Ozone Limb Sensor Restored on NPOESS Prepratory Project

COLORADO SPRINGS, Colo. — The U.S. decision to resume construction of a key ozone satellite sensor, though good news in the view of U.S. scientists, still leaves open the question of whether the United States will launch a series of such sensors on its future weather satellites, according to officials with NASA and the National Oceanic and Atmospheric Administration (NOAA) .

Scientists will use the readings to monitor the health of the planet’s radiation-absorbing ozone layer following the ban on ozone-damaging chlorofluorocarbons. Ozone is also a heat-trapping greenhouse gas, and so the readings could better define its role in global warming, NOAA and NASA scientists said.

The Ozone Mapping and Profiler Suite is the instrument package that would gather those readings. It is scheduled to fly on the satellites of the new National Polar-orbiting Operational Environmental Satellite System (NPOESS) and on a NASA-managed pathfinder satellite called the NPOESS Preparatory Project scheduled for launch in 2009. Last year, during a cost-cutting overhaul of the NPOESS program, managers cut a key part of the ozone suite: a sensor that would measure the vertical distribution of ozone in the atmosphere by looking off toward the curve, or limb, of the Earth.

NOAA Administrator Conrad Lautenbacher announced April 11 that the limb sensor will be added back to the NPOESS Preparatory Project’s ozone suite partly at the insistence of climate scientists. He spoke during a press briefing at the National Space Symposium 2007 in Colorado Springs.

Managers in charge of developing the NPOESS constellation, however, are still weighing whether or how to restore the limb sensor to that program , said NASA and NOAA scientists. NPOESS is a joint effort of NOAA and the U.S. Air Force, with NASA playing a secondary role. The NPOESS satellites are slated to start launching around 2013 and will provide global weather forecasting information for civilian and military users for years to come.

Even with the uncertainty regarding long-term measurements from the limb sensor, scientists were happy with Lautenbacher’s announcement.

“This is a nice step. One would hope that in the future we could either get that back on [NPOESS] or find a way to compensate for that,” said Paul Newman, a physicist and ozone expert at NASA’s Goddard Space Flight Center in Greenbelt, Md. “When they took [the limb sensor] off there were a lot of people who were really upset by that. I think there’s a lot of joy in the fact that it’s coming back now.”

An NPOESS official, who asked not to be named, said many options are being explored to find a solution beyond the NPOESS Preparatory Project . The solution would not necessarily involve NPOESS directly, this official suggested. “What are our international partners doing? What are the Europeans going to fly that might have similar measurements? Are there free flyers? We’re looking at everything,” the official said.

The multi-billion-dollar NPOESS constellation originally was supposed to carry sophisticated climate-science sensors in addition to its primary weather-forecasting instruments for NOAA and the Defense Department. A huge NPOESS cost overrun forced managers to overhaul the constellation by cutting some climate sensors from the spacecraft designs. One of the casualties was the first limb sensor, already under construction at Ball Aerospace and Technologies Corp. of Boulder, Colo., for the NPOESS Preparatory Project spacecraft.

Plans for subsequent limb sensors were erased from the NPOESS blueprint as well. A downward-looking, or nadir, sensor was retained in the ozone packages for both NPOESS and the pathfinder satellite, however. The nadir sensor will produce global maps of the atmosphere’s total ozone content, but cannot see the detailed, vertical distribution of the gas, NOAA and NASA scientists explained. Scientists lobbied vigorously for restoration of the limb instrument. The sensor was embraced in the first-ever decadal survey of U.S. Earth science priorities, Lautenbacher noted.

The National Research Council’s Spa ce Studies Board patterned that survey after the astronomical decadal surveys that have been used so effectively to build political support for programs in that discipline. The limb instrument “was one of the instruments that the decadal survey recommended that we look at restoring and find options for,” Lautenbacher said.

NASA and NOAA would share the $10 million cost of completing the instrument for the pathfinder spacecraft, which will test key NPOESS sensors and continue climate science measurements begun with NASA’s Earth Observing System satellites, Lautembacher said.

In terms of climate change, “ozone is a very nice little greenhouse gas” because it traps infrared radiation, Newman said. Climate scientists need to know the distribution of ozone in the troposphere, the weather region of the atmosphere, if they are to improve climate predictions, he said. Newman’s expertise is studying the seasonal ozone hole of Antarctica, and the new ozone sensors will offer a major improvement in detail. “One of the first signs of recovery will be the change of the [ozone layer] shape over Antarctica,” Newman said.