A synthetic aperture radar correlation system including a moving diffuser located at the image plane of a radar processor is described. The output of the moving diffuser is supplied to a lens whose impulse response is at least as wide as that of the overall processing system. A significant reduction in clutter results is given

Jain, A.

English

NASA Technical Reports Server

Reply to Ann of
NASA
National Aeronautics and
Space Administration
Washington, D.C.
20546
TO: NST-44/Scientific and Technical Information Division
Attn: Shirley Peigare
FROM: GP-4/Office of Assistant General Counsel
for Patent Matters
SUBJECT: Announcement of NASA-Owned U.S. Patents in STAR
In accordance with the procedures agreed upon by Code GP-4 and
Code NST-44, the attached NASA-owned U.S. Patent is being
forwarded for abstracting and announcement in NASA STAR.
The following information is provided:
U.S. Patent No. : 4,371,873
California Institute of Technology
Government or 120! E. California Blvd.
Contractor Employee: Pasadena. CA 9Q4Q6
NASA Case No. : NPO-14.035-1
NOTE - If this patent covers an invention made by a contractor
employee under a NASA contract, the following is applicable:
YES / v / NO
Pursuant to Section 305(a) of the National Aeronautics and Space
Act, the name of the Administrator of NASA appears on the first
page of the patent; however, the name of the actual inventor
(author) appears at the heading of Column No. 1 of the
specification, following the words "...with respect to an
invention of...."
(NftSA-Case-NPO- 1403C- !\
SYH1EETIC f lFERTDRE B f i M R CCBBEIl lTCB'Sa ten t N83-19968
4 p
 CSCL 171
Unclas
00/32 1J058
https://ntrs.nasa.gov/search.jsp?R=19830011697 2020-03-22T00:40:19+00:00Z
3 -/
United States Patent
Jain
[ii]
[45]
4,371,873
Feb. 1, 1983
[54] CLUTTER FREE SYNTHETIC APERTURE
RADAR CORRELATOR
[76] Inventor: Robert A. Frosch, Administrator of
the National Aeronautics and Space
Administration, with respect to an
invention of Atul Jain, Pasadena,
Calif.
[21] Appl. No.: 858,767
[22] Filed: Dec. 8,1977
[51] Int. C1.3 G01S 13/89; GDIS 13/90
[52] U.S. Cl 343/9 PS; 343/5 CM;
343/100 CL
[58] Field of Search 343/5 CM, 100 CL, 17,
343/9 PS
[56] References Cited
U.S. PATENT DOCUMENTS
3,344,419 9/1967
3,355,579 11/1967
3,398,269 8/1968
3,483,557 12/1969
3,526,893
3,560,973
3,889,226
9/1970
2/1971
6/1975
Lund 343/5 R
Robertson 343/100 CL X
Williams 343/100 CL X
Skenderoffet al 343/100 CL
Skenderoffetal 343/100 CL
Kazel 343/5 CM
Hildebrand 343/17 X
3,953,822 4/1976
4,019,184 4/1977
4,042,928 8/1977
Vilkomerson 343/17 X
Dorey 343/100 CL X
Altes 343/100 CL X
OTHER PUBLICATIONS
"Speckle Removal by a Slowly Moving Diffuser Asso-
ciated with a Motionless Diffuser", by S. Lowenthal et
al., Journal of Optical Society of America, vol. 61, No.
7, p. 847, (1971).
"A Speckle—Free Rear—Projection Screen Using
Two Closed Screens with Slow Relative Motion", J.
Optical Society of America, vol. 66, p. 176, (1976).
Primary Examiner—Malcolm F. Hubler
Attorney, Agent, or Firm—Monte F. Mott; John R.
Manning; Paul F. McCaul
[57] ABSTRACT
A synthetic aperture radar correlation system including
a moving diffuser located at the image plane of a radar
processor. The output of the moving diffuser is supplied
to a lens whose impulse response is at least as wide as
that of the overall processing system. A significant
reduction in clutter results.
6 Claims, 2 Drawing Figures
U.S. Patent Feb. 1, 1983 4,371,873
4,371,873
1 2
reimages the diffuser output image and outputs to the
CLUTTER FREE SYNTHETIC APERTURE RADAR film processing apparatus represented by the squarer 17.
CORRELATOR- • The following discussion illustrates in more detail the
operation of the radar system of FIG. 1.
BACKGROUND OF THE INVENTION 5
 In F1G. 1, the terrain reflectivity is p(x,y) and gener-
The subject invention relates to a synthetic aperture ally consists of a real term a(x,y) and a pure phase func-
radar correlator that allows high resolution processing lion exp [i<|>(x,y)] arising from the surface roughness of
of synthetic aperture radar data using the image capabil- the terrain. The reflectivity p(x,y) is convolved by the
ities of the full azimuth and doppler bandwidth without radar impulse response i|/(x,y) to give the results field in
the clutter noise degradation. 10 the radar image plane. This result is operated on by the
Clutter or speckle has been long known to limit the diffuser phase transmittance exp [i0(x,y)] and then con-
working resolution of images obtained with coherent volved by the impulse response of the output lens z(x,y)
radiation. Images obtained.by synthetic aperture radar
 and the output js amplitude squared by the recording
systems suffer from the same drawback and it has,
 process of ,he fi,m Each independent position of the
therefore, been of considerable interest to explore van- l:"
 diffuser corresponds to an independent 0^,y), and the
ous techniques for the reduction of the radar speckle in
 correspOnding images in the output plane are added
these images. The conventional technique of achieving
 Her tQ j(Je {he ^^ ,£ a ;
incoherent averaging of the radar speckle, which m-
 TQ sh(jw (hjs a mcn, does not ,e*d *o a
volves adding images obtained from different sections , . . . . . ° .. . . ,
of the radar bandwidth, has the disadvantage of a Urn- 20 blurnng of the radar image consider p(x,y) to be the
.ted number of possible looks due to the loss of resolu- delt" fu"ctlon. K*-*'*^ ™en hfle °«"P«' /ima*eJ?r
tion associated with the narrower, doppler bandwidth fach. 0*^< '.* f xlW^ exp [iflXx,y)]*z(x,y). The
used for each independent look. ' location and width of this function is independent of the
value of 0fa,y). To show that independent values of
SUMMARY OF THE INVENTION
 25 <M*.y) result in independent speckle patterns, one notes
It is therefore an object of the invention to improve first that the electric field for one particular value of
synthetic aperture radar systems. 0^\,y) in the output plane is
It is another object of the invention to substantially
increase the number of looks possible in a synthetic eAx.y)=\{p(x.yy\ii(x.yYi exp [ie^x,y)]*z(x.y)] (1)
aperture radar system without loss of resolution. 30 and the corresponding image intensity is u,<x,y) = eXx-
These and other objects and advantages are accom- ,y)e,*(x,y). If z(x,y) is wider than >J»(x,y) and e(x,y) and
plished according to the invention by using a diffuser in <Kx'y) have white noise Gaussian statistics then e,{x,y)
motion at the image plane of the radar processor and may be assumed to have Gaussian statistics. We can,
reimaging this diffuser with a lens whose impulse re- therefore, write the cross correlation of the intensities
sponse is at least as wide as the impulse response of the 35 uXx.y) and um(x,y), recorded for the two independent
radar system. While each independent position of the descriptions of the diffuser, 0K*,y) and 0m(x,y) respec-
diffuser corresponds to an independent look in the lively, as,
speckle averaging process, the final resolution of the
radar system is determined by the impulse response of <u^x.y)um'(x.y)> = <uj.x.y)> <um'(x.y)> + \ <-
this output lens. With this arrangement, the number of 40 cAx.y)em*(x,y)> \ (2)
looks can be made very large with a limited loss of
resolution where the brackets < > denote the expectation value.
By definition, therefore, the two intensities may be
BRIEF DESCRIPTION OF THE DRAWINGS considered uncorrelated if <e4(x,y)em*(x)y)> is negli-
FIG. 1 is a block diagram illustrating the use of a 45 gible. By substituting (1) for e^x.y) and em(x,y) and
moving diffuser to reduce speckle according to the rearranging terms, we obtain the result
preferred embodiment of the invention.
FIG. 2 illustrates an optical system simulating that of <l\X'y)em',(^>^<f(^P''py}^^(M'
,-,.-* , ,,<exp [i(e4x.y)-em(x.y))]>*R^O,0)] (3)
rlLr. 1.
50DETAILED DESCRIPTION OF THE where Ru,(0,0) is the auto correlation of «|/(x,y) for the
PREFERRED EMBODIMENT value (0,0) and Rz(0,0) the corresponding auto correla-
FIG. 1 illustrates a radar system including a conven- 'io,n of z(x.y) and the symbol * is the convolution sym-
tional radar processor 11, a moving diffuser 13 at the bo'" If ^x,y) and 0m(x,y) are independent, then <exp
image plane of the radar processor, an output lens 15, 55 ('^^,y)-em(x,y))}> is zero and the expression (3) is
and a squarer 17 which represents the recording process approximately zero. Thus, the speckle patterns from the
of the film. The construction and operation of a moving two '"dependent positions of the diffuser will be uncor-
diffuser 13, as an individual element, is well-known to related and a superposition of the intensities from mde-
those skilled in the art. By using such a diffuser 13 ac- pendent positions of the diffuser will result in an effec-
cording to the inventive embodiment of FIG. 1, a sur- 60 tive smoothing of the speckle noise,
prising improvement in the radar image occurs. An optical illustration of the concept of the invention
The radar processor 11 provides a radar image field is shown in FIG. 2. In this system, a rough target 19 is
plane. The moving diffuser 13 is located at the image illuminated by a laser system 21 and imaged by two
field plane of the processor 11. As the diffuser 13 lenses 23, 25. The lens system comprising lenses 23, 25
moves, it provides an output image to the lens 15, which 65 images the rough object 19 into the moving diffuser 27
has an impulse response at least as wide as the impulse at plane II. The image provided by the diffuser. 27 is
response of the overall radar system including proces- then reimaged by another lens 29. Such a system can be
sor 11, diffuser 13, lens 15 and squarer 17. The lens 15 used to optically simulate the preferred embodiment.
4,371,873
3 4
In conclusion, by pla.cing a diffuser in the image plane l A radar processing system supplied with terrain
. , , . . . . . . . . reflectivity and radar impulse response indications com-
of the radar processor, and reimaging this with a lens rising-
whose impulse response is not narrower than the im-
 a radar processor means for providing a radar image
pulse response of the radar system, an effective speckle 5 field plane;
averaging takes place if indepdent images obtained for a moving diffuser located at the image field plane of
said radar processor and providing an output im-
mdependent positions of the diffuser are added to-
 age. and
gether, with resulting improvement in feature discrimi- a lens means having an impulse response at least as
nation capability. The foregoing technique can be im- 10 wide as the '^pu\se response of the radar system,
, . , • , , , . . , , ,. . , , for reimaging said output image,
plemented ,n both the optical and the digital or analog
 2 The ^^ *f daim /father including means for
electronic processors. squaring the output of said lens means.
As may be appreciated, the just described preferred 3. The system of claim 1 wherein said lens means has
embodiment is subject to many adaptations and modifi- 15 an imPulse response wider than the impulse response of
the radar system.
cat.ons without departing from the scope and spirit of
 4 The system of claim j wherein said radar processor
the invention. means is an optical processor.
It is therefore to be understood that, within the scope 5- The system of claim 1 wherein said radar processor
of the appended claims, the invention may be practiced 20 means is an electronic digital processor. _K
 6. The system of claim 1 wherein said radar processor
other than as specifically described herein.
 means is an electronic analog processor.
What i s claimed i s : * . * * • *
25 , • ' '
30
35
40
45
50
55
4371873 U 126 BOX "282
COUPON* 0472047
ATTN:BRUNO/STAR
02/24 HL
 U953


Clutter free synthetic aperture radar correlator

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server