The constellations of spacecraft orbiting Earth high above our heads are essential to maintaining civilisation in the 21st century, particularly in their role in monitoring the changing climate.
Why satellites?
The Earth has been observed from space for many decades. The first picture of Earth was taken by a modified V2 rocket in 1946. Since then, astronauts and increasingly sophisticated types of artificial satellite have been monitoring our world.
U.S. Army, Public domain, via Wikimedia Commons
Image source
for Dark, grainy image of Earth's surface
These robotic sentinels monitor our atmosphere, the weather and climate, land use, pollution levels, forest fires, ocean levels and temperatures and many phenomena of both natural and human origin. Together with communications, timekeeping, global positioning and navigational satellites, these spacecraft are essential to life in the 21st century.
And observation from space is increasingly important to our understanding of climate change. The United Nations considers Earth observation and other satellite capabilities to be crucial in helping meet its goals for sustainable development.
Only satellites can conduct truly global surveys of Earth’s systems, and one of the key measurements satellites are particularly good at taking is sea surface temperature. Water covers 70% of the Earth’s surface, and the oceans, which carry heat from the tropics to the polar regions, act as a principal driver of climatic and weather patterns. Scientists use changes in sea surface temperature as a reliable way to measure how fast global temperatures are increasing.
Measuring sea temperature from space
One satellite instrument in particular has played a hugely important role in measuring sea surface temperatures, and that's the Along Track Scanning Radiometer (ATSR).
ATSRs were flown on two successive Earth observation satellites, ERS-1 and ERS-2. An advanced design, the Advanced Along Track Scanning Radiometer (AATSR) was then flown on Envisat. These three missions overlapped, providing 21 continuous years of sea surface temperature readings.
How does the ATSR work?
The rather opaque title of this instrument actually hints at how it works. All of the Earth’s surface radiates heat, whether at low or higher temperatures. A satellite’s radiometer can accurately detect that heat, emitted as infrared radiation, to fractions of a degree Celsius.
But the atmosphere, which sits between the Earth’s surface and the satellite, distorts these readings. In particular, the water vapour within the atmosphere has a distorting effect on measurements. The ATSRs and AATSR compensate for this effect by scanning the surface of the ocean both directly beneath the spacecraft and along the track of the satellite’s path. By knowing the distances travelled by these two sets of radiated heat, the satellite operators are able to calculate the effect the atmosphere is having on the readings and so arrive at far more accurate temperature measurements.
The two ‘smiley’ apertures allow the infrared heat into the instrument—one from below the satellite and one from ahead.
These two beams hit a revolving mirror which bounces the beams onto the detector. Infrared radiation is also emitted and then collected from onboard calibration units—one very cold and the other very hot. Knowing the exact temperatures of these onboard sources enables the AATSR to constantly attune and check the radiation collected from the surface of the Earth.
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Some of the performance data from the 10-year life of AATSR illustrates the remarkable ingenuity and sophistication of the instrument. The scanning mirror revolved 2.1 billion times with no anomalies; the calibration units remained stable at their required temperatures throughout the mission; and, excluding the preliminary phase of commissioning when all of the systems are tested, the AATSR gathered sea surface temperature data for 95% of the mission’s operating lifetime.
These ATSRs have set a gold standard for sea surface temperature measurements, contributing greatly to our understandings of how the oceans work. Scientists can now see how the oceans themselves are being affected by the greenhouse warming of the planet’s atmosphere, caused by increased carbon dioxide levels from centuries of fossil fuel burning.
What’s next for Earth observation satellites?
AATSR’s successor is now flying on a satellite called Sentinel 3. The instrument is called SLSTR (Sea and Land Surface Temperature Radiometer) as it can now include enhanced measurements of the land as well as the sea.
The two Sentinel 3 spacecraft are part of Copernicus, the most ambitious and sophisticated Earth observation programme to date. Although also linked to ground-based observation and measurement, the Copernicus programme will provide an unmatched space-born capability with some 14 satellites planned or already in orbit.
Between them, these satellites will be able to provide a range of observations including high-resolution imagery of vegetation, soil and water cover; ocean and global land monitoring (Sentinel-3A and 3B); atmospheric composition; high precision altimetry sea level measurements; and polar ice and snow cover topography.
Like ATSR, these instruments—and many others—will help us step up into space, so that we can better see what is happening below. To mitigate the effects of climate change on our planet, we have to first understand it, and space technologies like these are a crucial part of that effort.
Find out more
- European Space Agency, Space for our climate
