Note: Descriptions are shown in the official language in which they were submitted.
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RADAR SYSTEM AND METHOD FOR LOCATING AND IDENTIFYING OBJECTS BY THEIR NON-
LINEAR
ECHO SIGNALS
Field of the Invention
The invention generally relates to a method and system for detecting and
locating man-made objects by means of a "harmonic radar" utilizing the
objects non-linear response to incident electromagnetic waves.
Background of the Invention
Radar systems are widely used for detecting and locating objects. Radar
systems usually operate in the UHF (Ultra-High-Frequency) or microwave
part of the RF (Radio-Frequency) spectrum, and are used to determine the
position and/or movement of an object. There have been developed various
types of radar systems, for different purposes and applications.
Any radar system can locate a target by finding its direction and range.
The range to an object is determined by calculating the delay between a
pulse transmitted by the system, and the consequential receipt of the
reflection of said pulse from the object. This determination is based on the
known propagation velocity of the pulse c (the speed of light), when the air
is the medium. As a general rule, for obtaining a relatively good range
resolution, a very short pulse should be transmitted. The duration of the
pulse may be shortened by any known compression technique.
Metal junctions and electronic components, particularly those containing
semiconductors, have a non-linear response to the application of voltage
over them. This physical phenomenon is known in the art for several
decades, and has been used, for example, in systems for detecting and
locating mines. In these cases, the current / via a metal or a semiconductor
junction can be expressed by the following equation:
(1) / = /o + gy. + gy2 g3V3 +...
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The coefficient gr in this equation expresses conductance, while the other
terms represent the coefficients of the non-linear response of the object to
the application of voltage over it. Typically, the values of the coefficients
gn
decrease as the order of the coefficient index n increases. The said
phenomenon is used by object detection systems to remotely detect and
characterize man-made objects which are made of metals or contain
semiconductor components. Generally, such systems transmit
electromagnetic waves towards the object, that induce voltages over the
metal or the semiconductor junctions of it. These voltages induce currents
according to equation (1), which cause radiation that can be detected by a
receiving portion of the system. The radiation contains information
regarding the non-linear characteristics of the object, that are
characterized by the distinct relative levels of coefficients gn.
In most radar applications, a sinusoidal or a quasi-sinusoidal wave is
transmitted towards the object. Assuming that a wave Vcos(27ift) is
transmitted, the scattered non-linear response from a man-made object
contains harmonics of the transmitted frequency f, namely, 2f, 3f, 4f, 5f, ...
. The reflections from natural objects, however, are linear, i.e., include
only
the fundamental transmitted frequency f. Also, if the transmission
contains more than one frequency, the response contains various
combinations of the transmitted frequencies.
Man-made objects that do create harmonic scattering can be divided into
two classes:
One. Man-made metal objects: Due to an oxidized layer formed on
most metals, junctions of metals are essentially combinations of metal-
oxide-metal (MOM), which cause a symmetrical non-linear re-radiation of
the applied voltage (by "symmetrical", it is meant that /(V)=-1-(-V)). Due to
the symmetrical structure of man-made metal. objects, the harmonic
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response contains only odd harmonics: 3f, 5f, 7f.....
Two. Man-made semiconductor junctions: Semiconductor
junctions, existing, for example, in diodes and transistors, are essential to
the operation of electronic instruments, and cause non-linear re-radiation.
As is known, this non-linearity is not symmetrical, namely, 1/(V)1 #1/(¨V)1.
Therefore, the scattering from electronic components contain the
fundamental frequency and its entire harmonics, odd and even, i.e., 2f, 3f,
4f, 5f, 6f, 7f .
From the above, it is clear that a harmonic radar can detect man-made
objects, and can even distinguish between metal objects and objects that
contain electronic components.
A few general properties of the harmonic response should be noted:
One. Frequency dependence: Physically, there is no known low
boundary to the linear response from typical objects. In practice, it is
difficult to concentrate low frequency radiation on targets, and also the
coupling is not efficient (the term "coupling' refers to the ratio between the
voltage induced on a component inside the target object and between the
intensity of the electromagnetic field surrounding the target object). On
the other hand, parasitic capacitance of the junctions that are part of the
object, for example, PN junctions in semiconductor components within the
object, metal-semiconductor junctions, or metal-oxide-metal junctions,
tend to short the voltage in high frequencies. More particularly, as the
frequency increases, a lower amplitude response is scattered from a man-
made object. Harmonic radar systems using frequencies ranged from a few
tens of MHz up to about lOGHz have been applied for various radar
applications. However, second harmonic generation by metal-oxide-metal
(MOM) diode made by dissimilar metals or metal¨oxide semiconductor
(MOS) diodes have been demonstrated up to 30THz (teraherz) using CO2
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:PCT/IL 2005/000 748
ituz6-W0-00
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laser (by F. Kneubel et al., ETH, Zurich).
Two. Power Dependence: As a general rule, the re-radiation from
man-
made objects in each harmonic is proportional to the incident power raised by
the
u-th power, n is approximately equal to the harmonic order. For example, if
the
incident power is P, the scattering from the object in the second harmonic 21
is
approximately proportional to p2.
Three. Order Dependence: The higher the harmonic order, the lower the
response from the object.
Harmonic radar systems utilizing the above phenomena are used in the art,
generally for short-range detection and location of objects, typically in
ranges of
between several centimeters, up to several kilometers. Such systems, when
applied to detect objects in ranges of several kilometers require transmission
of
very high power.
WO 02/39140, US 6,163,259, and DE 2732465 are three additional publications
which disclose systems for detecting man made objects.
Parasitic harmonic signals that are developed in the transmitting unit cause
significant problems in distinguishing the searched objects from their
surrounding. For example, the operation of a transmitter is accompanied by
many non-linear electromagnetic components, particularly in the amplifier or
the
oscillator, that produces radiation. Also, metal junctions in the antenna and
the
transmission lines may create odd harmonics. Furthermore, corona formation in
the transmitter caused by the high transmitted power might create harmonics of
all orders. Harmonics of the fundamental transmitting frequency which are
produced in the transmitter are then transmitted towards the object, together
with the main transmission in the fundamental frequency. These transmitted
harmonic frequencies are scattered by all objects and may mask the expected
scattered harmonic signals from the non-linear objects, and prevent their
detection and investigation. Moreover, the transmitter platform may contain
metallic materials and electronic devices which may,
1 AMENDED SHEET
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by themselves generate harmonic signals, that may be confused with the
harmonics scattered by the objects. This is a particular problem when
using antennas having strong side-lobes, that cause problems in short-
range detection in any type of radar system.
It is an object of the invention to provide a method and system for
detecting and locating man-made objects using harmonic detection.
It is another object of the invention to provide a harmonic radar method
and system for detecting and locating man-made objects at close-range.
It is still an object of the invention to provide a harmonic radar method
and system enabling better distinction of the searched man-made objects
from their surroundings, particularly when parasitic harmonics are
generated at the transmitting and receiving units, and at the carrying
platform.
It is a particular object of the invention to provide a harmonic radar
method and system for locating man-made objects in very high precision,
namely, with high range and azimuth resolution.
It is still a particular object of the invention to provide a method and
system for precisely locating man-made objects buried in the ground.
It is still a particular object of the invention to provide a method and
system for remotely classifying man-made objects. The term "classifying"
refers herein to the ability to distinguish and classify the type of the
target
detected, for example, to distinguish between a mine, tank, missile, etc.
Other objects of the invention will become apparent when the description
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proceeds.
Summary of the Invention
The present invention relates to a radar system which comprises: (a) at
least two transmitting units for simultaneously, and in synchronization
transmitting electromagnetic radiation in distinct frequencies f1, f2,
f3 ... towards a space of interest; and (b) At least one receiving unit
tuned to a frequency of nfi + mf2 + qf,..., wherein n,m,q... being integers
not equal to zero, for receiving a non-linear response of said radiation
from objects located within the said space of interest.
Preferably, the following condition n ,m #1, and q 1... also applies.
Preferably, the following condition n ¨1 , m ¨1, q ¨1... also applies.
Preferably, the receiving unit is tuned to receive at least two
frequencies nifi + m1f2 + q1f3... , and n2fi + m2f2 + q2f3..., wherein
ni, ,qi...,n2,m2, q2..., are integers and at least n1 n2 or m1 m2.
Preferably, the system further comprises evaluating means for
evaluating the relative intensity of a response from an object in said
two received frequencies, and for classifying accordingly the type of
the object.
In an embodiment of the invention, the system may comprise two
transmitters for simultaneously and in synchronization transmitting
electromagnetic radiation in two distinct frequencies fp f2 towards a
space of interest, and one receiving unit for receiving at a frequency of
+ m1f2 , n,m, being integers not equal to zero.
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Preferably, each transmitting unit comprises a transmitter and a
transmitting antenna, and each receiving unit comprises a receiver
and a receiving antenna.
Preferably, the at least two antennas of the transmitting units and the
at least one antenna of the receiving unit are mounted on a same
carrier, and all said antennas are directed in a synchronized manner
to simultaneously scan essentially a same space of interest.
In one embodiment of the invention, the transmissions from the
transmitting antennas are orthogonally polarized one with respect to
the other, for eliminating the production of parasitic signal
components at the frequency to which the receiving unit is tuned.
Preferably, the said orthogonal polarization is used for eliminating
transmission of signals at the frequency of the receiving unit from any
of the transmitting units.
Preferably, the orthogonal polarization is performed by means of a
polarizing component located between one of the transmitters and its
corresponding antenna.
Preferably, the antennas of the transmitting units and of the at least
one receiving unit are all mounted on a same antenna pedestal, to be
directed simultaneously to a same direction.
Preferably, a multi-beam antenna is used at the receiving unit for
improving the direction resolution.
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The transmission frequencies fp f2, f3... may be selected to be in the
range of between 30 MHz to 30 THz, depending on the purpose for
which the system is used.
In one embodiment of the invention, fi , f2 , f3 ... are in the microwave
frequency range.
The invention also relates to a method for detecting man-made objects
comprising the steps of: (a) simultaneously transmitting from a first
transmitting unit and from a second transmitting unit, and optionally
from more transmitting units electromagnetic radiation in frequencies
of A , f2, f3 . . . respectively towards a space of interest; (b) receiving by
a receiving unit reflections from objects in said space of interest in a
frequency of n,f1+mf2+qf3... wherein n, m, q ... are integers not equal
to zero.
Preferably, the following condition n #1, in #1, q #1... also applies.
Preferably, the following condition n# ¨1, m# ¨1, q# ¨1... also
applies.
Preferably, the receiving unit is tuned to receive at at least two
frequencies 17/ jei +177/f2 qif,..., and n2f,+m2f2+q2f,..., wherein
ni,m,,q,...,n2,m2,q2..., are integers and at least n1# n2 or mi m2.
Preferably, the method of the invention further comprises the steps of:
(a) carrying out repeated experiments for determining a typical
response for each of various types of objects, while using selected
transmitted frequencies f1 , f2, f3 ... in each transmission unit
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respectively; (b) for each object type, recording in a database its typical
received response to said transmitted frequencies; and (c) when
receiving a response from a real object to a simultaneous transmission
of same selected frequencies, correlating the response as received with
each of said typical records in the database, to find the type having
most similarity, and classifying the object type accordingly.
Preferably, the electromagnetic radiation is transmitted in pulses.
Preferably, a receiving gate is provided at the receiver for enabling
reception during the expected time of receipt of a response from an
object, and disabling receipt in all other times.
Preferably, the pulses are short duration pulses, obtained by means of
compression.
Preferably, the electromagnetic radiation is a continuous
transmission.
Preferably, the transmitted frequencies are varied continuously, and
the receiving frequency being also varied to follow the said variation of
said two transmitting frequencies.
Preferably, the method also comprises the masking the leakage of
signal components in the frequency transmitted from any transmitting
unit to the other transmitting units.
Preferably, the method also comprises the providing to the receiving
unit indication regarding the time of the simultaneous transmission of
by the transmitting units for determining the location of the object.
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Brief Description of the Drawings
In the drawings:
- Fig. 1 shows a general structure of the harmonic radar system according
to one embodiment of the invention;
- Fig. 2 shows an antenna adapted for transmitting signals from two
transmitting units, and receiving a signal from as target which is
conveyed into a receiving unit, according to one embodiment of the
invention;
- Fig. 3 shows an object that is radiated by simultaneous transmissions
from two transmitting units having different transmission frequencies;
- Fig. 4 shows how a leakage of radiation between two transmitting units
is obtained by means of providing filters between each transmitter and
its corresponding antenna;
- Fig. 5 shows how a location of an object 50 is obtained by means of using
a plurality of receiving units and/or receiving antennas; and
- Fig. 6 shows how a classification of objects can be obtained by means of
providing a plurality of receiving channels, each tuned to a different
harmonic combination of the transmitted signals, and comaparing the
resulting signals with data obtained experimentally.
Detailed Description of Preferred Embodiments
As said, the invention relates to a radar system for detecting, locating, and
classifying man-made objects, that scatter signals at harmonic frequencies
of the transmitted frequency due to their non-linear scattering
characteristics, as mentioned in equation (1) above. Harmonic radar
systems of the prior art comprise at least one transmitting unit, and a
receiving unit. Each transmitting unit in harmonic radar systems of the
prior art transmits a signal (a short pulse or a continuous wave) at a
frequency f, and the receiving unit of such systems is tuned to receive a
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signal from the object at a harmonic frequency off, namely, a frequency nf,
wherein n is an integer. Generally, in view of the fact that the intensity of
harmonics re-radiated by the object decreases as the harmonic order
increases, it is common to tune the receiver to a low n-order harmonic. As
the receiving unit tries to detect low-intensity signals at a specific
harmonic of the transmitted frequency, any parasitic signal in said
harmonic frequency might cause confusion at the receiver in
distinguishing real objects from their surroundings. Unfortunately, it has
been found in harmonic radars of the prior art that signals in harmonic
frequencies nf of the fundamental frequency f do develop at the
transmitting unit/s and leak to the receiving unit, despite the means that
are generally provided in such systems to eliminate such leakage.
Harmonic components of the fundamental transmitting frequency that are
parasitically produced at the transmitter are then transmitted towards all
objects within the antenna beam width, together with the main signal at
the fundamental frequency. These transmitted harmonic frequencies,
scattered by all objects, may mask the expected harmonic signals from the
non-linear objects, and prevent their detection and investigation.
Moreover, the transmitting unit contains metallic materials which may, by
themselves re-radiate harmonic signals, that might be confused with the
real harmonics reflected from the objects. This is a particular problem in
antennas having strong side-lobes, that cause problems in short-range
detection in any type of radar system. The system of the invention provides
a solution to this problem, and significantly improves the distinction
between harmonics re-radiated by real objects and other parasitic
harmonics. Moreover, the method and system of the present invention
provides the ability to classify targets.
Fig. 1 illustrates a basic block diagram of a radar system for detecting
man-made objects, according to one embodiment of the invention.
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According to an embodiment of the invention the system comprises at least
two transmitters 1 and 2, transmitting simultaneously at different
frequencies fi and f2, respectively. Each transmitter feeds an antenna,
transmitter 1 feeds antenna 5, and transmitter 2 feeds antenna 7. The
transmitter may comprise a magnetron-type oscillator, CFA (Cross-Field
Amplifier), Klystron or TWT (Traveling Wave Tube) or Gyrotron
amplifiers, solid-state amplifiers, or any other kind of amplifier or
oscillator producing electromagnetic waves.
The system also comprises at least one receiving antenna 6, feeding at
least one receiver 3. Said receiver is tuned to at least one of the
frequencies that are produced by man-made objects as a result of said
simultaneous transmission, namely, nfl + inf2, wherein n and m are any
integers that are positive, negative, but not zero.
All the three antennas 5, 6, and 7 are directed so that they cover
simultaneously essentially a same area of interest. The antennas may be
fixed, or preferably mounted on a same pedestal for scanning
simultaneously the same area of interest. Alternatively, each antenna may
be mounted on a separate pedestal, while a central unit is used for keeping
all the antennas directed to the same direction during scanning. The
isolators 8 and 9 are used for preventing leakage of electromagnetic
radiation from one transmitter to the other, and between the transmitters
1 and 2 and the receiver 3.
Fig. 2, shows an embodiment of the invention in which of all the antennas
are mounted on a same pedestal. All the three antennas 105, 106, and 107
are mounted on a same frame 110, and rotated together by means of
common pedestal 104. The energy transmitted from transmitters 101 and
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102 to the antennas, and the received energy going from antenna 106 to
receiver 103, go through the pedestal by means of either a standard rotary
joint or flexible cables (not shown).
The transmission of transmitters 1 and 2 (Fig. 1) may be continuous or
pulsed. Pulse transmission is preferred when a range resolution is of high
importance. For pulse transmission, a timing pulse is provided by control
unit 20, which synchronizes the simultaneous transmission of transmitters
1 and 2. The result of the described antenna structure and simultaneous
transmission is that only in a limited volume of space the two signals in
the two transmitted frequencies A and f2 exist simultaneously. Fig. 3
describes that schematically. Antenna 205 has a beam width 305 to 405,
Antenna 207 has beam width 307 to 407, and the pulse duration in space
is indicated by numeral 300. So, at a given time, the only volume of space
that contains simultaneous transmission of ft and f2 is indicated by
numeral 200. Therefore, only man-made objects, such as object 50 that are
within the volume 300 will produce non-linear response at frequencies nfr
+ mf2. Receiving antenna 206 having a beam width between 306 to 406
defining a volume of space 200 that includes within it the space 300, is
used for receiving the non-linear response in frequency nfl + mf2 (as said,
n,m 4) from man-made object 500 which is located within space 300.
The received signal is filtered and amplified by receiver 3 (Fig. 1) that is
tuned in a manner known in the art to one selected frequency
f3 = nfi+ mf2. Once a signal at frequency nfi + mf2 is detected by receiver
3, the control unit 20 provides any type of suitable and conventional
indication on display 40.
Besides detection of a man made target, the system can also determine the
location of that target. The accuracy of the location of the target is
relative
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to the minimal beam width each of each of the three antennas 205, 206,
and 207 (Fig. 3), and to the pulse duration 100. The receiver 3 in Fig. 1
delivers the detected target signal to control unit 20. The control unit
measures the delay T between the transmission of the pulse and the
detection of a signal from the target. The said delay T, multiplied by light
velocity c, and divided by 2 is the range to the target. The delay T may be
measured by conventional techniques, such as linear frequency modulation
(LFM), when chirped transmitters are employed.
Control unit 20 can display the direction of the target OB and the range on
display 40. The direction to the target OB is the determined by the
overlapping directions of the three antennas 205, 206, and 207.
The above method for locating an object by combining the separate
determination of the range and the azimuth to the object is well known in
the art, and essentially forms the basis of almost any radar system.
Therefore, it will not be discussed herein in more detail.
However, in the present invention, the use of at least two transmitted
frequencies fr and f2, which are simultaneously present only at a limited
space portion, ensures that only within that space portion electromagnetic
components in frequency combinations of nfi + mf2, while n# 0 and m # 0
can be created by man-made objects. In the prior art, where only one
frequency fr is transmitted, harmonics nfi can be created at the man-made
target, but almost always these harmonics are present also in the
transmitted signal, and that generally masks the target and limits the
detection sensitivity.
According to the present invention, the receiver is tuned to at least one of
the frequency combinations nfi + mf2, (n# 0 and in # 0. The coefficients m
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and n are positive or negative integers, and therefore the receiver is tuned
to, for example, f, f2, 2f, f2, f, 2f2 , .... etc. Moreover, according
to
the present invention, ft and f2 are selected so that P.fi nfi + inf2, and
Qf2 # nfi + mf2 for any integers P and Q, while nfl + mf2 is the frequency to
which the receiver is tuned. That selection assures that harmonics Pfi
and/or Qf2generated at the transmitters, will not mask the detection of the
desired signal having harmonic frequency/s of nfi + in [2.
EXAMPLE
The following is an example for an efficient frequency selection that
provides a system having high sensitivity to the detection of man-made
objects. According to this example the transmitting frequencies are
= 2.74GHz, f2=1.26GHz , and the reception frequency is
fr = 4.0GHz (being the sum f, + f2). In that case the harmonics created at
the transmitter 2 are in frequencies 5.48GHz, 8.22GHz, 10.96GHz, ... etc.,
and the harmonics created at the transmitter 4 are in the frequencies
2.52GHz, 3.78GHz, 5.04GHz, ... etc. It has been demonstrated that in this
example the receiving frequency, when properly selected, is remote enough
from each of the transmitting frequencies, and/or from any of their
harmonics. Therefore, the sensitivity and the ability of the system to
distinguish man-made target from their surroundings are improved.
In one preferred embodiment of the invention, the system of the invention
comprises more than two transmitting units, transmitting at more than
two frequencies, and more than one receiving units receiving at more than
one receiving frequency. Each receiving frequency is a combination of the
transmitting frequencies. Such a system is capable of obtaining more
information about the targets, and has an improved sensitivity and a
reduced false alarm rate.
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While it is better to separate transmitters and transmitting antennas,
which transmit high power, in order to reduce the risk of producing target-
like signals in the transmitted spectrum, in receiving, where power levels
are very low, a single antenna and receiver may be used instead of a
plurality of such units. To enable receiving and detection at more than on
frequency, a broad band antenna and front-end amplifier in the receiver
may be applied. Then by standard techniques, several frequencies may be
filtered each into a separate detection channel.
As said, in order to obtain a powerful harmonic return, it is desired to
transmit as high power as possible by the transmitters 1 and 2. Any
coupling between the antennas 5, 6, and 7, even a weak one, may cause
leakage of energy between the two transmitters, that will produce signal
components being combinations of a plurality of the transmitting
frequencies, and these signal components may radiate from the
transmitting antennas 5, and 7, and received by antenna 6. As said, it is
very important that such frequency combinations will be produced only or
mainly at the target. A reduction of the coupling between the antennas can
be obtained by the introduction of the buffers 8,9, (Fig. 1) between the
antennas made of absorbing materials. In another embodiment of the
invention, the two transmitting antennas 5, and 7 are built to produce
signals that are orthogonally polarized. This reduces the coupling between
the antennas 5 and 7 to a minimum. Furthermore, two filters 28 and 29
(shown in Fig. 4) that pass only the frequency of transmission of the
relevant transmitter may be correspondingly introduced between each
transmitter and its antenna in order to further eliminate leakage of energy
in one transmitting frequency from a first transmitter, to the second
transmitter transmitting in a second frequency.
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As said, the azimuth to the object depends on the directivity of the
antennas, and also on the receiving frequency. Therefore, it is preferable to
use a high receiving frequency, for example j + f2 = 4GHz . Furthermore,
as shown in Fig. 5, in a preferred embodiment of the invention the locating
of the object 50 is improved by means of using a plurality of receiving units
and/or receiving antennas. In the embodiment of Fig. 5, four receiving
units having respectively four antennas 51a - 51d are used. Each of said
antennas has a narrow receiving lobe 57a ¨ 57d slightly displaced one with
respect to the other. The received signals are correspondingly passed in the
corresponding receivers through filters 36a ¨ 36d, allowing signals only in
the receiving frequency to pass. The signals are then amplified by
amplifiers 37a ¨ 37d and conveyed to both the azimuth processing unit 49,
and to summing unit 69. The azimuth processing unit 49, which is given
the four signals 47a ¨ 47d compares the relative intensity of the signals
from the four amplifiers 37a ¨ 37d, and determines from it the azimuth to
the object relative to the lobes direction of each of the antennas. In Fig. 5,
there are four antenna lobes 57a-57d. When the object 50 is positioned
between the two lobes 57a and 57b as shown, only signals 47a and 47b will
essentially be received, and therefore it can be concluded that the object 50
is located within the azimuth covered by the antennas 55a and 55b, and
not within the azimuth covered by antennas 55c and 55d. Moreover, the
relative values of the signals 47a and 47b may provide even a better
distinction relating to the azimuth. The signals 47a ¨ 47d are also
summed by summing unit 69, and a pulse 62 resulting from this
summation is provided to the range processing unit 38. The range
processing unit 38 also receives a reference pulse from the control unit 20,
and by carrying out timing comparison between these two pulses it
determines in a conventional manner the range to the object 50.
As mentioned, the intensity of the scattered harmonic signals is
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approximately proportional to the H +17721 power of the intensity of the
transmitted signals. More specifically, if P1 andP2 are the transmitted
powers of the signals in frequencies f and f2 respectively, then the received
signal at frequency nfi + mf2is approximately proportional to Pilni = P21m1
Therefore, in the system of the invention there is a significant advantage
to the transmission of as high power as possible. Furthermore, the
required bandwidth at the receiver is inversely proportional to the
duration of the transmitted pulse. As the noise at the receiver is
proportional to the bandwidth, it can be noted that the level of the noise is
inversely proportional to the transmitted pulse duration. Therefore, it
follows that there is an advantage in the use of a narrow pulse with high
amplitude in comparison to a wider pulse with a same energy.
One common compact way for obtaining a very narrow pulse with high
amplitude is the application of pulse compression technique. For example,
a 1MW pulse having a duration of 27s can be compressed to a pulse of
100MW with duration of lOnsec. Such compression techniques are known
in the art. Moreover, it is obvious and known in the art that the use of
shorter pulses provides better range resolution, and this is another
motivation for the use of shorter pulses.
Furthermore, a range gate may be used in the receiver if the transmission
pulse duration is shorter than the time required for the pulse to reach the
target and return to the receiver. Such a gate when used, opens the
receiver during the time expected for the returned pulse to reach the
receiver, and prevents the clutter reception.
Design considerations: An exemplary case is provided herein, where
1 p sec pulses are transmitted by two transmitters, with a power of each
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pulse of about 1MVV (Pi = P2 = 1MW). If the pulses are compressed to
1Onsec each, while keeping the same energy in each pulse, and power of Pi
= P2 = 100MW, the power of the received signal at a frequency ji +A will
be 10,000 times stronger. In such a case, the receiver should use a
matched filter having a band width of about y
10n sec =100MHz instead of a
matched filter for a 1p sec pulse, i.e., having a bandwidth of
V
sec = 1MHz , as had to be originally used with no compression.
/lp
Therefore, it has been shown that the compression of the transmitted
pulses by a factor of 100 increases the power of the received pulses by a
factor of 10,000, while the noise is increased by a factor of only 100. In
other words, an improvement of 100 fold is obtained by the compression in
the signal-to-noise ratio and in the detection sensitivity.
On the other hand, a too much compression of the transmitted pulses, may
increase too much their spectral content, which may cause great
difficulties in separating the target signal from the transmitted signal. In
the above example, a 10 nsec pulse includes most its spectral content
within 100 MHz, and is well separated from the received signal. However,
if, for example, a I nsec pulse were used, its spectral width will be about
1GHz and the separation between the transmitted signal at a central
frequency of 2.5 GHz and the received signal at a central frequency of 4
GHz (also with a filter of 1 GHz width in the receiver) will not be
sufficient.
In some applications there may be advantage for using the harmonic radar
system of the invention in much higher frequencies, up to 30THz. One
application in the high frequency band (30GHz to 30THz) can be the
detection of defects in microelectronic components, similar to the detection
of concealed objects (for example, mines or explosive charges) in the lower
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band (30MHz to 30GHz).
Classification of Targets: The ability to classify targets is another
aspect of the present invention. As mentioned above, a metal object
creates mostly odd harmonics (weak even harmonics are re-radiated
by dissimilar metallic junctions), while solid-state junctions (PN),
typical of electronic components, produce all (even and odd)
harmonics. More specifically, if the transmitted frequencies are fi
and f2, a receipt of a signal at nfi + mf2 where n + m is even is a
strong indication of a target that contains electronics.
Moreover, different classes of targets have a different non-linear response
to incident frequencies. These differences are manifested in the relative
intensities of the scattered frequencies nfi + mf2, while operating with an
embodiment of the invention. By receiving more than one response, for
example, one in frequency 1//fi + m1f2 and a second in frequency n2f, + m2f2,
and comparing the intensities of these two response signals, it is possible
to classify the targets.
Other means for target classification may be the harmonic response of
targets to the incident power density, as at some power density level
target's harmonic response tends to saturate. Another indication may be
associated with temporal stability of the target's response.
In a preferred embodiment (Fig. 6), the receiving antenna 6 is a broad
band antenna, that covers a range of frequencies containing at least two
harmonics, nib + mif2 and n2li + m2f2, where at least n1#n2or m1 # m2.
The receiver has at least two channels 81-83, a first channel 81 for nib +
mif2 and a second channel 82 for n2fi + m2f2. The received signals pass
through the broadband amplifier A, and conveyed into splitter S. The
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splitter S maneuvers each signal according to its frequency into its
corresponding cannel 81, 82, or 83. Detectors 91, 92, and 93 detect the
corresponding signals, and the results are correspondingly provided into a
comparing and processing unit C. The unit C corresponding compares the
ratio of the outputs of these channels with several expected ratios of
specific classes of targets, as previously obtained experimentally and
stored in database B, to find one class, the ratio of which matches most.
The target is classified to the experimental class matches most. Obviously,
the ability to classify targets in a system according to the invention
increases as the number of channels increases.
Another way for obtaining more information on the targets, and for
classifying them is by varying the transmitted frequencies fi and f2, while
following in the receiver the harmonic response nifi + mif2.
Still another way for obtaining a better classification of targets in a system
of the invention is by transmitting more than two frequencies, for example
four frequencies I, f2, f3, and f4, and receiving a nonlinear response at a
frequency such as nf; +mf, +1f3,+kf4, while n, m, 1, and k are integers.
