Investigations into the feasibility of sensing ocean color from high altitude for determination of chlorophyll and sediment distributions were carried out using sensors on NASA aircraft, coordinated with surface measurements carried out by oceanographic vessels. Spectrometer measurements in 1971 and 1972 led to development of an imaging sensor now flying on a NASA U-2 and the Coastal Zone Color Scanner to fly on Nimbus G in 1978. Results of the U-2 effort show the imaging sensor to be of great value in sensing pollutants in the ocean

Hovis, W. A.

English

NASA Technical Reports Server

OCEAN COLOR IMAGERY - COASTAL ZONE COLOR SCANNER M-6
By Warren A. Hovis, NASA Goddard Space Flight Center
Greenbelt, Maryland
ABSTRACT
Investigations into the feasibility of sensing ocean color from high
altitude for determination of chlorophyll and sediment distributions have
been carried out using sensors on NASA aircraft, coordinated with surface
measurements carried out by oceanographic vessels. Spectrometer measure-
ments in 1971 and 1972 led to development of an imaging sensor now flying
on a NASA U-2 and the Coastal Zone Color Scanner to fly on Nimbus G in
1978. Results of the U-2 effort have shown the imaging sensor to also be
of great value in sensing pollutants in the ocean.
.INTRODUCTION
In preparation for the flight of the Coastal Zone Color Scanner (CZCS)
on Nimbus G in 1978, a breadboard scanner has been constructed for use on
a U-2 aircraft. The scanner has ten spectral bands ranging from 433 to
772 nm center wavelength with spectral bandwidths from 20 to 27 nanometers-.
The gain of each channel was set, based on the 1972 CV 990 Ocean Color
Expedition results, to be optimum for water as viewed from high altitude
with the exception of one channel, at 733 nanometers, that has a gain like
that of the LANDSAT MSS, optimized for land.
Provision has been made to adjust gains for time of day and seasonal
sun angle effects and to offset the contribution of atmospheric backscatter
to the signal. This allows the data collection system to be most effi-
ciently used for recording the effects of ocean color without wasting dy-
namic range on the contribution of the atmosphere.
Flights began in April 1974 and have covered targets off the West
Coast, lakes in California and Nevada, the Great Lakes, and the East Coast
from the New York Bight to Florida and the Gulf of Mexico from Key West to
Timbolier Bay.
The New York Bight has been of special interest because it provides an
opportunity to study both the sediment plume of a major river and the ef-
fects of large-scale waste dumps going on continually in the Bight.
Data from the Bight, taken on a number of overflights, show the sedi-
ment plume in several parts of the tidal phase and a gap, possibly caused
by a gyre, separating the plume from the dumping areas. Dispersion from
the dump areas, seen on several occasions, shows a predominant movement to
the Northeast and Southwest, toward Long Island and New Jersey, with little
motion to seaward in the first few meters visible from the plane. This
work has led to participation in an extensive prpgram called MESA (Marine
Ecosystems Analysis), a NOAA-led investigation of the New York Bight.
Investigations off Tampa, Florida were carried out, in cooperation
with the State of Florida, to begin assessment of the ability of multispec-
tral scanners to locate, identify and quantify so-called red tide blooms.
Algae blooms, already fairly well developed, were easily detected and a
1989
https://ntrs.nasa.gov/search.jsp?R=19760010491 2020-03-22T16:15:26+00:00Z
program involving laboratory, ship and U-2 movements set up to begin in the
Fall of 1975.
Investigations off Sanibel Island, Tampa Bay and Panama City, Florida,
and Tirnbolier Bay, Louisiana were carried out in November 1974 with surface
truth measurements carried out by personnel from Moody College, Texas A&M
University aboard the RV Calypso. These measurements are being compared
with the surface truth measurements to determine gradient levels that can
be detected and the quantitative accuracy that can be realized.
TECHNIQUES
The U-2 Ocean Color Scanner (OCS) senses simultaneously in ten spectral
bands with a spatial resolution of 3.8 milliradians from an aircraft alti-
tude of 19.8 km. Swath width is 20 nautical miles with a recording time of
2 hours available per flight on magnetic tape of the scanner output. The
scanner is supported by four Mitchell-Vinten 70mm cameras that are normally
loaded with black and white film in three of the cameras filtered for green,
yellow and red and one camera loaded with SO 242 color film with an ultra-
violet haze filter.
The OCS has ten spectral bands ranging from 433 to 772 nanometer center
wavelength as shown in Table I. The radiance for saturation is shown in
Table I for a gain of 1 for each channel. Gain may be increased separately
for each channel in steps of 1.5, 2 and 3 to allow for changing sun angle
due to seasonal changes or time of flight. All channels except 9 are op-
timized for water scenes, including atmospheric backscatter as seen from
19.8 km. Channel 9 has the same level of gain as the LANDSAT Band 6 that
covers 700 to 800 nanometers.
TABLE I
PARAMETERS OF THE U-2 OCEAN COLOR SCANNER
Channel
1
2
3
4
5
6
7
8
9
10
Center
Wavelength
(nm)
433
471
509
547-
583
620
662
698
733
772
Bandwidth
(nm)
22.5
21.5
27.5
24.5
25.0
26.0
22.0
20.5
22.5
23.0
Radiance
(Gain x 1)
mw/cm2y
40.1
26.0
23.6
14.7
11.8
10.0
7.55
5.0
11.9
3.47
All ten bands are recorded in the analog mode and, at this writing,
any four bands selected are digitized and multiplexed on the aircraft to-
gether with time code. A new digitizer, now on order, will allow all ten
channels to be digitized simultaneously on the aircraft. The data system
is operated by the U-2 pilot by on-off switch and a reset button for the
digitizer. Reset will not be required in the new system. Time of data
1990
collection is usually determined from a flight plan prepared in advance
although on some occasions the pilot has to exercise judgement such as
when overflying an event such as a red tide that cannot be precisely lo-
cated before flight.
Glint from the ocean surface is a serious problem that can obliterate
any information about the content of the water below the surface. Early
flights, in April 1974, showed that glint would appear in imagery, even in
flights flown in the morning with relatively low sun angles. It was deter-
mined that glint could largely be avoided by flying before 10:30 a.m. and
after 1:30 p.m., local sun time, and by flying directly toward or away from
the sun whenever possible.
After flights, data is formatted and processed to tapes of calibrated
radiance. When imagery is produced, an offset is applied to each channel
to remove the DC background due to atmospheric backscatter. Black and white
negatives are produced using a digital image processor and utilized to pro-
duce false color imagery. For analytical purposes, to develop algorithms,
the data is reproduced as graphs of radiance along scan lines and grid print
maps.
RESULTS
The first flights were conducted in April 1974 with tests from Wallops
Flight Center. A flight was conducted on April 27 with one pass over the
New York Bight on a-due North-South line. The pass over the mouth of the
Hudson clearly showed the Hudson River sediment plume and a waste dump with
dispersion. The result, shown in Figure 1, is a false color image using the
channels at 471, 583, and 662 nanometers.
The bright blue spot just south of Long Island is sun glint with ship
wakes, one in the square pattern of a dump barge. Figure 2 shows three
camera images from the color camera taken at 30-second intervals. The cir-
cular glint pattern, in the southeast corner of each image, varies in dia-
meter due probably to varying surface roughness. Since the scanner lines
run through the center of each image, from East to West, the glint pattern
appears on scanner imagery only when the glint pattern extends to the North
far enough to be seen by the scan line. Subsequent flights were flown di-
rectly toward'and away from the sun, whenever possible, and at times of low
solar elevation.
Since Nimbus G will fly in a high noon ascending mode orbit, a scan
tilt mechanism is incorporated into the Coastal Zone Color Scanner to tilt
the scan,-away from the sun glint in either hemisphere.
/
A flight conducted on July 22, 1974 utilized the flight line information
garnered from the first flight and avoided glint entirely. This flight, con-
ducted at very nearly the same time as the April flight, and hence at a
higher solar elevation, again shows the Hudson plume and an acid waste dump
in the Bight. This image, Figure 3, obtained at slack water on the high
tide, shows considerable detail but more detail can be seen by further en-
hancement. Figure 4 is a color enhanced image obtained by multiplying the
ratio of Channel 4 divided by Channel 5 times the ratio of Channel 7 divided
by Channel 5. .
In response to a request from the State of Florida, a program is being
developed to determine the optical properties of the Gymnodinium breve
1991
organism, known as "Red Tide" and to develop techniques for detecting algae
blooms early in the event. This program will begin intensive effort in the
summer of 1975 but s.ome preliminary work was done in 1974 to familiarize
personnel of the Department of Natural Resources of Florida with remote
sensing techniques.
On September 9, 1974 a flight was flown conducted over the Tampa Bay
area on the West Coast of Florida. No Gymnodinium breve was present but a
concentration of the blue-green organism Oscillatoria erythrea was seen in
the southwest corner of the image. Figure 5 shows the image, again made
with enhanced data from Channels 2, 5, and 7. In addition to the algae
bloom, sediments can be seen around the mouth of Tampa Bay, including a fan-
shaped plume from Johns Pass, north of Tampa Bay.
Several flights have been conducted over the Chesapeake Bay in order
to discriminate areas of high and low sediment concentration. In recent
years, the clam population of the Chesapeake has been contaminated with
bacterial levels so high that clamming was forbidden in many previously
productive areas. In fact, in the summer of 1972, following Hurricane
Agnes, all clamming was stopped in the Maryland portion of the Chesapeake.
Much of the pollution that enters the bay and is concentrated by clams
comes from the Susquehanna River that drains large areas of Pennsylvania
and New York before emptying into the Chesapeake. Bacteria laden particles
enter the Bay and proceed toward the Atlantic but are subject to tidal mo-
tion, as well as river flow, as soon as they enter the Bay. .It is desirable
to determine sediment patterns as a guide to flow of material from the
Susquehanna. Figure 6 shows a scanner image of the Upper Chesapeake Bay
from the mouth of the Susquehanna to Gibson Island. The brighter areas are
areas of high sediment concentration with anomalous areas of very low scat-
tering, hence very little suspended particulate matter off the mouth of the
Chester River and Bodkin Creek. These studies will be continued as often
as possible to build up a base of experience on sediment motion in the
Chesapeake.
In November 1974, an opportunity arose to conduct investigations in
conjunction with surface truth measurements being carried out, in the Gulf
of Mexico, by personnel from Texas A & M University aboard the RV Calypso.
The Calypso was available for about one week to carry out investigations
along the Gulf Coast from Key West, Florida to the Timbolier Bay' area in
Louisiana.
In cooperation with the Texas A & M personnel, a sampling plan was de-
vised to provide coverage of a reasonably large area in a short period of
time. A track was selected in the direction of the proposed aircraft flight
and ten nautical miles long. The Calypso carried out extensive sampling at
stations at the beginning and end of the ten-mile track and at the midpoint.
Small, inflatable outboard boats were deployed on either side of the larger
vessel collecting samples and making measurements at 1-mile intervals four
miles out on either side of the larger vessel. The small boats were tracked
by radar to verify location and each collected samples at 12 locations as
shown in Figure 7. In this manner, an 80-square mile area was sampled in
about 4 hours.
The large vessel track, A to B to C, is parallel to the aircraft flight
line and the small craft tracks perpendicular to the flight path but parallel
to the sensor scan line direction. This allows fairly easy comparison by
selecting one scan line that falls along a set of readings. Figure 8 shows
1992
the radiance values for four spectral channels (2, 4, 5 and 7) along a
position centered on 29°32'N, 85°28'W in the Gulf of Mexico off Panama
City, Florida. Also shown are the chlorophyll measurements at the surface
for each sampling point.
A preliminary algorithm has been applied to some of the measurements
using two channels at a time. Using only two channels, Channels 4 and 7
appear the best in a formula where
R7
_ = a + b log C
R4
where R^ = radiance (unenhanced) 547 nm
R-j = radiance (unenhanced) 662 nm
a = 0.309
b = .0166
C = chlorophyll Mg/M3
A comparison along the track is shown in Figure 9. The algorithm was
next applied to the scene to produce the image shown in Figure 10. This is
the first attempt at applying an algorithm to imagery and will be further
refined to utilize more than one channel.
CONCLUSIONS
A scanner with spectral channels and dynamic range optimized for ocean
color can detect subtle color changes even through the earth's atmosphere.
If the sensor has a reasonably high signal-to-noise ratio, much of the
atmospheric backscatter can be eliminated by subtracting a DC offset from
the total signal and preserving the modulation caused by color changes in
the ocean. Quantification of parameters such as chlorophyll concentration,
on a global scale, will require extensive research effort, prior to the
launch of the Nimbus G CZCS, to include high altitude aircraft scanner
flights coordinated with far more extensive surface truth measurements than
have been accomplished to date.
1993
FIGURE 1
False Color Image, New York Bight, April 27, 1974
showing Sediment Plume, Waste Dump, Sun Glint
1994
FIGURE 2
Color Photographs of New York Bight
showing Glint Patterns
1995
FIGURE 3
False Color Image, New York Bight, July 22, 1974
showing Hudson River Plume, Acid Waste Dump
1996
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Ocean color imagery: Coastal zone color scanner

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