Ocean-Tracking Receiver to Aid Weather Forecasts

Now, scientists at the University of Massachusetts Amherst and the Jet Propulsion Laboratory (JPL), Pasadena, are designing and building the next generation of orbiting tracker for NASA that will supply such data with unparalleled precision.

The 18-inch receiver being built at UMass Amherst, is part of the larger instrument expected to greatly enhance forecasting. It works by reflecting 35-GHz microwaves off the Earth’s surface from an orbit 600 miles above to track factors that long-range meteorologists use to predict climate phenomena. Knowing water temperature and current flow can help to give early warning of an El Niño effect, for example, which periodically triggers drought, floods, and other unusual weather events, costing billions of dollars.

The lead researcher building a critical component of the instrument, an interferometric receiver, is Paul Siqueira of the UMass Amherst Microwave Remote Sensing Laboratory and associate professor of electrical and computer engineering. He and colleagues recently received a $1.08-million, three-year NASA grant to design and build the receiver, which is expected to be launched with other supporting instruments aboard the space agency’s Surface Water and Ocean Topography (SWOT) satellite sometime between 2013 and 2016. It will provide a continually updated map of global water levels, topography and temperature for the oceans and for selected inland waters.

This latest interferometer project from UMass Amherst and JPL represents a significant improvement over a previous version that flew on board the space shuttle in 2000, and orbited at a lower altitude (140 miles). The new generation will carry more advanced electronics, and will be smaller, lighter and consume less power, hence cheaper to launch and operate.

The interferometer works by bouncing a microwave beam off the water surface below and measuring the difference in arrival time back at the antennas located 30 feet from each other on the satellite. The instrument takes extremely accurate readings of the water height at many points worldwide, according to Siqueira. “With both antennas receiving signals at nearly the same time, we measure the difference in time that it takes each signal to reach the antennas, and then with a simple geometric transformation, determine the height of that spot in the ocean.” The microwaves are extremely low power and will be harmless to people, wildlife and boats in the water, Siqueira points out.

With these data, water temperature can be calculated, given that a half-inch change in ocean height over a 100-foot vertical volume corresponds to a one-degree Fahrenheit change in temperature. “The warmer the water, the more oceans swell, and the more water goes into the atmosphere,” the engineer explains. “What you’re getting from the satellite measurements is a temperature map of the oceans that has been derived from its topography.” Mapping in this way can alert observers to changes in ocean temperature and currents – keys to predicting hurricane tracks, monitoring features such as the Gulf Stream and, ultimately, assessing the climate variables.

“The more detailed measurements you have from a satellite,” says Siqueira, “the more accurate global climate model you can create.”