Window on the World
Everyday the fine balance of our planet’s natural ecological system is gradually being altered by many of the normal activities associated with modern daily life that we all take for granted.
Today, therefore, we have an urgent need to monitor and understand both man-made and natural changes to our atmosphere, land and the oceans in every part of the world.
In many cases satellites are the only way to obtain suitable data. Satellites orbiting the Earth can gather comprehensive information. But visual systems that rely on daylight and clear conditions to ‘see’ the surface have a distinct disadvantage – they cease to function when it is cloudy or at night.
So European engineers, under the direction of ESA, developed a special instrument called a Synthetic Aperture Radar (SAR). As one of the key instruments on the ERS-1 and -2 satellites the SAR is a high-resolution, wide-swath imaging radar producing high quality images of the oceans, coastal zones, polar ice and land regions, irrespective of weather conditions and cloud coverage or whether it is day or night.
It works by bouncing microwaves off the Earth’s surface and measuring the reflected radiation. A SAR also has five different modes of operation, allowing a variety of image types and giving us a new and permanently open window on the world.
What is ERS?
The first SAR was launched into space by Europe’s Ariane-4 rocket in July 1991 as one of three main instruments on ESA’s ERS-1 spacecraft. It was followed by a second on ERS-2 in 1995.
At the time the two ERS satellites were the most sophisticated Earth observation spacecraft ever developed and launched in Europe.
ERS-1 completed its operation in 1999, overlapping with the new ERS-2 launched in 1995. These highly successful ESA satellites have collected a wealth of valuable data on the Earth’s land surfaces, oceans, and polar caps.
They have been called upon to monitor natural disasters such as severe flooding or earthquakes in remote parts of the world.
On a global scale they have expanded our understanding of the interaction between the oceans and atmosphere, ocean currents and changes in the Arctic and Antarctic ice, giving climatologists more confidence in assessing climate trends.
The ERS satellites have also kept a close eye on agricultural areas, forests, coastlines and marine pollution.
By detecting land-use changes, such as the destruction of tropical rain forests, ERS data has helped governments around the world police a broad range of local and global environmental problems.
For the good of Europe
As the most comprehensive modern day source of satellite environmental data, the ERS spacecraft have brought significant benefits to Europe and the rest of the world.
These benefits have ranged from improved public services, such as weather forecasting and crop monitoring, to the creation and sharing of commercial and industrial enterprises, and helping European scientists to excel in key areas of environmental research.
Who uses the data
Today, several hundred research groups worldwide, involving at least 2000 scientists, use ERS data to further their studies.
Much of this research has paved the way for the formation of a number of international initiatives, with countries across the globe working together to combat environmental problems.
Perhaps the most significant of these is the Inter-Governmental Panel on Climate Change (IPCC), which has led to the Montreal Protocol for worldwide reduction in the production of aerosol gases/chlorofluorocarbons (CFCs).
The satellites and their instruments
In 10 years Europe’s ERS-1/2 spacecraft have provided us with a very different view of planet Earth thanks to a suite of advanced and complementary instruments designed and developed by ESA.
The ERS-2 satellite circles the Earth at a height of 800 km and completes an orbit every 100 minutes, crossing each pole in the process. The special orbit – known as a polar orbit – means that the Earth is gradually rotating beneath the spacecraft and so on each different orbit a slightly different part of the ground is seen. Using this type of orbit the ERS-2 can, therefore, cover the entire globe in just three days.
Both ERS satellites were built with a core payload of two specialised radars and an infrared imaging sensor. The two spacecraft were designed as identical twins with one important difference – ERS-2 included an extra instrument designed to monitor ozone levels in the atmosphere.
Active Microwave Instrument (AMI)
The Active Microwave Instrument is the largest onboard system and combines the functions of a Synthetic Aperture Radar (SAR) and a wind scatterometer (SCATT). The AMI has three modes of operation: image mode and wave mode (performed by the SAR); and wind mode (by the SCATT). In image mode, the SAR produces highly detailed images of a 100 km wide strip of the Earth’s surface day and night and in all weather conditions. In its wind and wave modes, the instrument continuously measures global ocean surface wind speeds and directions, and provides information on the direction and shape of ocean wave patterns.
Radar Altimeter (RA)
This measures variations in the satellite’s height above sea level and ice with an accuracy of a few centimetres and helps provide data to know the satellite’s exact orbital position. As well as contributing data on the position of ice flows below, the instrument produces ocean surface wave height and wind speed information for climatologists.
Along-Track Scanning Radiometer (ATSR)
The ATSR consists of two instruments, an Imaging Infrared Radiometer (IIR) and a passive Microwave Sounder (MS). The infrared sensor provides detailed maps of the temperature at the surface of the seas and oceans, accurate to more than 0.5 degrees Celsius. It also measures cloud top temperatures, cloud cover and land surface temperatures (useful in monitoring forest fires). For ERS-2, the infrared capability was enhanced with the addition of visible channels which enable the estimation of vegetation cover. The Microwave Sounder is a passive radiometer providing measurements of the total water content of the atmosphere within a 20 km footprint.
Global Ozone Monitoring Experiment (GOME)
In the light of the increasing concern about atmospheric ozone levels, the GOME instrument was added to the ERS-2 payload. This ultraviolet and visible light spectrometer provides information on ozone, CFCs and trace gas levels. A more advanced version of GOME will be carried on the Metop spacecraft series – three polar orbiting satellites currently under development. These will produce high-resolution images, detailed vertical temperature and humidity profiles and temperatures of the land and ocean surface on a global basis.
Additional to the core payload on both ERS-1/2 are:
Precise Range and Range Rate Equipment (PRARE)
An all-weather microwave ranging system designed to provide measurements used for highly precise orbit determination and geodetic applications, such as movements of the Earth’s crust.
This optical device operates in the infrared and is used as a target by ground-based laser ranging stations to determine the precise altitude of the spacecraft.
ERS 1/2 Tandem Mission
A double success story
Towards the end of 1993 it was becoming clear that ERS-1 was still in very good condition technically and operationally, even though it was coming to the end of its original lifespan. ESA saw the advantage, in terms of both scientific research and technological achievement, in overlapping the work of ERS-2 with the remaining few months of life of ERS-1 by flying the two satellites together in a tandem mission.
Shortly after the launch of ERS-2 in 1995 ESA decided to link the two spacecraft in the first ever ‘tandem’ mission which lasted for nine months. During this time the increased frequency and level of data available to scientists offered a unique opportunity to observe changes over a very short space of time, as both satellites orbited Earth only 24 hours apart.
During 1999 the ERS-1 satellite finally ran out of fuel, far exceeding its planned lifetime. ERS-2 is expected to continue operating for several more years.
Benefits to Europe
How does Europe benefit?
Environmental problems require global cooperation. The political and commercial strength of a united European space power is more important today than ever before.
ESA provides just that strength. As its satellite data becomes more widely used across the globe, Europe is becoming firmly established as a major player in the commercial and environmental monitoring aspects of the space industry.
The ERS satellites and their data have provided many benefits to people in Europe and throughout the world in three key areas.
Improved our scientific capabilities and understanding about the planet we live on by:
- taking regular global measurements of the oceans, ice and land
- continually monitoring the delicate and important ozone layer and atmospheric gases for the study of pollution
- creating a huge amount of data for climate research
- establishing international cooperation amongst the world’s scientists, making Europe’s scientist’s key partners in many new areas of science.
They have contributed to our quality of life through enhanced public services:
- improving our ability to forecast the weather and monitor sea and ice, and agricultural practices
- building our knowledge to help sustain society in Europe and in developing nations
- helping to shape environmental decision-making in Europe and beyond for the benefit of Europeans and other nations.
Equally important, the development and operation of the ERS satellites has helped European industry remain competitive by:
- encouraging a truly independent European spacecraft and ground-systems industry
- creating and securing more employment in Europe
- sharing out industrial skills within Europe
- developing a European information service business to compete in the world market.
An environmental library
Our planet is constantly evolving around us. Volcanic discharges and forest fires are just some of the natural causes of change in our environment. But man’s everyday activities are speeding up the effects of change and altering the delicate ecological balance of our World.
Pollution from the cars we drive, the power stations that generate our electricity and the industrial processes that release toxins into the air is altering our atmosphere at an unprecedented rate.
To ensure we can understand and stop any further damage to the environment we need ongoing, coherent and precise data of the kind generated by ERS-1 and ERS-2.
Using the technology of the ERS satellites we have been able to observe all these elements on a daily basis, building up a library of information from which to learn and act upon.
Combating El Niño
El Niño is the weather phenomenon responsible for some of the world’s most drastic and devastating disasters. From summer droughts in Australia, Africa, Brazil, Asia and Central America to milder winters and severe flooding in other parts of the World, El Niño is now recognised by scientists as one of the world’s most serious environmental threats.
Monitoring particular aspects of the environment can provide answers to many of the mysteries surrounding El Niño. ERS onboard instruments can measure changes in sea temperature and levels – two key environmental changes associated with the El Niño event.
The huge amounts of satellite-generated data now available to scientists ensures that they are more able to understand the signs and causes of this extraordinary natural phenomenon and are better able to help governments and communities predict and prepare for the future.
Observation of our polar ice caps plays an important role in helping us assess changes in our climate. Fears of melting ice in polar regions are well-founded - it would result in widespread flooding across the globe with many of our coastal cities disappearing beneath rising sea levels.
Fortunately, ERS data can help scientists see whether ice is melting or accumulating. Detailed maps of the ice sheets can be drawn up on a continuous basis to watch for emerging adverse patterns or trends.
Oil is one of man’s most valuable resources. We use it to power our cars, create our electricity and produce many of the plastic and cosmetic products that we all rely on. A key element of our modern world, it is both our saviour and our enemy.
As we become increasingly reliant on oil to drive our economies we are finding it more and more difficult to find new sources. The ERS-2 Radar Altimeter is able to produce topographic maps of the sea floor to help us identify new oil supplies.
At the other end of the spectrum, ERS monitoring is helping us to prevent the devastation that oil pollution can bring to our coastal, sea and marine environments. The highly sophisticated Synthetic Aperture Radar (SAR) can detect oil spills on the ocean surface.
This high vantage point can help special oil spill teams in clearing accidental oil disasters more efficiently and it can also act as a policing presence. Thanks to ERS satellite monitoring, oil pollution monitoring is now well established. Even illegal discharges, principally from ships clearing their tanks at night, can be identified and the offenders prosecuted.
The ozone layer is a composition of atmospheric gases above the Earth’s surface that acts like a sunscreen, filtering all the harmful rays of the sun and preventing the planet from overheating.
Since the early 1990s, scientists around the world have been concerned about the number of holes appearing in this protective layer. If the Earth warms up too much, agricultural crops could fail, water supplies could become scarce, and diseases and conditions such as skin cancer could increase.
To help us monitor these effects, ERS-2 includes an ultraviolet and visible light spectrometer, called the Global Monitoring Ozone Experiment (GOME) for atmospheric ozone level research.
By keeping ongoing information on ozone levels we can help to prevent further damage to the environment by restricting offending chemicals and pollutants or finding alternatives.
Watching over our future
None of us know for certain what the future may hold. But what we do know is that ESA’s ERS satellites have spent many years keeping a watchful eye on our planet to make sure we do not go too far wrong.
ERS-1 has only recently been decommissioned after nine years of service – three times its planned lifetime. In this time it has provided us with information and images of the Earth never seen before.
ERS-2 continues to watch over our planet and now has a new ally in Envisat, ESA’s new ‘super satellite’ which has begun its five-year mission to enlighten us further about our changing world.
Beyond Envisat, ESA’s Living Planet Programme will involve the use of smaller satellites on specialised missions to help us expand our knowledge of our beautiful but fragile world.