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Data Processing Techniques of Remote Sensing for Oceanography
361
DATA PROCESSING TECHNIQUES OF REMOTE SENSING FOR OCEANOGRAPHY Y . TOZAWA,l
M. SF\TO,’ M . II)KA,l
K . MUNEYAMA,2
Y . SASAKI,‘
I. ASANUMA,’
and
T. I C H I Y E 3 ’Tokyo S c i e n t i f i c Center, IBM-Japan, 5-19, Sanbancho, Chiyodaka, Tokyo 102 2Japan M a r i n e Science and Technology Center, 2-15, Natsushima, Yokosuka, 237 Japan 3Department o f Oceanography, Texas A&M U n i v e r s i t y , C o l l e g e S t a t i o n , TX 77843
ABSTRACT S a t e l l i t e remote s e n s i n g can p r o v i d e oceanographers w i t h p o w e r f u l t o o l s f o r c o l l e c t i n g d a t a , s i n c e i t covers a wide area i n s t a n t a n e o u s l y and as f r e q u e n t l y as s e v e r a l t i m e s a day. I n o r d e r t o e x t r a c t m e a n i n a f u l i n f o r m a t i o n f r o m d a t a c o l l e c t e d by s a t e l l i t e s , s p e c i a l d a t a p r o c e s s i n g i s an a b s o l u t e n e c e s s i t y . Techniques f o r d a t a p r o c e s s i n g a r e d i f f e r e n t , depending on c h a r a c t e r i s t i c s o f t h e d a t a , measurement p r i n c i p l e s and t h e oceanographic r e q u i r e m e n t s . Three systems o f s a t e l l i t e sensing, AVHRR, CZCS and SAR, a r e p a r t i c u l a r l y u s e f u l f o r oceanography b u t need s p e c i a l t e c h n i q u e s f o r d a t a p r o c e s s i n g . Several t e c h n i q u e s developed by JAMSTEC and IBM-Japan f o r t h e s e systems a r e d e s c r i b e d w i t h examples a p p l i e d t o oceanography.
INTRODUCTION The Japan M a r i n e Science and Technology Center (JAMSTEC) and IBM-Japan, Tokyo S c i e n t i f i c Center have been c o o p e r a t i n g s i n c e 1980 on r e s e a r c h conc e r n i n g a p p l i c a t i o n o f remote s e n s i n g t o oceanography (JAMSTEC and IBM-Japan, 1982).
S a t e l l i t e remote s e n s i n g t e c h n i q u e s have b r o u g h t new approaches t o
t h e c o l l e c t i o n o f oceanographic d a t a .
The t e c h n i q u e s have t h e f o l l o w i n g
advantages o v e r c o n v e n t i o n a l oceanography methods. wide a r e a o f t h e ocean a l m o s t i n s t a n t a n e o u s l y .
F i r s t , t h e y can c o v e r a
Second, t h e y can m o n i t o r
many p a r t s o f t h e w o r l d oceans w i t h f r e q u e n c i e s never achieved b e f o r e by c o n v e n t i o n a l methods, t h a t i s , about t w i c e d a i l y .
The f i r s t advantage l e a d s
t o i n f o r m a t i o n on s p a t i a l c h a r a c t e r i s t i c s o f t h e ocean.
The second advantage
can be u t i l i z e d i n u n d e r s t a n d i n g ocean dynamics by measuring time-dependent s t a t e s o f t h e ocean. Remote s e n s i n g i n v o l v e s o b s e r v a t i o n t e c h n i q u e s which use e l e c t r o m a g n e t i c waves.
Sources o f e l e c t r o m a g n e t i c waves may be r e f l e c t e d s u n l i g h t , n a t u r a l l y
r a d i a t i v e o b j e c t s o r r e t u r n i n g radar s i g n a l s from objects.
I n order t o
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u t i l i z e t h e sensor d a t a f o r oceanographic purposes, i t i s necessary t o process i t b y v a r i o u s methods s i n c e t h e s e d a t a c o n t a i n a l o w s i g n a l - t o - n o i s e r a t i o .
F o r p a s s i v e s e n s o r s , s o u r c e s o f n o i s e i n c l u d e c l o u d s and a e r o s o l s . I n t h i s paper, d a t a processing procedures f o r t h r e e s a t e l l i t e sensor systems w i l l b e d i s c u s s e d :
( 1 ) AVHRR (Advanced V e r y H i g h R e s o l u t i o n R a d i o m e t e r )
o f NOAA - p o l a r o r b i t i n g m e t e o r o l o g i c a l s a t e l l i t e o f NOAA 6 and 7, ( 2 ) CZCS ( C o a s t a l Zone C o l o r S c a n n e r ) o f Nimbus-7 s a t e l l i t e and ( 3 ) SAR ( S y n t h e t i c A p e r t u r e R a d a r ) o f SEASAT.
AVHRR d a t a w e r e r e c e i v e d b y Japan M e t e o r o l o g i c a l
S a t e l l i t e Center.
o a t a p r o c e s s i n g needs f o r o c e a n o g r a p h i c a p p l i c a t i o n s T e c h n i q u e s used f o r r e m o t e s e n s i n g i n l a n d a p p l i c a t i o n s , such as LANDSAT, emphasize image p r o c e s s i n g o r classification/clustering.
However, t h e s e
t e c h n i q u e s a r e n o t as i m p o r t a n t f o r o c e a n o g r a p h i c a p p l i c a t i o n s .
This i s
because o c e a n o g r a p h i c r e q u i r e m e n t s and measured p a r a m e t e r s d i f f e r
from
those l a n d use sensor systems. The t h r e e s e n s o r s y s t e m s d i f f e r f r o i i i e a c h o t h e r b o t h i n t h e i r measurement p r i n c i p l e s and i n t e n d e d oceanographic purposes.
Techniques i m p o r t a n t f o r
each s y s t e m a r e : AVHRR:
(1) c a l i b r a t i o n , ( 2 ) geometric correction,
(3) atmospheric
c o r r e c t i o n and ( 4 ) g e n e r a t i o n o f sea s u r f a c e t e m p e r a t u r e (SST) maps. CZCS:
( 1 ) atmospheric modeling, i n c l u d i n g aerosol d i s t r i b u t i o n s , and ( 2 ) g e o m e t r i c c o r r e c t i o n .
SAR:
( 1 ) image r e c o n s t r u c t i o n and ( 2 ) wave l e n g t h d e t e r m i n a t i o n .
D e t a i l s o f these items a r e d e s c r i b e d below.
NOAA AVHRR The AVHRR i s a f o u r - o r f i v e - c h a n n e l f i f t h channels r e c e i v i n g a t 3.5-3.9
radiometer, w i t h t h e t h i r d through
,m, 10.5-11.5
Ilm and 11.5-12.5
llm i n t h e
t h e r m a l i n f r a r e d , a l t h o u g h t h e f i f t h c h a n n e l i s a v a i l a b l e o n l y o n t h e NOAA-7 s a t e l l i t e (Bernstein, 1982). f o u r t h channel.
The SST map f o r o u r s t u d y i s d e r i v e d f r o m t h e
The t h e r m a l band c h a n n e l s sense r a d i a t i o n f r o m t h e sea s u r -
f a c e and n o t r e f l e c t e d s u n l i g h t . t i m e SST d a t a .
T h e r e f o r e t h e AVHRR c a n p r o v i d e e v e n n i g h t
The h i g h e s t h o r i z o n t a l r e s o l u t i o n i s 1 . 1 km and t h e s w a t h
i s u p t o 3000 kin. One scene f r o m c l o u d - f r e e AVHRR d a t a c a n g e n e r a t e a SST map c o v e r i n g a wide area.
The SST i s a p r i m a r y i n d i c a t o r o f w a t e r mass d i s t r i b u t i o n i n
many a r e a s , p a r t i c u l a r l y i n c o l d e r seasons.
The AVHRR d a t a i s u s u a l l y w i d e
enough t o c o v e r m e s o s c a l e p r o c e s s e s i n t h e u p p e r l a y e r o f t h e ocean.
From
363
4
Atmospheric, correction Geometric correction
J.G
Optical depth
'
1
SST map
Fig. 1.
Block diagram of a system f o r processing the AVIiRR d a t a .
Fio. 2 . An o r i g i n al AVHRR C h . 4 image on April 2 6 , 1980 o f t h e KuroshioOyashio c o n f l u e nt region e a s t of t h e Sanriku D i s t r i c t . The strong d i s t o r t i o n due t o t h e e a r t h ' s curvature and t h e scan geometry makes the image almost u s e l e ss f o r oceanographic a p p l i c a t i o n .
364
a t i m e s e r i e s o f such d a t a , we can o b t a i n i n f o r m a t i o n e s s e n t i a l t o ocean dynami cs. There a r e two m a j o r purposes of p r o c e s s i n g t h e AVHRR d a t a .
One i s t o
determine t h e s p a t i a l d i s t . r i b u t i o n of t h e w a t e r masses t h r o u g h c o n s t r u c t i o n o f SST maps.
The o t h e r i s t o d e t e r m i n e t h e m o t i o n o f t h e w a t e r by t r a c i n g
successive SST maps.
The SST determined f r o m AVHRR i n f r a r e d channels d i f f e r s
i n a b s o l u t e magnitude by 1" K from t h e SST measured d i r e c t l y ( B e r n s t e i n , 1982); t h e d i f f e r e n c e i s caused m a i n l y by w a t e r vapor i n t h e atmosphere (Deschamps and P h u l p i n , 1980).
However, i n o r d e r t o d e t e r m i n e s p a t i a l d i s -
t r i b u t i o n s o f w a t e r masses from t h e SST, r e l a t i v e temperatures o r temperature gradients are s u f f i c i e n t .
R e l a t i v e temperatures can be determined a c c u r a t e l y
i f t h e sensors a r e c a l i b r a t e d f r e q u e n t l y .
T h i s i s necessary because charac-
t e r i s t i c s o f t h e sensors v a r y w i t h i n m i n u t e s . Water movement can be i n f e r r e d f r o m s u c c e s s i v e SST maps which inlay be cons t r u c t e d r o u g h l y t w i c e d a i l y i f t h e atmospheric c o n d i t i o n s a r e f a v o r a b l e . However, images r e c e i v e d by t h e AVHRR a r e d i s t o r t e d by t h e scan geometry o f the s a t e l l i t e .
T h e r e f o r e , t o m a i n t a i n c o n t i n u i t y i n t h e geometry o f succes-
s i v e images, g e o m e t r i c a l c o r r e c t i o n i s necessary.
Tozawa (198213) developed
an a l g o r i t h m f o r such c o r r e c t i o n which i s a c c u r a t e and e f f i c i e n t when a p p l i e d t o many images, though i t i s c o m p l i c a t e d by t h e e a r t h ' s r o t a t i o n . F i g u r e 1 shows a b l o c k diagram o f o u r system f o r c o n s t r u c t i n g a SST map from t h e AVHRR d a t a ( I i s a k a
g aJ.,
1980).
I n order t o increase t h e
accuracy i n t h e a b s o l u t e magnitude o f t h e SST, atmospheric c o r r e c t i o n s (McCnaghy, 1979) may be a p p l i e d i f sea t r u t h i s a v a i l a b l e .
Atmospheric c o r -
r e c t i o n s , i n c l u d i n g i n p u t o f sea t r u t h , a r e complex i n e x e c u t i o n .
Opera-
t i o n a l l y , i t i s necessary t o use one method f o r t h e e n t i r e image p r o c e s s i n g (Tozawa, 1983a). channel 4 .
F i g u r e 2 i s an unprocessed image r e c e i v e d f r o m AVHRR
F i g u r e 3 shows a r e l a t i v e SST map i n t h e M e r c a t o r ' s c o o r d i n a t e
system a f t e r g e o m e t r i c c o r r e c t i o n .
The map c l e a r l y i n d i c a t e s c o m p l i c a t e d
p a t t e r n s o f SST's e a s t o f t h e S a n r i k u D i s t r i c t o f Japan, where t h e warm Kuroshio and t h e c o l d Oyashio meet each o t h e r .
Particularly striking i s a
branch o f warm w a t e r e x t e n d i n g n o r t h w a r d f r o m ' t h e main f l o w o f t h e K u r o s h i o . The branch t u r n s west, t h e n n o r t h a g a i n and f i n a l l y makes an a n t i c y c l o n i c loop, e n t r a i n i n g t h e c o l d Oyashio w a t e r .
The K u r o s h i o main c u r r e n t seems
t o f l o w eastward from t h e l o c a t i o n o f b r a n c h i n g .
The Tsugaru Warm C u r r e n t
can a l s o be r e c o g n i z e d a l o n g t h e e a s t c o a s t o f t h e S a n r i k u D i s t r ' c t as a narrow band o f s l i g h t l y warm w a t e r .
Eddies and s t r e a k s r e v e a l e d i n t h i s
p i c t u r e a r e d i f f i c u l t t o observe b y c o n v e n t i o n a l h y d r o g r a p h i c methods. Coastal l i n e s a r e o v e r l a y e d i n F i g u r e 3 t o show geographic l o c a t i o n s o f t h e w a t e r mass d i s t r i b u t i o n .
These l i n e s a r e o b t a i n e d by a p p l y i n g o u r geometric
365
F i g . 3. An AVHRR image o f t h e same a r e a as F i o . 2 b u t d i g i t a l l y p r o c e s s e d w i t h g e o m e t r i c a l c o r r e c t i o n . The image was r e c e i v e d o n A p r i l 28, 1981. The c o a s t l i n e s o v c r l a y e d s u g o e s t a c c u r a c y of t k p e o m e t r i c a l c o r r e c t i o n .
F i o . 4. S u r f a c e i s o t h e r m map p r o c e s s e d a n d d i g i t i z e d from a n AVHRR image o n May 23, 1980 f o r t h e K u r o s h i o e a s t o f Honshu, Japan. The a r e a c o v e r e d i s s o u t h o f t h e one o f F i g . 2 and 3. The crowded i s o t h e r m s i n d i c a t e t h e n o r t h e r n f r o n t o f the Kuroshio.
366
Fig. 5 .
An original bathymetric chart..
The depths a r e i n meters.
Fig. 6 . Depth contours d i g i t a l l y processed from Fig5.These a r e t o be overlayed automatically on o t h e r s a t e l l i t e images.
367
c o r r e c t i o n t o t h e o r i g i n a l image.
Comparison w i t h a c t u a l c o a s t l i n e s i l l u s -
t r a t e s t h e accuracy o f o u r systeni. I n o r d e r t o i n t e r p r e t niesoscale processes i n t h e ocean more a c c u r a t e l y , i t i s necessary t o o v e r l a y o t h e r iniages, such as CZCS o r t o p o g r a p h i c a l images,
on AVHRR images.
When more t h a n one image i s superposed on an AVHRR image,
t h e l a t t e r becomes masked u n l e s s i t i s c o n v e r t e d t o a l i n e drawing.
We
developed a t e c h n i q u e t o t r a n s f o r m an AVHRR image t o an i s o t h e r m map (Sato
9fl., 1983).
F i g u r e 4 i s one example o f t h i s t e c h n i q u e .
D i f f i c u l t y i n p r o c e s s i n g AVHRR images may o c c u r i f a c l o u d even inuch s m a l l e r t h a n t h e f i e l d o f view i s p r e s e n t as seen i n t h e s t u d i e s o f Deschamps and P h u l p i n (1980) and documented by many a u t h o r s i n c l u d i n g B e r n s t e i n (1982). Since v i s i b l e (0.6-0.7
pm)
and near i n f r a r e d (0.7-1.1
urn) channels o f t h e
AVHRR can d i s c r i m i n a t e c l o u d areas, o u r t e c h n i q u e a u t o m a t i c a l l y e l i m i n a t e s t h e s e areas u s i n g t h e s e channels when t r a n s f o r m i n g an AVHRR image t o an i s o t h e r m map.
T h i s i s shown w i t h i s o t h e r m s i n t e r r u p t e d by c l o u d s .
I t i s found t h a t o c e a n i c c i r c u l a t i o n i s s t r o n g l y i n f l u e n c e d by b o t t o m
topography i n t h e n o r t h w a r d i n t r u s i o n o f t h e K u r o s h i o west o f Kyushu (Muneyama et
fl., 1984).
I n o r d e r t o i n t e r p r e t sea s u r f a c e processes d y n a m i c a l l y , i t
i s o f t e n necessary t o superpose b a t h y m e t r i c d a t a on o t h e r images.
A tech-
n i q u e f o r a u t o m a t i c i n p u t o f b a t h y m e t r i c d a t a has been developed (Tozawa _ et _ al.,
1983; I o k a
aJ.,
1983).
An example o f t h i s t e c h n i q u e i s shown by
F i g u r e 6 which i s made f r o m t h e o r i g i n a l b a t h y m e t r i c c h a r t o f F i g u r e 5 . CZCS __ CZCS i s designed t o measure t h e n e a r - s u r f a c e c o n c e n t r a t i o n o f p h y t o p l a n k t o n
pigment ( c h l o r o p h y l l a and p h a e o p h y t i n ) ( H o v i s
e aJ.,
1980).
The pigment
i s produced by t h e p h o t o s y n t h e t i c a c t i v i t y o f p h y t o p l a n k t o n and i t s concent r a t i o n i s c l o s e l y r e l a t e d t o t h e p r i m a r y p r o d u c t i v i t y o f t h e ocean.
CZCS
sensors measure s u n l i g h t r e f l e c t e d by suspended m a t e r i a l s , which a r e m a i n l y p h y t o p l a n k t o n and i n o r g a n i c s o l i d s .
Because o f t h e l o w r e f l e c t i v i t y of
t h e s e m a t e r i a l s , t h e sensors must measure w i t h h i g h s e n s i t i v i t y i n a s e v e r e l y r e s t r i c t e d s p e c t r a l range, o n l y f o u r n a r r o w s p e c t r a l bands f o r CZCS ( C l a r k , 1982).
However, more t h a n n i n e t y p e r c e n t o f t h e r a d i a t i o n r e c e i v e d by t h e
sensors comes f r o m r e f l e c t i o n by t h e a e r o s o l c o n t a i n e d i n t h e i n t e r v e n i n g atmosphere (Gordon, 1982).
I t i s necessary t o e l i m i n a t e t h e atmospheric
e f f e c t s i n o r d e r t o e x t r a c t a c c u r a t e i n f o r m a t i o n on pigment c o n c e n t r a t i o n f r o m t h e s a t e l l i t e imagery. A t e c h n i q u e has been developed f o r removal o f t h e s e a t m o s p h e r i c e f f e c t s
f r o m CZCS d a t a (Sasaki
g fi., 1983).
The t e c h n i q u e i s based on a new model
f o r t h e r a d i a t i o n c h a r a c t e r i s t i c s o f t h e atmosphere.
The model was developed
368
Fig. 7 . An original CZCS image of the Yellow Sea on February 1 6 , 1980. The orio,inal i s in c o l o r . The shades i n t h e p i c t u r e may be mainly due t o aerosol in the atmosphere.
Fig. 8. The image processed from Fig. 7 by use of t h e atmospheric correction of Sasaki algorithm (Sasaki e t a l . , 1983). The shades show the s u r f a c e pigment concentration.
369
from t h e o r e t i c a l c o n s i d e r a t i o n s and has been t e s t e d i n v a r i o u s f i e l d e x p e r i ments.
An example of t h i s atmospheric c o r r e c t i o n a l g o r i t h m i s shown by
F i g u r e s 7 and 8.
F i g u r e 7 i s an unprocessed CZCS image which shows a s u r f a c e p i c t u r e o f t h e Y e l l o w Sea ( o r i g i n a l i n c o l o r ) . However, most o f t h e sea s u r f a c e c o l o r p a t t e r n s a r e p r o b a b l y due t o a e r o s o l d i s t r i b u t i o n s i n t h e atmosphere.
F i g u r e 8 i s an image processed w i t h t h e Sasaki a l g o r i t h m f o r
atmospheric c o r r e c t i o n .
I t shows t h e d i s t r i b u t i o n o f pigment c o n c e n t r a t i o n ,
which i n d i c a t e s mesoscale m i x i n g o f d i f f e r e n t t y p e s o f c o a s t a l waters i n t h e Ye1 l o w Sea. The s a t e l l i t e images o b t a i n e d w i t h AVHRR and CZCS a r e e q u i v a l e n t t o t h e f l o w v i s u a l i z a t i o n s i n e x p e r i m e n t a l hydrodynamics (Werl6, 1973).
As p i c t u r e s
o f f l o w v i s u a l i z a t i o n , t h e s a t e l l i t e images v i v i d l y r e v e a l mesoscale processes i n t h e ocean.
However, i t i s a l s o i m p o r t a n t t o compare t h e s e images w i t h
t h e sea t r u t h i n o r d e r t o assess p h y s i c a l and dynamical processes q u a n t i t a t i v e l y , as f l o w v i s u a l i z a t i o n must be supplemented by measurements w i t h h o t w i r e anernometry o r o t h e r i n s t r u m e n t a t i o n .
Much e f f o r t has been made i n t h i s
d i r e c t i o n by oceanographers i n c o o p e r a t i o n w i t h p h y s i c i s t s , e n g i n e e r s and computer s c i e n t i s t s .
One example o f s t u d y i n g t h e w a t e r mass d i s t r i b u t i o n
o f f S a n r i k u as shown i n F i g u r e 3 i s g i v e n by Vastano and B e r n s t e i n (1984). SAR SAR i s expected t o be a p o w e r f u l t o o l f o r oceanography.
by t h e p u b l i c a t i o n o f a monograph on SAR (Beal
aJ.,
i s s u e o f t h e J o u r n a l o f Geophysical Research ( K i r w a n
T h i s i s evidenced
1981) and i n a s p e c i a l
g fi., 1983)
f o r SEASAT
remote sensing, one q u a r t e r o f which was devoted t o SAR, i n s p i t e o f t h e f a c t t h a t t h e s a t e l l i t e o p e r a t e d f o r o n l y t h r e e months i n 1978.
The reasons
f o r t h i s s e n s o r ' s p o t e n t i a l f o r wide oceanographic use a r e as f o l l o w s (Pravdo g& aJ.,
1983):
t h e SAR d a t a a r e a v a i l a b l e i n a l l weather c o n d i t i o n s ,
s i n c e t h e SAR uses r a d a r which sends and r e c e i v e s microwaves t h r o u g h c l o u d c o v e r , and a l s o i t can measure sea s u r f a c e roughness o v e r l a r g e ocean areas a t frequent time intervals.
The SAR i s capable o f o b s e r v i n g n o t o n l y s u r f a c e
g r a v i t y waves w h i c h a r e p r i m a r i l y causes o f t h e sea s u r f a c e roughness, b u t a l s o i n t e r n a l waves, sea i c e movement and o c e a n i c mesoscale eddies (Fu and H o l t , 1982). Though t h e SAR has a d a t a a c q u i s i t i o n r a t e (110 megabits p e r second), o r d e r s o f magnitude h i g h e r t h a n o t h e r p a s s i v e sensors and r a d a r s on SEASAT and o t h e r s a t e l l i t e s , i t s image i s a l s o e x t r e m e l y n o i s y .
Therefore, i n
o r d e r t o e x t r a c t i n f o r m a t i o n on t h e sea s u r f a c e w i t h t h e ground r e s o l u t i o n o f 25 m, a s o p h i s t i c a t e d image-forming process i s necessary. I n g e n e r a l , t h i s can be done by two k i n d s o f c o r r e l a t i o n methods, r e f e r r e d
tg
as o p t i c a l
370
F i g . 9. An image w i t h SEASAT SAR o f t h e sea s u r f a c e o f a J A S I N ( J o i n t A i r Sea I n t e r a c t i o n ) Experiment area i n t h e n o r t h A t l a n t i c Ocean on Auoust 19, 1978.
F i g . 10. A f i l t e r e d and smoothed p o r t i o n o f F i g . 3 w i t h a t e c h n i q u e o f JAMSTEC and IBN-Japan (1982) f o r d e t e c t i n g s u r f a c e g r a v i t y waves.
371
and d i g i t a l c o r r e l a t i o n s . and IBM-Japan,
Our t e c h n i q u e i s i n t h e l a t t e r c a t e g o r y (JAMSTEC
1982) and i s e x e m p l i f i e d i n F i g u r e s 9 and 10.
F i g u r e 9 shows
a SAR image o f a sea s u r f a c e area t e n by t e n k i l o m e t e r s which i s l o c a t e d n o r t h w e s t o f S c o t l a n d i n t h e J A S I N ( J o i n t Air-Sea I n t e r a c t i o n ) Experiment r e g i o n (Vesecky
eg d., 1983).
The image does n o t r e v e a l any s p e c i a l f e a t u r e s
o f t h e sea s u r f a c e because o f s p e c k l e n o i s e .
The f o l l o w i n g procedure i s
a p p l i e d t o e x t r a c t s u r f a c e wave c h a r a c t e r i s t i c s f r o m F i g u r e 9: ( 1 ) a p p l i c a t i o n o f two-dimensional FFT ( f a s t F o u r i e r t r a n s f o r m ) , ( 2 ) i d e n t i f i c a t i o n o f h i g h i n t e n s i t y p o i n t s c o r r e s p o n d i n g t o t h e wave d i r e c t i o n i n t h e f r e q u e n c y domain, ( 3 ) masking t h e h i g h f r e q u e n c y p a r t a l o n g t h e h i g h i n t e n s i t y p o i n t s ,
( 4 ) a p p l i c a t i o n o f t h e i n v e r s e FFT. The r e s u l t o f t h i s procedure i s i l l u s t r a t e d i n F i g u r e 10, which shows l o n g c r e s t e d s w e l l s w i t h wave l e n g t h s o f a b o u t 250 m.
I t a l s o shows p a t t e r n s of
s h o r t - c r e s t e d waves superposed on t h e l o n g - c r e s t e d waves. CONCLUSION S a t e l l i t e remote s e n s i n g i s g r e a t l y advantageous o v e r c o n v e n t i o n a l methods
I t can c o v e r wide areas a l m o s t i n s t a n -
o f d a t a a c q u i s i t i o n i n oceanography.
t a n e o u s l y and w i t h f r e q u e n c i e s o r d e r s o f magnitude h i g h e r t h a n t h e convent i o n a l methods.
However, t h e d a t a o b t a i n e d by s a t e l l i t e remote s e n s i n g
o f t e n c o n t a i n n o i s e f r o m v a r i o u s sources and do n o t p r o v i d e i n f o r m a t i o n d i r e c t l y u s e f u l t o oceanographers.
I n o r d e r t o e x t r a c t u s e f u l oceanographic
d a t a , d i f f e r e n t t e c h n i q u e s must be developed f o r d a t a p r o c e s s i n g .
This
paper has d e s c r i b e d examples o f such t e c h n i q u e s developed f o r d i g i t a l p r o c e s s i n g o f AVHRR, CZCS and SAR d a t a which a r e d i f f e r e n t f r o m t h o s e a p p l i c a b l e t o t h e l a n d - o r i e n t e d remote s e n s i n g .
The r e s e a r c h f o r d e v e l o p i n g t h e s e sea-
o r i e n t e d t e c h n i q u e s has been c a r r i e d o u t o n l y i n t h e p a s t decade and many new t e c h n i q u e s a r e needed.
F o r d e v e l o p i n g e f f e c t i v e techniques,
it i s
i m p o r t a n t t o understand oceanographic r e q u i r e m e n t s , p r i n c i p l e s o f t h e measurements and c h a r a c t e r i s t i c s o f t h e d a t a .
A l s o , as o f t e n as p o s s i b l e , any d a t a
o b t a i n e d by s a t e l l i t e remote s e n s i n g s h o u l d be compared o r c a l i b r a t e d w i t h data c o l l e c t e d i n t h e f i e l d .
W i t h o u t such comparison o r c a l i b r a t i o n , s a t e l -
l i t e d a t a may n o t be u s a b l e i n s p i t e o f s o p h i s t i c a t e d d a t a p r o c e s s i n g . ACKNOWLEDGMENTS
D r . John K l i n c k o f t h e Department o f Oceanography, Texas A&M U n i v e r s i t y r e a d t h e m a n u s c r i p t s c r i t i c a l l y and D r . Toshiko I c h i y e o f t h e Department o f Chemistry, Harvard U n i v e r s i t y and D r . P e t e r M i r a u o f t h e Department o f Chemistry, U n i v e r s i t y o f C a l i f o r n i a - San Diego checked t h e E n g l i s h usage
312
i n technical writing.
REFERENCES
Beal, R.C., DeLeonibus, and Katz, I. ( E d i t o r s ) , 1981. Spaceborne S y n t h e t i c A p e r t u r e Radar f o r Oceanography. Johns Hopkins U n i v e r s i t y Press, B a l t i m o r e , MD, 216 pp. Beal, R.C., T i l l e y , D.G. and Monaldo, F.M., 1983. Large- and s m a l l - s c a l e s p a t i a l e v o l u t i o n o f d i g i t a l l y processed ocean wave s p e c t r a f r o m SEASAT S y n t h e t i c A p e r t u r e Radar. J . Geophys. Res., 88: 1761-1778. B e r n s t e i n , R.L., 1982. Sea s u r f a c e t e m p e r a t u r e e s t i m a t i o n u s i n g t h e NOAA 6 s a t e l l i t e Advanced Very High R e s o l u t i o n Radiometer. J. Geophys. Res., 87: 9455-9466. C l a r k , D . K . , 1981. P h y t o p l a n k t o n pigment a l g o r i t h m s f o r t h e Nimbus-7 CZCS. I n : J.F.R. Gower ( E d i t o r ) , Oceanography f r o m Space. Plenum Press, New York, 227-238. Dechamps, P.Y. and P h u l p i n , T., 1980. Atmospheric c o r r e c t i o n o f i n f r a r e d measurements of sea s u r f a c e t e m p e r a t u r e u s i n g channels a t 3.7, 11, and 12 L j M . Boundary Layer M e t e o r o l . , 18: 131-143. Fu, L.-L. and H o l t , B . , 1982. SEASAT views oceans and sea i c e w i t h S y n t h e t i c A p e r t u r e Radar. JPL P u b l i c a t i o n 81-120, NASA and JET P r o p u l s i o n Lab. C.I.T., 200 pp. Gordon, H.R., 1981. A p r e l i m i n a r y assessment o f t h e Nimbus-7 CZCS atmosp h e r i c c o r r e c t i o n a l g o r i t h m i n a h o r i z o n t a l l y inhomogeneous atmosphere. I n : J.F.R. Gower ( E d i t o r ) , Oceanography f r o m Space. Plenum Press, New York, 257-266. Hovis, W.A., C l a r k , D.K., Anderson, F., A u s t i n , R.W., Wilson, W.H., Baker, E.T., B a l l , D., Gordon, H.R., M u e l l e r , J . L . , El-Sayed, S.Z., Sturm, B., W r i g l e y , R . C . and Yentsch, C . X . , 1980. Nimbus-7 Coastal Zone C o l o r Scanner: System d e s c r i p t i o n and i n i t i a l imagery. Science, 210: 60-63. I i s a k a , J., S a i t o h , S., Muneyama, K., Sasaki, Y . and Tozawa, Y., 1981. SST e s t i m a t i o n by NOAA AVHRR and i t s a p p l i c a t i o n t o o c e a n i c f r o n t e x t r a c t i o n . Proc. o f 2nd A s i a n Conference o f Remote Sensing, R-4. I o k a , M., Sato, M. and Tozawa, Y., 1983. Automatic h a n d l i n g o f b a t h y m e t r i c c h a r t s . ( I n Japanese). Proc. o f 2 7 t h Conference o f IPSJ, 949-950. JAMSTEC (Japan M a r i n e Science and Technology C e n t e r ) and IBM-Japan Tokyo S c i e n t i f i c Center, 1982. Oceanography f r o m Space. ( I n Japanese) 312 pp. Kirwan, A.D., Ahrens, T.J. and Born, G.H. ( E d i t o r s ) , 1983. SEASAT S p e c i a l I s s u e 11: S c i e n t i f i c R e s u l t s , J o u r n a l o f Geophysical Research, 88: 15291952. McCnaghy, D.C., 1980. Measuring sea s u r f a c e t e m p e r a t u r e f r o m s a t e l l i t e s : A ground t r u t h approach, remote s e n s i n g o f environment 10: 307-310. Muneyama, K., Asanuma, I . , Sasaki, Y., S a i t o h , S., Tozawa, Y., and I c h i y e , T., 1984. An a p p l i c a t i o n o f t h e NOAA AVHRR f o r oceanography i n t h e E a s t China Sea. I n : T. I c h i y e ( E d i t o r ) , Ocean Hydrodynamics o f Japan and E a s t China Seas. E l s e v i e r , Amsterdam. Pravdo, S.H., Huneycutt, B., H o l t , B.M. and Held, D.N., 1983. Seasat Synt h e t i c - A p e r t u r e Radar Data U s e r ' s Manual, NASA. Sasaki, Y . , Asanuma, I . , Muneyama, K . , Emori, Y . , Yasuda, Y., I i s a k a , J. and Tozawa, Y . , 1983. Development o f atmospheric c o r r e c t i o n a l g o r i t h m s f o r Nimbus-7 CZCS, LANDSAT and MOS-1 MESSR, Proc. o f 7 t h I n t e r n a t i o n a l Symposium on Remote Sensing o f Environment. Tozawa, Y . , 1983a. A s p e c i f i c a t i o n o f image p r o c e s s i n g f o r remote sending. Proc. o f I n t e r G r a p h i c s '83, B8-3. Tozawa, Y . , 1983b. F a s t g e o m e t r i c c o r r e c t i o n o f NOAA AVHRR, Proc. o f 9 t h I n t e r n a t i o n a l Symposium o f Machine P r o c e s s i n g o f Remotely Sensed Data, 46-53.
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Tozawa, Y . , I o k a , M . and Sato, M . , 1983. Automatic i n p u t o f w a t e r d e p t h map. Proc. o f I E E E Workshop on CAPAIDM, 194-201. Vastano, A . C . and B e r n s t e i n , R.L., 1984. Mesoscale f e a t u r e s a l o n g t h e F i r s t Oyashio I n t r u s i o n . J . Geophys. Res., 89: 587-596. Vesecky, J.F., A s s a l , H.M. and S t e w a r t , R.H., 1981. Remote s e n s i n g o f t h e ocean wave h e i g h t spectrum u s i n g S y n t h e t i c - A p e r t u r e - R a d a r images. I n : J.F.R. Gower ( E d i t o r ) , Oceanography f r o m Space. Plenum Press, New York, 449-458. Werl6, H., 1973. Hydrodynamic f l o w v i s u a l i z a t i o n . Ann. Rev. o f F l u i d Mech., 5: 361-382.
DATA PROCESSING TECHNIQUES OF REMOTE SENSING FOR OCEANOGRAPHY Y . TOZAWA,l
M. SF\TO,’ M . II)KA,l
K . MUNEYAMA,2
Y . SASAKI,‘
I. ASANUMA,’
and
T. I C H I Y E 3 ’Tokyo S c i e n t i f i c Center, IBM-Japan, 5-19, Sanbancho, Chiyodaka, Tokyo 102 2Japan M a r i n e Science and Technology Center, 2-15, Natsushima, Yokosuka, 237 Japan 3Department o f Oceanography, Texas A&M U n i v e r s i t y , C o l l e g e S t a t i o n , TX 77843
ABSTRACT S a t e l l i t e remote s e n s i n g can p r o v i d e oceanographers w i t h p o w e r f u l t o o l s f o r c o l l e c t i n g d a t a , s i n c e i t covers a wide area i n s t a n t a n e o u s l y and as f r e q u e n t l y as s e v e r a l t i m e s a day. I n o r d e r t o e x t r a c t m e a n i n a f u l i n f o r m a t i o n f r o m d a t a c o l l e c t e d by s a t e l l i t e s , s p e c i a l d a t a p r o c e s s i n g i s an a b s o l u t e n e c e s s i t y . Techniques f o r d a t a p r o c e s s i n g a r e d i f f e r e n t , depending on c h a r a c t e r i s t i c s o f t h e d a t a , measurement p r i n c i p l e s and t h e oceanographic r e q u i r e m e n t s . Three systems o f s a t e l l i t e sensing, AVHRR, CZCS and SAR, a r e p a r t i c u l a r l y u s e f u l f o r oceanography b u t need s p e c i a l t e c h n i q u e s f o r d a t a p r o c e s s i n g . Several t e c h n i q u e s developed by JAMSTEC and IBM-Japan f o r t h e s e systems a r e d e s c r i b e d w i t h examples a p p l i e d t o oceanography.
INTRODUCTION The Japan M a r i n e Science and Technology Center (JAMSTEC) and IBM-Japan, Tokyo S c i e n t i f i c Center have been c o o p e r a t i n g s i n c e 1980 on r e s e a r c h conc e r n i n g a p p l i c a t i o n o f remote s e n s i n g t o oceanography (JAMSTEC and IBM-Japan, 1982).
S a t e l l i t e remote s e n s i n g t e c h n i q u e s have b r o u g h t new approaches t o
t h e c o l l e c t i o n o f oceanographic d a t a .
The t e c h n i q u e s have t h e f o l l o w i n g
advantages o v e r c o n v e n t i o n a l oceanography methods. wide a r e a o f t h e ocean a l m o s t i n s t a n t a n e o u s l y .
F i r s t , t h e y can c o v e r a
Second, t h e y can m o n i t o r
many p a r t s o f t h e w o r l d oceans w i t h f r e q u e n c i e s never achieved b e f o r e by c o n v e n t i o n a l methods, t h a t i s , about t w i c e d a i l y .
The f i r s t advantage l e a d s
t o i n f o r m a t i o n on s p a t i a l c h a r a c t e r i s t i c s o f t h e ocean.
The second advantage
can be u t i l i z e d i n u n d e r s t a n d i n g ocean dynamics by measuring time-dependent s t a t e s o f t h e ocean. Remote s e n s i n g i n v o l v e s o b s e r v a t i o n t e c h n i q u e s which use e l e c t r o m a g n e t i c waves.
Sources o f e l e c t r o m a g n e t i c waves may be r e f l e c t e d s u n l i g h t , n a t u r a l l y
r a d i a t i v e o b j e c t s o r r e t u r n i n g radar s i g n a l s from objects.
I n order t o
362
u t i l i z e t h e sensor d a t a f o r oceanographic purposes, i t i s necessary t o process i t b y v a r i o u s methods s i n c e t h e s e d a t a c o n t a i n a l o w s i g n a l - t o - n o i s e r a t i o .
F o r p a s s i v e s e n s o r s , s o u r c e s o f n o i s e i n c l u d e c l o u d s and a e r o s o l s . I n t h i s paper, d a t a processing procedures f o r t h r e e s a t e l l i t e sensor systems w i l l b e d i s c u s s e d :
( 1 ) AVHRR (Advanced V e r y H i g h R e s o l u t i o n R a d i o m e t e r )
o f NOAA - p o l a r o r b i t i n g m e t e o r o l o g i c a l s a t e l l i t e o f NOAA 6 and 7, ( 2 ) CZCS ( C o a s t a l Zone C o l o r S c a n n e r ) o f Nimbus-7 s a t e l l i t e and ( 3 ) SAR ( S y n t h e t i c A p e r t u r e R a d a r ) o f SEASAT.
AVHRR d a t a w e r e r e c e i v e d b y Japan M e t e o r o l o g i c a l
S a t e l l i t e Center.
o a t a p r o c e s s i n g needs f o r o c e a n o g r a p h i c a p p l i c a t i o n s T e c h n i q u e s used f o r r e m o t e s e n s i n g i n l a n d a p p l i c a t i o n s , such as LANDSAT, emphasize image p r o c e s s i n g o r classification/clustering.
However, t h e s e
t e c h n i q u e s a r e n o t as i m p o r t a n t f o r o c e a n o g r a p h i c a p p l i c a t i o n s .
This i s
because o c e a n o g r a p h i c r e q u i r e m e n t s and measured p a r a m e t e r s d i f f e r
from
those l a n d use sensor systems. The t h r e e s e n s o r s y s t e m s d i f f e r f r o i i i e a c h o t h e r b o t h i n t h e i r measurement p r i n c i p l e s and i n t e n d e d oceanographic purposes.
Techniques i m p o r t a n t f o r
each s y s t e m a r e : AVHRR:
(1) c a l i b r a t i o n , ( 2 ) geometric correction,
(3) atmospheric
c o r r e c t i o n and ( 4 ) g e n e r a t i o n o f sea s u r f a c e t e m p e r a t u r e (SST) maps. CZCS:
( 1 ) atmospheric modeling, i n c l u d i n g aerosol d i s t r i b u t i o n s , and ( 2 ) g e o m e t r i c c o r r e c t i o n .
SAR:
( 1 ) image r e c o n s t r u c t i o n and ( 2 ) wave l e n g t h d e t e r m i n a t i o n .
D e t a i l s o f these items a r e d e s c r i b e d below.
NOAA AVHRR The AVHRR i s a f o u r - o r f i v e - c h a n n e l f i f t h channels r e c e i v i n g a t 3.5-3.9
radiometer, w i t h t h e t h i r d through
,m, 10.5-11.5
Ilm and 11.5-12.5
llm i n t h e
t h e r m a l i n f r a r e d , a l t h o u g h t h e f i f t h c h a n n e l i s a v a i l a b l e o n l y o n t h e NOAA-7 s a t e l l i t e (Bernstein, 1982). f o u r t h channel.
The SST map f o r o u r s t u d y i s d e r i v e d f r o m t h e
The t h e r m a l band c h a n n e l s sense r a d i a t i o n f r o m t h e sea s u r -
f a c e and n o t r e f l e c t e d s u n l i g h t . t i m e SST d a t a .
T h e r e f o r e t h e AVHRR c a n p r o v i d e e v e n n i g h t
The h i g h e s t h o r i z o n t a l r e s o l u t i o n i s 1 . 1 km and t h e s w a t h
i s u p t o 3000 kin. One scene f r o m c l o u d - f r e e AVHRR d a t a c a n g e n e r a t e a SST map c o v e r i n g a wide area.
The SST i s a p r i m a r y i n d i c a t o r o f w a t e r mass d i s t r i b u t i o n i n
many a r e a s , p a r t i c u l a r l y i n c o l d e r seasons.
The AVHRR d a t a i s u s u a l l y w i d e
enough t o c o v e r m e s o s c a l e p r o c e s s e s i n t h e u p p e r l a y e r o f t h e ocean.
From
363
4
Atmospheric, correction Geometric correction
J.G
Optical depth
'
1
SST map
Fig. 1.
Block diagram of a system f o r processing the AVIiRR d a t a .
Fio. 2 . An o r i g i n al AVHRR C h . 4 image on April 2 6 , 1980 o f t h e KuroshioOyashio c o n f l u e nt region e a s t of t h e Sanriku D i s t r i c t . The strong d i s t o r t i o n due t o t h e e a r t h ' s curvature and t h e scan geometry makes the image almost u s e l e ss f o r oceanographic a p p l i c a t i o n .
364
a t i m e s e r i e s o f such d a t a , we can o b t a i n i n f o r m a t i o n e s s e n t i a l t o ocean dynami cs. There a r e two m a j o r purposes of p r o c e s s i n g t h e AVHRR d a t a .
One i s t o
determine t h e s p a t i a l d i s t . r i b u t i o n of t h e w a t e r masses t h r o u g h c o n s t r u c t i o n o f SST maps.
The o t h e r i s t o d e t e r m i n e t h e m o t i o n o f t h e w a t e r by t r a c i n g
successive SST maps.
The SST determined f r o m AVHRR i n f r a r e d channels d i f f e r s
i n a b s o l u t e magnitude by 1" K from t h e SST measured d i r e c t l y ( B e r n s t e i n , 1982); t h e d i f f e r e n c e i s caused m a i n l y by w a t e r vapor i n t h e atmosphere (Deschamps and P h u l p i n , 1980).
However, i n o r d e r t o d e t e r m i n e s p a t i a l d i s -
t r i b u t i o n s o f w a t e r masses from t h e SST, r e l a t i v e temperatures o r temperature gradients are s u f f i c i e n t .
R e l a t i v e temperatures can be determined a c c u r a t e l y
i f t h e sensors a r e c a l i b r a t e d f r e q u e n t l y .
T h i s i s necessary because charac-
t e r i s t i c s o f t h e sensors v a r y w i t h i n m i n u t e s . Water movement can be i n f e r r e d f r o m s u c c e s s i v e SST maps which inlay be cons t r u c t e d r o u g h l y t w i c e d a i l y i f t h e atmospheric c o n d i t i o n s a r e f a v o r a b l e . However, images r e c e i v e d by t h e AVHRR a r e d i s t o r t e d by t h e scan geometry o f the s a t e l l i t e .
T h e r e f o r e , t o m a i n t a i n c o n t i n u i t y i n t h e geometry o f succes-
s i v e images, g e o m e t r i c a l c o r r e c t i o n i s necessary.
Tozawa (198213) developed
an a l g o r i t h m f o r such c o r r e c t i o n which i s a c c u r a t e and e f f i c i e n t when a p p l i e d t o many images, though i t i s c o m p l i c a t e d by t h e e a r t h ' s r o t a t i o n . F i g u r e 1 shows a b l o c k diagram o f o u r system f o r c o n s t r u c t i n g a SST map from t h e AVHRR d a t a ( I i s a k a
g aJ.,
1980).
I n order t o increase t h e
accuracy i n t h e a b s o l u t e magnitude o f t h e SST, atmospheric c o r r e c t i o n s (McCnaghy, 1979) may be a p p l i e d i f sea t r u t h i s a v a i l a b l e .
Atmospheric c o r -
r e c t i o n s , i n c l u d i n g i n p u t o f sea t r u t h , a r e complex i n e x e c u t i o n .
Opera-
t i o n a l l y , i t i s necessary t o use one method f o r t h e e n t i r e image p r o c e s s i n g (Tozawa, 1983a). channel 4 .
F i g u r e 2 i s an unprocessed image r e c e i v e d f r o m AVHRR
F i g u r e 3 shows a r e l a t i v e SST map i n t h e M e r c a t o r ' s c o o r d i n a t e
system a f t e r g e o m e t r i c c o r r e c t i o n .
The map c l e a r l y i n d i c a t e s c o m p l i c a t e d
p a t t e r n s o f SST's e a s t o f t h e S a n r i k u D i s t r i c t o f Japan, where t h e warm Kuroshio and t h e c o l d Oyashio meet each o t h e r .
Particularly striking i s a
branch o f warm w a t e r e x t e n d i n g n o r t h w a r d f r o m ' t h e main f l o w o f t h e K u r o s h i o . The branch t u r n s west, t h e n n o r t h a g a i n and f i n a l l y makes an a n t i c y c l o n i c loop, e n t r a i n i n g t h e c o l d Oyashio w a t e r .
The K u r o s h i o main c u r r e n t seems
t o f l o w eastward from t h e l o c a t i o n o f b r a n c h i n g .
The Tsugaru Warm C u r r e n t
can a l s o be r e c o g n i z e d a l o n g t h e e a s t c o a s t o f t h e S a n r i k u D i s t r ' c t as a narrow band o f s l i g h t l y warm w a t e r .
Eddies and s t r e a k s r e v e a l e d i n t h i s
p i c t u r e a r e d i f f i c u l t t o observe b y c o n v e n t i o n a l h y d r o g r a p h i c methods. Coastal l i n e s a r e o v e r l a y e d i n F i g u r e 3 t o show geographic l o c a t i o n s o f t h e w a t e r mass d i s t r i b u t i o n .
These l i n e s a r e o b t a i n e d by a p p l y i n g o u r geometric
365
F i g . 3. An AVHRR image o f t h e same a r e a as F i o . 2 b u t d i g i t a l l y p r o c e s s e d w i t h g e o m e t r i c a l c o r r e c t i o n . The image was r e c e i v e d o n A p r i l 28, 1981. The c o a s t l i n e s o v c r l a y e d s u g o e s t a c c u r a c y of t k p e o m e t r i c a l c o r r e c t i o n .
F i o . 4. S u r f a c e i s o t h e r m map p r o c e s s e d a n d d i g i t i z e d from a n AVHRR image o n May 23, 1980 f o r t h e K u r o s h i o e a s t o f Honshu, Japan. The a r e a c o v e r e d i s s o u t h o f t h e one o f F i g . 2 and 3. The crowded i s o t h e r m s i n d i c a t e t h e n o r t h e r n f r o n t o f the Kuroshio.
366
Fig. 5 .
An original bathymetric chart..
The depths a r e i n meters.
Fig. 6 . Depth contours d i g i t a l l y processed from Fig5.These a r e t o be overlayed automatically on o t h e r s a t e l l i t e images.
367
c o r r e c t i o n t o t h e o r i g i n a l image.
Comparison w i t h a c t u a l c o a s t l i n e s i l l u s -
t r a t e s t h e accuracy o f o u r systeni. I n o r d e r t o i n t e r p r e t niesoscale processes i n t h e ocean more a c c u r a t e l y , i t i s necessary t o o v e r l a y o t h e r iniages, such as CZCS o r t o p o g r a p h i c a l images,
on AVHRR images.
When more t h a n one image i s superposed on an AVHRR image,
t h e l a t t e r becomes masked u n l e s s i t i s c o n v e r t e d t o a l i n e drawing.
We
developed a t e c h n i q u e t o t r a n s f o r m an AVHRR image t o an i s o t h e r m map (Sato
9fl., 1983).
F i g u r e 4 i s one example o f t h i s t e c h n i q u e .
D i f f i c u l t y i n p r o c e s s i n g AVHRR images may o c c u r i f a c l o u d even inuch s m a l l e r t h a n t h e f i e l d o f view i s p r e s e n t as seen i n t h e s t u d i e s o f Deschamps and P h u l p i n (1980) and documented by many a u t h o r s i n c l u d i n g B e r n s t e i n (1982). Since v i s i b l e (0.6-0.7
pm)
and near i n f r a r e d (0.7-1.1
urn) channels o f t h e
AVHRR can d i s c r i m i n a t e c l o u d areas, o u r t e c h n i q u e a u t o m a t i c a l l y e l i m i n a t e s t h e s e areas u s i n g t h e s e channels when t r a n s f o r m i n g an AVHRR image t o an i s o t h e r m map.
T h i s i s shown w i t h i s o t h e r m s i n t e r r u p t e d by c l o u d s .
I t i s found t h a t o c e a n i c c i r c u l a t i o n i s s t r o n g l y i n f l u e n c e d by b o t t o m
topography i n t h e n o r t h w a r d i n t r u s i o n o f t h e K u r o s h i o west o f Kyushu (Muneyama et
fl., 1984).
I n o r d e r t o i n t e r p r e t sea s u r f a c e processes d y n a m i c a l l y , i t
i s o f t e n necessary t o superpose b a t h y m e t r i c d a t a on o t h e r images.
A tech-
n i q u e f o r a u t o m a t i c i n p u t o f b a t h y m e t r i c d a t a has been developed (Tozawa _ et _ al.,
1983; I o k a
aJ.,
1983).
An example o f t h i s t e c h n i q u e i s shown by
F i g u r e 6 which i s made f r o m t h e o r i g i n a l b a t h y m e t r i c c h a r t o f F i g u r e 5 . CZCS __ CZCS i s designed t o measure t h e n e a r - s u r f a c e c o n c e n t r a t i o n o f p h y t o p l a n k t o n
pigment ( c h l o r o p h y l l a and p h a e o p h y t i n ) ( H o v i s
e aJ.,
1980).
The pigment
i s produced by t h e p h o t o s y n t h e t i c a c t i v i t y o f p h y t o p l a n k t o n and i t s concent r a t i o n i s c l o s e l y r e l a t e d t o t h e p r i m a r y p r o d u c t i v i t y o f t h e ocean.
CZCS
sensors measure s u n l i g h t r e f l e c t e d by suspended m a t e r i a l s , which a r e m a i n l y p h y t o p l a n k t o n and i n o r g a n i c s o l i d s .
Because o f t h e l o w r e f l e c t i v i t y of
t h e s e m a t e r i a l s , t h e sensors must measure w i t h h i g h s e n s i t i v i t y i n a s e v e r e l y r e s t r i c t e d s p e c t r a l range, o n l y f o u r n a r r o w s p e c t r a l bands f o r CZCS ( C l a r k , 1982).
However, more t h a n n i n e t y p e r c e n t o f t h e r a d i a t i o n r e c e i v e d by t h e
sensors comes f r o m r e f l e c t i o n by t h e a e r o s o l c o n t a i n e d i n t h e i n t e r v e n i n g atmosphere (Gordon, 1982).
I t i s necessary t o e l i m i n a t e t h e atmospheric
e f f e c t s i n o r d e r t o e x t r a c t a c c u r a t e i n f o r m a t i o n on pigment c o n c e n t r a t i o n f r o m t h e s a t e l l i t e imagery. A t e c h n i q u e has been developed f o r removal o f t h e s e a t m o s p h e r i c e f f e c t s
f r o m CZCS d a t a (Sasaki
g fi., 1983).
The t e c h n i q u e i s based on a new model
f o r t h e r a d i a t i o n c h a r a c t e r i s t i c s o f t h e atmosphere.
The model was developed
368
Fig. 7 . An original CZCS image of the Yellow Sea on February 1 6 , 1980. The orio,inal i s in c o l o r . The shades i n t h e p i c t u r e may be mainly due t o aerosol in the atmosphere.
Fig. 8. The image processed from Fig. 7 by use of t h e atmospheric correction of Sasaki algorithm (Sasaki e t a l . , 1983). The shades show the s u r f a c e pigment concentration.
369
from t h e o r e t i c a l c o n s i d e r a t i o n s and has been t e s t e d i n v a r i o u s f i e l d e x p e r i ments.
An example of t h i s atmospheric c o r r e c t i o n a l g o r i t h m i s shown by
F i g u r e s 7 and 8.
F i g u r e 7 i s an unprocessed CZCS image which shows a s u r f a c e p i c t u r e o f t h e Y e l l o w Sea ( o r i g i n a l i n c o l o r ) . However, most o f t h e sea s u r f a c e c o l o r p a t t e r n s a r e p r o b a b l y due t o a e r o s o l d i s t r i b u t i o n s i n t h e atmosphere.
F i g u r e 8 i s an image processed w i t h t h e Sasaki a l g o r i t h m f o r
atmospheric c o r r e c t i o n .
I t shows t h e d i s t r i b u t i o n o f pigment c o n c e n t r a t i o n ,
which i n d i c a t e s mesoscale m i x i n g o f d i f f e r e n t t y p e s o f c o a s t a l waters i n t h e Ye1 l o w Sea. The s a t e l l i t e images o b t a i n e d w i t h AVHRR and CZCS a r e e q u i v a l e n t t o t h e f l o w v i s u a l i z a t i o n s i n e x p e r i m e n t a l hydrodynamics (Werl6, 1973).
As p i c t u r e s
o f f l o w v i s u a l i z a t i o n , t h e s a t e l l i t e images v i v i d l y r e v e a l mesoscale processes i n t h e ocean.
However, i t i s a l s o i m p o r t a n t t o compare t h e s e images w i t h
t h e sea t r u t h i n o r d e r t o assess p h y s i c a l and dynamical processes q u a n t i t a t i v e l y , as f l o w v i s u a l i z a t i o n must be supplemented by measurements w i t h h o t w i r e anernometry o r o t h e r i n s t r u m e n t a t i o n .
Much e f f o r t has been made i n t h i s
d i r e c t i o n by oceanographers i n c o o p e r a t i o n w i t h p h y s i c i s t s , e n g i n e e r s and computer s c i e n t i s t s .
One example o f s t u d y i n g t h e w a t e r mass d i s t r i b u t i o n
o f f S a n r i k u as shown i n F i g u r e 3 i s g i v e n by Vastano and B e r n s t e i n (1984). SAR SAR i s expected t o be a p o w e r f u l t o o l f o r oceanography.
by t h e p u b l i c a t i o n o f a monograph on SAR (Beal
aJ.,
i s s u e o f t h e J o u r n a l o f Geophysical Research ( K i r w a n
T h i s i s evidenced
1981) and i n a s p e c i a l
g fi., 1983)
f o r SEASAT
remote sensing, one q u a r t e r o f which was devoted t o SAR, i n s p i t e o f t h e f a c t t h a t t h e s a t e l l i t e o p e r a t e d f o r o n l y t h r e e months i n 1978.
The reasons
f o r t h i s s e n s o r ' s p o t e n t i a l f o r wide oceanographic use a r e as f o l l o w s (Pravdo g& aJ.,
1983):
t h e SAR d a t a a r e a v a i l a b l e i n a l l weather c o n d i t i o n s ,
s i n c e t h e SAR uses r a d a r which sends and r e c e i v e s microwaves t h r o u g h c l o u d c o v e r , and a l s o i t can measure sea s u r f a c e roughness o v e r l a r g e ocean areas a t frequent time intervals.
The SAR i s capable o f o b s e r v i n g n o t o n l y s u r f a c e
g r a v i t y waves w h i c h a r e p r i m a r i l y causes o f t h e sea s u r f a c e roughness, b u t a l s o i n t e r n a l waves, sea i c e movement and o c e a n i c mesoscale eddies (Fu and H o l t , 1982). Though t h e SAR has a d a t a a c q u i s i t i o n r a t e (110 megabits p e r second), o r d e r s o f magnitude h i g h e r t h a n o t h e r p a s s i v e sensors and r a d a r s on SEASAT and o t h e r s a t e l l i t e s , i t s image i s a l s o e x t r e m e l y n o i s y .
Therefore, i n
o r d e r t o e x t r a c t i n f o r m a t i o n on t h e sea s u r f a c e w i t h t h e ground r e s o l u t i o n o f 25 m, a s o p h i s t i c a t e d image-forming process i s necessary. I n g e n e r a l , t h i s can be done by two k i n d s o f c o r r e l a t i o n methods, r e f e r r e d
tg
as o p t i c a l
370
F i g . 9. An image w i t h SEASAT SAR o f t h e sea s u r f a c e o f a J A S I N ( J o i n t A i r Sea I n t e r a c t i o n ) Experiment area i n t h e n o r t h A t l a n t i c Ocean on Auoust 19, 1978.
F i g . 10. A f i l t e r e d and smoothed p o r t i o n o f F i g . 3 w i t h a t e c h n i q u e o f JAMSTEC and IBN-Japan (1982) f o r d e t e c t i n g s u r f a c e g r a v i t y waves.
371
and d i g i t a l c o r r e l a t i o n s . and IBM-Japan,
Our t e c h n i q u e i s i n t h e l a t t e r c a t e g o r y (JAMSTEC
1982) and i s e x e m p l i f i e d i n F i g u r e s 9 and 10.
F i g u r e 9 shows
a SAR image o f a sea s u r f a c e area t e n by t e n k i l o m e t e r s which i s l o c a t e d n o r t h w e s t o f S c o t l a n d i n t h e J A S I N ( J o i n t Air-Sea I n t e r a c t i o n ) Experiment r e g i o n (Vesecky
eg d., 1983).
The image does n o t r e v e a l any s p e c i a l f e a t u r e s
o f t h e sea s u r f a c e because o f s p e c k l e n o i s e .
The f o l l o w i n g procedure i s
a p p l i e d t o e x t r a c t s u r f a c e wave c h a r a c t e r i s t i c s f r o m F i g u r e 9: ( 1 ) a p p l i c a t i o n o f two-dimensional FFT ( f a s t F o u r i e r t r a n s f o r m ) , ( 2 ) i d e n t i f i c a t i o n o f h i g h i n t e n s i t y p o i n t s c o r r e s p o n d i n g t o t h e wave d i r e c t i o n i n t h e f r e q u e n c y domain, ( 3 ) masking t h e h i g h f r e q u e n c y p a r t a l o n g t h e h i g h i n t e n s i t y p o i n t s ,
( 4 ) a p p l i c a t i o n o f t h e i n v e r s e FFT. The r e s u l t o f t h i s procedure i s i l l u s t r a t e d i n F i g u r e 10, which shows l o n g c r e s t e d s w e l l s w i t h wave l e n g t h s o f a b o u t 250 m.
I t a l s o shows p a t t e r n s of
s h o r t - c r e s t e d waves superposed on t h e l o n g - c r e s t e d waves. CONCLUSION S a t e l l i t e remote s e n s i n g i s g r e a t l y advantageous o v e r c o n v e n t i o n a l methods
I t can c o v e r wide areas a l m o s t i n s t a n -
o f d a t a a c q u i s i t i o n i n oceanography.
t a n e o u s l y and w i t h f r e q u e n c i e s o r d e r s o f magnitude h i g h e r t h a n t h e convent i o n a l methods.
However, t h e d a t a o b t a i n e d by s a t e l l i t e remote s e n s i n g
o f t e n c o n t a i n n o i s e f r o m v a r i o u s sources and do n o t p r o v i d e i n f o r m a t i o n d i r e c t l y u s e f u l t o oceanographers.
I n o r d e r t o e x t r a c t u s e f u l oceanographic
d a t a , d i f f e r e n t t e c h n i q u e s must be developed f o r d a t a p r o c e s s i n g .
This
paper has d e s c r i b e d examples o f such t e c h n i q u e s developed f o r d i g i t a l p r o c e s s i n g o f AVHRR, CZCS and SAR d a t a which a r e d i f f e r e n t f r o m t h o s e a p p l i c a b l e t o t h e l a n d - o r i e n t e d remote s e n s i n g .
The r e s e a r c h f o r d e v e l o p i n g t h e s e sea-
o r i e n t e d t e c h n i q u e s has been c a r r i e d o u t o n l y i n t h e p a s t decade and many new t e c h n i q u e s a r e needed.
F o r d e v e l o p i n g e f f e c t i v e techniques,
it i s
i m p o r t a n t t o understand oceanographic r e q u i r e m e n t s , p r i n c i p l e s o f t h e measurements and c h a r a c t e r i s t i c s o f t h e d a t a .
A l s o , as o f t e n as p o s s i b l e , any d a t a
o b t a i n e d by s a t e l l i t e remote s e n s i n g s h o u l d be compared o r c a l i b r a t e d w i t h data c o l l e c t e d i n t h e f i e l d .
W i t h o u t such comparison o r c a l i b r a t i o n , s a t e l -
l i t e d a t a may n o t be u s a b l e i n s p i t e o f s o p h i s t i c a t e d d a t a p r o c e s s i n g . ACKNOWLEDGMENTS
D r . John K l i n c k o f t h e Department o f Oceanography, Texas A&M U n i v e r s i t y r e a d t h e m a n u s c r i p t s c r i t i c a l l y and D r . Toshiko I c h i y e o f t h e Department o f Chemistry, Harvard U n i v e r s i t y and D r . P e t e r M i r a u o f t h e Department o f Chemistry, U n i v e r s i t y o f C a l i f o r n i a - San Diego checked t h e E n g l i s h usage
312
i n technical writing.
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