National Aeronautics and Space Administration

Wallops Flight Facility

Ocean Color


The Airborne Oceanographic Lidar (AOL) is an advanced laser fluorosensor that is flown along with up-looking and down-looking spectroradiometers and an infrared radiometer that measures sea surface temperature. The AOL group at Wallops Flight Facility (WFF) participates in numerous collaborative studies with investigators from a variety of other agencies and academic institutions shown in Table 1. WFF has been involved in the development of airborne laser fluorosensing since its very inception in the mid 1970’s. With the implementation of the AOL sensor in 1977, the AOL group demonstrated many potential uses of airborne laser fluorosensing including measuring laser-induced fluorescence from chlorophyll and phycoerythrin pigments in marine phytoplankton, chromophoric dissolved organic matter (CDOM), oil film thickness and typing, and tracking fluorescent tracer dye. The AOL also measures the Raman backscatter from seawater. The water Raman backscatter signal is used to normalize the laser-induced fluorescence signals, thus providing a built-in correction for horizontal spatial variability in the attenuation properties of the ocean surface layer. During the past 25 years, the AOL sensor suite has been the principal remote sensing tool for many oceanographic experiments as shown in Table 2.

Since the launch of the SeaWiFS ocean color satellite the role of the AOL has changed. The AOL is now used in experiments such as the summer monsoon season in the JGOFS Arabian Sea Experiment where pervasive cloud cover during the summer monsoons precluded effective use of satellite ocean color imagery. With the launch of MODIS, the value of the AOL sensor suite has been increasingly demonstrated for developing and testing ocean color algorithms. In a single flight the AOL sensor suite gathers tens of thousands of laser-induced pigment measurements paired with ocean color radiance and solar irradiance spectra. These paired measurements acquired over a variety of water mass types with differing amounts of CDOM and concentrations of phytoplankton biomass facilitate the ocean color algorithm development and testing. Ocean color satellite under flights further allows the validation of products being distributed by the SeaWiFS and MODIS sensor teams.

Table 1: Current Collaborations

NOAA Coastal Services Center (CSC). SeaWiFS and MODIS Validation underflights of NASA AOL and ADAS AOL/Pump&Probe for development and validation for conversion of inherent optical properties to carbon concentration. In cooperation with NOAA EstHab (Estuarine Habitat) program. Drs. Dave Eslinger and Mary Culver
U. Delaware Research Vessel Cape Henlopen. SeaWiFS and MODIS Validation cruise(s) to obtain dissolved and particulate organic carbon (including chlorophyll) vs depth. Dr. Jon Sharp PI for cruises. Used NASA’s Shipboard Laser Fluorimeter (SLF) as supporting instrumentation.
Univ. MD Horn Point Laboratory. Pigments, dissolved organic carbon, small vessel in Chesapeake Bay and aboard Univ Del Cape Henlopen. Numerous collaborative PI’s from this Horn Point Lab.
Univ MD College Park Dr. Neil Blough (and Tony Vodacek/now Univ. Rochester)Dissolved organic matter, CDOM bleaching algorithms.
Marine Science Consortium (near Wallops) 50 foot vessel for day cruises/validationStudent involvement/ outreach.
Old Dominion University, PI Dr. Glen Cota, Measures Inherent Optical Properties in near coastal zone with their new vessel now under construction.
Horn Point Marine Lab Phytoplankton taxonomy and photosynthesis investigations. Dr. Hugh McIntyre.
Naval Research Lab CDOM chemistry and fate. Dr. Tom Boyd.

Table 2: Airborne Laser Experiments

1998-2002 Estuarine Habitat Field Experiment (NOAA/NASA)
2000 Northeast Gulf of Mexico Experiment (NEGOM) (NASA/NOAA/NSF)
1996 Airborne Polarization Lidar Exp. (Russia/NASA)
1995 Arabian Sea Experiment (NSF/NASA)
1993 IRONEX/Galapagos (NSF/NASA)
1992 Monterey Bay Experiment (NASA/NSF)
1991 Tampa Bay Experiment (TAMBAX) Gulf of Mexico (NASA/NSF)
1991 Atlantic Richfield Oil Company (ARCO) Exp. Santa Barbara Channel
1991 California Cooperative Fisheries Investigation (CalCOFI) (NASA)
1991 Pacific/Atlantic/Gulf of Mexico CDOM Mapping (ARCO/NASA)
1989-90 Bay Watch Program Chesapeake Bay (NOAA)
1983-88 Shelf Edge Exchange Processes (SEEP) experiment. (DOE/NASA)
1987 Subsurface Scattering Layer Experiment Mid Atlantic Bight (NOSC-Navy)
1987 Fall Exchange (FLEX) experiment South Atlantic Bight (DOE/NASA)
1985 Spring Exchange (SPREX) experiment South Atlantic Bight (DOE/NASA)
1982-83 Warm Core Ring (WCR) Experiment (NSF/NASA)
1981 Nantucket Shoals Experiment (NOAA/NASA)
1980 SuperFlux ( NOAA/NASA Chesapeake Bay Mapping Study)
1979 Atlantic Oil Spill Mapping (NASA)
1979 Maritime Remote Sensing Experiment (MARSEN) German Bight (NASA)
1978 Atlantic Oil Spill Mapping (NASA/Canada)

Present Activities:

Estuarine Habitat Studies

The AOL sensor suite has been utilized in a joint program with NOAA’s Coastal Services Center (CSC) located in Charleston, SC. This program which was initiated in 1996 is designed to investigate the application of airborne sensor technology to near shore coastal and estuarine bodies of water where it is difficult to apply satellite ocean color technology due to the kilometer size footprint of these sensors. These investigations have typically focused on specific events such as gauging the effects on the Pamlico and Albemarle Sounds (PAS) in response to the deluge of runoff into these systems as a result of Hurricane Floyd in September, 1999. Shown are progressive panels showing the distribution of chlorophyll and CDOM in the PAS system following the passage of Floyd over a period of 6 weeks. A follow-up survey of the PAS system in October, 2000 indicates the normal distribution of these parameters during the fall when there has been no hurricane perturbation.

A comparison of the October, 2000 chlorophyll distribution plot with that of the September 25 survey indicate that the high chlorophyll concentration normally found in the lower reaches of the tributaries to Pamlico Sound was now displaced into southwestern portion of the sound. The elevated chlorophyll is seen to be further displaced into the central portion of the Pamlico by the October 15 survey with higher chlorophyll now evident in the nearshore waters flanking Oregon Inlet north of Cape Hatteras. By the October 31 survey, lower levels of chlorophyll are now seen in the nearshore coastal waters and the area of elevated chlorophyll was again in the southwestern portion of Pamlico Sound although the lower portions of the tributaries remained uncharacteristically lower in concentration of the pigment. The sequence indicates that the distribution of chlorophyll within the PAS system had not completely recovered from the effects of Hurricane Floyd over a six week period, but much of the sound and nearshore coastal water appeared near normal or lower than normal in chlorophyll concentration. It is felt that the increased nutrient loading resulting from the high runoff from Hurricane Floyd would have resulted in considerably more phytoplankton growth had the event occurred in July or August when the water temperature and PAR would have both been higher.

In 2002 several of the Estuarine Habitat missions were focused on the Delaware Bay and coastal New Jersey. These missions were coordinated with sampling by scientists from the University of Delaware and the New Jersey Department of Marine Water Monitoring. The plots of chlorophyll, phycoerythrin, and SST from the March 19 mission shows the influence of water temperature on the distribution of the planktonic photopigments, especially within the Delaware Bay. The elevated chlorophyll levels are generally associated with the warmer surface water flanking the middle section of the bay while the phycoerythrin bearing organisms appear to be more concentrated in the cooler central portion of the middle section of the bay. Notice the higher concentrations of CDOM in the upper portion of the Delaware Bay, in the coastal lagoons along the New Jersey coast and in the lower portion of the Raritan Bay near the top of the figure.

Additional Estuarine Habitat surveys have been flown to map distribution of these parameters in the Chesapeake Bay, New York Bight, and coastal waters of MD, DE, NC, SC, GA, FL, and MS.

Figure 1. Post Hurricane Floyd Survey – Pamlico and Albemarle Sounds

Fig. 1a September 25, 1999 Fig. 1b October 2, 1999
Fig. 1c October 15, 1999 Fig. 1d October 31, 1999
Fig. 1e October 11, 2000

Figure 2. Delaware Bay and Coastal New Jersey Survey March 19, 2002

Fig 2a. Chlorophyll Fig. 2b. Phycoerythrin Fig. 2c SST

Ocean Color Satellite Product Validation

Each year during the joint Estuarine Habitat program with NOAA CSC additional surveys have been conducted in coastal, shelf, and offshore watermasses in order to validate satellite ocean color products. Results recently obtained during validation of the MODIS chlorophyll fluorescence line height (FLH) product are shown Figures 3. Figure 4(top) shows the chl FLH image of the Mid Atlantic Bight (MAB). The distribution of chl patches is apparent as is the striping in the in the MODIS FLH image. Note the location of the three flight lines occupied with the AOL sensor suite. Figure 4(bottom) is a comparison between the AOL laser-induced chlorophyll fluorescence and the corresponding MODIS FLH extracted from the satellite image. The good agreement between the profiles is reflected in the scatter plot and the high 0.851 R2 correlation coefficient. A comparison between the AOL laser-induced CDOM fluorescence and the corresponding MODIS FLH extracted from the satellite image indicates the independence of the FLH product from CDOM. The noncoherent relationship between CDOM fluorescence and the MODIS FLH product is reflected in the scatter plot and the low 0.309 R2 correlation coefficient. This shows that FLH is not influenced by CDOM, the principal absorber in coastal regions.

Figure 3. MODIS Fluorescence Line Height Image with AOL Flight Track
Figure 4. Profiles of MODIS FLH plotted with (top) AOL CHL fluorescence/Raman and (bottom) AOL CDOM fluorescence/ Raman.

Algorithm Development and Application to Satellite Ocean Color Imagery

The capability of the AOL sensor suite to acquire thousands of paired laser-induced fluorescence measurements with corresponding oceanic radiance spectra gathered with the Airborne Diode Array Spectrometer (ADAS) has led to the development of a successful inverse modeling approach for recovery of the three major inherent optical properties (IOP’s) of sea water using an ocean color radiative transfer model. Following the successful demonstration of the extraction of these IOP’s, aph (absorption of phytoplankton), acdom (absorption of CDOM), and bb (total constituent backscatter), from the airborne ADAS spectra, the inverse modeling approach was applied to SeaWiFS imagery with considerable success. Figure 5 (A) shows an acdom of a SeaWiFS scene from the MAB along with the ground track of AOL instrument suite. The aph and Total Constituent Backscatter plots are shown in (B) and (C), respectively. Figure 6 (a,b,c) are the profiles SeaWiFS radiances at 412, 490, and 555 nm compared to Radiances from the airborne ADAS sensor for Line A in Figure 5. Panels (d,e,f) are profiles along Line A (Figure 5) of the three IOP’s developed from SeaWiFS and ADAS radiances plotted along with (d) airborne laser-induced chlorophyll fluorescence, (e) airborne laser-induced CDOM fluorescence, and (f) sea surface temperature from the airborne Heimann IR radiometer. Features crossed by the profile are annotated within the Panel (f).

Figure 5. IOP Images Extracted from October 6, 1997 SeaWiFS Scene
Figure 6. Panels (a,b,c) SeaWiFS Radiances at 412, 490, and 555 nm compared to Radiances from the airborne ADAS sensor. Panels (d,e,f) are the three iops developed from SeaWiFS and ADAS radiances plotted along with (d) airborne laser-induced chlorophyll fluorescence, (e) airborne laser-induced CDOM fluorescence, and (f) sea surface temperature from the airborne Heimann IR radiometer.

The inverse modeling approach has been applied to all of the SeaWiFS imagery from 1997 to near present. The resulting products have revealed interesting seasonal and regional patterns that are currently under investigation. An example can be seen in Figure 7 which is a 25-day composite of the SeaWiFS acdom product showing the western North Atlantic Ocean. This image of acdom was acquired during the late summer when the CDOM in the surface layer had been bleached resulting in reduced absorption. The track of Hurricane Gert (from the National Hurricane Center) is shown on the figure. Note the elevated acdom to the right side of the hurricane track indicating the vertical advection of unbleached acdom from below the thermocline into the surface layer resulting from mixing due to internal waves generated by the hurricane. Other noteworthy examples include upwelling in the Gulf of Tehuantepek offshore of Mexico and off Central America resulting from cold air outbreaks, off Peru and Ecuador, and off the coast of southern Argentina.

Figure 7. SeaWiFS acdom image showing track of Hurricane Gert and CDOM upwelling.

MODIS imagery has heretofore been plagued with problems that have only recently been reasonably corrected. In the near future we expect to similarly process the improved MODIS imagery.

Future Direction:

The AOL sensor suite will continue to validate satellite ocean color data products and improve algorithms for retrieving these products. A redesign of the AOL lidar receiver will provide a continuous spectrum of channels from 400 to 750 nm while retaining the same high signal to noise ratio of the present AOL. A shipboard version of the AOL, referred to as the Shipboard Laser Fluorometer (SLF), has been put into service. The SLF, which acquires essentially the same laser-induced fluorescence products as the AOL, can be put on ships of opportunity providing additional ocean color satellite validation and high resolution information for the scientists on these research vessels.

The AOL will also continue to provide a test bed for Pump and Probe P&P technology development headed by Alex Chekalyuk. The P&P technology is discussed separately within this document.


The AOL program has had numerous Summer Faculty Research participants. Most recently, Dr. Sima Bagheri from the New Jersey Technical Institute is currently working with the project as a Summer Faculty Research participant. Dr. Frank Hoge served on the evaluation panel for Emma Rochelle Newell at Horn Point who recently received her Ph.D. degree from the University of Maryland. AOL personnel have frequently given presentations to high school and college classes participating at the nearby Marine Science Consortium.


Lead Investigator: Frank Hoge