Gauging Wind Direction is Worth a Second Look
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March 26, 1997: A new way of looking at the "body heat" of the world's oceans may let satellites measure wind direction at sea level by combining microwave images of the ocean taken from two different angles.
Project scientist Robbie Hood of the Space Sciences Laboratory at Marshall Space Flight Center said the effect is based on what many boaters already know: wind-driven waves look different if viewed upwind versus downwind.
Measuring wind direction will help in understanding how the Earth's climate system works, and contribute to improved weather forecasts. Principal Investigator for this project is Dr. Roy Spencer, a scientist at NASA/Marshall. Both work at the Global Hydrology and Climate Center, a NASA-university partnership.
"While there are existing satellites that measure wind speed and direction at the ocean surface," Hood said, "they use radars that cost ten times as much, and their spatial coverage is not as extensive, as the passive microwave radiometers we are testing for this application".
An object does not have to be red hot to glow. Everything emits electromagnetic energy at all wavelengths. The human body emits infrared radiation that can be detected by special cameras, for example.
The NASA scientists are measuring microwave emissions by the ocean surface at a frequency of 37 gigahertz (37 GHz), about 300 times higher than the FM radio band. For years weather satellites have carried microwave sounding units that measure the microwave emissions of oxygen at 60 GHz as an aid to measuring air temperature. The microwave signals are measured at different polarizations, similar to the use of polarized sunglasses to limit glare off of a water surface.
Listening to the surf
If the ocean is very smooth, it emits less microwave energy. As it roughens with wave action, more energy is emitted by small waves near the wavelength of the radiation (0.8 cm) and by foam generated by the wave action. This provides the basis for the measurement of wind speed.
But which way is the wind blowing?
Engineers at Marshall built the C-STAR, the Conically-Scanning Two-look Airborne Radiometer for flight tests aboard NASA's ER-2 high-altitude research jet.
|C-STAR is shown (left) mounted upside down for checkout on the ground and (right) in the belly of the ER-2 aircraft. The twin mirrors are mounted at the end of a short boom. The circular objects at the top (right) are the antennas. The white object is a calibration heat source.
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"With C-STAR, we hope to measure how the wind is deforming the ocean surface and from that measure the wind speed and direction," Hood said.
The idea is simple. Foam tends to collect on the lee side of waves. Looking at the same piece of ocean from different angles, and comparing the two signals, should tell which direction the wind is blowing from.
A different angle on the problem
Getting the different angles was done by the "conically scanning" aspect of C-STAR. The radiometers were put on a turntable that spins once every 10 seconds to look ahead and then behind of the ER-2 aircraft. The aft-looking scan covers ocean seen 4 minutes earlier by a forward scan.
In addition, C-STAR also used polarizing filters - similar to polarizing sunglasses - at four different angles (vertical, horizontal, and left and right diagonals) to help determine how rough and how foamy the ocean surface is.
C-STAR flew four times on the ER-2 operating out of NASA's Ames Research Center and flying along the coast of California in February. Data from the flights were converted into images in which one can see images of Catalina and other islands, and wind shadows on the lee sides of the islands.
When the color of the images is enhanced, Hood said, differences appear that should yield wind direction information.
|Technician Doug Huie of Mevatec, an SSL support contractor, inspects C-STAR before it is mounted in its pallet (left) and after integration in the belly of the ER-2 (right). The white object at left is an aerodynamic shroud to protect the antenna.
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"The wind direction effect is very small," she said, "on the order of a 1 to 2 K change in brightness temperature." C-STAR measures an "apparent" microwave temperature, or brightness temperature, on the Kelvin absolute temperature scale where a difference of 1 K equals almost 2 deg. F.
Hood and others now must refine the data and analyze it in detail to see if they proved their theory.
"We're pretty confident that we captured what we wanted to on this series of flights," she said.
The data will be analyzed over the next few weeks, and an additional flight is being considered to follow up on the results.