Remote Sensing Puts Focus on Climate Change

MARIE FREEBODY, CONTRIBUTING EDITOR

As temperatures and carbon dioxide concentrations rise to levels not previously recorded, shifts in weather and other environmental changes are impacting the global landscape. With the U.N. recording one climate-related crisis every week in 2019, such as the forest fires in the U.S. and bushfires in Australia, the consequences of such a globally disrupted climate are many and difficult to track. This is so because of variables such as altitude, movement, and the size of equipment used to measure these phenomena.

The near-infrared band from the multispectral instrument (MSI) of Sentinel-2 reveals the status of vegetation (in various tones of red) and also reveals burned areas (lack of vegetation, in black). Courtesy of USGS/ESA.

Optical technologies such as remote sensing can not only gauge the extent of problems, but can also help target solutions. Remote sensing can be used to monitor shoreline changes for coastal mapping and erosion prevention; examine ocean circulation and current systems; measure ocean temperature and wave heights; track sea ice; trace hurricanes, earthquakes, erosion, and flooding; and monitor land use to help decide how best to protect natural resources.

“Global environmental, meteorological, and climate demands are huge. They are driving quality [and] precision and generating vast amounts of data,” said Ralph Cordey, manager of Earth observation business development at Airbus Defence and Space, based in Hertfordshire, England. “We see drivers for making quantitative measurements of things like sea surface temperature, ocean color for plankton blooms, forest height and biomass, soil moisture, wind speed at different heights in the atmosphere, and the concentration of pollutant gases in the atmosphere.”

A wide range of sensing techniques to gather relevant data, each with exacting levels of accuracy, are necessitated by such a diverse array of environmental demands. There is a push in both industry and government to exploit various optical technologies and use all parts of the spectrum from the UV to the far-IR to capture as much information as possible.

Many of the latest advancements in sensing can be found on board aerial systems and satellites, where greater coverage provides a more detailed picture, often using a combination of optical imaging, lidar, and spectroscopy.

A view of Berlin from Sentinel-2 at the highest resolution achievable with the visible bands of the MSI (10 m) (left). In comparison, the same location can be seen from the U.S. Landsat-8’s OLI instrument (30 m) (right). Images contain modified Copernicus Sentinel data (2017-2019). Courtesy of ESA.

“We are already designing and building the future — or at least systems in space that will be operating for several decades to come,” Cordey said. “Those include missions with the sensitivity to monitor the sources of carbon dioxide in our atmosphere and distinguish between natural and man-made sources.”

Today’s commercial sensors, both in orbit and in production, are being designed for finer spatial resolutions — significantly below 1 m. The operation of multiple satellites at one time, known as constellations, allows more rapid imaging opportunities. Evidence of this variety can be seen in the satellite and topographical information featured in the media following disasters.

A satellite image containing modified Copernicus Sentinel data (2017-2019). Courtesy of ESA.

“There is a spectrum of solutions between ‘many and small’ and ‘fewer more performant’ imaging systems — and the distinctions along that path may become less distinct,” Cordey said. “It’s a kind of graduation that leads up to the powerful global mapping machines procured by institutions — such as Europe’s Sentinel-2 satellites.”

With huge swaths of data now being produced, compared with what was generated in the early days of optical remote sensing, the technological infrastructure on the ground for processing and disseminating data has been ramped up dramatically.

Sentinels in space

The Sentinel-5 Precursor (Sentinel-5P) is the latest satellite to launch as part of Europe’s Copernicus series of satellites. The initiative is headed by the European Commission in partnership with the European Space Agency (ESA) and aims to pull together vast amounts of global data from satellites and from ground-based, airborne, and seaborne measurement systems to provide a comprehensive picture of Earth’s health.

ESA coordinates the delivery of data from upward of 30 satellites and is currently developing a new family of satellites, called Sentinels, specifically for the Copernicus program. So far, three complete two-satellite constellations are in orbit, plus an additional single satellite, the Sentinel-5P.

Various bands from the MSI of Sentinel-2 are used to reveal different processes on the ground. In July 2017, wildfires broke out in Italy, including blazes on the slopes of Mount Vesuvius, near Naples, that led some to believe the volcano was erupting again. A natural color band combination that uses the visible bands, plus highlights from the shortwave infrared part of the spectrum, shows the extent of the disaster and smoke (top). Shortwave infrared bands penetrate the smoke of the fire (bottom). Courtesy of USGS/ESA.

On board this latest satellite is a multiwaveband spectrometer, covering the UV to the shortwave infrared (270 to 2385 nm), that can identify pollutant gases in the atmosphere, including nitrogen dioxide, sulfur dioxide, and carbon monoxide.

Troublingly, massive concentrations of carbon monoxide were recorded in August 2019, a marked increase from the previous month. The Atmospheric Infrared Sounder (AIRS) on NASA’s Aqua satellite tracked carbon monoxide plume blooms in the northwest Amazon region, drifting south and east in a more concentrated plume toward Sao Paulo.

Josef Aschbacher, director of ESA’s Earth observation programs, said, “Over the last months, we have observed more and more vegetation fires on our planet — in Brazil, Siberia, Greenland, Africa, Spain, Greece, and many other places. Our Earth observation satellites keep a close eye on them in order to inform people and politicians with undisputed facts about our changing planet.”

OPUS’ RSD5000 is deployed on the roadside to measure pollutants emitted by the exhausts of motor vehicles. Courtesy of OPUS Remote Sensing Europe (OPUS RSE).

The watchful sentries carry out their duties every day at a few kilometers spatial resolution over the entire globe. The satellites’ data is also used in the Copernicus Atmosphere Monitoring Service, operated by the European Centre for Medium-Range Weather Forecasts (ECMWF) in Reading, England, which provides detailed monitoring and forecasts of atmospheric pollution.

“This mission will be pursued with the Sentinel-5 instruments to be flown on board the next generation of MetOp satellites,” said Philippe Goudy, head of ESA’s Earth observation projects at the European Space Research and Technology Centre in Noordwijk, Netherlands. “In geostationary orbit, the next generation of Meteosat satellites will embark Sentinel-4 spectrometers for air quality and an infrared atmospheric sounder.”

Spectroimagery is used to generate pictures where each pixel has precise spectral content, indicating everything from the nature of the soil and vegetation to its water content.

The Airbus-built Sentinel-5P satellite in one of Airbus’ cleanrooms in Stevenage, England. Courtesy of Airbus/Max Alexander.

“Observation of spectra from the top of the atmosphere has been widely expanding since the beginning of the 21st century,” Goudy said. “In specific spectral regions this allows [us] to access the concentration of the constituents of interest — mostly pollutants and greenhouse gases.”

At wavelengths where transmission is poor, the satellite senses higher-altitude layers. Where transmission is optimal, the whole column down to the surface is quantified. Measurements in between allow scientists to determine the contribution of various altitude layers, a technique that is widely used to compute temperature and humidity profiles. This data is then fed into numerical weather prediction models for accurate forecasting.

At present, due to signal-to-noise limitations, spectrometry from space is restricted to satellites flying in low Earth orbit (LEO). To fully observe the globe, LEO satellites must orbit from pole to pole, with the natural rotation of Earth ensuring that a different region is covered on each pass.

Although imaging spectrometers are powerful for isolating minor constituents of the atmosphere via their absorption fingerprints, the spectrometers are tricky and costly to build because they require high spectral resolution and sensor sensitivity, and at the same time need to be as small and lightweight as possible. However, another optical sensing technique exists that has not yet been fully exploited in space: lidar.

Lidar sensors in space

Lidar is a technology at the forefront of remote sensing from space. It is the technique of choice for precise distance measurement (photogrammetry), such as over ice sheets — as in the U.S. ICESat program — or for sensing atmospheric constituents in which the impact on the spectra is too weak to allow accurate measurement.

“I see further growth in lidar sensing, because it is a controlled and precise method for examining aspects of the world, be it forests, ice caps, or the composition or motions of the atmosphere,” Airbus’ Cordey said.

Today, the French/U.S. CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) mission uses lidar to measure the vertical profile of Earth’s atmosphere from around the world. Data collected on the location, altitude, and optical properties of clouds and aerosols enables researchers to better understand climate mechanisms and the role that clouds and aerosols play in climate change.

“Lidars in space remain a challenge because of their high thermal dissipation, their sensitivity to alignment, and the reliability of the active components,” said ESA’s Goudy. “A world premiere has been achieved recently with the launch in 2018 of the Aeolus mission, which measures wind speed using the Doppler shift of a lidar signal reflected on molecules and aerosols.”

ESA’s Aeolus satellite is the world’s first lidar mission to use Doppler techniques to measure wind speed in the atmosphere at various heights. In essence, it fires a series of UV pulses into the atmosphere from its orbit and picks up light reflected by molecules and aerosols via its large telescope.

“It has led to a greater understanding of how powerful lasers can be operated in space in ways that avoid damage or contamination to the optical elements that they use,” Cordey said. “By getting over hurdles in development, it is now providing new valuable data for meteorologists who previously had to rely on tracking clouds or even releasing balloons.”

Emissions policy changes

In September 2015, a scandal erupted that not only made international headlines, but also brought remote sensing into prominence and led to a complete overhaul of emission control policies. The scandal was “Dieselgate,” in which the U.S. Environmental Protection Agency found that carmaker Volks- wagen had intentionally programmed its turbocharged direct injection (TDI) diesel engines to activate their emissions controls only during laboratory emissions testing.

The programming software, which has since been found in the diesel engines of many other carmakers, works to suppress emissions of nitrogen oxide during vehicle tests, while the engines emit 40× more in real-world driving.

The scandal heightened awareness of the higher levels of pollution emitted by diesel-powered vehicles and prompted officials to conclude that the only proper legislative response is to ensure that cars are tested on the roads throughout their lifetimes.

This year, the European Union will adopt a new regulation that will set new limits on the pollutants emitted by diesel vehicles. To this end, the EU is currently collecting data to establish these limits carefully — which means big business for optical sensing specialists.

“European legislation is a driver these days, as the new market surveillance regulation includes remote sensing as a tool to monitor the real emissions of the automotive market and encourages the different EU member states to control these emissions on a regular basis,” said Javier Buhigas, head of technical consultancy at OPUS Remote Sensing Europe (OPUS RSE). “Citizen pressure, due to environmental and climate change crisis, is also another driver, pushing authorities to apply more control over manufacturers and prevent future frauds like Dieselgate.”

OPUS RSE, based in Madrid, is currently the only worldwide ISO 17025 accredited laboratory for the remote measurement of real traffic emissions. Today, its customers come in the form of local, regional, or national authorities that wish to monitor the real-world emissions from traffic, as well as some private companies, such as those running fleets of buses or trucks. Transport firms are drawn to using remote sensing to monitor the health of their fleets and identify vehicles in need of repair.

“Optical remote sensing of exhaust emissions of real-driving traffic, the basic principles of its science, has been refined substantially for commercial applications over the last 20 years,” Buhigas said. “Now we can measure more pollutants and [deploy] user-friendly devices for quick enforcement on the roads or wide coverage monitoring.”

OPUS is currently improving the scope of the pollutants that it can accurately measure from the exhaust plume of normally circulating vehicles, including nitrogen oxide and ammonia. “We are also improving data analysis and IT capabilities, including real-time data transfer and analysis,” Buhigas said.

“The future of remote sensing of real-driving emissions has a lot of potential in the next couple of decades. The electrification of the market is being extremely slow and combustion engines will be around us for decades,” he said. “In the meantime, more control and regulation of real-driving emissions will expand around the world.”