periods and urbanisation.    

 
Professor Alfredo Huete was interviewed for A Question of Balance by Ruby Vincnt. All images provided by Professor Huets. Summary text by Victor Barry, October 2015.

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