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Engineers Use Microwaves to Track Weather

By Helen M. Wise

Using a process called microwave remote sensing, two researchers in the UMass Department of Electrical and Computer Engineering are working hard to make forecasting the weather easier.

Their technique could also be used to understand other fast-changing natural phenomena, such as ocean currents and sea-ice, a goal usually frustrated by the very dynamics of the complex meteorological processes involved.

Engineers Robert E. McIntosh and Calvin T. Swift explain: Taking measurements by conventional means at or near the relevant surface can be slow, circumscribed by practical difficulties, and sometimes downright dangerous. Too, sometimes before sufficient data can be collected to make the situation – such as a developing hurricane, for instance – intelligible, it has already changed dramatically. Under these circumstances, remote sensing, i.e. a method that allows researchers to look at what is happening from farther away rather than close-up and piecemeal, may yield better information.

Nor can the researchers depend on an infrared remote sensing camera, which requires crystal-clear conditions in order to record what’s below, often a forlorn hope in weather systems.

The technique the researchers are working on is known as radiometry. Operating on a microwave frequency in the range of four to eight gigahertz, the equipment can “see” through darkness and clouds. As the sun beams its broad-band energy to earth, the antenna on the radiometer (for present test purposes protruding from the belly of a specially converted turbo-prop aircraft) picks up the much smaller emissions picked reflected from the earth’s surface and tries to make geophysical sense of them.

What the instruments receive is an array of “signatures” characteristic of various kinds of cover – for instance, specific crops, fresh and salt water, different kinds of ice. Moisture has little attenuating effect on signals, which means that the equipment is able to function in adverse weather conditions.

Passive System

For their first project, McIntosh and Swift spent 18 months building a passive system that merely reads signals reflected from earth, as opposed to an active radar system which sends down its own beam of microwave radiation and reads the return. Departmental colleague David M. Pozar designed the aluminum antenna, built on campus.

After assembling the components, including $25,000 worth of microwave equipment, the researchers are now busy with the task of fine calibration, a six-month exercise to gauge the system stability, a critical factor given the need for the equipment to withstand operational stresses, especially aircraft vibration.

Friends for the past decade, both McIntosh and Swift started their careers in the field of plasma physics. About seven years ago, McIntosh, who’s taught at UMass for 16 years, became interested in measuring inospheric effects with high-frequency signals (a kind of upwards-looking remote sensing).

Swift first worked on re-entry projects at NASA’s Langley Research Center in Hampton, Virginia, in 1962. By 1962 he had become involved in his first microwave project, a radiometer mounted on the Buzzard’s Bay Bridge.

Four years ago, he invited McIntosh down to NASA/Langley to collaborate on remote sensing work. In 1981, Swift joined the UMass faculty, helping to set up the Microwave Remote Sensing Laboratory, one of several facilities in the School of Engineering to promote research in microwave engineering.

Massachusetts itself is a leader in the microwave industry, which grew up after World War II. Swift was recently elected a Fellow in the Institute of Electrical and Electronics Engineers in honor of his contributions to microwave remote sensing of the oceans. 

Practical applications of microwave remote sensing are wide-ranging, the collaborators say, with radiometry techniques being used to monitor ice formations, severe storms, and such ocean conditions as wind speeds and current, along with surface roughness. Hitherto, information on icing conditions in polar regions has never been comprehensive and up-to-date. Ice is a fast-changing solid which, in addition to taking the dramatic forms of islands, bergs, and floes, at certain seasons may also present a complicated jigsaw of brash ice (small floating fragments) and frazil (gray slush), with small “leads” (channels) either open or partially re-frozen in between.

As oil companies step up their offshore drilling activities in Arctic regions, they need regular and active pictures of icing conditions, since operational delays prove exceedingly costly.

Swift, whose group at NASA logged thousands of hours flying “ice missions” over the Beaufort Sea, the Great Lakes, and the coast of northern Norway, has developed an algorithm to determine from their radiometry signatures the age of ice formations, from first-year to multi-year (potentially the most damaging to offshore rigs).

Weather satellites have since furnished the data to verify his calculations. Climatologists and oceanographers also have an interest in the information gleaned from this kind of remote sensing operation, because it tells them more about the phenomenon of heat transfer – in which heat energy from warmer waters beneath sea-ice rises through cracks to help generate climatic patterns in the atmosphere above.

While still working for NASA, Swift accumulated a lot of data on storm systems, and in August 1980 his group was “lucky enough” to reconnoiter Hurricane Allen, a so-called 500-year Caribbean storm. Flown in a C-130 aircraft of NOAA, the instruments picked up surface wind speeds over the entire rain column and also measured rain rates. Swift is still analyzing this data which, he notes, correlates handily with information collected  by more conventional means from a low-flying aircraft which almost got trapped within the hurricane’s eye wall.

Satellite Applications, Too

In terms of oceanic monitoring, their present equipment is capable of registering water temperatures to within one degree Centigrade and salinity to better than one part per thousand in coastal work. Both Swift and McIntosh had considerable experience in such remote sensing operations, having flown over such areas as Chesapeake Bay and around the coast of Puerto Rico to detect fresh water outflows. They’re now hoping to supplement their present capability with a C-band radar (active) system operating on the same frequency as their radiometer, with support from NASA, their long-term sponsor, the Office of Naval Research, and industry.

As they start to assemble parts for this project, they’re becoming increasingly interested in satellite applications of microwave remote sensing, especially for the synoptic view of oceanic conditions they offer. For a start, Swift hopes to develop an algorithm for deciphering data from an oceanographic satellite scheduled for launching in 1985. Carrying a passive microwave system for defense and meteorological surveillance, it will also furnish data to civilians working in conjunction with NASA and the U.S. Navy.

Giving added impetus to the development of an active system at UMass is the European plan to launch a microwave remote sensing satellite toward the end of the decade.

To date, McIntosh and Swift are uncertain whether or not their system’s C-band operating frequency will pick up wind speed and direction ith sufficient resolution, but having both their active and passive  systems on the same frequency should prove “very helpful” for other reasons, they say. They envision their system installed in a satellite in geostationary orbit – in other words, traveling at the same speed as the earth’s rotation, and thus remaining over a particular spot on the surface to take repeated signatures over a wide area.

Among the pieces of useful information provided by the return signals would be the movement of ocean currents. These can be detected by the Doppler effect: Areas of surface water that are moving toward or away from the observation point produce a slight shift in frequency, from which change the speed of the current is calculated. Water temperature and salinity, two other important variables, can also be monitored since these co-vary with the water’s conductivity. Information on wind conditions comes from monitoring capillary waves, the small swells produced when moving air ruffles the surface of the water.

As helpful as the multiple capacity of this orbiting system would be its ability to scan an area roughly equal to two-thirds of the North Atlantic in 90 minutes, with readings taken in six-mile intervals. A far cry from the oceanographer dropping abuoy over the side of a boat and measuring its movement – a necessary exercise in “ground truthing,” but one which itself mechanically interferes with the water, the researchers point out.

A mass of current satellite data on oceanic conditions would obviously supplement the hitherto piecemeal basic research conducted in the past. Just as important, it should make possible some interesting applications McIntosh and Swift would like to pursue. They have in mind, for example, a synoptic study of the Gulf Stream, an approximately 50-mile-wide, ever-shifting current with many eddies peeling off.

To the north, oceanographers have collected so-called warm core rings, and to the south, cold core rings, important for the marine life they harbor.

To be able to pinpoint these highly productive areas at intervals throughout the day would obviously help commercial fishing boats to locate the most plentiful catches. Too, McIntosh and Swift would like to study ocean currents and wind speeds over the large area of the North Atlantic “observed” by the microwave system. Passed on to trans-Atlantic shipping, this information could result in more fuel-conscious routes – and in a much more precise forecasting of the destination of oil spills – two factors which would more than pay for the cost of the satellite and its launching, they believe.

A special bonus of university research in microwave remote sensing is the way in which it transforms what might seem like “esoteric work in electromagnetism”,” according to Swift,” into practical projects in such interdisciplinary fields as oceanography, climatology, glaciology, and engineering design. This technology will also play a role in interplanetary exploration, the researchers say, with NASA already planning to map the surface of the cloud-covered planet Venus with a radar-imaging system within the next couple of years.

So far, up to 15 students have been involved at any one time with various aspects of the UMass project, and two, Kathleen Ryan and Douglas DoHerity, are writing their master’s theses on this topic. Recently a number of the students accompanied the two professors to the Army Corps of Engineers’ Cold Regions Research and Engineering Laboratory in Hanover, N.H., to continue calibration work on the passive system, using tanks of frozen brine to simulate field conditions.

The team planned to spend the early part of the summer on further calibration tests, this time over tanks of smooth water back at UMass.

After checking the special aircraft modifications being made for their equipment, they expect to be flying storm missions for NOAA out of Miami, beginning around the start of the official hurricane season in July. As a result of their experience, some of the 20 students involved in the summer’s work are likely to find jobs in the microwave industry, as have some of their predecessors.

Given the accuracy of the data collected so far by McIntosh and Swift, they are optimistic that radiometry techniques will play a vital role in telling us more about the earth we live on and will become a major tool in exploring our near neighbors, the planets.