From: Canadian Space Agency
Posted: Monday, March 26, 2001
Ever since the dawn of the space age, we have grown accustomed to the ``big picture'' imagery of far-distant planets and moons, which have come to be studied as whole entities. Increasingly, astronomers are interpreting data from telescopes and space probes to identify the features of other bodies in space.
But what of our own planet? Actually, a Canadian Earth observation satellite, RADARSAT-1, is being used to develop a better understanding of the globe from pole to pole. Imagery produced by this renowned leading-edge Earth observation satellite is helping to answer crucial scientific questions about the rate and extent of global climate change. Indeed, RADARSAT-1 data collected so far show that Antarctica, one of the most remote parts of our world, is changing and possibly in response to changing climate. This is important because the Antarctic ice cap is a huge reservoir of fresh water and changes in that reservoir directly change global sea level.
Following its launch in 1995, RADARSAT-1 embarked in 1997 on the first Antarctic Mapping Mission. This scientific initiative was a collaboration between the Canadian Space Agency (CSA), NASA, the Byrd Polar Research Center at The Ohio State University, California's Jet Propulsion Laboratory, Colorado's Vexcel Corporation and the Alaska SAR Facility.
Ken Jezek, Principal Investigator of the mission, at the Byrd Center, developed an unprecedented high-resolution mosaic map of the entire frozen continent in 1999. The mission produced the first complete image of the continent, providing a baseline from which future changes could be measured. It also demonstrated the capacity of RADARSAT-1 to measure ice flow, using its Synthetic Aperture Radar (SAR) in an interferometric mode (InSAR), over several repeat passes.
The Antarctic Mapping Mission revealed features of Antarctica in a way that great polar explorers of the past, like Shackleton, Scott and Amundsen, could only dream of. Images produced during the mission showed the rocky tops of volcanoes surrounded by the West Antarctic ice sheet, giant fields of snow dunes that zigzag across East Antarctica, and even the surface outline of subglacial Lake Vostok. These images also revealed that a network of ice streams, one ice stream of which is 800-kilometres long and reaches speeds up to one kilometer per year, was transporting close to 80 cubic kilometers of ice into the sea every year.
In November 2000, another Antarctic Mapping Mission was completed. This second mission aimed at producing even higher resolution image mosaics of Antarctica north of 80 degrees and using radar interferometry to obtain the first complete measurement of surface velocity around the perimeter of the continent. This new look at Antarctica will help scientists better understand the forces that drive the ice sheet and the response of the ice sheet to climate change.
Although it will take several years to process the entire data set captured during this second mission, images have already been compared with earlier data and the comparisons graphically portray the retreat of ice shelves in the Antarctic Peninsula which many scientists consider to be a consequence of global warming. But that is not the whole story. With the continental scale observations from these two scientific missions, it is now possible for the first time to inspect a variety of different environmental regimes and the local behavior of the ice sheet margin. Not surprisingly, the situation now seems more complex.
Early analyses show that in just three years the Amery Ice Shelf has advanced five kilometres, while the Shirase Glacier, located in the Indian Ocean sector of the continent, has retreated twelve kilometres, and an enormous tabular iceberg (10,915 km2) calved from the Ross Ice Shelf. Is this variability due to the forces of external climate on the great ice sheet or is it due to natural and episodic instabilities that arise from the forces that control complex glacier flow? The new velocity measurements from the second mission will help answer that question. For example, initial analysis of InSAR data of the Lambert Glacier, one of the world's longest ice streams, reveal a sinuous network of tributary glacier that eventually feed the Amery ice shelf. Understanding the interplay between these tributaries as well as tributaries within other similar systems in East and West Antarctica is of prime scientific importance in understanding how ice is transported through the ice sheet and out into the sea.
Canada's RADARSAT-1 has played a major role in these Antarctic missions and is a technological success story. Traveling at a speed of close to 7 kilometres a second, in a near-circular polar orbit, about 800 kilometres above the surface of the Earth, it goes around the Earth once every 100 minutes, completing fourteen orbits per day. RADARSAT has the unique ability to train its synthetic aperture radar on the Earth, penetrating cloud cover and the shroud of darkness. This is a special advantage in the polar regions, where the darkness of the winter night is measured not in hours, but in months. SAR is particularly well-suited for providing high-resolution images of subtle differences in surface features, such as ice and geological formations. RADARSAT can be rotated, and is equipped with tape recorders, so that it can collect data, before downloading it to ground stations.
But RADARSAT is also a human success story. The way the satellite was handled by its Canadian operators during these two scientific missions is a testimony to their resourcefulness and ingenuity. The Canadian Space Agency performed the difficult feat of rotating RADARSAT 180 degree during the first Antarctic Mapping Mission, in order to provide the needed angle of incidence for Antarctic imaging. During the follow-on mission, the Canadian Space Agency controlled the 800-kilometre orbit of RADARSAT-1 in a very precise manner by periodically firing the spacecraft's onboard thrusters so as to position the satellite within a few hundred meters of the same position occupied 24 days earlier. All the while, CSA coordinated the capture and downlinking of data to a network of stations distributed around the world.
The two scientific missions have produced a new baseline dataset for the scientific community one that will prove invaluable in monitoring the state of the Antarctic ice cap. This same leading-edge technology is also being brought to bear on studies of polar ice in Canada's Arctic region and has been used to produce the first, high resolution radar map of all of Canada.
RADARSAT-1 is being exploited beyond its nominal lifetime. RADARSAT-2, currently under construction for the Canadian Space Agency by MacDonald Dettwiler and Associates and scheduled for launch in 2003, will build on a legacy of service to commercial, government and academic users. These unique spacecraft, and the team of highly skilled professionals operating them, are an important resource, providing key data for clients in the fields of mapping, geology, oceanography, ice surveillance, agriculture, natural resources exploration, supporting disaster and relief efforts worldwide and helping scientists improve their understanding and measure the effects of global warming on our planet Earth.
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