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Introduction

The NASA Airborne Oceanographic LIDAR (AOL) project is involved in several different ways of remotely measuring chlorophyll and other biological and chemical substances in the world's oceans. The AOL uses sensors that are flown in aircraft to make these measurements.

Why measure and study chlorophyll in the ocean?

There is an excellent website called Living Ocean on the NASA SeaWifs site that can answer basic questions on why what the NASA AOL measures is important to everyone. After visiting the Living Ocean site, please return here to see how the AOL project supports satellite measurements of chlorophyll.

Why use aircraft to do oceanography?

Aircraft have several advantages as a platform to study the ocean. Aircraft can fly over long stretches of ocean very quickly (as compared to a ship), therefore the measurements made by the aircraft instruments have less changes due to time than shipborne measurements. Aircraft can also fly in weather that makes satellite measurements impossible (for example under cloud cover). On the other hand, ships can make subsurface measurements that aircraft cannot, and can carry a wider variety of instruments to study the ocean. Satellite sensors have advantages in measuring large areas of the ocean very quickly, and repeating measurements over a number of years. The three types of platforms actually work well with each other, with the measurements from all three validating and supporting each other.

How does the AOL measure chlorophyll in the ocean from an aircraft?

The AOL fluorosensor uses a pulse of laser light fired from the aircraft down into the ocean. The laser light hits the single celled plants in the ocean. The chlorophyll inside the plants absorbs the laser light, and fluoresces, giving off red light, in much the same way as certain paints glow under a "black light". A telescope onboard the aircraft collects the light from the plants, and electronic equipment converts the signal into numbers a computer can record.

The AOL also carries spectrometers similar to the type used by satellite sensors, that measure the color of the sunlight reflected from the ocean. The spectrometers used onboard the aircraft have more channels than the satellite spectrometers, so the data collected by the aircraft can be analyzed in greater detail, and improved methods of processing satellite data developed.

The combination of using two completely different methods of measuring chlorophyll from the aircraft allows comparison of the results by each method, and a better understanding of how both sunlight and laser light interact with ocean water.

How long has the AOL been making these measurements?

The present NASA Airborne Oceanographic Lidar (known as AOL3), evolved from several earlier versions of airborne laser instruments dating back to 1977. To read about the history and development of the NASA AOL project, please visit the history page.

What other instruments are available through the AOL program?

The AOL program also has developed the NASA Shipboard Laser Fluorometer (SLF) which is carried onboard research ships, and automatically makes laser measurements of plankton in the ocean. The SLF data is useful in verifying the airborne and satellite measurements.
The AOL program maintains a NASA GSFC Code 921 AERONET CSPOT sensor at the WFF airport. The CSPOT makes aerosol measurements by viewing the sky and sun on clear days, and also operates automatically. The CSPOT data can be useful in evaluating data from the satellite-like sensors onboard the AOL3 aircraft.

Details

The NASA AOL Fluorosensor is a laser fluorospectrometer (and associated instruments) which is carried onboard aircraft as small as twin engine planes. The AOLFL measures a variety of reflected and induced light properties, from which a number of oceanographic surface water properties can be derived.

The primary AOL3 sensor is a dual wavelength laser fluorospectrometer. This sensor transmits two laser wavelengths, one UV (355nm) and one green (532nm) to the ocean surface from the aircraft. These laser frequencies interact with the water molecules, causing a shift in the laser frequency. This Raman shift of the 355nm and 532 nm laser radiation allows normalization of other light measurements to compensate for changes in water clarity. If biological organisms containing chlorophyll and/or phycoerythrin are present in the water, the 532nm laser light is absorbed, and reemitted as particular bands of fluorescence. These fluorescent signals can be normalized to the water Raman signal, and have been shown to agree well with shipboard measurements of the same pigments. The 355nm laser radiation causes fluorescence of some of the dissolved organic material in the water. The water Raman normalized of this dissolved organic material is labeled CDOM or FDOM is the AOL3 data product library.

The AOL3 also carries several spectrometers which measure the downwelling sunlight incident on the top of the aircraft, and the reflected sunlight from the ocean surface. From these measurements, various algorithms can determine chlorophyll concentrations in the same manner as CZCS satellite. The ability of the AOLFL sensors to make oceanographic optical component measurements by both laser fluorescence techiques and by reflected spectra techniques allows the validity of each technique to be tested.

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Author: EG&G/Richard Mitchell
NASA Official: 614.2/Dr. Frank E. Hoge
Last Updated: 07/09/2008
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