science, health & technology

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Tale of two instruments
by Brandt Irion

Our project manager poked his head into the doorway of our temporary laboratory building in the NASA hangar at the San Jose, Costa Rica airport. “No flight today, the flight has been canceled.” “Are they doing this just to make our lives difficult?” I wondered. What a letdown! We had arisen early for nothing. It was about 6:00 a.m. My lab partner and I had been up since 3:30 a.m. in order to make a pre-flight check of our atmospheric research instruments. The instruments were bolted to the wing of a NASA research aircraft called the WB-57. The WB-57 had landed in the rain the day before and the slick runway had worn through the five plys of the right hand tire as it touched down. The tire exploded with a bang that a bus driver told me he heard half a mile away. Fortunately there was no damage to the aircraft. Now the NASA project manager decided not to risk another wet landing and scrubbed the flight.

We were taking part in the Costa Rica Aura Validation Experiment (CR-AVE) mission. The Aura Satellite, launched July 15, 2004, carries instruments that map global ozone. Ozone (O3) is a gas that forms naturally in the stratosphere when individual oxygen atoms combine with oxygen molecules (O2). Ozone molecules found in the stratosphere are formed when oxygen molecules are broken up by the sun’s ultraviolet (UV) radiation. The stratosphere is that layer of our atmosphere that begins around six miles above the earth’s surface and continues upward to about 31 miles. This stratospheric ozone is important to all living organisms on the earth's surface because it protects them from harmful UV radiation from the sun.

One of the instruments aboard NASA's Aura Satellite is called the Ozone Monitoring Instrument (OMI). The OMI was built by a group of Dutch and Finnish companies. It was put on the satellite by The Netherlands Agency for Aerospace Programs (NIVR) and the Finnish Meteorological Institute (FMI). This instrument measures the quantity of ozone throughout the atmosphere and creates a map of the earth much like a topographic map that shows scientists where the Stratospheric Ozone Layer is thickest and where it is the thinnest.

How accurate is the OMI? Should we trust what it is telling us? Scientists answer these questions by taking similar ozone measurements on the ground, from weather balloons, and from aircraft. They compare those measurements with OMI’s measurements. This process is called validation. AVE stands for Aura Validation Experiment. It is the aircraft-based validation of what instruments aboard the Aura (including OMI) are telling us. The Aura Validation Experiment (AVE) was originally conceived by scientists from Pasadena, California’s Jet Propulsion Laboratory (JPL) and NASA in response to the Aura instruments’ need for good validation data from other sources like aircraft. Funding for the entire series of AVE projects comes from NASA. There have been AVEs in New Hampshire, South Texas, and Costa Rica so far. CR-AVE is the particular AVE that took place in Costa Rica from January 12 through February 11, 2006.

The main thrust of CR-AVE was to validate data that the instruments aboard the Aura are providing. Also, scientists hoped to get some good basic atmospheric research in along the way. Flights took place about every other day. The NASA WB-57 typically took off from San Jose airport in the morning of the flight day and flew South to Panama, Colombia, Ecuador, and the Pacific Ocean near the Galapagos Islands. In doing so, it arrived at a rendezvous point at which it was right underneath the Aura Satellite. After about 5 hours of flying time it returned to San Jose to land. There were about 27 research instruments bolted to the wings and fuselage of the WB-57. Each instrument had a team of two to five people to maintain and download data from it. Instruments were contributed to CR-AVE from major universities, JPL, the National Center for Atmospheric Research (NCAR), the National Oceanic and Atmospheric Administration (NOAA), and private companies. Costa Rica's partners in the project were the Direccion General de Aviacion Civil, Centro Nacional de Alta Tecnologia (CENAT).

A typical day for an instrument team was to arise at about 4:00 a.m. and drive or take the bus to the airport. They would go the hanger and make a preflight checkout of their instruments which were already installed on the aircraft. Then they were free to relax for about eight hours while the pilot and flight crew prepared the WB-57 for takeoff and while the plane was in the air. They would return at a pre-arranged landing time, meet the WB-57 and download the data that their instruments had recorded during the five-hour flight. If all went well and the instrument behaved correctly then they could relax, analyze their data, and archive it. If the instrument had problems then they knew they had a long night and next day of troubleshooting ahead of them until the next flight. The troubleshooting and repairing of a malfunctioning instrument against a deadline of the next flight was the most brutal aspect of the CR-AVE project for our instrument team.

The WB-57 that carried our instruments is one of two such aircraft owned by NASA. This model of aircraft started out as the Canberra, a bomber used by the Royal Air Force in the 1950s. Dozens of B-57s were built by the Glenn L. Martin company for the US Air Force. B-57s saw wartime service in Vietnam. Now two of these bombers do research and photography missions for NASA from their hangar near the Johnson Space Center in Houston, Texas.

The WB-57 was loaded with 27 atmospheric research instruments for the CR-AVE project. Photo courtesy of NASA

This article focuses on two instruments: the Two Dimensional Stereo Particle Imaging Probe (2DS) and the CCD Actinic Flux Spectroradiometer (CAFS).

2DS

The 2DS is an airborne laser-based instrument that measures the size, shape and concentration of cloud particles.

The 2DS has two identical optical channels: the horizontal channel and the vertical channel. Four tubular aluminum arms jut out in front of the instrument. Each arm has a window in it through which a laser beam passes. Two of the arms shown here comprise the horizontal channel and two comprise the vertical channel. Photo courtesy of SPEC Inc

The 2DS has two identical optical channels: Two arms comprise the vertical channel and two comprise the horizontal channel. For each channel the principle of operation is the same: A laser beam shoots out of the channel's transmit arm and across the gap between the arms to the receiving arm. It then bounces off mirrors and goes through lenses to illuminate an array of light sensitive diodes. A light sensitive diode is a small electronic device whose electrical characteristics change when light falls on it. Each of the tiny diodes in the array sends a signal to the 2DS's computer if it is receiving laser light and does not send the signal if it is in the shadow of a cloud particle. Any cloud particle that passes between the transmit arm and the receiving arm will cast a shadow on the array of diodes. The array of diodes sends an image of the cloud particle to the 2DS's computer based on the shadow the particle casts on it

Any picture made electronically consists of thousands of small picture elements or “pixels.” The smaller the pixel size in relation to the picture size, the greater the resolution and quality of the image. In the case of the 2DS, each pixel is a small square that is 12.5 microns per side. A micron is one thousandth of a millimeter. Cloud particles as small as 50 microns and as large as 1600 microns in diameter can be imaged by the 2DS. The larger the particle that is imaged, the greater the resolution. To get an idea of the sizes of cloud particles that we are talking about, a hair from a blonde person's head is about 20 microns in diameter.

The 2DS mounts on the wing or fuselage of an aircraft and takes pictures of cloud particles as the aircraft flies at up to 450 miles per hour through clouds. It can take three dimensional pictures of particles that happen to pass through that small area in the center of the sampling arms where the horizontal and vertical laser beams intersect. It has a special computer program that can put together the horizontal view with the vertical view to create a 3-D picture of the particle. Making this fast, high resolution instrument was a huge achievement for the optical and electrical engineers at SPEC Inc, the manufacturer.

These raw images of cloud crystals taken by the 2DS will later have the vertical lines removed by data processing software. Photo courtesy of SPEC Inc

In the lower reaches of the stratosphere just above the troposphere a cloud called sub-visible cirrus is sometimes found. These clouds are not visible from the ground because they are so thin. They CAN be seen from an aircraft looking on-edge (see photo). The WB-57 flew in these very high altitude very thin clouds near the equator. Scientists are interested in these clouds because they may have an effect on incoming radiation from the sun and thus earth's climate and weather. The 2DS made high resolution images and particle size distributions of particles in these clouds. Sub-visible cirrus clouds are made up of ice crystals. There may be particles of all sizes cohabiting inside one cloud. Particle size distributions are often in the form of graphs that depict how many cloud particles of each size were found per liter of cloud. High resolution images and particle size distributions from the 2DS will help scientists learn what these clouds are made of and help them determine if they have any effect on incoming radiation from the sun.

The WB-57 flew through sub-visible cirrus clouds near the equator. Images taken by the 2DS will help scientists determine what these clouds are made of and the effect they have, if any, on incoming radiation from the sun. Photo courtesy of NASA

Is it easy and trouble-free to mount an instrument like the 2DS on an aircraft wing and send it into the stratosphere at an altitude of about 60,000 feet? We did have some challenges.

Associated with the 2DS instrument is a computer that is enclosed in a pressure-tight metal box. One issue that we had with the instrument was that the metal box had a small air leak. This meant that as the WB-57 climbed from San Jose which is 3,770 feet above sea level into the stratosphere at 60,000 feet above sea level, most of the nitrogen gas which we put inside the computer box leaked out. This caused overheating because the cooling fans inside the computer case had no nitrogen to blow around to cool the computer. Also, the computer used a hard disk drive for data storage just like a desktop personal computer. The disk was damaged by operating in the low atmospheric pressure environment. We found that a rubber O ring seal had become displaced and was letting nitrogen gas leak out. Also a valve fitting was leaking. We fixed the leaks in the metal computer box and replaced the hard disk drive early on in the project and had no further issues with pressure leaks.

There were other minor issues with the 2DS instrument but by the middle of the series of CR-AVE flights most of them were solved and the 2DS was providing us with lots of useful data on high altitude clouds.

CAFS

NCAR in Boulder, Colorado contributed the CAFS instrument to the project. The CAFS was more directly involved in the Aura Validation part of the project than the 2DS. It had a sensor mounted on the top of the WB-57 that faced upward toward the sun. It took radiation measurements from which “Column of Ozone” was determined. “Column of Ozone” means “the number of ozone molecules between the CAFS and the sun”. Since the CAFS was flying near the boundary between the troposphere and the stratosphere, that is a measure of how much ozone was in the stratosphere above the CAFS.

Imagine glancing at the sun through a piece of green stained glass. The sun would appear green. You know that the sunlight on the sun’s side of the glass is white. You also know that white light contains the full visible spectrum of light which includes all colors of the rainbow. Since the light on the side of the glass facing your eyes is green you can deduce that the glass must be absorbing the reds and oranges and allowing only blues, greens, and yellows to pass.

The green glass is absorbing reds and oranges but it isn't perfect and doesn't absorb all of them. Some small amount of reds and oranges come through. If you could measure that tiny amount of reds and oranges that are getting through then you could subtract that measurement from the known intensity of reds and oranges on the sun-side of the glass. The difference is a measure of how much the reds and oranges are being attenuated, or diminished in intensity, by the stained glass.

The CAFS measures ozone in much the same way. It measures radiation intensity from the sun at wavelengths from 292 to 400 nanometers (nm). These wavelengths represent the sunlight “colors” ultraviolet through violet. The ultraviolet “colors” cannot be seen by the human eye. Ozone absorbs each wavelength in this range by a known amount. Also, scientists know what the intensity of each wavelength is at the outer reaches of our atmosphere before the sunlight hits the ozone. In this analogy, the ozone in the stratosphere is like the stained glass.

Researchers use CAFS to measure the intensity of each wavelength of sunlight in the 292 to 400 nanometer range in the lower stratosphere. They then subtract that measurement from the intensity of the wavelength that is known to exist at the outer reaches of our atmosphere. Using this difference in intensity, they can determine how much that wavelength has been absorbed in its journey through the stratosphere. Since they know how many ozone molecules it takes to attenuate or reduce the intensity of that wavelength by a given amount they can determine how many ozone molecules are above the aircraft.

Actually, the math is more complex because there are other gases in the atmosphere that also absorb solar radiation in this band of wavelengths, but this is basically how it is done.

Measurements from the CAFS instrument will be compared to measurements taken at the same time and at the same place (although higher in altitude) by the Ozone Monitoring Instrument (OMI) aboard the Aura satellite. The OMI looked down and the CAFS looked up at the same parcel of atmosphere. This process is called validation. It gives us an idea of the OMI’s accuracy. The CAFS was a major player in CR-AVE.

Measurements of ozone in the atmosphere, such as those provided by the OMI and other instruments aboard the Aura, are used by researchers to further their understanding of atmospheric processes. These measurements also help scientists keep tabs on the thickness and distribution of the Stratospheric Ozone Layer in the long term.

Ozone in the troposphere is considered a pollutant. The ozone layer in the stratosphere protects living things on the earth’s surface from the damaging effects of solar UV radiation. The ozone layer in the stratosphere is not uniform. It is thinner in some parts of the world and thicker in others. It ebbs and flows and moves around with the seasons. One day your town may have a thick protective layer of ozone above it and another day that layer may be thin. On thin ozone days UV radiation from the sun is more intense. The US National Weather Service forecasters use OMI data to forecast high UV index days. On such days people are urged to wear shirts and hats and to put on sunscreen. Man made compounds such as chlorofluorocarbons (CFCs) that were released into the atmosphere have been shown to be catalysts for the destruction of ozone (A catalyst accelerates a chemical reaction without itself being consumed or transformed). Because these man made chemicals are not consumed in the reaction, one CFC molecule can destroy many ozone molecules.

Scientists are cautious about declaring that there is a definite link between the recent depletion of  stratospheric ozone and skin cancer in humans. However there is scientific evidence that suggests depleted ozone layers and the subsequently more intense UV radiation can cause a higher incidence of skin cancer.

The WB-57 flight crew wear pressurized suits for protection in case there is accidental loss of cockpit pressure. The WB-57 flew to an altitude of 60,000 feet. That is about five miles higher than the altitude at which commercial aircraft fly. Photo courtesy of NASA

During the CR-AVE project the WB-57 served as an airborne platform for about 27 instruments. These instruments measured pollutants, naturally occurring chemicals, temperature, cloud thickness, and other properties of the atmosphere. This has been the tale of just two of those instruments, a cloud particle imager and an Ozone Column measuring instrument.

Data from these 27 instruments that took part in the CR-AVE project is now being analyzed and will help scientists understand atmospheric processes. It will also give them an idea of how accurate the data is from the OMI and other instruments aboard the Aura Satellite.

To Go Beyond…

NASA’s CR-AVE web site: http://cloud1.arc.nasa.gov/ave-costarica2/

NASA’s news articles about CR-AVE: http://www.nasa.gov/centers/goddard/news/topstory/2006/crave.html and http://www.nasa.gov/vision/earth/environment/crave.html

Information on the Aura satellite: http://www.nasa.gov/mission_pages/aura/main/index.html

Information on the 2DS instrument: http://www.specinc.com/2DS_operation.htm

You may e-mail the author, Brandt Irion, at birion1@juno.com

Also in this section:
The lone psychiatrist in a Third World setting
Two instruments used in the CR-AVE project
STRI social sciences lecture: social inequality needs no market
The La Niña effect