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Ocean Surface Topography from Space
JPL Develops Next Generation Radiometer For Newest Ocean Observing Mission
November 01, 2007

AMR instrument
This photo shows the Advanced Microwave Radiometer (AMR).

The Ocean Surface Topography Mission on the Jason-2 spacecraft has some big shoes to fill. Its job is to continue one of the most important on-going chronicles of Earth's changing climate--detailed measurements of global sea level begun in 1993 by TOPEX/Poseidon and carried into the present by Jason-1. This unprecedented view of the ocean has led to a new understanding of Earth's climate.

Scheduled for launch in June 2008, Jason-2 will extend the now 15-year-long record of the ocean's highs and lows into two decades. Researchers say continuing these observations is critical for learning more about how the ocean and climate work together, especially over long time periods, and improving our ability to anticipate the future.

While Jason-2 will make the same measurements as its illustrious predecessors, this latest ocean-watching satellite comes with some improvements. One is its Advanced Microwave Radiometer (AMR), designed and built at JPL.

"We've made a lot of advances in electronics since the TOPEX microwave radiometer, which was basically made out of spare parts from Seasat," says Amarit Kitiyakara, AMR instrument manager. Launched in 1978, the pioneering Seasat mission proved the value of space-based ocean observations. "The Jason-2 AMR uses state-of-the-art monolithic microwave integrated circuit technologies for the electronics and a new antenna design. These have reduced its mass and power requirements and will provide improved performance and reliability."

Like TOPEX/Poseidon and Jason-1, Jason-2 uses a radar altimeter to measure the height of the sea surface. The altimeter sends a pulse of radar to the ocean's surface and times how long it takes for the signal to bounce back. To determine just how far the signal has traveled-the exact distance between the satellite and the ocean surface-requires knowing how fast it has gone. That's why the altimeter is twinned with a microwave radiometer.

Just as poor road conditions can slow down a speeding car, so water vapor in the atmosphere can delay a radar signal. Looking straight down from the spacecraft, the radiometer can measure exactly how much water vapor and liquid water is in the air below. It senses three microwave wavelengths: the signature thermal signals emitted by water vapor, clouds and the ocean surface roughed up by wind. "The combination of these three channels allows the radiometer to see just what we want it to see," says Kitiyakara. "No other instrument can do this so accurately."

"The AMR is like a very sensitive radio receiver," he adds. "The amount of energy that we're measuring is so small, it's almost like being able to measure the mood of a person."

"The radiometer measures radiance, which we convert to an equivalent temperature called brightness temperature," says Shannon Brown, the instrument science lead. To be sure that its readings are correct, the radiometer has to be calibrated. "A scanning radiometer can look to a fixed external target for hot and cold reference points to calibrate itself," he explains, "but the AMR, like the radiometers on TOPEX and Jason-1, is fixed looking downward. So, it has an internal reference temperature to calibrate itself against."

The Jason-2 radiometer will be able to correct for any changes in calibration over time due to stress or aging, however, much more quickly than its predecessors. "We've developed an autonomous radiometer calibration system for AMR," says Brown. "It's a continuous, on-going system that allows corrections to be made before the data are processed."

Line drawing of Jason-2
A line drawing of the Jason-2 spacecraft.

The satellite's raw measurements are collected and processed every 30 days into a uniform format, called the Geophysical Data Record, or GDR. Over time, Brown says, the radiometer's measurements may still need to be fine-tuned and some data may have to be reprocessed. With the new system, however, scientists will get better data from the very start of the mission.

In addition to improved electronics, Jason-2's Advanced Microwave Radiometer has a larger antenna than its predecessors. The bigger the antenna, the smaller the features it will be able to sense. This increase in resolution will be particularly useful near the coast.

"When you're looking at water vapor in the atmosphere, the scale in the open ocean is very large, basically the size of a storm," says Jason-2 project scientist Lee-Leung Fu. "But on the coast, where the water meets the land, it is a different situation. There is a rapid change in moisture content. We've all experienced it-sudden patches of fog or overcast skies. The scale of water vapor is much smaller, perhaps ten times smaller, than in the open ocean."

"The scale of the oceanography is also smaller," says Fu. "An El Niño or La Niña may cover hundreds or thousands of kilometers, but coastal phenomena are more on the order of ten kilometers. "We expect that the AMR will be able to measure the change of water vapor over much smaller scales than does the Jason-1 radiometer, and thus will much improve the ability of computing coastal currents from radar altimeter measurement."

While the radiometer's primary function is to determine the amount of water vapor in the atmosphere to get the most accurate sea surface height measurement from the satellite's altimeter, there is another benefit from the data it collects.

This image shows the antenna in an acoustic chamber.
This image shows the antenna in an acoustic chamber.
"Water vapor is the most important greenhouse gas," says Brown. Along with the long-term record of sea level, TOPEX/Poseidon and Jason-1 have gathered an equally long record of global water vapor. "There's a concern that as sea surface temperatures rise," says Brown, "there is a positive feedback. There is less water vapor over cooler areas of the ocean and more water vapor over warmer water. With more water vapor, the climate will warm even more."

While the Advanced Microwave Radiometer was built specifically for Jason-2, its designers have the long-term in view. "This is a stand-alone design," says Kitiyakara. "You could bolt it on to any spacecraft. The design anticipates that this mission will move from an experimental to an operational mode."

That's the plan for Jason-3. NASA and the French space agency, CNES, which jointly developed and operated the TOPEX/Poseidon and Jason-1 missions, share responsibility for Jason-2 with the U.S. National Oceanic and Atmospheric Administration and the European Organisation for the Exploitation of Meteorological Satellites. These two weather agencies will take the lead for Jason-3, the next in the series.

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