Improvements in remote sensing capabilities hinge very directly upon attaining an understanding of the physical processes contributing to the measurements. In order to devise new measurement strategies and to learn better techniques for processing remotely gathered data, it is necessary to understand and to characterize the complex radiative interactions of the atmosphere-surface system. In particular, it is important to understand the role of atmospheric structure, ground reflectance inhomogeneity and ground bidirectional reflectance type. The goals, then, are to model, analyze, and parameterize the observable effects of three dimensional atmospheric structure and composition and two dimensional variations in ground albedo and bidirectional reflectance. To achieve these goals, a Monte Carlo radiative transfer code is employed to model and analyze the effects of many of the complications which are present in nature

Pearce, W. A.

English

NASA Technical Reports Server

THE CHARACTERIZATION OF SURFACE VARIEGAT1,ON EFFECTS 
ON REMOTE SENSING 
William A. Pearce 
EG&G Washington Analytical Services Center, Inc. 
5000 Philadelphia Way 
Lanham, Maryland 20706 
Improvements in remote sensing capabilities hinge very directly 
upon attaining an understanding of the physical processes contributing 
to the measurements. In order to devise new measurement strategies and 
to learn better techniques for processing remotely gathered data, we 
need to understand and to characterize the complex radiative 
interactions of the atmosphere-surface system. In particular, it is 
important to understand the role of atmospheric structure, ground 
reflectance inhomogeneity and ground bidirectional reflectance type. 
Our goals, then, are tc model, analyze, and parameterize the observable 
effects of three dimensional atmospheric structure and composition and 
two dimensional variations in ground albedo and bidirectional 
reflectance. 
To achieve these goals, we employ a Monte Carlo radiative 
transfer code to model and analyze the effects of many of the 
complications which are present in nature. We can, for 'example, treat 
finite clouds as well as vertical atmospheric structure. We can model 
the effects of two dimensional structure in ground albedo and 
directional reflectance. We can examine alternate data acquisition 
strategies perhaps involving variations in detector viewing direction, 
solar illumination direction, or polarization sensitivity. Most 
importantly, for the purposes of analysis and characterization, we can 
divide the detected radiance into components. The radiance transmitted 
directly through the atmosphere from the ground to the detector is the 
https://ntrs.nasa.gov/search.jsp?R=19860004336 2020-03-20T16:37:13+00:00Z
component which c a r r i e s  t h e  most s i g n i f i c a n t  informat ion about the  
c h a r a c t e r  of t h e  s u r f a c e .  The rad iance  a r i s i n g  s o l e l y  from atmospher ic  
s c a t t e r i n g  is  a term which, i n  e f f e c t ,  should be s u b t r a c t e d  froro. t h e  
d a t a  a s  it  has  no bea r ing  on t h e  s u r f a c e  charac te r .  L a s t l y ,  t h e r e  is a  
ground d i f f u s e  component comprised of r a d i a t i o n  which, having s c a t t e r e d  
from t h e  ground, i s  a tmospher ica l ly  s c a t t e r e d  p r i o r  t o  d e t e c t i o n .  This 
l a s t  term i s  r e s p o n s i b l e  f o r  non-local adjacency e f f e c t s .  Its i n f l u e n c e  
on the  d a t a  u s u a l l y  may be desc r ibed  by an atmospheric spread func t ion .  
S p e c i f i c a l l y ,  thn  o b j e c t i v e s  of the  p resen t  s tudy  a r e  t o  
5.nves t i g a t e  t h e  s e n s i t i v i t y  of t h e  radiance  components and t h e  
atmospheric spread f u n c t i o n  to :  v a r i a t i o n s  i n  t h e  v e r t i c a l  a e r o s o l  
p r o f i l e ;  v a r i a t i o n s  i n  t h e  a e r o s o l  s i z e  d i s t r i b u t i o n  (and,  hence, i t s  
s c a t t e r i n g  d i s t r i b u t i o n ) ;  v a r i a t i o n s  i n  t h e  d e t e c t o r  look d i r e c t i o n  and 
s o l a r  i l l u m i n a t i o n  d i r e c t i o n ;  v a r i a t i o n s  i n  ground r e f l e c t a n c e  type and 
b i d i r e c t i o n a l  r e f l e c t i v i t y ;  the presence of clouds i n  t h e  v i c i n i t y  of 
the  po in t  of view. 
RESULTS 
A s  t h e  modal r a d i u s  of the  a tmospher ic  a e r o s o l  s i z e  d i s t r i b u t i o n  
i n c r e a s e s ,  the a s s o c i a t e d  s c a t t e r i n g  phase func t ion  is  enhanced both i n  
the  forward and backward d i r e c t i o n s .  For remote sens ing ,  i t  is  the  
changes i n  the forward s c a t t e r i n g  peak which a r e  more important .  Our 
s imula t ions  show s i g n i f i c a n t  s e n s i t i v i t y  i n  t h e  i n n e r  regions  of t h e  
spread func t ion  (wi th in  250 meters of t h e  point  of view). Figure  1 
i l l u s t r a t e s  t h i s ,  showing t h e  atmospheric modulation t r a n s f e r  f u n c t i o n ,  
o r  MTF ( t h e  F o u r i e r  t r ans fo rm of t h e  atmospheric l i n e  spread f u n c t i o n )  
f o r  t h r e e  log-normal s i z e  d i s t r i b u t i o n s  wi th  modal r a d i i  of 0.13,  0.26, 
and 1.04 micrometers (bottom curve ,  middle curve ,  and top curve ,  
r e s p e c t i v e l y ) .  
For Lambertian ground r e f l e c t a n c e ,  t h e r e  is  r e l a t i v e l y  l i t t l e  
s e n s i t i v i t y  t o  d e t e c t o r  nad i r  angle f o r  nad i r  angles  less than about 
40 degrees.  For l a r g e r  n a d i r  ang les ,  however, t h e  atmospheric l i n e  
spread f u n c t i o n  becomes asymmetric and depends upon t h e  i l l u m i n a t i o n  
angle.  Figure  2 shows t h e  atmospheric l i n e  spread f u n c t i o n  f o r  a 
d e t e c t o r  nad i r  ang le  of 50 degrees ( d e t e c t o r  po in t ing  roughly i n t o  the  
sun) ,  whi le  the  dashed curve is  f o r  180 degree r e c e i v e r  azimuth. The 
s o l a r  z e n i t h  ang le  f o r  both cases  is  47 degrees.  The l i n e  spread 
func t ion  here  is computed from the  g r a d i e n t  of the  i n t e n s i t y  i n  a scan 
ac ross  a. f i x e d  a lbedo boundary. The asymmetry is due p r imar i ly  t o  the  
forward-backward asymmetry of the  a e r o s o l  s c a t t e r i n g  phase funct ion.  
Var ia t ions  i n  t h e  form of the v e r t i c a l  a e r o s o l  d e n s i t y  p r o f i l e  
tend t o  s c a l e  t h e  range of t h e  atmospheric spread func t ion .  This 
s c a l i n g  appears t o  be roughly l i n e a r  i n  the  average a e r o s o l  a l t i t u d e .  
Clouds i n  t h e  v i c i n i t y  of the po in t  of view can i n f l u e n c e  tile 
de tec ted  radiance .  I n  p a r t i c u l a r ,  the  shadow of a cloud produces much 
t h e  same i n t e n s i t y  g r a d i e n t  when scanning ac ross  i t s  edge a s  is  obtained 
f o r  a high c o n t r a s t  ground a lbedo boundary. Figure  3 shows a d e t a i l e d  
comparison of the  rad iance  components f o r  two cases .  The s o l i d  l i nes  
show (from top t o  bottom on the  r i g h t  s i d e  of the  Eigura) t o t a l  
radiance ,  ground d i r e c t  radiance  and atmospheric radiance  as de tec ted  
from n a d i r  whi le  scanning ac ross  a Lambertian albedo boundary (albedou 
0.4 and 0.07). The symbols correspond t o  a s i m i l a r  scan ac ross  a cloud 
shadow boundary ( t h e  ground a lbedo i s  0.4, the s o l a r  z e n i t h  ang le  i a  
47 degrees ,  and t h e  a l t i t u d e  of the cloud top is 7 km.). The  p'liiacs 
i n d i c a t e  t o t a l  r ad iance ;  t h e  s o l i d  squares  show the  ground d i r ec t  
radiance;  t h e  open squares  show the  ground d i f f u s e  component; and tlw 
crosses  mark the  atmospheric radiance .  Here, the  atmospheric component 
v a r i e s  because t h e  cloud shades t h e  s c a t t e r i n g  atmosphere a s  well as tire 
ground. The ground d i r e c t  r ad iance  v a r i e s  l e s s  ab rup t ly  on the  dark 
s i d e  of t h e  shadow due t o  cloud t ransparency and t r ans lucence ,  The 
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