Wind and the Wires: A History of Scatterometry

News

By Katie Oberthaler and Ashley Thompson
Spring 2010

Radar Scatterometers take snapshots of the oceans’ winds many times a day from high-flying satellites. They help to measure strong storms, like hurricanes, model global climate, study sea ice, and even navigate international sailing contests. This little-known technology evolved from an even more esoteric history. Before CReSIS scientists flew multi-channel radars over the Arctic and Antarctica, their forerunners were engineering the first scatterometers in space.

In 1965, Richard Moore and Willard Pierson might have been the only two people in the United States who weren’t shooting for the moon.

NASA had just successfully sent a man into Earth’s orbit. Four years later, Neil Armstrong would plant the American flag on the rocky surface of the moon. Yet Moore, then a professor of electrical engineering at The University of Kansas, and Pierson, an oceanographer at New York University, were eager to get their feet wet with a new technology: scatterometry.

Scatterometry uses microwave radar techniques to measure wind speeds and directions near the surface of the ocean and earth.

Pacific Ocean winds

Pacific Ocean winds as captured by QuickScat on January 8, 2004. Ocean surfaces with rough waves send a stronger signal back to radar systems than smooth surfaces, allowing scientists to determine wind behavior with scatterometers. Photo credit: NASA/JPL

Up until the 1960s, naval ships and floating buoys used radar sensors to detect submarine periscopes and other nearby vessels by collecting backscattering data. Around that time, Richard Moore and three other professors had just founded the Remote Sensing Lab at KU, a predecessor of CReSIS. The group believed that they could harness this backscattering to directly measure ocean wind speed and direction. Because earth-based radars provided limited coverage, the group aimed to launch a system into space.

Up in the Air

In November of 1963, Moore received a phone call from Peter Badgley, a NASA representative who was working on the short-lived Manned Space Science Program. He wanted to pool together various instruments to study the earth on a wide scale. “Badgley was really the spark that started this whole program of space remote sensing in this country,” said Moore.

Moore joined a team of scientists in Washington, D.C. from the microwave radar side. He convinced the team to base the microwave radar systems at the Remote Sensing Lab. “I said, ‘Oh, we can do it in Kansas!’” Moore remembers.

Prior to scatterometry, sensors had used radiometry, a passive microwave radar. Although scatterometry provides less detailed images than radiometry, the team thought that a scatterometer would expand the spatial coverage of data collection and enhance imaging radars.

“You don’t want the detailed information if you’re measuring winds at sea,” said Moore. “What are you going to do with 5 meter by 5 meter patches over the whole ocean? You’d be overwhelmed. You want things that cover 10 kilometers or more.”

In 1965, the microwave radar team gave a presentation to the National Academy in San Diego to include a scatterometer on The Department of Defense’s Manned Orbiting Laboratory. The Department of Defense aimed to launch a small, man-inhabited capsule into space to provide surveillance on Soviet Russia and China as well as conduct experimental science tests as a part of the Manned Orbiting Program. Moore hoped the DOD would include scatterometry as one of those tests, but the National Academy did not pick up their proposal.

“They were interested in the moon. Part of our story was that we were going to be remote-sensing the moon, but most of the members of [our] team were interested in looking at the earth, not the moon,” Moore explained.

The Manned-Orbiting Laboratory project never left the ground, folding within the year due to budget problems. However, the project did produce results: Moore remembers coining the word “scatterometry” after his presentation to the National Academy.

“We had been using all these different names for it. I said, ‘Let’s come up with a simple word,’ and that’s what I came up with. And the world seems to have adopted it, because it makes sense because it’s something that measures scattering,” said Moore.

Moore’s next project would make that term ubiquitous among space programs. The Remote Sensing Lab partnered with NASA and the Langley Research Center in 1965 to build a 15 gigahertz scatterometer, one of the first in the world. Building a system at that frequency was just a theory at the time. The U.S. Navy had demonstrated that backscattering could only measure light winds up to 15 knots. Badgley, along with Moore and Pierson, thought they could push those limits.

Linwood Jones supervised the project at Langley. Jones, now Director of the Central Florida Remote Sensing Lab, said the group did not fold in the face of theory.

“We wanted to take good ideas that weren’t ready for space flight and develop them to fly on future satellites,” said Jones. “We were solutions looking for a problem.”

Jones helped Moore and Pierson test their scatterometer. Under the Advanced Applications Flight Experiment, they tested the scatterometer on a NASA C-130 flying over the Gulf of Mexico. Their testing blew the U.S. Navy’s data predictions out of the water. Moore and Pierson demonstrated that the system could actually measure strong storms.

Around the same time as these tests, the scatterometer finally saw its maiden voyage into space. The Langley Research Center helped NASA launch the scatterometer onto America’s first space station, Skylab, in 1973. Comparing the data for the next year against those from ship-based radars, the team verified their prediction that scatterometry could accurately measure ocean winds from space.

Starts, Stops, and Smooth Sailing

The following decades refined scatterometry and expanded its uses beyond Moore’s original idea. A series of satellites proved his initial hunch that scatterometry could study ocean dynamics, but not without technical setbacks. NASA launched the Ku-Band Seasat scatterometer in 1978, only to have a technical malfunction truncate its operation prematurely. However, its six-month tenure demonstrated that Synthetic Aperture Radar (SAR)-specific scatterometers could detect ocean wind vectors. Seasat provided the first global coverage of those wind vectors.

A flurry of European and American scatterometers in the 1980s and early 90s continued to add antennas and improve beam movement to cover wider areas multiple times. The longest-operating scatterometer was launched in 1999 from NASA’s Jet Propulsion Lab. The 13.4 gigahertz SeaWinds Scatterometer aboard the QuickScat satellite operated until just last year when the mechanism rotating its antenna stopped working. SeaWinds lasted seven years longer than anticipated and gathered oceanographic and atmospheric data on 90 percent of the world every day.

Scatterometers have helped meteorologists pinpoint and predict the location and dynamics of cyclones, hurricanes, and basic weather fronts. As scatterometry's global tracking expands, so does its global application. European and Asian research groups were quick to develop their own scatterometers after Skylab launched. Systems like Europe's ASCAT and India's OceanSat-2 have filled the void left by QuickScat with their own devices.

“The scatterometers have revolutionized weather forecasting,” said Jones. “If you want to predict weather, it is a global phenomenon. Scatterometry provides a fourth of a million observations per day. It uniformly samples the globe.”

Hurricane Dora

QuickScat captures an image of Hurricane Dora in the eastern Pacific Ocean in 1999. Tropical storms in the Pacific are difficult to monitor from other platforms; the continuous coverage offered by current scatterometers helps meteorologist track the development and movement of such storms and provides better warnings about their intensity. Arrows show wind direction. Photo Credit: NASA/JPL

Specifically, QuickScat allowed weather forecasters to create a new wind category called the extratropical cyclone hurricane force-wind. Cyclones in the North Pacific and Atlantic Oceans can reach large scales in the winter time and pose hazards to both commerce and livelihoods. Scientists always knew that these storms could produce destructive, hurricane-like winds that intensified quickly. However, they did not possess solid data for their frequency or intensity in these areas of the globe before QuickScat. The monitoring of these winds will now help forecasters predict and track these bursts of wind.

Scientists have also noticed that wind can play role in ocean upwelling, which can bring nutrients to coastal regions and supports marine biodiversity. Wind data is also used to inform commercial fishing endeavors and offshore oil productions. As such, studying the interactions between ocean winds and the water cycle may help scientists map important evaporation and absorption dynamics in the ocean.

Paul Chang leads the ocean winds science team at the National Oceanic and Atmospheric Administration (NOAA). His job demonstrates that scatterometry doesn’t necessarily cater exclusively to the meteorological crowd. He received requests for QuickScat data sets from all kinds of groups.

"I've gotten various emails from different folks interested in putting in wind farms. The level of detail that they look at some of this data is very impressive," said Chang.

Perhaps the most adventurous use of scatterometry comes from the international sailing competitors looking to dodge dicey waves and stiff competition in the Southern Ocean. A Global Challenge Around-the-World sailing race team used QuickScat data to find optimal winds and to understand a cold front they encountered near Brazil. SeaWinds also helped the 2008 Beijing Olympic organizers to prepare for their sailing events in Qindao, China.

Eye on the Ice

When Dr. Moore and his colleagues invented scatterometry in 1965, they did not imagine that their creation would be counseling regatta fleets. Yet, just as KU's electrical engineering efforts have tended toward ice sheet studies, so has scatterometry's applications. Its sensitivity to liquid water's motion and behavior makes it a valuable instrument in glacier studies. Dr. David Long, director of BYU's Center for Remote Sensing, has long been applying scatterometry to polar ice sheets.

Melting snow in Greenland

Scatterometry’s sensitivity provides a better understanding of melting areas by pinpointing where the melt water has percolated back to snow. This photo demonstrates a 15-day time lapse of melting in Greenland in 1999. Photo credit: NASA/BYU

Scatterometry’s sensitivity provides a better understanding of melting areas by pinpointing where the melt water has percolated back to snow. This photo demonstrates a 15-day time lapse of melting in Greenland in 1999. Photo credit: NASA/BYU

"Those areas where water percolates back into snow where it refreezes - we call them ice lenses. They increase backscatter, so we can see where melting has occurred and when using data dating back to the early 1990s that map spatial extent of melt in Greenland."

Applications for this technology extend far beyond the Poles. BYU currently hosts the largest database for iceberg measurements in the world. Since the early 1990s, radar scatterometry has tracked the movements of large icebergs every single day. The National Ice Center drew on data from QuickScat when it operated. Since QuickScat's flame went out, Long, BYU and other interested institutions have turned to the European Space Agency's A-Scat and its polar ice applications for data. Currently-orbiting systems record Arctic and Antarctic ice data twice a day.

The demise of QuickSCAT leaves the status of American scatterometry in limbo. Systems like ASCAT and OceanSat-2 provide some of the coverage that QuickSCAT previously captured. NOAA is pursuing a collaboration with NASA and the Japanese Exploration Agency to launch a dual frequency scatterometer by 2017. The new scatterometer will employ both Ku-band and C-band radar to estimate wind and rain phenomena simultaneously. Previously, rain events have contaminated wind measurements. This dual frequency could also broaden coverage and provide a different depth of snow penetration.

Scatterometry is one field that clearly doesn't suffer from having too many hands in one pot. Broader coverage and international collaboration will continue to improve weather models worldwide. As ice and climate patterns shift, an armada of international scatterometers will only uncover more phenomena to study.

"It's proven very useful, and looking at ocean winds and sea ice applications in the Arctic region opens up more for commerce and exploration. Scatterometry will be useful for looking at the ice and the wind at the same time," said Chang. "At those high latitudes, you'll have a lot of repeat coverage. There's a lot of overlap near the poles."

Chang also said that NOAA hopes to employ scatterometry to study fluxes in global heat transfer and ocean circulation in the future.

The uses for scatterometry have skyrocketed since Moore offered to develop it at the University of Kansas. Moore, now a professor emeritus of electrical engineering at KU, still contributes to CReSIS studies. Although scatterometry is no longer practiced just by Kansas scientists, Moore's efforts proved that there is still plenty to study right here on Earth.


Story Sources:

Moore, Richard K and Linwood W. Jones. "Satellite Scatterometer Wind Vector Measurements - the Legacy of the Seasat Satellite Scatterometer" IEEE GeoSci Rem Sens Newsletter, Issue-321, Sept. 2004

Chang, Paul. "Operational Use and Impact of Satelliete Remotely Sensed Ocean Surface Vector Winds in the Marine Warning and Forecasting Environment." Oceanography 22.2, 2009: 66-79

Buis, Alan. "NASA Assessing New Roles for Ailing QuikScat Satellite ." QuickScat Mission Status Report. 23 Nov 2009. www.nasa.gov.