The opportunity to use a large space antenna system for remote sensing applications permits the creation of an advanced ocean sensing radar that combines the abilities of previously developed techniques. The 15 meter antenna will permit much higher angular and spatial resolution at the surface that will lead to techniques of observing ocean wave heights and the directional spectrum that had not previously been feasible from space. At the same time, sensors to measure ocean surface winds can be in operation and the data from both can be combined to increase the accuracy of each individual sensor. The existing capabilities and sensor techniques with typical data characteristics for the individual measurement of sea surface quantities are outlined

Johnson, J. W.

Weissman, D. E.

English

NASA Technical Reports Server

ADVANCED TWO-FREQUENCY OCEAN SENSING RADAR USING 
H I G H  RESOLUTION ANTENNA BEAMS 
D . E .  Weissman 
Hofstra University 
Hempstead, New York 
J.W. Johnson 
Hampton, Virginia 
NASA Langley Research Center 
Large Space Antenna Systems Technology - 1984 
December 4 - 6 ,  1984 
109 
https://ntrs.nasa.gov/search.jsp?R=19850015510 2020-03-22T19:03:10+00:00Z
INTRODUCTION 
The oppor tun i ty  t o  use a large space  antenna system f o r  remote sens ing  
a p p l i c a t i o n s  permi ts  t h e  c r e a t i o n  of  a n  advanced ocean sens ing  radar 
combines t h e  a b i l i t i e s  of  p rev ious ly  developed techniques .  The 15 meter 
antenna w i l l  permit much h ighe r  angu la r  and s p a t i a l  r e s o l u t i o n  a t  t h e  s u r f a c e  
tha t  w i l l  lead t o  t echn iques  of  observ ing  ocean wave h e i g h t s  and t h e  d i r e c t i o n a l  
spectrum t h a t  had not p rev ious ly  been feasible from space .  A t  t h e  same time, 
s e n s o r s  t o  measure ocean s u r f a c e  winds can be i n  o p e r a t i o n  and t h e  data from 
both can be  combined t o  i n c r e a s e  t h e  accuracy  of  each i n d i v i d u a l  s enso r .  T h i s  
paper o u t l i n e s  t h e  e x i s t i n g  c a p a b i l i t i e s  and senso r  t echn iques  w i t h  t y p i c a l  
data c h a r a c t e r i s t i c s  f o r  t h e  i n d i v i d u a l  measurement of  sea s u r f a c e  q u a n t i t i e s .  
t h a t  
APPLICATION NEEDS 
A f t e r  t h e  SEASAT Sca t t e romete r  demonstrated t h e  p o t e n t i a l  o f  measuring 
global winds from space on a real-time basis,  p l ans  f o r  more advanced ocean 
sens ing  satel l i tes  moved r a p i d l y  ahead. While i n s t rumen ta t ion ,  an tenna  systems, 
and a lgo r i thms  are be ing  r e f i n e d  f o r  f u t u r e  s c a t t e r o m e t e r s ,  a fundamental l i m -  
i t a t i o n  w i l l  be t h a t  t h e  radar c r o s s  s e c t i o n  of t h e  s u r f a c e  w i l l  no t  depend on 
wind speed a lone .  This  l i m i t a t i o n  appea r s  as a scatter i n  t h e  c a l i b r a t i o n  data 
f i g . 1 )  which limits t h e  p r e c i s i o n  o f  i n v e r t i n g  o p e r a t i o n a l  data i n t o  wind speed. 
Recent s t u d i e s  (ref.  1 )  have shown t h a t  t h e  radar c r o s s  s e c t i o n  of  s u r f a c e  w i l l  
depend on sea state (RMS wave s l o p e  o r  height) ,  and air-sea tempera ture  differ-  
ence i n  a d d i t i o n  t o  wind speed. Sensors  t h a t  can measure t h e  s u r f a c e  wave spec- 
trum a l s o  depend on wind speed ( r e f s . 2 , 3 ) .  
t h a t  can measure both q u a n t i t i e s  from space ,  s i n c e  each senso r  ou tpu t  depends 
on two v a r i a b l e s .  Then t h e  two unknowns can be i n f e r r e d  from t h e  two indep- 
endent equa t ions  ( o r  a l g o r i t h m s ) .  Th i s  idea i s  not  p a r t  o f  e x i s t i n g  p l a n s  f o r  
f u t u r e  sa te l l i tes ,  so  t h e  p o s s i b i l i t y  of  developing a new approach w i t h  a 
shu t t l e -based  radar and an tenna  system is  ve ry  a t t r a c t i v e .  A large space  
an tenna  would be a s i g n i f i c a n t  b e n e f i t  t o  a n  ocean observ ing  system because of  
its a b i l i t y  t o  provide  s p a t i a l  r e s o l u t i o n  i n  a beam-limited mode. The e n t i r e  
radar system could  be based on p rocess ing  narrow bandwidth s i g n a l s ,  which 
is  u s u a l l y  s imple r  t h a n  o t h e r  approaches f o r  ach iev ing  s p a t i a l  r e s o l u t i o n .  
One s o l u t i o n  is t o  use a mul t i - sensor  
110 
SCATTEROMETER MEASUREMENTS 
SEASAT Sca t t e romete r  measurements have been processed and analyzed w i t h  
1). I nhe ren t  i n  these and o t h e r  data 
t h e  best  a v a i l a b l e  informat ion  on sea s u r f a c e  c o n d i t i o n s ,  w i t h  atmospheric 
c o r r e c t i o n  t o  n e u t r a l  s t a b i l i t y  (see ref. 
sets is  a random f l u c t u a t i o n  o r  data scatter about  a " b e s t  f i t t f  approximation 
f u n c t i o n  between t h e  radar c r o s s  s e c t i o n  and t h e  wind speed ( f i g .  1 ) .  T h i s  
i n d i c a t e s  t h a t  o t h e r  sea s u r f a c e  v a r i a b l e s  are i n f l u e n c i n g  t h e  backscattered 
power, i n  a d d i t i o n  t o  t h e  wind. Tower-based measurements y i e lded  backscatter 
data (ref.  1) a long  w i t h  s u r f a c e  wave measurements t h a t  show 
of  t h e  RMS wave s l o p e  on t h e  c r o s s  s e c t i o n .  Work is  i n  p rogres s  t o  f u r t h e r  
ana lyze  and i n t e r p r e t  t h i s  data s e t  w i t h  a p p l i c a t i o n  t o  f u t u r e  spaceborne 
Sca t t e romete r  measurements. 
a s t r o n g  effect  
dB 
20 
0 
-20 
-40 
-60 
0 
10 lo1 
Wind speed m/s 
lo2 
Figure 1 
111 
WAVE HEIGHT MEASUREMENTS 
An e x c e l l e n t  example of  sea state measurements u s ing  a narrowband 
beam-limited radar is t h e  n a d i r  looking  Dual Frequency C o r r e l a t i o n  Radar t h a t  
r e q u i r e s  a narrowbeam antenna t o  measure RMS waveheight (ref.  4 ) .  These prev- 
i o u s  measurement programs us ing  t h i s  radar from a n  aircraft p la t form showed 
t h e  c a p a b i l i t y  of  measuring wave he ight  over  a v a r i e t y  of  sea cond i t ions .  The 
known l i m i t a t i o n s  f o r  higher a l t i t u d e s  were more narrow beamwidths w i t h  a l t -  
i t u d e  and nea r  n a d i r  po in t ing  con t ro l .  The normalized c o r r e l a t i o n  f u n c t i o n  
between two c l o s e l y  spaced f r equenc ie s  decreases as t h e  RMS wave he igh t  
i n c r e a s e s  (see f ig .  2 - t h e o r e t i c a l  f u n c t i o n  fo r  aircraft  c o n d i t i o n s  w i t h  
d i f f e r e n t  sea s ta te  v a l u e s ) .  I n  t h e  p a s t ,  space a p p l i c a t i o n s  were cons idered  
bu t  t h e  r equ i r ed  antenna technology was not  a v a i l a b l e .  The c u r r e n t  new Large 
Space Antenna technology creates a n  oppor tuni ty  t o  achieve  t h e  r equ i r ed  beam- 
w i d t h  and near  n a d i r  p o i n t i n g  c o n t r o l  t h a t  can make t h e  system feasible. 
There is a lso  t h e  prospec t  t h a t  w i t h  a s u f f i c i e n t l y  narrow beamwidth, a 
wider swath than  t h e  "near nadi r"  r eg ion  w i l l  be accessible, making c o i n c i d e n t  
measurements w i t h  t h e  Sca t te rometer  poss ib l e .  
RADAR DECORRELATION WITH SURFACE ROUGHNESS 
1.0 
0.8 
0.6 
0.4 
0.2 
Beamwidth = 1.5 deg. 
Altitude = 10000 ft. 
o=O. lm 
0 4 10 15 20 25 30 
Af, MI42 
Figure 2 
I 112 
WAVE SPECTRUM MEASUREMENTS 
Ocean wave spectrum measurement w i t h  microwave sensors  is becoming a 
more accurate  art ,  w i t h  a sounder phys ica l  basis, as severa l  sensor techniques 
( synthe t ic  aper ture  radar, sho r t  pulse spectrometer, and Delta-K) continue t o  
improve. The measurement of ocean wave spectra from space is considered feas- 
i b l e .  Consistent and compatible wi th  t h e  present narrowband radar mode under 
discussion here is the Delta-K radar (ref. 2,3). This  instrument responds t o  
one ocean wavelength a t  a time, within a sweep of measurements t h a t  spans t h e  
ocean spectrum. One set  of r e s u l t s  is  shown i n  f ig .  3, where the radar infer red  
spectrum compares well w i t h  t h a t  obtained by a highly accurate radar system 
(sur face  contour radar). However accurate  r e s u l t s  w i t h  t h e  Delta4 instrument 
(and other  oblique incidence sensors) requi re  wind speed information i n  order  
t o  provide a reliable estimate for  t h e  modulation t r a n s f e r  function. The sig- 
n i f i can t  conclusion from these facts is t h a t  a wave sensor and Scatterometer 
(radar c ross  sec t ion)  
TWO-FREQUENCY 
lo2 
measurement can be combined wsynerg is t ica l lyw.  
RADAR SURFACE SPECTRUM MEASUREMENTS 
A RS LOE 11/ 12/ 80 
Two-f requency radar 
0 Surface contour radar 0 
\ 
\ 
' -4 
'\ k 
\ 
\ 
. 01 0.1 1.0 
Wavenumber, m - l  
Figure 3 
113 
SUMMARY 
This  paper seeks t o  call  a t t e n t i o n  t o  a coherent  set of  facts t h a t  are 
i n t e r e s t i n g  and impor tan t  t o  e x i s t i n g  and planned oceanographic remote s e n s i n g  
programs and which may open up a new senso r  approach i n  t h i s  f i e l d .  To demon- 
strate t h e  f e a s i b i l i t y  o f  u s i n g  a large space antenna  (up t o  15 meters i n  
diameter) from a s h u t t l e  p l a t fo rm,  a set of c a l c u l a t i o n s  were conducted u s i n g  
e x i s t i n g  t h e o r e t i c a l  models of  t h e  n a d i r  d u a l  frequency s c a t t e r o m e t e r  (ref.  4 ) .  
These are basically d e s i g n  c a l c u l a t i o n s  t o  de te rmine  t h e  necessary  s i z e  (dia- 
meter) o f  a s i n g l e  a p e r t u r e  an tenna  as a f u n c t i o n  o f  a l t i t u d e  which would be 
needed t o  ach ieve  a n  acceptable accuracy  ( a n  e r r o r  c o r r e l a t i o n  f a c t o r  o f  0.5 
can be removed from t h e  data w i t h  acceptable losses). The r e s u l t s  of  these 
c a l c u l a t i o n s  are shown i n  f ig.  4 f o r  a g iven  se t  o f  radar and sea parameters 
( f requency  = 14 GHz, RMS wave height  = 0.25 meter and 1.0 meter). These show 
t h a t  f o r  s h u t t l e  a l t i t udes  (200 t o  400 km) an tenna  diameters less than  15 
meters are capable o f  provid ing  t h e  des i red  performance. 
This survey  advoca tes  t h a t  i n t e r e s t i n g  and rewarding p o s s i b i l i t i e s  e x i s t  
i f  we cons ide r  combining i n d i v i d u a l  s e n s o r s  i n t o  mul t i - s enso r s  w i th  t h e  new 
large scale antenna  technology t h a t  is on t h e  horizon. We are i n  a f o r t u n a t e  
p o s i t i o n  because much data i s  a v a i l a b l e  from p rev ious  s t u d i e s  t h a t  could  be 
used i n  a more focussed and i n t e g r a t e d  s tudy  of t h i s  approach. And many of t h e  
earlier i n d i v i d u a l  system s t u d i e s  can s e r v e  as t h e  basis f o r  a new combined 
system s tudy .  
ANTENNA SIZE REQUIREMENTS 
FOR 50% ERROR AT R(bf)* = 0.5 
14 
10 
o = Sea state, m 
f = 14 G H z  
Antenna 
Diameter, 
m 
1 10 100 1000 
Altitude, km 
Figure 4 
I 114 
REFERENCES 
1. Keller, W.C., P l a n t ,  W.J., Weissman, D.P., The Dependence of  X-Band Micro- 
wave Sea Return on Atmospheric S t a b i l i t y  and Sea S t a t e ;  J .  Geophys. Res., 
Vol. g o ,  pp 1019-1029, Jan. 20, 1985 
2. Johnson, J.W. and Weissman, D.P., Two-Frequency Microwave Resonance Meas- 
urements from a n  Aircraft: A Q u a n t i t a t i v e  Estimate of t h e  D i r e c t i o n a l  
Ocean S u r f a c e  Spectrum; Radio Sc i . ,  Vol. 19, pp 841-854, May-June 1984 
3. Weissman, D.E. and Johnson, J.W., Measurements of  Ocean Wave S p e c t r a  and 
Modulation Transfer Function wi th  t h e  Airborne Two Frequency Sca t t e romete r . ,  
F r o n t i e r s  of  Remote Sensing o f  t h e  Oceans and Troposphere from A i r  and 
Space P la t forms ,  URSI Comm. F Symposium, May 1984 - NASA CP-2303 
4. Weissman. D.E. and Johnson, D.E., Dual Frequency C o r r e l a t i o n  Radar Measure- 
ments o f  t h e  Height S t a t i s t i c s  of Ocean Waves., IEEE Trans. Antennas and 
Propagat . ,  Vol AP-25, No. 1 ,  January  1977 


Advanced 2-frequency ocean sensing radar using high resolution antenna beams

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server