Synthetic aperture radar system

A synthetic aperture radar system is mounted in a moving platform. The synthetic aperture radar system includes a multi-beam antenna having a plurality of reception beams different in direction from one another, the multi-beam antenna being adapted to receive radar echoes from objects. The width of each of the reception beams is selected such that the band width of a Doppler shift contained in the radar echo of a moving object is broader than that of a Doppler shift contained in the radar echo of a stationary object. The radar echo is pulse compressed to improve the range resolution before the frequency thereof is shifted such that the center frequency of the Doppler shift due to the velocity of the moving platform becomes zero. After the frequency shifting, the radar echo is filtered to separate the radar echoes of the moving and stationary objects from each other. The radar echoes of the moving and stationary objects are respectively subjected to Fourier transform with respect to the distance between the moving platform and the objects. The spectrum of the radar echo from the moving object is further shifted such that the center frequency of the Doppler shift due to the velocity of the object becomes zero. These reception, pulse compression, frequency shift and Fourier transform are executed for each reception beam. The spectrums in the radar echoes of the moving and stationary objects are respectively synthesized for all the reception beams. After the synthesization, the spectrums are respectively multiplied by a reference spectrum in the complex manner. The results of the multiplication are respectively inverse transformed from the spectrums.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a synthetic aperture radar system 
utilizing the Doppler effect caused by the movement of a radar platform to 
improve the cross-range resolution. The present invention particularly 
concerns improvements in the capacity of such a synthetic aperture radar 
system for detecting a moving object. 
2. Description of the Related Art 
FIG. 4 is a block diagram of a synthetic aperture radar system disclosed in 
Donald R. Wehner, "High Resolution radar", Artech House, Section 6. More 
particularly, this synthetic aperture radar system is constructed by 
modifying such a structure as described in the above publication on page 
260 and shown in FIG. 6.41, according to the description of the same 
publication. 
The synthetic aperture radar system is adapted to detect an object by the 
use of a single antenna beam and includes means for transmitting radio 
waves. The transmitting means comprises a transmitter 1, a circulator 2 
and an antenna 3. The transmitter 1 generates radio waves modulated by 
pulse signals. The circulator 2 functions as a transmission/reception 
changeover circuit for supplying the output of the transmitter 1 to the 
antenna 3 and for supplying the output of the antenna 3 to a receiver 4. 
When the antenna 3 receives the radio waves from the transmitter 1 through 
the circulator 2, the antenna 3 radiates the radio waves. Since the radio 
waves have been modulated by the pulse signals, the synthetic aperture 
radar system will radiate the radio waves with a repetition interval 
determined by the pulse signals. 
The radio waves radiated from the antenna 3 are reflected by the objects, 
earth, sea and so on. The reflected radio waves, that is, echoes are 
inputted in the receiver 4 through the circulator 2. The receiver 4 
amplifies the received radio waves before they are subjected to phase 
detection. 
On the results from the phase detection, the receiver 4 generates a 
two-dimensional digital output signal which is represented by a range bin 
number m and a pulse hit number n. The range bin numbers are applied to 
the respective transmission timings while the pulse hit numbers represent 
the positions of the objects and others relating to the creation of the 
echoes in the reflection. In other words, each of the range bin numbers 
can specify a transmission timing or azimuth while each of the pulse hit 
numbers determines a range between the synthetic aperture radar system and 
an object or the like. 
The post-stage of the receiver 4 is connected to a pulse compression unit 5 
which performs a pulse compression to the two-dimensional digital signals 
from the receiver 4 on their correlation along the direction of range bin. 
Such a process improves the range resolution in the synthetic aperture 
radar system. 
The post-stage of the pulse compression unit 5 includes a circuit for 
improving the cross-range resolution of the synthetic aperture radar 
system. This circuit comprises Fourier transform units 6A and 6B, a 
reference signal generator 7, a complex multiplication unit 8 and an 
inverse Fourier transform unit 9. 
One of the Fourier transform units 6A Fourier-transforms the output of the 
pulse compression unit 5 for the pulse hit numbers n. The other Fourier 
transform unit 6B Fourier-transforms the output of the reference signal 
generator 7. The complex multiplication unit 8 multiplies the output of 
the Fourier transform unit 6A by the output of the Fourier-transform unit 
6B to form complex data. The complex data is then inversely 
Fourier-transformed by the inverse Fourier transform unit 9. Thus, the 
cross-range resolution can be improved. 
After the signals have been improved in range resolution and cross-range 
resolution, they are then provided to a square-law detection unit 10. The 
signals are subjected to square-law detection in the square-law detection 
unit 10. The square-law detection determines an electric power 
corresponding to each pixel in the screen of a display unit 11. The 
square-law detection unit 10 outputs the results to the display unit 11. 
As a result, the screen of the display unit 11 displays radar images 
representing the positions, distances, azimuths and the like of the object 
and others around the radar system. 
In such an arrangement, the range resolution is determined by the band 
width of transmitted pulse signals. The cross-range resolution is 
determined as follows: 
It is now assumed that the radar system is mounted in a moving platform 
such as aircraft or the like. It is further assumed that this moving 
platform moves straight at a velocity V as shown in FIG. 5 and that radio 
waves are radiated in a direction substantially perpendicular to the 
direction of movement of the platform. It is still further assumed that 
the transmitted radio waves are reflected by a stationary object such as 
the ground. 
In such a case, the distance R(t) between the moving platform and the 
object can be represented by the following equation (1): 
EQU R(t)=R.sub.0 -Vt cos .theta..sub.0 +V.sup.2 sin.sup.2 .theta..sub.0 t.sup.2 
/ (2R.sub.0) (1) 
where R.sub.0 is equal to R(0) and .theta..sub.0 is an expected angle of 
the object at time t=0, which angle is a reference in the direction of 
advance. 
When the moving platform moves relative to the stationary object, Doppler 
effect is created. An instantaneous value in the Doppler frequency, that 
is, instantaneous Doppler frequency f.sub.d (t) is represented by the use 
of a transmission wavelength .lambda. from an equation (2). If the 
synthetic aperture time is T, the band width B of the Doppler frequency 
and the cross-range resolution .DELTA.r are represented by equations (3) 
and (4), respectively. 
EQU f.sub.d (t)=2/.lambda.(V cos .theta..sub.0 -V.sup.2 sin.sup.2 .theta..sub.0 
t/R.sub.0) (2) 
EQU B=f.sub.d (-T/2)-f.sub.d (T/2) (3) 
EQU .DELTA.r=V sin .theta..sub.0 /B=.lambda.R.sub.0 /2VT sin .theta..sub.0( 4) 
A synthetic aperture radar for observing a continuous field of view has the 
maximum resolution when the value .theta..sub.0 is equal to 90 [deg]. In 
the normal operation, such a setting is selected. Since the synthetic 
aperture time T is given by an antenna beam width .theta..sub.B from an 
equation (5), the band width B of the Doppler frequency and the 
cross-range resolution .DELTA.r are given by equations (6) and (7): 
EQU T=R.sub.0 .theta..sub.B /V (5) 
EQU B=2V.theta..sub.B /.lambda.=V/.DELTA.r (6) 
EQU .DELTA.r=.lambda./2.theta..sub.B =B/B (7) 
When such a synthetic aperture radar is to be used to detect and image an 
object moving on the ground or sea, the band width of the Doppler 
frequency in the radar echo from the moving object is consistent with the 
reflected waves from the stationary object, but different in center 
frequency from that of the reflected waves. If the spectrums of the radar 
echo from the stationary and moving objects can be separated from each 
other in the frequency domain as shown in FIG. 6, the moving object can 
easily be detected and imaged. On the contrary, if the two spectrums are 
over-lapped on each other as shown in FIG. 7, it is difficult to detect 
and image the moving object. The overlap of the two spectrums is created 
when the velocity of the moving object is too low. In order to separate 
the spectrums from each other, it is therefore required to reduce the band 
width B of the Doppler frequency. Alternatively, it is required to 
increase the velocity of the moving object to be detected and to abandon 
the detection of an object moving at lower velocities. If the band width B 
of the Doppler frequency is decreased, it is noted that the cross-range 
resolution .DELTA.r is degraded as will be apparent from the equation (7). 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a synthetic aperture 
radar system which can detect a low-speed moving object without 
degradation of the cross-range resolution. Another object of the present 
invention is to enable the low-speed moving object to display without 
positional error. Still another object of the present invention is to 
accomplish the above objects without reduction of the transmission antenna 
gain. 
To this end, the present invention provides a synthetic aperture radar 
system mounted on a moving platform, comprising: 
a) a multi-beam antenna having a plurality of reception beams different in 
direction from one another, the width of each of said reception beams 
being set such that the band width of the Doppler shift contained in the 
radar echo from a moving object is broader than that in the radar echo 
from a stationary object; and 
b) means for improving the cross-range resolution of said radar system, 
said cross-range resolution improving means comprising: 
b1) moving object echo separating means for separating the radar echo of 
the moving object from the radar echo of the stationary object in radar 
echoes acquired by the multi-beam antenna for each reception beam; 
b2) moving object spectrum transforming means for transforming the radar 
echo from the moving object into a spectrum relating to the distance 
between the moving platform and the moving object for each reception beam; 
b3) moving object spectrum synthesizing means for synthesizing spectrums 
obtained by the moving object spectrum transforming means for respective 
reception beams; 
b4) reference spectrum generating means for generating a predetermined 
reference spectrum; 
b5) moving object side multiplying means for multiplying the spectrum 
synthesized by said moving object spectrum synthesizing means by the 
reference spectrum; and 
b6) moving object spectrum inverse transform means for inversely 
transforming the results of multipication by said moving object side 
multiplying means from the spectrum. 
In the present invention, the multi-beam antenna is used as a reception 
antenna (or transmission/reception antenna). The antenna has a plurality 
of reception beams different in direction from one another. Further, the 
width of each of the reception beams is set to be relatively narrow. More 
particularly, the band width of the Doppler shift contained in the radar 
echo from the moving object is set to be broader than that in the radar 
echo from the stationary object. The radar echo acquired by each of the 
reception beam is normally in the form of a radio wave. In this case, the 
radar echo is transformed into the form of digital data. 
In such an arrangement, a filtering enables the separation between the 
stationary and moving objects. More particularly, since the band width of 
the Doppler frequency is reduced by narrowing the width of each of the 
reception beams, the radar echo from the moving object can be separated 
from the radar echo from the stationary object in the radar echoes 
acquired by the multi-beam antenna. The resulting radar echo of the moving 
object is transformed into a spectrum relating to the distance between the 
moving platform and the moving object, for example, by Fourier transform. 
The separation between the stationary and moving objects and the transform 
to the spectrum are carried out for each reception beam. Thus, the 
spectrum of the radar echo from the moving object can be obtained for each 
reception beam. The present invention synthesizes such spectrums. After 
the synthesization, the synthesized spectrum is multiplied by the 
reference spectrum, with the result thereof being then subjected to 
inverse transform. 
By synthesizing the spectrum in such a manner, the apparent reception beam 
width is enlarged into the total width of the actual reception beams. This 
means that the cross-range resolution can remarkably be improved 
irrespective of the actual narrowed width of each of the reception beams 
(see the above equation (6)). 
Consequently, the present invention can detect a low-speed moving object 
without degradation of the cross-range resolution. 
Preferably, the radar system of the present invention is constructed such 
that the radar echo of the stationary object separated from that of the 
moving object will also be subjected to the spectrum transform, spectrum 
synthesization, multiplication and inverse spectrum transform for each 
reception beam. The radar system of the present invention can include a 
range resolution improving means. Since the radar system of the present 
invention includes a plurality of such reception beams, a plurality of 
such range resolution improving means may be provided corresponding to the 
number of the reception beams. The range resolution improving means may be 
means for compressing the radar echo in the direction of distance. 
The example procedure of separating the radar echo of the moving object 
from that of the stationary object is as follows: First of all, the 
frequency of the radar echo acquired by the multi-beam antenna is shifted, 
for reception beam, depending on the center frequency of the Doppler shift 
contained in the echo of the stationary object. This removes the Doppler 
shift due to the velocity of the moving platform from the radar echo. 
Thus, the radar echo will be transformed into a relatively low frequency. 
When the radar echo for each reception beam is high-pass filtered after 
the frequency shifting, the radar echo of the moving object having its 
frequency higher than that of the radar echo of the stationary object is 
extracted from each reception beam. If the radar echo for each reception 
beam is low-pass filtered after the frequency shifting, the radar echo of 
the stationary object having its frequency lower than that of the radar 
echo of the moving object will be extracted for each reception beam. 
The radar echo of the stationary object thus separated is preferably 
resampled to reduce the amount of data. After the resampling, the radar 
echo of the stationary object is transformed into a spectrum. The radar 
echo of the moving object is transformed into a spectrum relating to the 
distance before it is subjected to velocity compensation. The velocity 
compensation shifts the frequency of the spectrum from the radar echo of 
the moving object, depending on the center frequency of the Doppler shift 
contained in the echo of the moving object. Thus, the Doppler shift due to 
the velocity and particularly radial velocity of the moving object will be 
negated. 
After the synthesization of spectrum, the radar echoes of the moving and 
stationary objects are multiplied by the respective reference spectrums. 
The reference spectrums can be obtained from reference signal generating 
means and reference signal spectrum transforming means, all of which are 
common to these reference spectrums. The reference signal generating means 
generates predetermined reference signals which in turn are transformed 
into spectrums relating to the distance between the moving platform and 
the object by the reference signal/spectrum transforming means. This 
spectrum transform may be Fourier transform for the distance between the 
moving platform and the object. 
The result of inverse transform from the moving object spectrum inverse 
transform means is transformed as into information suitable for use in 
display, such information being then used to display information relating 
to the moving object. This enables the display of the low-speed moving 
object without positional error. The procedure may similarly be applied to 
the stationary object. 
Further, the information relating to the display may be synthesized and 
displayed according to the present invention. Namely, the moving and 
stationary objects can be displayed on one and the same screen. In such a 
case, the advantage of the present invention relating to the display 
without positional error can remarkably be provided. 
If there is used a transmission antenna having a transmission beam width 
which covers a plurality of reception beams of the multi-beam antenna, a 
transmitter generates a radio wave modulated by pulses, which wave is then 
supplied to the transmission antenna. At this time, the radio wave is 
transmitted by the broad transmission beam and the plurality of reception 
beams simultaneously acquire the radar echoes. 
The prevention of reduction of the transmission antenna gain, which is the 
third object of the present invention, can be attained by a time-sharing 
narrow beam transmission. More particularly, the multi-beam antenna is 
used not only the reception but also the transmission while a plurality of 
transmission beams are provided each corresponding to the respective one 
of the reception beams. When transmitting, one of transmission beams is 
suitably selected and the corresponding one of the reception beams will 
receive the echo. In such a manner, the transmission antenna gain can be 
assured. 
The present invention also provides a synthetic aperture radar system 
mounted in a moving platform, comprising: 
a) a multi-beam antenna having a plurality of reception beams different in 
direction from one another, the width of each of said reception beams 
being set such that the band width of the Doppler shift contained in the 
radar echo from a moving object is broader than that in the radar echo 
from a stationary object; 
b) a plurality of moving object spectrum extracting means each provided 
corresponding to the respective one of said reception beams, each of said 
moving object spectrum extracting means being adapted to extract the radar 
echo of the moving object from radar echoes acquired by the corresponding 
reception beam; and 
c) moving object spectrum synthesizing means for synthesizing the radar 
echoes of the moving object extracted by each of said moving object 
spectrum extracting means into a spectrum, 
d) whereby each of said moving object spectrum extracting means extracts 
the radar echo of the moving object from radar echoes acquired by the 
corresponding reception beam, utilizing the fact that the band width of 
the Doppler shift contained in the radar echo of the moving object is 
broader than that of the Doppler shift contained in the radar echo of the 
stationary object. 
The present invention further provides a synthetic aperture radar system 
mounted in a moving platform, comprising: 
a) a multi-beam antenna having a plurality of reception beams different in 
direction from one another, the width of each of said reception beams 
being set such that the band width of the Doppler shift contained in the 
radar echo from a moving object is broader than that in the radar echo 
from a stationary object; 
b) a plurality of moving object spectrum extracting means each provided 
corresponding to the respective one of said reception beams, each of said 
moving object spectrum extracting means being adapted to extract the radar 
echo of the moving object from radar echoes acquired by the corresponding 
reception beam; 
c) a plurality of stationary object spectrum extracting means each provided 
corresponding to the respective one of said reception beams, each of said 
stationary object spectrum extracting means being adapted to extract the 
radar echo of the stationary object from the radar echoes acquired by the 
corresponding reception beam; 
d) moving object spectrum synthesizing means for synthesizing the radar 
echoes of the moving object extracted by each of said moving object 
spectrum extracting means into a spectrum; and 
e) stationary object spectrum synthesizing means for synthesizing the radar 
echoes of the stationary object extracted by each of said stationary 
object spectrum extracting means into a spectrum, 
f) whereby each of said moving and stationary object spectrum extracting 
means extracts the radar echo of the moving or stationary object from 
radar echoes acquired by the corresponding reception beam, utilizing the 
fact that the band width of the Doppler shift contained in the radar echo 
of the moving object is broader than that of the Doppler shift contained 
in the radar echo of the stationary object. 
The cross-range resolution improving device of the present invention is 
used in a synthetic aperture radar system mounted in a moving platform and 
comprises: 
a) moving object echo separating means for separating the radar echo of a 
moving object from radar echoes acquired by a multi-beam antenna having a 
plurality of reception beams different in direction from one another for 
each reception beam, the width of each of said reception beams being set 
such that the band width of the Doppler shift contained in the radar echo 
of the moving object is broader than that of the Doppler shift contained 
in the radar echo of the stationary object; 
b) moving object spectrum transforming means for transforming the radar 
echo of the moving object into a spectrum relating to the distance between 
the moving platform and the object for each reception beam; 
c) moving object spectrum synthesizing means for synthesizing the spectrum 
obtained by said moving object spectrum transforming means; 
d) reference spectrum generating means for generating a predetermined 
reference spectrum; 
e) moving object side multiplying means for multiplying the spectrum 
synthesized by said moving object spectrum synthesizing means by the 
reference spectrum; and 
f) moving object spectrum inverse transform means for inverse transforming 
the result of multiplication of said moving object side multiplying means 
from the spectrum. 
The present invention further provides a synthetic aperture radar system 
mounted in a moving platform, comprising: 
a) a multi-beam antenna having a plurality of reception beams different in 
direction from one another, the width of each of said reception beams 
being set such that the band width of the Doppler shift contained in the 
radar echo from a moving object is broader than that in the radar echo 
from a stationary object; and 
b) means for improving the cross-range resolution of said radar system, 
said cross-range resolution improving means comprising: 
b1) stationary object echo separating means for separating the radar echo 
of the stationary object from radar echoes acquired by the multi-beam 
antenna for each reception beam; 
b2) stationary object spectrum transforming means for transforming the 
radar echo of the stationary object into a spectrum relating to the 
distance between the stationary platform and the object for each reception 
beam; 
b3) stationary object spectrum synthesizing means for synthesizing a 
spectrum obtained by the stationary object spectrum transforming means; 
b4) reference spectrum generating means for generating a predetermined 
reference spectrum; 
b5) stationary object side multiplying means for multiplying the spectrum 
synthesized by said stationary object spectrum synthesizing means by the 
reference spectrum; and 
b6) stationary object spectrum inverse transform means for inversely 
transforming the results of multiplication of said stationary object side 
multiplying means from the spectrum.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Some preferred embodiments of the present invention will now be described 
with reference to the drawings. 
First Embodiment 
FIG. 1 shows the first embodiment of a synthetic aperture radar system 
constructed in accordance with the present invention, which comprises a 
transmitter 1 for transmitting radio waves and a transmission antenna 13. 
The transmitter 1 generates radio waves modulated by pulse signals. The 
transmission antenna 13 radiates the modulated radio waves. 
The transmission beam of the transmission antenna 13 is set to include all 
beams formed by a multi-beam antenna 15, as shown in FIG. 2. The 
multi-beam antenna 15 functions to receive the radio waves (i.e. echoes) 
reflected by the object or the like and has a plurality of beams as shown 
in FIG. 2. The width .theta..sub.B of each of the beams (reception beams) 
in the multi-beam antenna 15 is set to satisfy the following relationship: 
EQU 2U/.lambda.&gt;2V.theta..sub.B /.lambda. (8) 
where V is the velocity of a radar platform and U is the radial velocity of 
an object. By setting the beam width .theta..sub.B under such a condition, 
a moving object can be distinguished from a stationary object. 
As described in connection with the equation (6), the radar echo from the 
stationary object contains a Doppler shift having its band width of 
2V.theta..sub.B /.lambda.. This Doppler shift is exclusively produced from 
the movement of the radar platform. 
The radar echo from the moving object futher contains a Doppler shift 
having a band width of 2U/.lambda.. The Doppler shift is exclusively 
produced by the movement of the object. 
When the above relationship (8) is valid, i.e when the band width of the 
Doppler shift caused by the velocity U is broader than that of the Doppler 
shift caused by the velocity V, both the Doppler shifts can be separated 
from each other by filtering the output of the multi-beam antenna 15. 
The post-stage of the multi-beam antenna 15 includes a circuit for 
separating the Doppler shifts respectively having velocities V and U from 
each other. More particularly, the post-stage of the multi-beam antenna 15 
includes a plurality of signal processing subsystems 14-1, . . . 14-N for 
each antenna beam. 
Each of the signal processing subsystems 14-i (where i=1, 2, . . . N) 
comprises a receiver 16-i, a pulse compression unit 17-i, a Doppler 
compensation unit 18-i, a low pass filter 19-i, a resampling unit 20-i, a 
Fourier transform unit 21-i, a high pass filter 22-i, a Fourier transform 
unit 23-i and a velocity compensation unit 24-i. 
The receiver 16-i amplifies the reception signal which is received by the 
corresponding beam of the multi-beam antenna 15, with the amplified signal 
being then subjected to phase detection. The receiver 16-i generates a 
two-dimensional digital signal from the result of the phase detection. The 
two-dimensional digital signal is represented by a range bin number m and 
a pulse hit number n. The pulse compression unit 17-i pulse compresses the 
two-dimensional digital signal from the receiver 16-i on the correlation 
along the direction of range bin. This improves the range resolution. 
When it is now assumed that the main lobe direction of a reception beam is 
.theta..sub.i, the center value of an instantaneous Doppler frequency, 
that is, a Doppler frequency of the radar echo coming from the direction 
.theta..sub.i becomes 2 V sin .theta..sub.i .lambda., as will be apparent 
from the equation (2). The Doppler compensation unit 18-i shifts the 
frequency in the output of the pulse compression unit 17-i such that the 
Doppler frequency in the radar echo coming from the direction 
.theta..sub.i becomes zero. By such a processing, that is, by the 
compensation of the Doppler frequency, the central value of the Doppler 
frequency in the echo from the stationary object can be handled as if it 
is zero. In other words, the center value of the Doppler shift due to the 
velocity V of the radar platform can be compensated into zero by the 
Doppler compensation unit 18-i. 
The low pass filter 19-i takes out only the radar echo of the stationary 
object by permitting only predetermined low-frequency components in the 
output of the Doppler compensation unit 18-i to pass therethrough. Thus, 
the band width of the output of the low pass filter 19-i is narrowed. In 
order to thin out data, the resampling unit 20-i samples the output of the 
low pass filter 190-i. The Fourier transform unit 21-i causes the output 
of the resampling unit 20-i, i.e. data thinned by the sampling to be 
subjected to Fourier transform. 
The high pass filter 22-i takes out only the radar echo of the moving 
object by permitting only predetermined high-frequency components in the 
output of the Doppler compensation unit 18-i to pass therethrough. The 
Fourier transform unit 23-i causes the output of the high pass filter 22-i 
to be subjected to Fourier transform. The output of the Fourier transform 
unit 23-i contains a Doppler shift due to the velocity U. The velocity 
compensation unit 24-i removes the Doppler shift due to the radial 
velocity U of the object from the output of the Fourier transform unit 
23-i. Thus, the moving object can be displayed without positional error on 
the screen. 
In such a manner, the Fourier transform unit 21-i of each of the signal 
processing subsystems outputs a spectrum relating to the radar echo of the 
stationary object while the velocity compensation unit 24-i outputs a 
spectrum relating to the radar echo of the moving object. The spectrum 
synthesizing unit 25 shown in FIG. 1 synthesizes spectrums relating to the 
radar echoes of the stationary objects while the other spectrum 
synthesizing unit 26 synthesizes spectrums relating to the radar echoes of 
the moving objects. By synthesizing these spectrums, the band widths of 
the radar echoes from the stationary and moving objects will be expanded 
by the number of reception beams equal to N. 
The post-stage of the spectrum synthesizing unit 25 is connected to a 
processing and display subsystem 27 while the post-stage of the spectrum 
synthesizing unit 26 is connected to a processing and display subsystems 
28. The processing and display subsystem 27 comprises a complex 
multiplication unit 29, an inverse Fourier transform unit 30, a square-law 
detection unit 31 and a display unit 32. Similarly, the processing and 
display subsystem 28 comprises a complex multiplication unit 33, an 
inverse Fourier transform unit 34, a square-law detection unit 35 and a 
display unit 36. 
Each of the complex multiplication units 29 and 33 causes the output of the 
spectrum synthesizing unit 25 or 26 to be multiplied by the output of the 
Fourier transform unit 37 in the complex manner. The Fourier transform 
unit 37 causes the output of a reference signal generator 38 to be 
subjected to Fourier transform. Each of the inverse Fourier transform 
units 30 and 34 causes the result of the complex multiplication unit 29 or 
33 to be subjected to inverse Fourier transform, the transformed output 
thereof being then provided to the respective one of the square-law 
detection units 31 and 35. Each of the square-law detection units 31 and 
35 cause the output of the inverse Fourier transform unit 30 or 34 to be 
subjected to square-law detection. 
In such a manner, it can be said that the processing and display subsystems 
27 and 28 improve, with the Fourier transform unit 37 and reference signal 
generator, the cross-range resolution in accordance with the same 
principle as in the prior art shown in FIG. 4. However, the first 
embodiment of the present invention is different from the prior art in 
that the processing and display subsystems 27 and 28 receive the spectrums 
obtained by synthesizing the spectrums relating to the reception beams of 
N in number. In the first embodiment, thus, the cross-range resolution can 
be improved as though the beam width .theta..sub.B is enlarged N times. In 
the first embodiment, further, the spectrums relating to the stationary 
and moving objects are separated from each other before they are subjected 
to such a processing operation. By supplying a square-law detected signal 
to the respective one of the separate display units 32 and 36, the echoes 
of the stationary and moving objects can separately be displayed. 
Furthermore, the echoes of the stationary and moving objects can 
simultaneously be displayed on the screen of the display unit 40 by 
synthesizing the square-law detected signals at an image composition unit 
39. 
In such a manner, the first embodiment of the present invention can detect 
any low-speed moving object without degradation of the cross-range 
resolution. The echo of the detected object can be displayed without 
positional error. 
Second Embodiment 
FIG. 3 shows the second embodiment of a synthetic aperture radar system 
constructed in accordance with the present invention. The second 
embodiment is obtained by improving the first embodiment and can prevent 
the transmission antenna gain and associated signal to noise power ratio 
of image from being reduced. In the following description, parts similar 
to those of the first embodiment are denoted by similar reference numerals 
and will not be further described herein. 
The second embodiment comprises a T/R timing controller 41 and a 
transmission beam changeover circuit 42. Each signal processing subsystem 
43-i (where i=1, 2, . . . N) is defined by the pre-stage of a 
corresponding signal processing subsystem 14-i further including a 
circulator 44-i. 
The T/R timing controller 41 informs a transmission timing to the 
transmitter 12, receiver 16-i and transmission beam changeover circuit 42. 
The transmitter 12 is responsive to this signal to generate radio waves to 
be transmitted. The transmission beam changeover circuit 42 supplies the 
generated wave to the multi-beam antenna 15 through the circulator 44-i at 
a time interval corresponding to the width of transmission pulse, in 
synchronism with the transmission timing from the T/R timing controller 
41. Only when the receiver 16-i receives the transmission timing from the 
T/R timing controller 41, the receiver 16-i executes the reception 
processing operation. The multi-beam antenna 15 performs the transmission 
and reception with a narrow beam corresponding to the receiver 16-i 
performing its reception processing operation. Therefore, each of the 
reception beams is used in a time sharing manner. 
In such a manner, the second embodiment can prevent the transmission 
antenna gain and associated signal to noise power ratio of image from 
being reduced since the narrow beam transmission is carried out in the 
time sharing manner.