- Earth observation satellites (EOSs) are spacecraft equipped with sensors to monitor Earth’s atmosphere and surface, providing data on weather, land-use patterns, vegetation, and water bodies.
- EOS data have revolutionised our understanding of global warming, climate change, environmental degradation and contributed to key findings about the rise in global temperatures, shrinking polar ice caps, and the impact of deforestation and urbanisation on ecosystems.
- EOSs can also track air pollution and water quality, and help monitor oil spills and extreme weather events.
Earth observation satellites (EOSs) are spacecraft that are placed in a temporary or permanent orbit around the Earth. These satellites are equipped with sensors, mostly image capturing ones, to collect data on the atmosphere and surface of the Earth, and help in monitoring weather events, land-use patterns, vegetation cover, ice cover, oceanic patterns, and climate change events. As of 2023, there are 322 in-orbit EOSs launched by 93 space organisations and agencies from around the world.
The first EOS was the Television Infrared Observation Satellite-1 (TIROS-1), which was launched in 1960 and had very basic imaging capabilities. The second generation of EOSs (Landsat 1-4, launched between 1970 and early 1980) had improved image resolution and multispectral imaging. The third generation of EOSs (Landsat 5-9, late 1980s-now) have high resolution imaging, advanced capabilities for environmental monitoring, and improved data processing.
India launched its first satellite, Bhaskara-I in 1979; the images obtained from this EOS provided important data on hydrology and forestry. Till date, India has launched 44 EOSs, with the latest one, EOS-07 being launched in early 2023. EOS-07 is expected to generate multispectral high-resolution images for monitoring agriculture, forestry, hydrology, and disaster management.
The soon-to-be launched (in March 2025) NISAR mission, a joint effort between the National Aeronautics and Space Administration (NASA) and the Indian Space Research Organisation (ISRO), will use radar to systematically map the Earth and produce fine-resolution images.
What types of sensors do earth observation satellites have?
Earth observation satellites can carry a variety of sensors that are broadly classified as either passive or active sensors and optical or microwave sensors. Passive sensors are usually spectrometers and radiometers that measure reflected solar radiations, whereas active sensors produce pulses of energy and measure the reflected radiation.
Passive optical sensors include spectrophotometers that measure visible light, infrared light, and thermal infrared emissions. Visible and infrared light sensors record information on the earth’s surface such as landforms, distribution of urban areas, and vegetation patterns. Thermal infrared emissions are used to monitor land temperatures, sea surface temperatures, volcanic activity, and fires.
LiDaR (Light Detection and Ranging) sensors are active sensors that emit light from a laser and can be used to measure forest height and the amount of particulate matter in air. Active microwave sensors such as the synthetic aperture radar have been used to study Antarctic icebergs and track oil spills.
What can we find out from earth observation satellite data?
Data received from earth observation satellites have been instrumental in demonstrating the effects of global warming and climate change. Based on data from satellites and other sources, it is now clear that the global surface temperature of the Earth has risen by 1.28 degrees Celsius (as of 2024) compared to the 1880s, when temperature record-keeping first began. Further corrections to satellite data analyses indicate that global warming was nearly 140% faster since 1998.
Similarly, satellite imagery has shown that the Arctic sea ice coverage has been shrinking at a rate of 12.2% per decade, alerting scientists to the melting of the polar ice caps. This is further supported by EOS data showing global changes in water dynamics, notably sea-level rise on the Earth, and variations in surface water reservoirs.
EOSs have also been invaluable in monitoring the Earth’s oceans by tracking oceanic carbon-cycles, water quality, and even studying marine wildlife and animal migration patterns.
Other key findings that EOS data have contributed to, are in monitoring greenhouse gases, carbon-cycle fluxes, the ozone layer, land-use changes including deforestation and urbanisation, in the prediction of extreme weather events and providing early warning for disasters.
A relatively recent and major phenomenon deduced through EOS data was the effect of the COVID-19 lockdown on reducing global carbon and nitrogen dioxide (NO2) emissions.
What can these satellites tell us about land-use changes?
EOS data has been invaluable for tracking land-use changes, especially loss of tropical forests, illegal deforestation, desertification, urban growth, and monitoring agricultural land.
In India, satellite data have been used to understand and monitor patterns of deforestation and rapid urbanisation in the Himalayas as well as in monitoring the forest cover dynamics in the Western and Eastern Ghats. Illegal logging has also been documented using satellite imagery in Arunachal Pradesh and Karnataka has recently harnessed satellite surveillance to tackle forest encroachment.
In addition, the long-term ecosystem degradation effects of operations such as opencast mining and coal mining, and contamination of groundwater due to seawater sand mining have been shown using satellite data. Riverbank erosion in the Himalayan foothills, Godavari, and the lower Ganges have also been monitored using Landsat imagery.
EOS-derived images have further been used in mapping urban sprawl and changes in urban areas and peri-urban and rural areas. These studies show that built-up areas contribute most to increases in land surface temperatures; such work can aid in understanding the microclimates of cities, especially emerging urban heat islands in fast-growing cities and help in urban/suburban planning to improve the environmental quality of cities.
How does EOS track pollution?
Data from EOSs have been widely used in tracking the levels of air pollutants, specifically carbon emissions, NO2, sulfur dioxide (SO2), methane, ozone, and particulate matter.
Recent studies using EOS data have shown that in India, NO2, which was once thought to be a major air pollutant in cities due to its presence in vehicle exhaust, is just as prevalent in rural areas. Similarly, 16 years of satellite and emissions data on SO2 levels in India, show a slight dip between 2016 and 2019 due to reductions in emissions and an increase in atmospheric water vapour that chemically ‘mops up’ the gas. Trends in stubble burning in north India and the resultant effects on the air quality of the surrounding areas are now being tracked in real-time to help frame policies to mitigate this practice.
The COVID-19 lockdown provided several research groups the opportunity to demonstrate that reduced travel/transportation is important for improving air quality, in national, regional, and city-wide scales in India, using a combination of data from surface and satellite sources. Likewise, data from the satellites Sentinel-2 and Landast-8 were also used to show improvements in the water quality of the Ganges (water turbidity dropped by 55%) and coastal wetlands in southern India and the Sundarbans and during the lockdown.
In addition to monitoring air and water pollution, EOSs can also been used to detect and monitor oil spills along the Indian coast.
Can EOSs help in addressing climate change impacts?
A major fallout of climate change is an increase in the number of extreme weather events that lead to disasters. EOSs are invaluable in monitoring these and serve as space-based early warning systems.
In India, satellite data has been used to not only track cyclones, glacial movements, earthquakes, forest fires, landslides, and extreme rainfall events, but also to map areas prone to flooding and understand rockslides. These have been critical for the development of early warning systems to minimise loss of life.
Satellite data are also being used to predict changes in land characteristics in India. Climate change-induced desertification of eastern and northeastern India, along with a decrease in aridity in north-western and southern peninsular India have been predicted using data on rainfall, evaporation, transpiration, and soil from various sources such as the Climate Research Unit and the National Remote Sensing Centre archive, Bhuvan.
Recent analyses using satellite data on the trade-offs between food, water, and air quality in northwest India, has shown that increased paddy cultivation in this area has led to groundwater depletion and worsening air quality indices, pointing to an urgent need to reconcile food security with preservation of water and air quality.
What does the future hold for the satellites?
With scientists exploring the use of satellite data to predict outbreaks and mortality rates of diseases such as malaria and dengue, healthcare decisions are also likely to be impacted by EOS data. In addition, EOSs have been useful in the management of key industries. For example, in India and Nepal, satellite data are being used to help governments and farmers take informed agricultural decisions. Data from Oceansat and INSAT (Indian National Satellite System) are being used to identify potential fishing zones, manage inland and offshore fisheries, and track illegal fishing.
Currently, earth observation data are immensely valuable for disaster management and informing policies on mitigating climate change.
“Weather and climate science depends heavily on observations to deepen our understanding of the science and inform policy decisions. Without sufficient high-quality observations, major gaps will emerge in our ability to forecast weather and develop climate models for future predictions. In addition, India’s ambitious National Clean Air Program requires high-resolution spatiotemporal data for tracking pollution trends. Achieving this would be difficult without such satellite data”, says V. Vinoj, a Professor from the Indian Institute of Technology, Bhubaneswar. “A well-calibrated EOS in space, working in conjunction with ground-based measurements, represents a one-time investment with substantial returns. Satellite infrastructure will provide valuable, long-term data with high spatiotemporal resolution, making it a cost-effective solution for both advancing scientific knowledge and informing policy”, he adds.
Unfortunately, cutting-edge research now suggests that greenhouse gases may limit the number of satellites that can orbit the Earth. The study indicates that the thermosphere, which is the atmospheric layer where most satellites establish orbits, is shrinking due to greenhouse gases. Decommissioned EOSs are typically equipped with propulsion systems that enable ground control to lower them to approximately 450 km after their missions conclude. At this altitude, atmospheric drag gradually pulls them down to Earth, usually within one to two years, explains Adithya Kothandapani, co-founder of SkyServe, a startup that provides computing services to satellite operators. But now, the decreased density of the thermosphere reduces atmospheric drag on satellites, which in turn, increases their lifespan in orbit which means they may not come to lower heights sooner.
In effect, this can increase space junk and debris that can litter sought-after regions of the thermosphere and increase the changes of in-orbit collisions.
Kothandapani adds that even modelling how atmospheric drag pulls satellites down to the Earth is also affected by GHGs. Therefore, predicting the upper and lower bounds of the timelines for a satellite to drop to Earth have become more difficult due to fluctuations cause by GHG emissions.
For now, India has ambitious plans for the space sector. The Indian space economy is expected to grow from 8.4 billion dollars to 44 billion dollars in the next eight years, and a recent report from ISRO estimates that the Indian economy sees a multiplier effect of 2.5 dollars for every dollar generated by the country’s space sector. With such promising prospects, EOSs are likely to be key components of India’s future economy.
Read more: Glaciers in Eastern Himalayas see marked retreat, finds study that used satellite images
Banner image: A multi-spectral image of Bhidaurya, Uttar Pradesh, taken by the earth observation satellite Cartosat -2 by ISRO. Image by ISRO.
Credits
