Sensing Technology Keeps a Remote Eye over India

Lynn Savage, lynn.savage@photonics.com

In India and other developing nations, the interactions of people with the soil, water, and plant and animal life around them have taken on an urgency in recent years as the country’s population grows, its cities expand and its dependence on natural resources threatens to swallow the entire subcontinent.

To study and react to rapid societal, economic and health-related changes within its borders, India has embraced satellite-based remote imaging technology. Relying primarily upon imaging and spectroscopy, remote sensing provides a multispectral look at all of the activities that affect the health and well-being of the country’s populace.

The most exciting aspect of remote sensing is that it gives a synoptic picture of the Earth’s surface features – and since the pictures do not lie, this adds beauty to the Earth’s resources, said Atiqur Rahman, an associate professor of urban environment management at Jamia Millia Islamia in New Delhi. Rahman and his colleagues have been using remote sensing technologies since the early 1990s to map the effects of environmental shifts and other problems on impoverished urban areas in India.

Measurements of particulate matter (PM) are the chief concern of scientists studying air pollution, with the density of PM_2.5 and PM₁₀ particles (roughly 2.5 and 10 µm in diameter, respectively) accepted as standard measures of air quality by the World Health Organization (WHO). Such particulates have profound effects on mortality and morbidity caused by cardiovascular and respiratory diseases, such as heart failure, asthma and lung cancer. The WHO benchmark for PM₁₀ is an annual mean of 20 µg/m³. New Delhi is consistently well over 200 µg/m³; India on the whole, about 84 µg/m³. Throughout the country, some 120,000 people die each year from causes attributable to air pollution.

Air quality data flows

Delhi is the tenth most polluted city in the world. In 2000, the Indian Supreme Court mandated that buses, taxis and other public transportation within the city be converted from gas or diesel engines to compressed natural gas. Nonetheless, air pollution skyrocketed, likely because reductions in pollution from public transportation were offset by a tremendous increase in private vehicles, which still run on gas and diesel, according to a report by Naresh Kumar of the University of Iowa in Iowa City and Andrew D. Foster of Brown University in Providence, R.I.

Using data from the NASA-run Moderate-resolution Imaging Spectroradiometer (MODIS), Kumar and his associates found that there is a definable correlation between the factor known as aerosol optical depth (AOD) and PM₁₀. In a study of air quality over Kanpur City in India, they found that a 1 percent change in AOD was associated with a 0.85 percent change in PM₁₀.

AOD runs on the principle that the wavelength of an optical signal changes due to interactions with aerosols, whether they originate from human-driven or natural causes. Human-generated pollutants are difficult to identify with exactitude among other particulates, but reasonable estimates based on meteorological conditions and seasonal differences are possible.

In addition to tracking pollution, remote sensing data is used to assess farm acreage and crop yields, to manage urban areas and coastal regions alike, and to monitor forests, deserts and water resources, including rivers and aquifers. Some scientists use it to study climate change, the socioeconomic effects of urban sprawl and the changes in health resulting from enforcement of new pollution laws.

In a report for the Urbanization and Global Environmental Change project, Rahman and his colleague Maik Netzband of Ruhr University in Bochum, Germany, wrote that remote sensing brings a wealth of assistance to social science research, such as population modeling. However, they noted, “a generally applicable and operational mapping of [urban] settlements has proven difficult.”

As more people head from rural areas to urban centers, populations in existing areas become denser, agricultural land along the borders gets paved over, and the city grows. In Delhi, increased urbanization has led to increased heat zones, with the mean surface temperature in the entire city rising as much as 2 °C between 2001 and 2005 because of changes from agricultural land use to new buildings and roads. Researchers hope to use remote sensing data to abate heat zones and other negative effects of rampant urbanization, such as altered patterns of water use and availability, the scarcity of urban services and housing, and poor infrastructure.

Eyes in the sky

Remote sensing instruments generally are installed on aircraft that pass over specific regions, or on satellites in either geosynchronous or heliosynchronous orbits. Generally, the devices are imagers using the visible to near-IR bands or spectrometers working in a number of wavelengths, including multispectral and hyperspectral sensors that provide deep knowledge at every pixel.

The most recent remote sensing payload – Resourcesat-2 – was launched into polar orbit on April 20, 2011. India now has a total of 10 such satellites in operation, one of the largest such networks extant, according to P.G. Roy, dean of the Indian Institute of Remote Sensing. Begun in 1966, the organization has been charged with preparing young scientists to work in the fields of remote sensing and geographic information systems.

Resourcesat-2 is one of several Earth-orbiting satellites that help monitor India’s air quality, urban sprawl and other health factors. Courtesy of Indian Space Research Organization.

The Indian Remote Sensing (IRS) satellite program supports what is perhaps the largest group of remote-sensor-bearing spacecraft in use, including the IRS, Oceansat, Resourcesat, IMS, Technology Experiment Satellite, Cartosat and Risat (Radar Imaging Satellite) missions. Plans are in place for additional Resourcesat, Cartosat and Oceansat launches. In all, India is second only to the US in terms of spending on space research as a percentage of gross domestic product (0.10 percent).

“India has very good remote sensing platforms, like Resourcesat-1 and -2 as well as Cartosat-1 and -2,” Rahman said.

However, nobody believes that remote sensing technology has reached its peak.

“The improvement that I would like to see,” Kumar said, “will involve enhanced spatial resolution.” Currently, he added, MODIS offers daily global coverage at a resolution of 250 m at best; other sensors provide 30 m resolution but on cycles that require 10 to 16 days of orbit. “If the spatial resolution of MODIS data that have daily global coverage is improved to 30 m, it will serve the purpose,” he said.

“The scope of remote sensing is immense, and we [have] yet to utilize its full potential,” Roy added. The challenge of the coming decade will be using all of the satellite data that is accumulating.