   NASA'S AIRBORNE OCEANOGRAPHIC LIDAR: A TWO EXCITATION FREQUENCY LASER
                               FLUOROSENSOR*

                              C. Wayne Wright
                        Goddard Space Flight Center
                     Wallops Flight Facility, Code 972
                       Wallops Island, VA 23337, USA

                                 Abstract

NASA has recently designed its AOL to acquire individual laser-induced
fluorescence (LIF) spectra from two excitation frequencies emitted from a
single laser transmitter. The backscattered laser-induced fluorescence
(LIF) signal from each of the separate two footprints pass through the same
optical train to form separate spectral images upon the focal plane of the
AOL spectrometer. Other major modifications a redesign of the AOL
spectrometer to provide substantial reduction of scattered light and the
inclusion of a narrow band (notch) holographic filter to reject 532nm
radiation from the spectrometer. Results from initial mission show good
signal-to-noise characteristics and has demonstrated high precision
resolution for the measurement of chromophoric dissolved organic matter,
chlorophyll, and phycoerythrin (an axillary pigment found in marine
phytoplankton). The most significant result of these recent engineering
modifications has been the development of the capability of the AOL to
capture clean LIF signals from the two phycoerythrin pigments,
phycourobilin and phycoerythrobilin.

                              1. INTRODUCTION

The NASA Airborne Oceanographic Lidar (AOL) has been flown since 1978 in
experiments designed to demonstrate the potential of an airborne laser
fluorosensor to measure phytoplankton chlorophyll [Hoge and Swift, 1981;
Smith et. al., 1987; Walsh et. al., 1988], thin oil films[Hoge and Swift,
1980], chromophoric dissolved organic carbon fluorescence[Hoge et. al.,
1995a], and fluorescence from tracer dyes [Hoge and Swift, 1981]. The AOL
has been an important component of many large scale oceanographic
experiments providing remote measurements of phytoplankton pigments and
chromophoric dissolved organic carbon (CDOM) even under cloud covered
conditions where satellite remote sensing would have precluded coverage.
These field studies the North Atlantic Bloom [Yoder et. al., 1993; Hoge and
Swift, 1993], Equatorial Pacific, Iron Enrichment [Martin et. al., 1994],
and the Arabian Sea components of the international Joint Global Ocean Flux
Studies (JGOFS). The NASA AOL is flown on a four-engine P-3B turboprop
aircraft. Both the sensor and aircraft are located and maintained at
Goddard Space Flight Center's Wallops Flight Facility.

The AOL Fluorosensor instrumentation acquires measurements of laser-induced
fluorescence (LIF) from certain oceanic pigments. These chlorophyll and
phycoerythrin from marine phytoplankton and chromophoric dissolved organic
matter. The water-Raman backscatter is also acquired and is used to
normalize the laser-induced fluorescence data for surface layer spatial
differences in water attenuation properties [Bristow et. al., 1981].
Excitation at 355 nm from a frequency tripled Spectra Physics DCR Nd:YAG
laser excites CDOM over a broad region of the blue-green spectrum. The
water-Raman from 355 nm excitation occurs in the ~404 nm band. Excitation
at 532 nm from a frequency doubled Spectra Physics DCR Nd:YAG laser excites
fluorescence from chlorophyll in a band centered on ~685 nm and
phycoerythrin bearing phytoplankton in the spectral region from ~450 nm to
500 nm. The water-Raman from 532 nm excitation occurs in the ~650 nm band.

This paper describes the AOL fluorosensor which was redesigned in 1995. The
sensor redesign, which is described in more detail in the ensuing section,
now utilizes a single Spectra Physics DCR Nd:YAG laser to simultaneously
provide both the 355 and 532 nm radiation on each laser pulse. Both the
frequency doubled (532nm) and frequency tripled (355nm) pulsed laser
radiation from a single Nd:YAG laser are directed from the aircraft to
illuminate separate footprints on the ocean surface which are displaced by
several meters when the aircraft is operated at the nominal mission
altitude of 150 m . The backscattered laser-induced fluorescence (LIF)
signal from each of the two footprints pass through the same telescope,
diffractive grating, and lens system to form separate spectral images upon
the focal plane of the AOL spectrometer. Additional changes to the
fluorosensor spectrometer provide almost complete rejection of the 532 nm
reflected radiation using a holographic (notch) filter and substantially
reduced internal scattered light. Beyond the obvious reduction from two
laser transmitters to a single laser transmitter, the sensor redesign has
resulted in improved signal-to-noise and has permitted the resolution of a
shorter wavelength phycoerythrin pigment fluorescence. These results are
presented later in this paper.

The redesigned AOL sensor has been flown in 5 missions in the Middle
Atlantic Bight during the spring bloom of 1995 as well as in the JGOFS
Arabian Sea Experiment during the 1995 summer monsoons. The most
significant result of the engineering changes was development of the
capability of the AOL to capture clean LIF signals from the two
phycoerythrin pigments, phycourobilin and phycoerythrobilin, contained in
marine phytoplankton. The emission peaks of these to pigments are ~565 and
~580nm respectively.

In addition to the laser-induced fluorescence, the instrument collects
passive (solar-induced) ocean color radiance data, and sea surface
temperature (from an auxiliary infrared radiometer). The combined active
fluorescence and passive ocean color radiance measurements are used to
develop and validate algorithms for use in ocean color scanners such as
SeaWiFS, which is planned for launch in summer, 1996.

                         2. INSTRUMENT DESCRIPTION

The AOL sensor has been in active operation since 1977. The last major
upgrade of the AOL was in 1985. The 1985 modifications were largely
confined to the sensor's electronic and analog signal processing
components. These instrument changes, described in Hoge et. al., 1986a and
1986b, permitted calibration using an illuminated calibration sphere [Hovis
and Knoll, 1983) and the simultaneous acquisition of laser-induced spectra
and background solar-induced ocean color spectra. The recent sensor
modifications described in this paper involve considerable modifications to
the laser transmitter optics, receiver optical train, and the development
of new calibration and operational software. These modifications and the
present function of these sensor components are described in detail in this
section.

2.1. Single Laser, Multi-wavelength pulse transmitter

The Spectra Physics DCR2-20 Nd YAG laser 1064 nm fundamental radiation is
transformed through a harmonic generator to produce ~300 mjoule/pulse
energy at 532 nm and ~100 mjoule/pulse energy at 355 nm. The present design
of the AOL uses both the 532 and the 355 nm radiation contained in each
pulse. The excess 1064 radiation is trapped and absorbed, while the 532 and
355 pulses are selected and directed to the ocean surface by dichroic beam
split mirrors. An additional 532 nm dichroic beam split mirror was required
to remove 532 energy which was present in the 355 nm pulse. The dichroic
filters/mirrors are thus used in this application to both separate the
laser frequencies and to direct the to outgoing laser frequencies to
separate spots on the ocean surface. Since the 355 and 532 nm pulses
illuminate different spots on the surface, the resulting fluorescence
appears at different positions on the receiver focal plane and the
spectrometer focal plane. It is this characteristic that we exploit to
simultaneously process both spectra with a single receiver/telescope.

2.2. Multi-pulse LIF receiver optics

A nadir pointing 30cm diameter telescope equipped with double computer
controlled field stops views the ocean surface through an open port in the
belly of the aircraft. The fluorosensor spectrometer is positioned directly
behind the telescope. The entire telescope/spectrometer is mounted on a
custom-built optical table which holds the receiver optics in rigid
alignment even during turbulent flights. The backscattered laser and laser-
induced fluorescence and water-Raman radiation pass through a holographic
notch filter to remove any 532 nm backscattered signal and are spectrally
dispersed with a diffractive grading. The resulting spectral image from
each of the laser footprints fall at two different locations on the focal
plane of the fluorosensor spectrometer. At each of these locations an array
of contiguous fiber optical channels have been placed to permit the
acquisition of spectra between ~400 and 800 nm in 60 individual wavelength
channels. Each fiber optic is 2 meters in length, and is rectangular on one
end and circular on the other. The rectangular ends of the fibers
(measuring 0.043 x 0.40 inches) are packaged into machined aluminum
cassettes to rigidly hold and protect the fragile fibers. The round end of
each fiber is captured in a metal ferrule. The bandwidth of each fiber
channel is ~4.2nm FWMH. Although designed to be viewed separately by
individual photomultiplier tubes (PMT's), groups of three fibers are
presently viewed by each of 32 PMT's providing an effective spectral
resolution of ~ 12.6 nm . A fourth fiber is included in each PMT cluster.
The input end of the additional fiber is set to view radiation from a
pulsed LED which is under computer control to provide in-flight calibration
reference.

Substantial effort has been made with this system to reduce optical scatter
and cross- channel interference. The principal source of optical scatter
within the fluorosensing spectrometer has been the 532 nm signal resulting
from strong Fresnel reflection from the water surface in addition to
substantial backscattered laser radiation from the ocean volume. The
holographic notch filter has been quite effective in rejecting the 532 nm
radiation from the spectrometer. We have been able to make further use of
the holographic notch filter by mounting a PMT to view the rejected 532 nm
radiation reflected from the face of the filter. The signal from this PMT
is used to provide the timing pulse for measuring the slant range to the
water surface and for initiating the integration of the laser-induced
spectra from the ocean.

                                                       [Graph of Figure 1]
2.3. DATA SYSTEM
                                                    Figure 1
The AOL data system consists three   Block Diagram of the AOL Data Statem
rack mounted 486/dx2 66mhz computers and a SUN workstation networked
together. Figure 1. is a block diagram of the data system. The PCs
interface directly to data acquisition hardware and control every aspect of
the instrument. Each PC uses Microsoft MS-DOS 6.2 and Sun Micro System
PC-NFS software. Custom programs for each PC were written using Borland
Turbo-C version 3.0 for DOS. The data-system can be broken down into the
following subsystems: Navigation, CAMAC data, High Voltage Control, Passive
data, and the Sun workstation. The Navigation subsystem broadcasts time and
aircraft position data over the LAN. Other subsystems use this broadcast
data as needed. Each of the subsystems has a real-time clock interface
which can be manually or automatically synchronized to the GPS time. Once
in sync, the real-time clock provides each subsystem with GPS real-time of
day resolved to 32 microseconds. The real-time clock is designed around an
inexpensive, off-the-shelf (OTS) PC interface card. It utilizes the
accurate 1hz TTL pulse from a GPS receiver for synchronization. Each
subsystem responds to a common set of data control commands entered either
from a local keyboard or input from other systems on the LAN. Instrument
specific commands are entered from the local keyboard. Each subsystem can
either store its data locally on an internal hard drive or remotely on a
Networked File System (NFS) drive. The system is generally operated using
the NFS drive which is physically connected to the SUN workstation.

2.3.1. THE HIGH VOLTAGE/ PASSIVE SUBSYSTEM

The High Voltage (HV) and CAMAC subsystems are central to the AOL
operation. Stable, regulated, and noise free high voltage is critical to
high performance PMT operation and is even more important when using arrays
of PMTs as is done with the AOL. The AOL as redesigned in 1985 (Hoge et.
al., 1986a) was equipped with a Lecroy HV4032 high voltage power supply
which was computer controlled via a slow RS-232 data link. System
calibration and operation was totally dependent on this power supply and
calibration speed was completely determined by the communications speed and
regulation loop bandwidth of the internal microprocessor chip located
within the power supply. The HV4032 had been a source of often erratic
operation, and even when operating correctly, this component made
calibration a slow process. In our recent redesign of the AOL we eliminated
the internal microprocessor of the high voltage power supply (which was the
principal problem) and interfaced an external PC directly to the HV4032's
internal HV power supply modules. The PC was then programmed to calibrate
the PMT output using passive data acquired from the PMT's when illuminated
by radiation from the calibration spheres or the LED calibration array. The
regulation and passive data acquisition procedures operate on an interrupt
driven basis in the PC leaving substantial CPU cycles available for other
functions. This PC also generates laser-fire command pulses, calibration
signals and controls for a mini-sphere and the LED calibration pulse
modules. Since this PC directly manages the high voltage regulation loop
and has direct access to the PMT passive signals, it provides the perfect
foundation to execute calibration. Using this PC HV as described, has
reduced time required for calibration from ~30 minutes to less than 30
seconds. A real-time display of high voltage, passive data, and calibration
spectra was also developed to assist the sensor operator.

2.3.2. CAMAC DATA SUBSYSTEM

The CAMAC data subsystem computer controls the single AOL CAMAC crate,
extracts LIF spectra, laser pulse time of flight (slant range), laser
power, and other data from it. The CAMAC is outfitted with six Lecroy 2249A
charge digitizers (CD's) which convert the LIF spectra into digital values
between 0 and 1023 counts. Three 2249A CD's digitize the spectra from the
355nm stimulated spectra and the other three 2249 CD's digitize the spectra
stimulated by the 532nm pulse. Both spectra arrive at the digitizer at
virtually the same time within +/- 5ns. A Lecroy 2249SG is employed to
convert transmit energy and surface return laser wavelength signals into
digital values. Each of the digitizers have 12 analog input channels. The
2249A digitizes all 12 channels upon a single trigger signal. Each analog
channel of the 2249SG has its own trigger input. The time difference
between laser firing pulse and the returning spectra and surface return
pulses is typically one microsecond at the nominal 150m operational
altitude of the AOL. The pulse width of the return laser-induced signal is
typically less than 20ns. The 2249 digitizers are set to integrate over a
30 ns interval which easily encompasses the ~20 ns over which the laser-
induce spectral signals are received.

A Lecroy 4208 Octal Time to Digital converter (TDC) measures the laser
pulse time of flight (slant range) for either the 355nm or 532nm signals.
Normal operation is with the 532nm channel. This channel also generates all
high speed trigger and timing signals for system. A Lecroy 4222 Digital
Delay module is configured and used as a range gate.

2.3.3. NAVIGATION SUBSYSTEM

This subsystem interfaces to the aircraft's Inertial navigation Unit (INS)
via a high speed ARINC 429 databus and to a 12 channel GPS receiver via an
RS-232 link. The INS provides aircraft position, attitude, heading, and
wind speed and direction. The GPS furnishes a more accurate position
measurement but at a lower sample rate (2Hz). The INS and GPS data are
displayed for the operator in the form of a map with the aircraft track
plotted as the flight progresses. The other parameters are displayed in
alphanumeric form. The data are also formatted and recorded to either a
networked drive or the local hard disk. Selected data are broadcast over
the local area network so it may be used by the other systems to merge into
their data streams.

                                3. RESULTS

The redesigned AOL fluorosensor was initially flown    [Graph of Figure 2]
on March 31, 1995 in a mission which was coordinated
with researchers from the Universities of Delaware          Figure 2
and Maryland who were conducting sampling from the University of Delaware
research vessel, Cape Henlopen. The location of the aircraft ground track
is shown in Figure 2. The line was first occupied from northwest to
southeast and then reoccupied from southeast to northwest. The ship sampled
the same line beginning on April 1 with a west to east traverse and
completed the east to west traverse on April 3. The same line was also
overflown by the aircraft on April 3.

Cross-sections of CDOM and chlorophyll LIF are shown   [Graph of Figure 3]
plotted in Figure 3a and 3b, respectively. The CDOM
is plotted the LIF at 450 nm ratioed to the                 Figure 3
water-Raman band at 404 nm f(450)/r(404). The chlorophyll LIF at 685 nm was
normalized to the water-Raman band centered near 650 nm f(685)/r(650) and
then converted into units of concentration with available chlorophyll a
measurements from the ship using an "eyeball" best fit. Sea surface
temperature (SST) from an auxiliary EG&G Heimann infrared radiometer is
presented in Figure 3c to facilitate interpretation of the LIF profiles.
The northwest to southeast profiles are plotted using small grey symbols
while the southeast to northwest profiles are plotted using small black
symbols. Laboratory measurements of CDOM fluorescence and chlorophyll
concentration from near surface samples acquired from the R/V Cape Henlopen
are shown as "diamond" symbols. No supporting SST observations from the
ship were available at the time this manuscript was prepared.

The CDOM f(450)/r(404) profiles in Figure 3a show rapid decline along the
inner and middle shelf located on the western portion of the profile
followed by a monotonous level over the outer shelf and slope water-masses
and a decline to near baseline (but measurable) levels in the Gulf Stream
and Sargasso Sea water-masses at the eastern end of the profile. The
western wall of the Gulf Stream can be seen just west of 73oW in the SST
profile in Figure 3c. The eastern wall of the Gulf Stream is located near
72.25oW. The source of CDOM on the Atlantic shelf within the Mid-Atlantic
Bight is primarily from terrestrial and estuarine sources. The observed
inshore to offshore CDOM distribution pattern is consistent with numerous
other ship and aircraft observations [Hoge et. al., 1995a]. Recent analysis
of satellite Coastal Zone Color Scanner (CZCS) imagery also reveals a
similar CDOM distribution pattern [Hoge et. al., 1995b]. The f(450)/r(404).
The northwest to southeast and southeast to northwest profiles show
remarkable agreement to each other over the entire flight line. The initial
portion of the northwest to southeast profile (west of 74oW) is noticeably
noisy. The noise in this initial flight test of the redesigned AOL was due
to an incorrect gain setting in the AOL receiver which was subsequently
adjusted near 74oW. The laboratory f(450)/r(404) measurements derived from
the ship samples show reasonable agreement with both of the airborne
profiles. Approximately one-half of the ship samples were obtained on the
outbound cruise which began on April 1 and the other half was obtained on
the inbound cruise which concluded on April 3. The CDOM f(450)/r(404)
measurements are consistent with previously reported measurements [Hoge et.
al., 1995a; Hoge et. al., 1995b].

The chlorophyll profiles in Figure 3b show elevated levels of chlorophyll
immediately east of 75oW located near the mouth of the Delaware Bay.
Reduced levels of surface layer chlorophyll are indicated over the middle
and outter shelf water mass between 74.4oW and 74.8oW. Elevated patches of
chlorophyll were observed over the remaining shelf and slope water-masses
with the large (~20 km across) patch located near 74oW reaching ~6
g/liter. Notice that there is a distinct correspondence between the
distribution of chlorophyll patches and inflections in the SST (in Figure
3c) within the patchy chlorophyll region between ~74.3oW and the western
wall of the Gulf Stream located west of 73oW. Low levels of chlorophyll
were observed in the Gulf Stream and Sargasso Sea water-masses. The
repeating chlorophyll profiles also show remarkable agreement with one
another even down to relatively 1-5 km small-scale features located within
the major chlorophyll features which are ~20 km across. The difference in
the repeating chlorophyll profiles at the western edge of the graph is
attributed to tidal advection which took place in the intervening ~3 hours
between observations in this region. The laboratory chlorophyll
measurements shows reasonable agreement with the aircraft profiles over
most of the sampling line considering the one to three day separation
between the airborne and ship surveys. Profiles of water-Raman normalized
phycoerythrin pigment LIF are plotted in Figure 4. The phycoerythrin
phycoerythrin pigment LIF are plotted in Figure 4. The phycoerythrin
observations from the northwest to southeast flight    [Graph of Figure 4]
line occupation is shown in Figure 4a while the
repeating southeast to northwest profile is presented       Figure 4
in Figure 4b. In each panel the f(566)/r(650) (PUB rich) profile is plotted
with small black symbols and the f(693)/r(650) profile (PEB rich) profile
is represented with grey symbols. No supporting ship truth measurements
exist with which to compare the airborne measurements. In a relative sense,
the f(693)/r(650) (PEB rich) signal was observed to be strongest over shelf
and slope water-masses while the f(566)/r(650) (PUB rich) signal was
strongest within the Gulf Stream and Sargasso Sea water-masses. These
observations are consistent with known distribution patterns for the two
phycoerythrin pigments [reference]. The agreement between the f(566)/r(650)
and f(693)/r(650) profiles in Figure 4a with those in Figure 4b is again
remarkable with even relatively small scale features down to 1 km in width
duplicated in both profiles.

                              4. CONCLUSIONS

The redesigned AOL has demonstrated that high resolution measurement of
CDOM, phycoerythrin and chlorophyll LIF can be remotely measured from an
aircraft platform. Repeating passes acquired on a mission flown on March
31, 1995 showed high reproducibility and a high signal-to-noise ratio. The
redesigned sensor was able to use a single Nd:YAG laser to produce
simultaneous frequency doubled (532 nm) and tripled (355 nm) laser
excitation pulses on the ocean surface and to separately image the
backscattered LIF and water-Raman signals from each laser at separate areas
of the receiver focal plane. The addition of a narrow band (notch) Bragg
diffraction transmission filter to reject 532nm radiation from the
spectrometer coupled with substantial reduction of stray light scatter
within the AOL spectrometer resulted in the capability to resolve LIF in
the 566 nm band permitting the separate measurement of PUB rich
phycoerythrin and PEB rich phycoerythrin. The availability of the two
phycoerythrin pigment measurements from the AOL permits the investigation
of the potential to resolve the pigments from passive ocean color radiance
spectra gathered simultaneously with the AOL's auxiliary Airborne Data
Acquisition System (ADAS) using previously demonstrated active-passive
correlation spectroscopy (APCS) techniques [Hoge and Swift, 1986; Hoge and
Swift, 1993] or inverse modeling techniques [Hoge et. al., 1995b].

Although the present AOL fluorosensor configuration permits the collection
of LIF in 96 channels of ~4 nm in width, the present computer controlled
voltage supply for the photomultiplier tubes (PMT's) is limited to 32
channels which requires the output of every three contiguous light-guides
to be viewed by a single PMT reducing the effective resolution of the LIF
spectra to ~12 nm. Future plans call for the implementation of additional
computer controlled PMT voltage settings permitting the full 4 nm
resolution of the LIF spectra.

                                References:

M. D. Bristow, Nielsen, D. Bundy, and F. Furtek, "Use of Water-Raman
Emission to Correct Airborne Laser Fluorosensor Data for Effects of Water
Optical Attenuation", Applied Optics, Vol. 20, pp. 2889-2906, 1981.

F. E. Hoge and R.N. Swift, "Oil Film Thickness Measurement Using Airborne
Laser-Induced Oil Fluorescence Backscatter," Applied Optics, Vol. 22, No.
21, pp. 3269-3281 (1980).

F. E. Hoge and R.N. Swift, "Airborne Simultaneous Spectroscopic Detection
of Laser-Induced Water Raman Backscatter and Fluorescence from Chlorophyll
a and Other Naturally Occurring Pigments," Applied Optics, Vol. 20, No. l8,
3l97, September l98l.

F. E. Hoge, R.E. Berry, and R. N. Swift, Active-passive airborne ocean
color measurement: l. Instrumentation, Applied Optics., Vol. 25, 39-47,
1986a.

F. E. Hoge, R.N. Swift, and J. K. Yungel, Active-passive airborne ocean
color measurement: 2. Applications, Applied Optics., Vol. 25, 48-57, 1986b.

F. E. Hoge and R.N. Swift, "The Influence of Chlorophyll Pigment Upon
Upwelling Spectral Radiances from the North Atlantic Ocean: An
Active-Passive Correlation Spectroscopy Study", Deep Sea Research, Vol. 40,
No. 1/2, pp. 265-277, 1993.

Frank E. Hoge, Anthony Vodacek, Robert N. Swift, James K Yungel, and Neil
V. Blough, "Inherent Optical Properties of the Ocean: Retrieval of the
Absorption Coefficient of Chromorphic Dissolved Organic Matter from
Airborne Laser Spectral Fluorescence Measurements", Applied Optics, Vol.
34, No. 30, pp.7032-7038, October 1995.

F. E. Hoge, M.E. Williams, R.N. Swift, J.K. Yungel, and A. Vodacek,
"Satellite Retrieval of the Absorption Coefficient of Chromophoric
Dissolved Organic Matter in Continental Margins", Journal of Geophysical
Research, Vol. 100, No. C12, pp. 24847-24854, December, 1995.

W.A. Hovis and J.S. Knoll, "Characteristics of an Internally Illuminated
Calibration Sphere", Applied Optics, Vol. 22, pp. 4004-4007, 1983.

J.H. Martin, K.H. Coale, K.S. Johnson, S.E. Fitzwater, R.M. Gordon, S.J.
Tanner, C.N. Hunter, V.A. Elrod, J.L. Nowicki, T.L. Coley, R.T. Barber, S.
Lindley, A.J. Watson, K. Van Scoy, C.S. Law, M.I. Liddicoat, R. Ling, T.
Stanton, J. Stockel, C. Collins, A. Anderson, R. Bidigare, M. Ondrusek, M.
Latasa, F.J. Millero, K. Lee, W. Yao, J.Z. Zhang, G. Friederich, C.
Sakamoto, F. Chavez, K. Buck, Z. Kolber, R. Greene, P. Falkowski, S.W.
Chisholm, F. Hoge, R. Swift, J. Yungel, S. Turner, P. Nightingale, A.
Hutton, P. Liss & N.W. Tindale, "Testing the Iron Hypothesis in Ecosystems
of the Equatorial Pacific Ocean", Nature, Vol. 371, pp. 123-129, 1994.

R.C., Smith, O.B. Brown, F. E. Hoge, K.S. Baker, R.H. Evans, R.N. Swift,
and W.E. Esaias, "Multiplatform Sampling (Ship, Aircraft, and Satellite) of
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2068-2081, June 1987.

John J. Walsh, Creighton D. Wirick, Leonard J. Pietrafesa, Terry E.
Whitledge, Frank E. Hoge, and Robert N. Swift, "High Frequency Sampling of
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* Presented at the Second International Airborne Remote Sensing Conference
and Exhibition, San Francisco, California, 24-27 June 1996.

Presently 12 of the PMT's are used to capture the laser-induced spectra
from the 355 nm pulse from 400 nm to 550 nm and the remaining 20 PMT's are
used to capture the laser-induced spectra from the 532 nm pulse from 550 nm
to 800 nm.

The 6 inch diameter mini-sphere is similar in design to the larger
calibration spheres described by Hovis and Knoll, 1983. It is located so
that the radiation from the sphere is introduced at the focal plane of the
telescope and thus travels through the entire remaining optical train and
to the PMT's via the fibers using the same path as radiation from the
larger external laboratory illuminated sphere or radiance from the ocean.
The mini-sphere is used to calibrate the PMT passive output during flights
while the strobed LED's are used to calibrate the active (laser bandwidth)
PMT output.
