The effects of soil moisture and other factors on soil surface emissivity are reviewed and design concepts for a multibeam microwave radiometer with a 15 m antenna are described. Characteristic antenna gain and radiation patterns are shown and losses due to reflector roughness are estimated

Lawrence, R. W.

Shiue, J. C.

English

NASA Technical Reports Server

O R B I T I N G  MULTI-BEAM MICROWAVE RADIOMETER FOR SOIL MOISTURE 
REMOTE SFNSING 
J. C. Shiue 
NASA Goddard Space F l i g h t  Center 
Greenbel t ,  Mary1 and 
R. W. Lawrence 
NASA Langley Research Center 
Hampton , V i  r g i  n i  a 
Large Space Antenna Systems Technology - 1984 
December 4-6, 1984 
73 
https://ntrs.nasa.gov/search.jsp?R=19850015507 2020-03-22T19:03:51+00:00Z
I MTRODUCTIOP 
Knowledge o f  s o i l  mo is tu re  i s  u s e f u l  t o  many s c i e n t i f i c  d i s c i p l i n e s .  For 
example, i n  atmospheric c i r c u l a t i o n  model ing f o r  meteorology and c l  imatology, s o i l  
mo is tu re  i s  an impor tant  boundary c o n d i t i o n  which a f f e c t s  the  e v a p o t r a n s p i r a t i o n  
process, which i n  t u r n  i n f l u e n c e s  humid i ty ,  c loud f o r m a t i o n  and p r e c i p i t a t i o n .  I n  
hydrolog.y, knowledge o f  s o i l  mo is tu re  i s  needed f o r  r e s e r v o i r  management, drought  
survey, and f l o o d  p r e d i c t i o n .  I n  a g r i c u l t u r e  a p p l i c a t i o n s ,  t i m e l y  mo is tu re  
i n f o r m a t i o n  can be used t o  a i d  p r e p l a n t i n g  and i r r i g a t i o n  scheduling, and r o u t i n e  
s o i l  mo is tu re  survey can h e l p  determine crop s t r e s s  and increase t h e  crop y i e l d  
f o r e c a s t  accuracy. 
The presence o f  water molecules, which are h i g h l y  po la r ,  increases t h e  
d i e l e c t r i c  constant  o f  the  s o i l ,  which, i n  t u r n ,  causes a r e d u c t i o n  i n  s o i l  sur face  
e m i s s i v i t y .  Microwave e m i s s i v i t y  can be ascer ta ined b.y measuring t h e  microwave 
b r i g h t n e s s  temperature and p h y s i c a l  ( thermal )  temperature o f  t h e  s o i l  sur face.  
Microwave rad iometers a re  used t o  measure t h e  br igh tness  temperature whereas 
p h y s i c a l  temperature i s  f r e q u e n t l y  obta ined from an i n f r a r e d  rad iometer  such as t h e  
M u l t i - S p e c t r a l  Scanner (MSS) o r  Thematic Mapper aboard t h e  Landsats. 
The "sensing depth" o f  pass ive microwave ( d e f i n e d  here as t h e  depth below t h e  
sur face  such t h a t  63 percent  o f  t h e  power rece ived by t h e  rad iometer  emanated from 
t h e  volume between t h i s  depth and t h e  t o p  sur face)  decreases w i t h  i n c r e a s i n g  
sensing frequency. Conseauently, i n  o rder  t o  penet ra te  deep i n t o  t h e  s o i l ,  low 
frequencies are p r e f e r r e d .  However, because t h e  r e s o l u t i o n  o f  pass ive microwave i s  
determined b y  t h e  r e a l  aper tu re  antenna, low f requency imp1 i e s  l a r g e  antenna 
aper ture,  e s p e c i a l l y  f rom sate1 1 i t e s .  Moreover, a t  f requencies below L-band, t h e r e  
are r a d i o  f requency i n t e r f e r e n c e s  (RFIs) ,  both  man-made and e x t r a - t e r r e s t r i a l  i n  
nature.  As a good compromise, t h e  21 cm hydrogen band (1.413 HGz), p r o t e c t e d  f o r  
r a d i o  astronomy purposes, i s  t h e  pr ime f requency f o r  r a d i o m e t r i c  remote sensing o f  
s o i l  mo is tu re  from space. 
I 74 
THE EFFECT OF SOIL MOISTURE ON E M I S S I V I T Y  
F i g u r e  1 shows, f o r  t h e  uppermost 25 cm o f  s o i l ,  some exper imental  da ta  o f  
measured s o i l  sur face  e m i s s i v i t y  a t  1.4 GHz vs s o i l  mo is tu re  together  w i t h  two 
l i n e a r  regress ion  l i n e s ,  The bare surface ( s o l i d  l i n e )  has more " s e n s i t i v i t y "  t o  
s o i l  mo is tu re  change. For one percent  change i n  s o i l  mo is tu re  ( b y  d r y  weight) ,  t h e  
corresponding change i n  e m i s s i v i t y  i s  about 1.3 percent .  The dashed l i n e  i s  f rom 
t h e  combinat ion o f  several  types o f  vegeta t ion  covered sur face  and i t s  s e n s i t i v i t y  
i s  g r e a t l y  reduced. 
I n  terms o f  microwave b r i g h t n e s s  temperature,  t h e  dynamic range o f  bare 
sur face  i s  about 90 K, and t h a t  of t h e  mixed vegeta t ion  as shown i n  F ig .  1 i s  about 
20 K. As we s h a l l  see l a t e r ,  a l a r g e  spaceborne microwave rad iometer  has a t y p i c a l  
measurement accuracy o f  about 3 K; hence, one can a s c e r t a i n  several  l e v e l s  o f  s o i l  
mo is tu re  from passive microwave, even i n  t h e  vegeta t ion  covered areas. 
1 .o 
0.9 
0.8 
0.7 
0.6 
FREQUENCY: 1.4 CHt 
TB=0.93-0.013 x SM (Bare Field Data) 
Tg = 0.67 - 0.0032 x SM 
- \/,,g / r = 0.58 (VEGETATED ) 
.. 
Q 
V 
Incidence Angle 10 
0 
V 
\ 
- + 
+ Bare 
@ Soybean 
Dry Corn 
AlOcm Grass 
v 30 cm Grass 
WURIUTION : HORIZOhTAL 
0-2.5 cm SOIL MOISTURE, % BY DRY WEIGHT 
Figure  1 (Adapted f rom r e f .  1) 
75 
OTHER FACTORS INFLUENCING EMISSIVITY 
However, microwave e m i s s i v i t y  alone i s  n o t  enough f o r  s o i l  m o i s t u r e  
determinat ion,  because t h e r e  are o t h e r  f a c t o r s  t h a t  tend t o  compl icate t h e  
i n t e r p r e t a t i o n  o f  t h e  s o i l  mois ture.  Therefore,  o t h e r  a n c i l l a r y  da ta  are a l s o  
needed t o  he lp  improve s o i l  mo is tu re  r e t r i e v a l .  Sur face geometric c h a r a c t e r i s t i c s ,  
such as vegeta t ion  canopy and sur face  roughness, w i l l  reduce t h e  s e n s i t i v i t y  o f  
e m i s s i v i t y  t o  s o i l  mo is tu re  change. O f  t h e  two f a c t o r s ,  vegeta t ion  canopy i s  t h e  
more ser ious  one. Th is  e f f e c t  must be removed b e f o r e  meaningful  i n t e r p r e t a t i o n  
from vegetated areas can be made, because vegeta t ion  can reduce t h e  s e n s i t i v i t y  by 
up t o  50 percent .  By us ing measurements from v i s i b l e  and near TR wavelengths, one 
can d i f f e r e n t i a t e  bare from vegetated areas, and a lso  a s c e r t a i n  t h e  biomass o f  t h e  
vegetat ion.  The roughness e f f e c t  can be est imated w i t h  e i t h e r  dual p o l a r i z a t i o n  o r  
m u l t i p l e  f requency techniques. I n  general,  t h i s  e f f e c t  i s  no t  as severe as t h a t  o f  
vegeta t ion  canopy. 
The microwave e m i s s i v i t y  i s  a lso  d i f f e r e n t  f o r  d i f f e r e n t  s o i l  t e x t u r e ,  due t o  
t h e  d i f f e r e n t  b i n d i n g  f o r c e  o f  host s o i l  p a r t i c l e s .  However, s ince  t h e  p r i m a r y  
i n t e r e s t  f rom most s c i e n t i f i c  o r  a p p l i c a t i o n  p o i n t s  o f  view i s  t h e  a v a i l a b l e  water 
f rom t h e  s o i l ,  t h i s  e f f e c t  becomes unimportant.  It t u r n s  ou t  t h a t  t h e  e m i s s i v i t y  
as a f u n c t i o n  o f  " a v a i l a b l e  water"  ( f i e l d  c a p a c i t y )  i s  n e a r l y  independent o f  s o i l  
t e x t  ure . 
F i g u r e  7 d e p i c t s  t h e  t h e o r e t i c a l  sensing depth vs f requency f o r  severa l  s o i l  
mo is tu re  conten ts  and v e r t i c a l  p r o f i l e s .  
\ 
- 
- 
%"v 
ill23 
10125 
29/15 
Figure  2 (From r e f .  2 )  
I 76 
A MULTI-BEAM MICROWAVE RADIOMETER WITH 15-M ANTENNA 
For  s o i l  mo is tu re  sensing, bo th  h i g h  s p a t i a l  r e s o l u t i o n  and l a r g e  swath w i d t h  
are d e s i r a b l e  system parameters. It i s  a l so  impor tan t  t h a t  t he  radiometer map a 
g iven area con t inuous ly .  However, a t  t imes n o t  a l l  t h ree  c o n d i t i o n s  can be f u l l y  
achieved and t r a d e - o f f s  must be made. Since mechanical scanning i s  i m p r a c t i c a l  f o r  
such a l a r g e  antenna, a m u l t i p l e - f e e d  design i s  used t o  achieve the l a r a e  swath 
w id th .  
F i o u r e  3 shows the s ide  and f r o n t  views o f  t he  c u r r e n t  quad aper tu re  design 
&meter hoop/column antenna, as c u r r e n t l y  designed. The f r o n t  view shows the  f o u r  
6.09-meter sub-apertures. 
The c u r r e n t  des i  gn i s  a mu1 ti p l  e beam/mul ti p l  e quadrant o f f  s e t  r e f 1  e c t o r  
system. Four separate areas of i l l u m i n a t i o n  (sub-aper tu res)  on t he  pa ren t  
r e f l e c t o r  a re  shown. The surface i s  shaped as i f  it were f o u r  o f f s e t s ;  thus, the 
p a r e n t  r e f l e c t o r  i s  cusped. 
As a r e s u l t  o f  the l a r g e  phys i ca l  s i z e  of the feed a r rays  needed, o n l y  two 
s ix -meter  sub-apertures o f  the 15-meter paren t  ape r tu re  are planned. Feed a r rays  
then are ab le  t o  produce a t o t a l  of 29 beams across the ground t r a c k  o f  t he  sensor 
p l a t f o r m .  The r e f l e c t o r  shape w i l l  i n c l u d e  two sub-apertures r a t h e r  than the f o u r  
c u r r e n t l y  under cons t ruc t i on .  The r e s u l t i n g  arrangement i s  c a l l e d  a "pushbroom" 
c o n f i g u r a t i o n .  
I U D W  
\ 
Urm- 
--\ 
F igu re  3 
77 
BEAM FORVATION 
Only two sub-apertures are  used f o r  the  rad iometer  mission, as shown i n  
F i g u r e  4. The feed a r rays  are  l o c a t e d  between the  two sub-apertures so as t o  
min imize the  blockage. The top  a r ray  conta ins  14 separate feeds and i l l u m i n a t e s  
sub-aperture #l. S i m i l a r l y ,  the  bottom feed a r ray  conta ins  15 separate feeds and 
i l l u m i n a t e s  sub-aperture #3. 
The center  o f  the  bottom feed a r ray  generates beam #15 which i s  po in ted  a t  
n a d i r .  The two feeds a t  t he  end o f  t h i s  feed a r ray  y i e l d  the  two edge beams, #1 
and #29. The remaining 12 feeds o f  t h i s  a r ray  produce the  o ther  odd numbered 
beams. The top  feed a r ray  produces 14 even numbered beams, #2 through #28, whose 
p o s i t i o n s  are  i n te rspe rsed  among the  15 beams produced by the  bottom feed ar ray .  I 
Beam 
#29 
F igure  4 
78 
FOOTPRINT LOCATION FOR THE 15-M HOnP-COLUMN PIJSHRROOM RADIOMFTFR 
F igure  S shows one-hal f  o f  t h e  in te rspersed beams, from #If; ( n a d i r )  t o  #29.  
The remain inq beams #1 t o  #14 are t h e  m i r r o r  imaae o f  F iaure  5, symmetrical about 
t h e  v e r t i c a l  90 degree l i n e .  
Degrees across t r a c k  
F i g u r e  5 
79 
ANTENNA G A I N  PATTEPN OF THE CENTER BEAM 
The secondary r a d i a t i o n  (antenna ga in )  p a t t e r n  o f  t h e  n a d i r  beam (#15) i s  
shown i n  F igure  6a. The pr imary  r a d i a t i o n  p a t t e r n  o f  t h e  feed i s  assumed t o  have a 
shape o f  "cos ine 8'' r a i s e d  t o  1.5, where 0 i s  t h e  angle from the  c e n t e r  o f  t h e  6.09 
m sub-aperture. The f i v e  separate p a t t e r n s  correspond t o  t h e  d i f f e r e n t  r e f l e c t o r  
surface roughness s i z e s  o f  0, 50, 69, 90, and 110 m i l s  (PMS).  The c o r r e l a t i o n  
l e n g t h  C i s  assumed t o  be 10 inches. It i s  seen t h a t  t h e  phase e r r o r s  due t o  
random sur face  roughness tend t o  f i l l  t h e  space between t h e  f a r  o u t s i d e  lobes; t h e  
main lobe p a t t e r n  i s  h a r d l y  changed. F i s u r e  6b i s  a l s o  t h e  secondary r a d i a t i o n  
p a t t e r n  o f  the  n a d i r  beam, w i t h  t h e  angle extended t o  90 degrees. 
I 
0.0 
-40.0 
-80.0 
f. 
8 
2 
G 
= d -120.0 
r 
i 
I 
I I ' r l  
i ' 
-160.0 
I 
-200.0 
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 0.0 10.0 20.0 3.0 40.0 50.0 60.0 70.0 80.0 41.0 
RGLE lDEGREE31 WLE IEGREES) 
F i g u r e  6 
80 
ANTENNA R A D I A T I O N  PATTERFJ 
Figure 7a i s  a p l o t  of the contour of the secondary rad ia t ion  pattern o f  the 
Figure 7b i s  a similar p lo t  for the 
Figure 7c i s  a p l o t  of Beam # 1 5 ' s  r a d i a t i o n  pattern out t o  
n a d i r  beam (#15) w i t h i n  2.5 degrees of nadi r .  
main lobe of Beam #29. 
10 degrees. The contour interval i s  1 dB. 
2.5 
1.3 
In 
W 
(5, 
W 
E 0.0 
n 
-1.3 
-2.5 
ea. B 90.0 91.3 92.5 87.5 
Degrees 
v, 
W 
W 
L 
(5, 
W 
n 
-10.0 v 
8s. 0 sa. 0 95.0 1m.o M). a 
Degrees 
(4 
Figure 7 
Degrees 
( b )  
81  
LOSS DUE TO REFLECTOR ROUGHNESS 
Figure 8 shows the e f fec t  of re f lec tor  surface roughness on beam eff ic iency.  
For a 100 mil RMS, the resul t ing reduction in beam eff ic iency due t o  surface 
roughness will be about 2 percent, i f  one s t a r t s  w i t h  an ideal ly  smooth r e f l ec to r  
surface with a beam eff ic iency of 90 percent. 
Figure 8 
82 1 
SUMMARY OF SYSTEM PARAMETERS OF THE RADIOMETER 
From a 300 km o r b i t ,  t h e  2.25 degree beam w i d t h  would y i e l d  a n a d i r  s p z t i a l  
r e s o l u t i o n  o f  13.1 km a t  n a d i r .  For t h e  swath edge beams (W1 and #29) ,  t h e  
f o o t p r i n t  s i z e  i s  degraded t o  18.7 km x 14.7 km. The rad iometer  w i l l  y i e l d  a swath 
o f  458.4 km. The n a d i r  angle a t  t h e  edge o f  t h e  swath i s  about 37 degrees, and t h e  
corresponding inc idence angle i s  39 degrees. The t i m e  i t  takes  f o r  t h e  
s u b - s a t e l l i t e  p o i n t  t o  move a f o o t p r i n t  d iameter i s  1.77 sec. From a 300 km o r b i t ,  
t h e  e q u a t o r i a l  d r i f t  i s  about 2500 km per  o r b i t .  Wi th  t h e  458 km swath w i d t h ,  t h e  
rad iometer  can produce a g loba l  s o i l  mo is tu re  map i n  6 days. 
The system parameters and accuracy budget o f  t h e  microwave rad iometer  are as 
g iven i n  Table 1. 
Frequency: 
Bandwidth 
E f f e c t i v e  Antenna 
Aper ture P i  ameter: 
Number o f  Beams: 
P o l a r i z a t i o n :  
Beam E f f i c i e n c y :  
Temperature S e n s i t i v i t y :  
I n t e g r a t i o n  Time: 
I .F. Bandwidth: 
System Noise F igure:  
Antenna Temperature: 
Feed Loss: 
Mesh Loss: 
C a l i b r a t i o n  Accuracy Est imate:  
Antenna Mesh ( em i s s i v i  ty/ t r an sm i s s i v i  t y )  
Feed Loss 
Receiver C a l i b r a t i o n  E r r o r  
RSS 
1.413 GHz 
27 MHz 
6.09 m 
i19 
H o r i z o n t a l  
85 t o  90% 
0.36 K* 
1 sec. (assumed) 
13.5 MHz 
2 dP (dsb)  
280 K 
1.5 dB 
0.17 dB 
2.3 t o  2.8 K 
0.3 t o  0.6 K 
2.0 K 
3.1 t o  3.5 K 
* 
Does n o t  i n c l u d e  s p a t i a l  antenna e f fec ts  such as t h e  i n v e r s i o n  
f rom antenna b r i g h t n e s s  temperature o r  t h e  d e f i n e d  r e s o l u t i o n  
c e l l .  
Table 1 
83 
SUMMARY OF RADIOMETER SYSTEM REOUIREMENTS 
The power, weight, and communications requirements o f  t h e  29 channel 
microwave rad iometer  are g iven i n  Table 2. 
POWER REQUIREMENTS 
Assembly 
F r o n t  -end 
Video 
S igna l  Processing 
T o t a l  Power Per Channel 
DC/DC Converter E f f i c i e n c y  
Tota l  Power f o r  29 Channels 
Misc.  Power 
T o t a l  Power 
Feed Panel: 
Rad i omet e r  
Front-end 
Thermal 
Video 
S igna l  Proc. 
Enclosure 
Power Per Radiometer 
1.8 ld 
2.7 W 
3.5 w 
9.0 w 
WEIGHT REOUIREMENTS 
150 oz 
16 oz  
16 oz 
10 oz 
18 oz 
80 oz 
80% 
290 W 
10 w 
300 W *  
Tota l  w t  Per Channel : 290 oz 
T o t a l  wt f o r  29 Channels 525 l b  
I n t e r c o n n e c t i n g  S t r u c t u r e  75 l b  
To t  a1 We i g h t  600 l b *  
DATA RATE REOIJ I REMENT 
R ad i ome t e r Duty Cyc 1 e 64 BPS 
Reference Temperature 16 BPS 
M u l t i p l e x e d  Phys ica l  Temperature 16 BPS 
T o t a l  Per Channel 96 RPS 
Coding Overhead: 28 Coded 8 B i t  Word/Sec 
T o t a l  B i t  Rate: (29 Beams) 6496 BPS 
* 
Does n o t  i n c l u d e  requirements f o r  thermal c o n t r o l  system. 
Table 2 
84 
1 
REFERENCES 
1. Wang, J.  R., J .  C. Shiue, and J. E.  McMurtrey 111, Microwave remote sensing o f  
o f  s o i l  mo is tu re  conten t  over  bare  and vegetated f i e l d s ,  Geophys. Res. L e t t . ,  
- 7, 801-804, 1980. 
2. Njoku, E .  G., J. P .  Schei lde,  and A. B. Kahle, J o i n t  microwave and i n f r a r e d  
s t u d i e s  f o r  s o i l  mo is tu re  de terminat ion ,  Rept. SM-YO-00495, J e t  Propu ls ion  
Laboratory ,  Sept. 1980. 
85 


Orbiting multi-beam microwave radiometer for soil moisture remote sensing

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