Lasers probe global climate from space

With the orbiting of ICESat planned for this winter, a laser altimeter on board will become perhaps the single most important tool in the effort to understand global warming. The Geoscience Laser Altimeter (GLAS) will provide climate researchers with vital data needed to get a full picture of what is happening to Earth's climate, what is causing higher temperatures, and therefore what might lie ahead.

The only instrument on ICESat, GLAS will serve many purposes in monitoring the icecaps, sea ice, clouds, aerosols and vegetation.¹ Its basic operation is simple and like that of any laser range finder. A 300-W average power diode-pumped, Q-switched Nd:YAG laser generates 40 pulses per second of both 1064-nm infrared (IR) and 532-nm green radiation. Each 5-ns-long pulse is focused by a 1-m mirror (see Fig. 1) and directed straight downward. The reflected light is then collected by the telescope, with the time delay measuring the distance to the reflecting surface. The green pulse is used to measure the height of clouds and aerosols, while the IR pulse, which penetrates clouds, is used to measure the height of the icecap, sea-ice, and land-based vegetation.

Using signals from the Global Positioning Satellites, the altitude of ICESat itself can be determined to better than 1 cm and the mean altitude of ice caps to a similar accuracy, although errors in any single measurement can depend on the slope of the ice over the 70-m-wide laser spot. The spots will be spaced at 175-m intervals along the path of the satellite, which will crisscross Earth at an average 15-km spacing.

The extremely accurate measurements of the icecap altitude are needed because climatologists currently do not know whether the Greenland and much larger Antarctic ice caps are shrinking or growing. If global warming is leading to shrinking ice caps, this may indicate that the warming process is accelerating because the ice caps themselves cool the Earth by reflecting sunlight. If the ice caps are stable or growing (perhaps because of increased snowfall), it could indicate that global warming is a temporary aberration in climate that will be reversed by Earth's own stabilizing mechanisms.

The 1-cm accuracy from GLAS will reduce the uncertainty in measuring the ice mass by more than fivefold over the current situation, allowing at the same time indirect measurement of global sea-level changes to an accuracy of 0.4 mm/year. Researchers will then be able to determine the balance between ice accumulation and melting to about 5% accuracy, as compared with the current 30% uncertainty in this key ratio. The new satellite will provide similar accuracy in measuring the thickness of sea ice over the arctic ocean, where measurement uncertainties have been even greater, leading to some alarmist, and inaccurate, reports in the popular press of the possibility of an ice-free North Pole.

The satellite's measurement of clouds and aerosols—such as dust clouds—in the atmosphere are equally important in solving the global warming puzzle. One of the greatest uncertainties in determining global climate is the effect of clouds and dust on Earth's radiation balance. On the one hand clouds reflect sunlight, cooling the Earth, but they can also absorb IR radiation from the ground, providing a degree of warming insulation. By measuring the height of clouds and aerosols and their relative reflectivity, data provided by GLAS will go a long way toward improving models of cloud action and to aiding the understanding of how cloud-cover changes affect global temperature (see Fig. 2).

FIGURE 2. In addition to measuring ice-cap thickness, the GLAS laser altimeter will measure the altitude and reflectivity of clouds and aerosols, producing data like that shown in this simulation. Even cirrus clouds that are invisible to other techniques, but that are important in climate formation, can be detected.

On land, GLAS will measure snow cover, and provide a map of vegetation coverage and height—data crucial to understanding the balance of carbon dioxide absorption and production. It will also produce the most accurate topographic map of land masses with a vertical accuracy at each point of about 50 cm.

Preparing LIDAR from space

A global map of wind patterns is essential for both climate and short-term weather prediction. Scientists have long known that space-based LIDAR would be an excellent way to obtain such global coverage with high resolution. LIDAR, widely used on the ground and from aircraft, bounces laser pulses off aerosols in the atmosphere and measures velocities from the Doppler shift of the return signal. But the development of a space-based LIDAR has proven difficult, despite considerable effort by NASA and its contractors.

The most important challenge is the need to continuously scan the laser beam. To derive horizontal wind speeds, the laser beam has to be directed at an angle from the vertical and for thorough coverage of all wind directions, the beam has to be continuously rotated around the vertical axis. The problem is that rapidly spinning a 1-m telescope is not easy in space, unless serious measures are taken to reduce the mass of the device.

Working with the University of Alabama in the Huntsville Center for Applied Optics, the Marshall Space Flight Center spent several years developing a prototype LIDAR instrument to be used on the space shuttle as a proof of concept. The device, the Space Readiness Coherent LIDAR Experiment (SPARCLE) used a stationary telescope and laser, but scanned the output beam by rotating a wedge of silicon.² The primary mirror of this test device was only 25 cm in diameter.

SPARCLE was designed as a compromise between the high performance required for space-based LIDAR and the need to reduce costs. Eventually, however, NASA abandoned this approach for a number of reasons. A 1-m rotating silicon wedge was deemed impractical both because of the difficulties of growing and cutting such a large perfect crystal and because of the mechanical stress produced by rapidly rotating such an asymmetrical shape.

NASA and the University of Alabama's efforts are now taking a different approach.³ By reducing the mass of the telescope itself, using high-strength metal alloy shells instead of bulk materials as structural elements, this LOMASS design (light weight optics using metal alloy shells and surfaces) seeks to avoid the problems of the asymmetrical rotating wedge (see Fig. 3). Studies indicate that the primary mirror assembly in this design will have only 10% of the mass of the SPARCLE primary with the same optical performance.

FIGURE 3. In the LOMASS concept for space-based LIDAR, the telescope that focuses the laser beam will be made out of hollow shells of high-strength alloys, much lighter than standard bulk materials.

Monitoring ozone from ground and space

In addition to measuring wind speed, LIDAR can measure the concentration of pollutants and trace gases in the atmosphere. Differential Absorption LIDAR (DIAL) measures the absorption at wavelengths that are absorbed by a given gas, thus measuring the concentration of these gases. Absorption is measured simultaneously at two related wavelengths and the difference indicates gas concentration. Again, the time delay of the return signals gives a measurement of distance, so a laser instrument can profile the density of trace gases within a slice of atmosphere.

Systems manufactured by Optech Inc. (Toronto, Canada ) are among the first commercialized products in this field. These DIAL systems, with 50-W excimer lasers measuring at 308 and 350 nm in the near ultraviolet (UV) have been used for monitoring zones from ground-based stations as part of the Network for the Detection of Stratospheric Change (NDSC), an international effort to monitor the stratosphere. These systems, pointing straight up, are now operating at stations in Toronto and Eureka in Canada and at Andoya, Norway.

Optech is now working with NASA and the Canadian Space Agency to develop ORACLE, a space-based DIAL system for ozone monitoring. ORACLE will provide global coverage and be able to monitor the ozone concentration at all levels of the atmosphere between ground and 50-km altitude. As with SPARCLE and LOMASS, a key consideration in the design of ORACLE is weight reduction. Although ORACLE does not require the side-looking scanning of the wind monitoring LIDAR, because it looks straight down, it requires a much larger mirror, some 2 m in diameter to achieve the accuracy needed in ozone measurements. Optech is still testing a number of alternative light-weight materials for this task.

Once ORACLE and a future wind-monitoring satellite join GLAS in space, much of the critical data needed to understand global warming and climate change will be obtained by laser-based instruments.