Antenna Array Beamforming for Low Probability of Intercept Radars
by
Daniel Goad
A thesis submitted to the Graduate Faculty of
Auburn University
in partial ful llment of the
requirements for the Degree of
Master of Science
Auburn, Alabama
December 14, 2013
Keywords: Radar, Low Probability of Intercept, LPI, Antenna Arrays, Beamforming
Approved by
Lloyd Riggs, Chair, Professor of Electrical and Computer Engineering
Michael Baginski, Associate Professor of Electrical and Computer Engineering
Shumin Wang, Associate Professor of Electrical and Computer Engineering
Stuart Wentworth, Associate Professor of Electrical and Computer Engineering
Abstract
A radar system?s focus on low probability of intercept (LPI) performance has
become increasingly important as systems designed for electronic support measures
(ESM) and electronic counter measures (ECM) continue to become more prevalent.
Due to the inherent two-way versus one-way propagation loss of a transmitted signal,
radar systems are often highly visible to intercept receivers, and thus have a high
probability of detection. A novel transmit array beamforming approach has been
introduced that o ers signi cant LPI performance gains for radar systems using a
one-dimensional phased antenna array. This method replaces the traditional high-
gain scanned beam with a set of low-gain, spoiled beams scanned across the same
observation area. A weighted summation of these spoiled beams can result in a re-
turn equivalent to that of the traditional high-gain pattern. As a result, the antenna
performance of the radar system remains unchanged while the peak gain of the trans-
mitted signal is reduced considerably. This LPI technique is expanded for the case
of a two-dimensional antenna array. With this added dimension, the computational
complexity of the method is increased, as the pattern now changes with respect to
both and . Simulation results show that the developed technique is still applicable
for a two-dimensional array. A carefully calculated set of complex coe cients can
be applied across the set of low-gain basis patterns, which are simply the high-gain
patterns spoiled by a certain phase shift, in a weighted summation. The results of
this summation can be shown to provide nearly identical returns when compared to
that of a traditional high-gain single beam scanned across the observation area. The
high-gain transient power is replaced by lower power signals with an increased in-
tegration time, resulting in the same total energy on the target, and thus the same
ii
detection performance. The simulation results show that the intercept area, the area
in which a hostile intercept receiver can detect the transmitted signal, can be reduced
signi cantly due to the low gain of the transmitted spoiled patterns. For example,
the intercept area is reduced by as much as 96% in the case of a 32x32 element array.
The LPI bene ts of this technique - signi cantly reducing the range at which a hos-
tile receiver can intercept the radar beam while maintaining the range at which the
radar can detect the target - are of obvious bene t in the ongoing battle of electronic
warfare.
iii
Acknowledgments
First and foremost, I owe everything to God for all he has done for me. He has
blessed my life tremendously by allowing me to attend Auburn University for both
my undergraduate and graduate degrees.
I credit Daniel Lawrence for developing and publishing the novel approach ex-
plored in this thesis. I would like to thank him for allowing me to expand his work
for my thesis and for the assistance he provided in that process.
I would like to thank Dr. Lloyd Riggs for being my advisor, for his aid in this
research and for his support throughout my undergraduate and graduate years. In
addition, I thank the other members serving on my advising committee: Dr. Baginski,
Dr. Wentworth, and Dr. Wang.
I would like to express my gratitude to the many faculty members in the Depart-
ment of Electrical Engineering at Auburn University who have taught and advised
me during my time at Auburn. Their teaching and guidance have equipped me with
the tools necessary to succeed both academically and professionally.
I would like to thank Kevin Nash and Pete Kirkland for the valuable work experi-
ence they provided during my years working at SMDC as a coop student. This work,
and their mentoring, introduced me to the eld of radar analysis which provided focus
for my graduate studies.
Finally, I wish to thank my parents, Ed and Melinda Goad, for raising me in a
Godly home and for homeschooling me for twelve years. They equipped me academi-
cally and instilled within me a work ethic that has allowed me to pursue my academic
goals successfully.
iv
Table of Contents
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Electronic Warfare . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.2 Growing ECM Threat . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Introduction to Low Probability of Intercept . . . . . . . . . . . . . . 3
1.2.1 Inherent Weakness of Monostatic Radars . . . . . . . . . . . . 3
1.2.2 Goal of LPI Development . . . . . . . . . . . . . . . . . . . . 4
1.3 Existing LPI Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3.1 Reducing Transmitted Energy Density . . . . . . . . . . . . . 5
1.3.2 Continuous Wave Radar . . . . . . . . . . . . . . . . . . . . . 6
1.3.3 Noise Radar . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.3.4 Frequency Hopping . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3.5 Other Methods . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.4 A Novel Approach to LPI . . . . . . . . . . . . . . . . . . . . . . . . 8
2 Original LPI Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1 Theoretical Development of the Original Approach . . . . . . . . . . 10
3 Expansion of the 2D Method Into 3D . . . . . . . . . . . . . . . . . . . . 21
3.1 Theoretical Calculations . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.2 Calculation of Phase Shift Values . . . . . . . . . . . . . . . . . . . . 31
4 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
v
4.1 Simulation Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4.2 Simulation Results for an 8x8 Element Array . . . . . . . . . . . . . . 36
4.3 Simulation Results for a 32x32 Element Array . . . . . . . . . . . . . 39
5 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
5.1 Implementation Into an Existing Radar System . . . . . . . . . . . . 44
5.2 Two-way Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
5.3 Computational Limitations . . . . . . . . . . . . . . . . . . . . . . . . 46
5.4 Hardware Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 47
5.5 Doppler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
5.6 Areas for Future Research . . . . . . . . . . . . . . . . . . . . . . . . 48
6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
A MATLAB Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
A.1 Optimize phase scan 2D.m . . . . . . . . . . . . . . . . . . . . . . . . 54
A.2 minimizeGain 2D scale.m . . . . . . . . . . . . . . . . . . . . . . . . 55
A.3 minimizeGain 2D.m . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
A.4 Beamer 2D.m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
vi
List of Figures
2.1 N-element linear phased array antenna . . . . . . . . . . . . . . . . . . . 11
2.2 Quadratic Phase Shift . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3 Fundamental Array Pattern and Basis Pattern . . . . . . . . . . . . . . . 14
2.4 Basis Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.5 Scanned Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.1 2-Dimensional quadratic phase shift values . . . . . . . . . . . . . . . . . 31
3.2 Alpha values used to create basis patterns for the two-dimensional array 32
3.3 Fundamental basis pattern for an 8x8 array pattern . . . . . . . . . . . . 33
4.1 Fundamental array pattern for an 8x8 element array . . . . . . . . . . . 36
4.2 Fundamental array pattern for an 8x8 element array - XZ Plane . . . . . 37
4.3 Fundamental basis pattern for an 8x8 element array - XZ Plane . . . . . 37
4.4 Recreated fundamental array pattern for an 8x8 element array . . . . . . 38
4.5 Recreated array pattern for an 8x8 element array with = 15 and = 15 39
4.6 Fundamental array pattern for a 32x32 element array . . . . . . . . . . . 40
4.7 Fundamental array pattern for a 32x32 element array - XZ Plane . . . . 40
vii
4.8 Fundamental basis pattern for a 32x32 element array . . . . . . . . . . . 41
4.9 Recreated fundamental array pattern for a 32x32 element array . . . . . 42
4.10 Recreated array pattern for a 32x32 element array with = 26 and = 44 42
4.11 Recreated array pattern for a 32x32 element array with = 26 and
= 44 - XZ Plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.12 Recreated array pattern for a 32x32 element array with = 26 and
= 44 - YZ Plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
viii
Chapter 1
Introduction
On the modern battle eld, radar systems have become a vital component of
warfare and can provide a signi cant military advantage to whoever possesses them.
There are many critical uses of radar systems including both active and passive
surveillance and detection for o ensive purposes. These radar systems can assume
many di erent shapes and forms and can be mounted on a wide range of platforms
such as missiles, aircraft, and sea and land based observation platforms. Regardless
of location, purpose, and scope, however, all radar systems share a high vulnerability
to detection and exploitation by opposing systems.
1.1 Background
1.1.1 Electronic Warfare
The term electronic warfare (EW) is used to classify military action to identify,
prevent, or exploit hostile use of the electromagnetic spectrum. EW can be further
divided into two categories: electronic support measures and electronic countermea-
sures. Electronic support measures (ESM) involve actions taken to search for, identify,
and analyze detected radar signals. Although ESM are by de nition passive, they can
provide a source of EW information required to conduct counter measures. Electronic
counter measures (ECM) involve actions taken to prevent or reduce hostile use of the
electromagnetic spectrum, or actions that actively seek to exploit the hostile radar
system. A further distinction, electronic counter-countermeasures (ECCM) involves
actions taken to ensure friendly use of the electromagnetic spectrum despite hostile
ECM e orts [1].
1
1.1.2 Growing ECM Threat
The increasing prevalence of ESM and ECM systems poses a great threat to any
system relying on radar performance. For example, aircraft often face great risk from
enemy defenses if they are detected by hostile ESM or ECM systems. Stationary radar
systems also face threats from Anti-Radiation Missiles (ARM). The threat posed by
these missiles has two aspects that must be considered. First, it is to the advantage
of the radar to avoid for as long as possible any reconnaissance of hostile ARM or
ECM systems. Second, in order to protect itself from incoming missiles, the radar
must work to deceive the ARM without interrupting search operations [2].
The operation of a radar system can also be severely hampered by noise jamming
and deception jamming e orts by ECM systems. Noise jamming involves deliberate
radiation in order to disturb the normal operation of a radar, while deception jamming
involves an attempt to deceive the radar using methods such as range deception or
velocity deception [3].
Due to the great risk associated with these threats, it should be a goal of every
radar design facing these dangers to attempt in some capacity to avoid detection by
a hostile system. The steps in the deployment of an ECM system can be listed as
follows:
1. Search in frequency, azimuth, and elevation
2. Detect an incoming radar signal
3. Identify the signal by its emission characteristics and assess priority of the signal
4. Select the proper ECM to employ
5. Initiate the ECM operation
Any delay in any of these steps could prevent timely ECM initiation, providing
an advantage to the detected system; therefore, it is bene cial to design radar systems
2
with ECCM properties in an attempt to decrease the threat caused by hostile systems
[4].
1.2 Introduction to Low Probability of Intercept
Because of the increased threat of ESM and ECM systems, a great focus has
been placed on developing radar systems designed to combat the dangers of detection.
Known as low probability of intercept (LPI) radar systems, these sensors have been
designed to reduce the potential for detection and exploitation by ESM and ECM
systems.
1.2.1 Inherent Weakness of Monostatic Radars
An inherent weakness of any monostatic radar system attempting to avoid de-
tection by an intercept receiver involves the di erence in propagation loss between
the radar and the receiver. The theoretical performance of such a radar system can
be de ned by the radar range equation [5]. For one way propagation of a transmitted
beam, the power density Qi at a point at a distance R away from the transmitting
source can be calculated as
Qi = PtGt4 R2 (1.1)
The power re ected by the target back towards the radar can be expressed by the
product of the incident power density and the radar cross section, , of the target.
When considering radar propagation, this re ected power must be taken into account
to compensate for the propagation losses of the wave travelling to the target and back
to the transmitter. The resulting power density Qr received at the transmitter can
be calculated as
Qr = Prefl4 R2 = PtGt (4 )2R4 (1.2)
3
From these equations, it can be seen that while one-way propagation loss is
proportional to 1=R2, two-way propagation loss is proportional to 1=R4, meaning
that the power received by a radar system is reduced by the power seen by the target
by a factor of 1=R2. This di erence bene ts the ESM receiver greatly, as it will always
have the advantage over the radar in terms of received power. In strategic terms, this
means that the intercept receiver in most cases will be able to detect the signal of the
radar system before it itself is detected.
1.2.2 Goal of LPI Development
It is important to note that, as active sensors, all traditional radar systems
must have a nite probability of intercept [6]. That is, there is always a minimum
range between the radar and the ESM system where the detection threshold of the
intercepting receiver is exceeded. Therefore, it is not a feasible goal to completely
avoid detection by a hostile system, but rather to delay that detection as long as
possible. The quiet range of a radar can be de ned as the range that the radar can
detect a target without interception from a hostile ESM system [7]. The primary
underlying goal of LPI, therefore, is to focus on increasing this range as much as is
practical for a given radar system.
1.3 Existing LPI Techniques
In order to overcome the inherent disadvantage of a radar system due to propa-
gation losses, a number of techniques have been developed to attempt to reduce the
visibility of the radar to any hostile ESM systems to enhance LPI performance. One
of the primary methods of reducing the visibility of a radar system involves spreading
the transmitted energy, either over time, frequency, or space. It should be noted that
technically there exists a distinction between such spread spectrum techniques and
true low probability of intercept techniques [8]. The principal idea of true LPI radar
4
is to avoid interception by mismatching the waveform of the radar with the waveforms
that the ESM system is expecting to receive. As a result, the development of such a
system requires the designer to consider the ESM and ECM systems the radar wishes
to avoid, and a complete assessment of the LPI performance must include analysis
of both the radar and the hostile systems [7]. Although this technical distinction
between approaches exists, the term LPI is used universally to describe any system
attempting to reduce its probability of intercept by a hostile system.
1.3.1 Reducing Transmitted Energy Density
As mentioned above, in general the capacity to reduce the visibility of a radar
system involves reducing the energy density of the transmitted signal. This can be
accomplished by spreading the energy over a longer time by using high duty cycle, or
even continuous wave, waveforms, spreading it over a wider bandwidth, or spreading
it in space, reducing the transmit antenna gain by spreading the energy over a wider
angle [6]. Although there are many ways to implement these spreading techniques,
the concept of high duty cycle, wideband waveforms is generally accepted as advan-
tageous to reducing visibility. By increasing the time duration of a waveform, the
peak power can be lowered while maintaining the same average power. By increasing
the bandwidth of the waveform, the power spectral density can be lowered, reducing
the probability of narrowband interception [9]. According to [10], one of the most
e ective techniques for reducing the probability of detection by an ESM system is to
implement ultra wide bandwidth pulses, causing the radar?s transmitted signal to be
mismatched to what the intercept receiver is expecting.
The authors of [11] discuss the many advantages of wideband radars, which in-
clude providing better target identi cation, and a greater reliability of detection.
They can also provide better velocity tracking, as the accuracy of wideband mea-
surements is less a ected by target maneuvering than narrowband measurements.
5
Wideband radars can also provide better secrecy and electromagnetic compatibility,
and also allow some level of immunity from interference; since the signal energy is
distributed through the spectrum, any jamming signal must be distributed as well,
requiring signi cantly more power to e ectively maintain jamming capability. How-
ever, it is also noted in [11] that excessive widening of the signal bandwidth can lead
to a decrease in detection quality if the bandwidth is increased such that individual
scatterers on the target are resolved in range.
1.3.2 Continuous Wave Radar
As discussed above, waveforms with high duty cycle or pulse repetition frequency
(PRF) allow the transmitted energy to be spread over time, resulting in increased
LPI performance. The PRF of a waveform could be increased to the extreme case
of becoming a continuous-wave (CW) transmission. A signi cant advantage of a
CW system is the ease and accuracy with which such systems are able to process
Doppler shifts. A disadvantage of CW radars, however, is their inability to measure
range. One solution to this de ciency is the frequency modulated continuous-wave
(FMCW) radar, which generates a range beat by changing the transmitter frequency
[12]. FMCW is a simple way of giving a radar an extremely high time bandwidth
product. This results in a high resistance to interception by ESM systems, due the
impracticality of matching the ESM receiver to the radar?s sweep pattern or e ectively
jamming the system [13].
Many believe that a CW waveform is the ideal waveform for LPI radar, as the
peak power of such a system is much lower than that of a pulsed radar. Although the
advantages of a CW, or FMCW, waveform are great, these systems also face certain
limitations. CW systems can be either monostatic, meaning a single antenna for both
transmit and receive, or bistatic, with separate antennas. Monostatic systems su er
6
from leakage due to transmitting and receiving simultaneously. A bistatic arrange-
ment eliminates this problem by separating the transmit and receive antennas by
some distance; however, this separation introduces other issues, such as the di culty
in correctly synchronizing time and direction between the two antennas [14].
1.3.3 Noise Radar
LPI development in radar systems with pulse or chirp waveforms is becoming
increasingly di cult, as these waveforms are so well de ned and therefore are easier
to exploit with ESM systems. As a result, some researchers have begun focusing
on the development of noise radars. Also known as random signal radars, these are
systems whose transmitting signal is modulated by a lower frequency noise, or is itself
microwave noise [15]. An ideal noise waveform is random by nature, resulting in a
nonperiodic waveform. This makes interception extremely di cult, as each successive
pulse is uncorrelated [16]. It has been shown in [17] that both phase and frequency
modulated noise radar can result in a wider output bandwidth and sidebands that
are suppressed signi cantly more than the modulated signal of a traditional radar
system.
Random signal radars often work in continuous-wave mode. This is due to the
advantanges of CW radar over conventional pulsed radar in regards to LPI perfor-
mance, and also the ease with which random signal radar can be operated in CW
mode. However, the inherent disadvantages of CW radar, such as leakage in the case
of a monostatic setup, also apply to these random signal radars. This leakage, and its
constraing on operating range, can be the most di cult weakness to overcome when
developing random signal radars [15].
7
1.3.4 Frequency Hopping
Another area of research involves frequency hopping. If the total illumination
time on a target is longer than the coherent processing interval required by the radar,
the carrier frequency of the transmitted signal may be changed to allow a new coherent
processing interval to begin. Such frequency agility greatly increases the di culty
of interception by an ESM system, as the interceptor receiver must now cover the
entire frequency band implemented by the radar [18]. Much research has been done
and many papers have been written on development in this area, such as techniques
based on the application of spread spectrum-frequency hopping methods [19], and
modulators for burst-by-burst carrier frequency hopping in TDMA systems [20].
1.3.5 Other Methods
Many other methods exist to reduce visibility and enhance LPI performance. In
[21], a novel approach has been proposed involving antenna hopping. In this paper,
the author argues that frequency or phase modulation can be imposed on a signal by
the phase shift resulting from switching either receiver input or the transmitter output
among a set of antennas. Some researchers, seeing a potential need for radars without
a scanning transmit main beam, have developed what are known as omnidirectional
radars. These systems, such as the one discussed in [2], require the transmitter to
illuminate the search area continuously, due to the lack of a main scanning beam. Al-
though these systems have very low transmit gain, they are dependent upon multiple
receiving beams to provide continuous coverage of the observation space.
1.4 A Novel Approach to LPI
In [22], a novel technique is developed to provide low probability of interception
for radar systems with phased array antennas. This method involves replacing the
traditional high-gain antenna beam used to scan a search region with a weighted
8
summation of a set of low-gain, "spoiled" beams. These spoiled beams are created
by simply adding a certain phase shift pattern across the array to reduce gain; thus,
this technique could in theory be applied to an existing array with minimal modi ca-
tions. The goal of this technique is to reduce the peak gain of the transmitted pattern
while maintaining the same performance as a traditional scanned radar system with a
high-gain main beam. It should be noted that this method is not designed to increase
the quiet gain of the radar, that is, the range at which the radar can detect a target
without interception from a hostile ESM system; therefore, such a system still has
no guarantee of being able to detect a target before being detected itself. Rather,
the technique simply aims to reduce as much as possible the intercept range of an
ESM receiver, thus reducing the probability of intercept. It can be shown using (1.1)
that a reduction of 10 dB in the gain of the transmitting antenna reduces the maxi-
mum intercept range of the ESM system by a factor of 1=p10. This translates into a
90% reduction in the intercept area of the ESM receiver. If the detection range and
performance of the transmitting radar can be maintained while also drastically reduc-
ing the intercept range of any potential hostile ESM systems, then the LPI bene ts
of such a technique would be substantial. The author of [22] claims to accomplish
this feat by e ectively replacing the traditional high gain transient sweep with lower
power beams radiated persistently over the observation area, as demonstrated in the
following chapter.
9
Chapter 2
Original LPI Approach
In [22], the author develops a beam-spoiling technique designed to increase LPI
performance of a radar system. The main approach of this method involves sequen-
tially forming a series of low-gain spoiled beams over the desired search area instead
of scanning with a single high-gain transmit beam. The author claims that after
transmitting and receiving the spoiled beams, the set of formed low-gain beams can
be weighted and combined to achieve the same detection results as a single high-
gain beam, e ectively replacing high transmit power with increased scan times. This
technique would reduce the peak power radiated in any one direction while still main-
taining the same antenna performance as a traditional scanned radar system.
2.1 Theoretical Development of the Original Approach
The high-gain pattern synthesis approach presented in [22] applies to a one-
dimensional linear phased antenna array, as shown in Figure 2.1. The far- eld radi-
ation pattern of such an array with N elements can be expressed as
f0( ) = 1 +ej dsin( ) +ej2 dsin( ) +:::+ej(N 1) dsin( ) (2.1)
where = 2 = is the free-space propagation constant, d is the array element spacing,
and is the spatial angle measured from the broadside direction. To reduce the
complexity of the calculations, it is bene cial to make the substitution = dsin( ).
If varies from 90 to 90 , then will vary from j d to j d. The step size of this
array will determine the precision of the array pattern. In [22], d is set at one half
10
Figure 2.1: N-element linear phased array antenna
of a wavelength (d = =2), but because of the substitution of into the equations,
the element spacing and frequency can be changed without increasing the complexity
of the mathematics. With the substitution, the fundamental array pattern can be
rewritten as
f0( ) = 1 +ej +ej2 + +ej(N 1) (2.2)
This array pattern results in a main lobe with high gain directed broadside to the
array. This main lobe can be scanned by applying a linear phase progression across
the elements of the array. In [22], a phase scan of = 2 =N is selected. This allows
a total of N scanned patterns to be produced; that is, N distinct far- eld patterns
exist with the center of the main lobe pointing in N even increments between 90
and 90 . The number of scanned patterns, and hence the density of the scanned area,
can either be increased or reduced as desired by changing the value of N in . The
11
set of scanned patterns can be written as
f1( ) = 1 +ej ej +ej2 ej2 + +ej(N 1) ej(N 1)
f2( ) = 1 +ej2 ej +ej4 ej2 + +ej2(N 1) ej(N 1)
...
fN 1( ) = 1 +ej(N 1) ej + +ej(N 1)(N 1) ej(N 1) (2.3)
For purposes of LPI applications, it is desirable to reduce the peak power trans-
mitted in any one direction to reduce the potential for detection by ECM systems,
while still maintaining the same range and coverage. The author of [22] proposes that
this can be accomplished by creating a set of low-gain, spoiled basis patterns which
can then be weighted and combined to form a far- eld pattern with gain equivalent
to that of the original fundamental array pattern. For optimal performance in this
scheme, the low-gain patterns should have low gain and broad beamwidth. It is possi-
ble to create such a spoiled beam by applying a certain phase shift to each element of
the array. In [22], a quadratic phase shift, shown in Figure 2.2, is applied across the
elements of the array used to create the low gain pattern. This phase shift pattern
both defocuses the beam and reduces the gain of the array.
Such an antenna array pattern can be expressed as
g0( ) = 1 +ej 1ej +ej 2ej2 + +ej N 1ej(N 1) ; (2.4)
where 1; 2;::: N 1 are shown in Figure 2.2. When comparing (2.2) and (2.4), it is
obvious that the basis pattern g0 is simply the fundamental pattern f0 "spoiled" by
applying the phase shift pattern. Figure 2.3 shows the fundamental array pattern g0
and the fundamental basis pattern f0. In these simulations, the gain of the main lobe
12
Figure 2.2: Quadratic Phase Shift
13
Figure 2.3: Fundamental Array Pattern and Basis Pattern
of the f0 is approximately 15 dB, while the maximum gain of g0 is approximately 1.7
dB.
As in the case of the fundamental array pattern, a set of N low-gain patterns
can be formed by applying the same linear phase progression, which can be written
as
g1( ) = 1 +ej 1ej ej + +ej N 1ej(N 1) ej(N 1)
g2( ) = 1 +ej 1ej2 ej + +ej N 1ej2(N 1) ej(N 1)
...
gN 1( ) = 1 +ej 1ej(N 1) ej + +ej N 1ej(N 1)(N 1) ej(N 1) (2.5)
14
Figure 2.4: Basis Patterns
Again, when comparing (2.3) and (2.5), it is apparent that the set of basis patterns
are simply the equivalent steered patterns spoiled by the phase shift pattern shown
in Figure 2.2. As a result, each basis pattern gn is steered to a speci c angle of ,
just as the original steered pattern; however, due to the spoiling e ect of the applied
phase shift, the gain of the main lobe is reduced, resulting in the patterns shown in
Figure 2.4. Although steered to di erent angles, the set of basis patterns appear to
be indistinguishable because of the spoiling e ect.
If it is assumed that the original fundamental array pattern can be written as a
linear combination of the set of basis patterns, then the reconstructed pattern can be
written as
f0( ) = !0;0g0( ) +!0;1g1( ) +!0;2g2( ) + +!0;N 1gN 1( ) (2.6)
15
!(0;n) represents the weighting of each basis pattern that is required for the sum of
the basis patterns to have a far- eld pattern equivalent to that of the fundamental
array. This process can be extended to reconstruct all N possible array patterns from
(2.3) as follows
f1( ) = !1;0g0( ) +!1;1g1( ) +!1;2g2( ) + +!1;N 1gN 1( )
= 1 +ej ej +ej2 ej2 + +ej(N 1) ej(N 1)
f2( ) = !2;0g0( ) +!2;1g1( ) +!2;2g2( ) + +!2;N 1gN 1( )
= 1 +ej2 ej +ej4 ej2 + +ej2(N 1) ej(N 1)
...
fN 1( ) = !N 1;0g0( ) +!N 1;1g1( ) +!N 1;2g2( ) + +!N 1;N 1gN 1( )
= 1 +ej(N 1) ej + +ej(N 1)(N 1) ej(N 1) (2.7)
In order to calculate the needed weights, a matrix equation can be set up by equating
the equal powers of ej of the high-gain patterns and basis patterns. For example,
for the fundamental pattern f0( ), an equation can be set up to nd the coe cients
!0;n
A
2
66
66
66
66
66
4
!0;0
!0;1
!0;2
...
!0;N 1
3
77
77
77
77
77
5
=
2
66
66
66
66
66
4
1
1
1
...
1
3
77
77
77
77
77
5
(2.8)
16
where the matrix A is de ned in (2.9).
A =
2
66
66
66
66
66
4
1 1 1 1
ej 1 ej( 1+ ) ej( 1+2 ) ej( 1+(N 1) )
ej 2 ej( 1+2 ) ej( 2+4 ) ej( 1+(N 1) )
... ... ...
ej N 1 ej( N 1+(N 1) ) ej( N 1+2(N 1) ) ej( N 1+(N 1)(N 1) )
3
77
77
77
77
77
5
(2.9)
17
Similar matrix equations can be set up relating each of the scanned beams to the
set of basis patterns, resulting in a total of N scanned beams
A
2
66
66
66
66
66
4
!1;0
!1;1
!1;2
...
!1;N 1
3
77
77
77
77
77
5
=
2
66
66
66
66
66
4
1
ej
ej2
...
ej(N 1)
3
77
77
77
77
77
5
A
2
66
66
66
66
66
4
!2;0
!2;1
!2;2
...
!2;N 1
3
77
77
77
77
77
5
=
2
66
66
66
66
66
4
1
ej2
ej4
...
ej2(N 1)
3
77
77
77
77
77
5
...
A
2
66
66
66
66
66
4
!N 1;0
!N 1;1
!N 1;2
...
!N 1;N 1
3
77
77
77
77
77
5
=
2
66
66
66
66
66
4
1
ej(N 1)
ej2(N 1)
...
ej(N 1)(N 1)
3
77
77
77
77
77
5
(2.10)
18
These equations can then be combined into a single matrix equation to solve for
all the coe cients simultaneously
2
66
66
66
66
66
4
!0;0 !1;0 !2;0 !N 1;0
!0;1 !1;1 !2;1 !N 1;1
!0;2 !1;2 !2;2 !N 1;2
... ... ...
!0;N 1 !1;N 1 !2;N 1 !N 1;N 1
3
77
77
77
77
77
5
= A 1
2
66
66
66
66
66
4
1 1 1 1
1 ej ) ej2 ej(N 1)
1 ej2 ) ej4 ej2(N 1)
... ... ...
1 ej(N 1) ) ej2(N 1) ej(N 1)(N 1)
3
77
77
77
77
77
5
(2.11)
The complex coe cient weights found allow high-gain patterns to be formed by linear
combinations of the N spoiled beams. Once these weights have been calculated, all
N of the steered high-gain patterns can be synthesized at once. Figure 2.5 shows the
reconstructed high-gain patterns steered to 0 deg, +30 deg, and -30 deg.
19
Figure 2.5: Scanned Patterns
20
Chapter 3
Expansion of the 2D Method Into 3D
Although the method developed in the previous chapter has obvious bene ts for
designing a LPI method of beamforming, a one-dimensional antenna array design,
which can only point in one plane, is not practical for a real world radar system.
Therefore, it is bene cial to explore whether this technique can be expanded into
three dimensions. In order to create a far- eld array pattern that is scannable in
both directions, a two-dimensional antenna array must be used instead of the original
one-dimensional array. The analysis for the two-dimensional array follows the same
theory as for the one-dimensional array; however, each element now has radiation
components in both the x and y planes, represented by and , respectively. Similarly
to the case of the one-dimensional array, we can make the substitutions n = dsin( )
and m = dsin( ). Throughout this theoretical development, n represents the rows
of the array of N elements, while m represents the columns of the array of M elements.
If the range 90 90 and 90 90 is considered, then both n and
m will vary from j d to j d, with the step size determining the precision of the
array pattern. This antenna con guration and range of angles allows the creation of
a far- eld array pattern that can be scanned to any point over a the hemisphere in
front of the array.
21
3.1 Theoretical Calculations
Allowing for changes in both and , the fundamental array pattern of a NxM
two-dimensional array can be written as
f0( ; ) = 1 +ej dsin( )ej dsin( ) +ej2 dsin( )ej2 dsin( )+
:::+ej(N 1) dsin( )ej(N 1) dsin( )
=
N 1X
n=0
M 1X
m=0
ejn nejm m
(3.1)
As in the case of the one-dimensional antenna array, a linear phase progression can
be applied across the array to steer the main lobe. If this phase scan is selected to be
= 2 =N,the a total of NM scanned beams can be formed, with N beam pointing
angles in and M beam pointing angles in . It should be noted that this selection of
the phase scan is made for convenience in showing the equations involved in developing
the technique. The value of , and thus the scanning angle increment, can be changed
with only minor modi cations to the equations developed below. The linear phase
progression can be applied across the elements of the two-dimensional array as follows:
First, an increasing phase progression of will be applied incrementally across the
rows of the array only, resulting in N beam formations that vary only in . A phase of
will then be applied across the columns of the array, and the same increasing phase
progression will be applied across the rows to create another N beams formations.
This process will be repeated M times until a total of NM beam formations have
22
been created. The set of scanned beams can be represented as
f1( n; m) =
N 1X
n=0
M 1X
m=0
ejn( n+ )ejm m
f2( n; m) =
N 1X
n=0
M 1X
m=0
ejn( n+2 )ejm m
f3( n; m) =
N 1X
n=0
M 1X
m=0
ejn( n+3 )ejm m
...
fN 1( n; m) =
N 1X
n=0
M 1X
m=0
ejn( n+(N 1) )ejm m
fN( n; m) =
N 1X
n=0
M 1X
m=0
ejn nejm( m+ )
fN+1( n; m) =
N 1X
n=0
M 1X
m=0
ejn( n+ )ejm( m+ )
...
fN2 1( n; m) =
N 1X
n=0
M 1X
m=0
ejn( n+(N 1) )ejm( m+(N 1) ) (3.2)
Now, the same assumption from the one-dimensional case is made: that the above
high-gain patterns can be created from a weighted combination of low-gain, spoiled
basis patterns. These basis patterns are similar to the basis patterns developed for the
two-dimensional array, except that now they must account for changes in the x and
y planes. Also, a new set of phase shifts must be developed for the two-dimensional
array. Again, each element will have a certain phase shift n;m applied to it, with the
ultimate goal of creating a defocused, spoiled far- eld pattern. The selection of the
values of is discussed further in Section 3.2. The fundamental basis pattern can be
23
written as
g0( ; ) =1 +ej 0;1ej dsin( )ej dsin( ) +ej 0;2ej2 dsin( )ej2 dsin( ) +:::
+ej N 1;N 1ej(N 1) dsin( )ej(N 1) dsin( )
=
N 1X
n=0
M 1X
m=0
ej n;mejn nejm m
(3.3)
Again, it should be noted that the only di erence between this pattern and the
fundamental array pattern from (3.2) is the extra phase shift n;m, which serves to
spoil the high gain beam pattern. As in the case of the high-gain array patterns, a set
of N2 low-gain basis patterns can be formed by applying a linear phase progression
across the elements of the array. The application of follows the same pattern
used for the high-gain patterns: incrementing the phase progression applied across
24
the rows, and then the columns, as observed in the following
g1( n; m) =
N 1X
n=0
M 1X
m=0
ej 0;1ejn( n+ )ejm m
g2( n; m) =
N 1X
n=0
M 1X
m=0
ej 0;2ejn( n+2 )ejm m
g3( n; m) =
N 1X
n=0
M 1X
m=0
ej 0;3ejn( n+3 )ejm m
...
gM 1( n; m) =
N 1X
n=0
M 1X
m=0
ej 0;Mejn( n+(N 1) )ejm m
gM( n; m) =
N 1X
n=0
M 1X
m=0
ej 0;Mejn nejm( m+ )
gM+1( n; m) =
N 1X
n=0
M 1X
m=0
ej 0;Mejn( n+ )ejm( m+ )
...
gN2 1( n; m) =
N 1X
n=0
M 1X
m=0
ej N 1;N 1ejn( n+(N 1) )ejm m+(N 1) (3.4)
The equations for both the high-gain patterns and the low-gain patterns can be
reduced to
fi( n; m) =
N 1X
n=0
M 1X
m=0
ejn( n+a )ejm( m+b ) (3.5)
gi( n; m) =
N 1X
n=0
M 1X
m=0
ej m;nejn( n+a )ejm( m+b ) (3.6)
25
where
i = Na+b+ 1
0 a N 1
0 b M 1
As before, the fundamental array pattern can be shown as a weighted combina-
tion of all N2 basis patterns
f0( ; ) = !0;0g0( ; ) +!0;1g1( ; ) + +!0;N2 1gN2 1( ; )
f1( ; ) = !1;0g0( ; ) +!1;1g1( ; ) + +!1;N2 1gN2 1( ; )
...
fN2 1( ; ) = !N2 1;0g0( ; ) +!N2 1;1g1( ; ) + +!N2 1;N2 1gN2 1( ; ) (3.7)
Expanding the high-gain pattern and the basis patterns and equating equal powers of
ej n and ej m allow the following matrix equation to be constructed for the coe cients
required to recreate the fundamental array pattern f0
A
2
66
66
66
66
66
4
!0;0
!0;1
!0;2
...
!0;N2 1
3
77
77
77
77
77
5
=
2
66
66
66
66
66
4
1
1
1
...
1
3
77
77
77
77
77
5
(3.8)
where A is de ned in (3.9). The phase shifts applied to the rows and columns in
this equation are represented by n and m, respectively. In this research, the phase
applied across the rows and the columns are considered to be equal, i.e. n = m;
26
however, the separate phases are shown in (3.9) to aid in the explanation of the
construction of the matrix A.
27
A
=
2 6666666666666666666666664
ej
0;
0
ej
0;
0
ej
0;
0
ej
0;
0
ej
0;
1
ej(
0
;1
+
m
)
ej(
0
;1
+2
m
)
ej(
0
;1
+(
N
1)
m
)
ej
0;
2
ej(
0
;2
+2
m
)
ej(
0
;2
+4
m
)
ej(
0
;2
+2(
N
1)
m
)
...
...
ej
0;N
1
ej(
0
;N
1+(
N
1)
m
)
ej(
0
;N
1+2(
N
1)
m
)
ej(
0
;N
1+(
N
1)(
N
1)
m
)
ej
1;
0
ej
1;
0
ej
1;
0
ej(
1
;0
+(
N
1)
n
)
ej
1;
1
ej(
1
;1
+
m
)
ej(
1
;1
+2
m
)
ej(
1
;1
+(
N
1)
n
+(
N
1)
m
)
...
...
ej
N
1;N
1
ej(
N
1;N
1+(
N
1)
)
ej(
N
1;N
1+2(
N
1)
)
ej(
N
1;N
1+(
N
1)(
N
1)
n
+(
N
1)(
N
1)
m
)3 7777777777777777777777775
(3.9)
28
This process can be repeated to form a total of N2 matrix equations
A
2
66
66
66
66
66
4
!1;0
!1;1
!1;2
...
!1;N2 1
3
77
77
77
77
77
5
=
2
66
66
66
66
66
4
1
ej
ej2
...
ej(N 1)
3
77
77
77
77
77
5
A
2
66
66
66
66
66
4
!2;0
!2;1
!2;2
...
!2;N 1
3
77
77
77
77
77
5
=
2
66
66
66
66
66
4
1
ej2
ej4
...
ej2(N 1)
3
77
77
77
77
77
5
...
A
2
66
66
66
66
66
4
!N 1;0
!N 1;1
!N 1;2
...
!N 1;N 1
3
77
77
77
77
77
5
=
2
66
66
66
66
66
4
1
ej(N 1)
ej2(N 1)
...
ej2(N 1)(N 1)
3
77
77
77
77
77
5
(3.10)
29
The above matrix equations can be combined into a single N2xN2 matrix equation
to solve for all equations simultaneously.
2
66
66
66
66
66
4
!0;0 !1;0 !2;0 !(N 1)2;0
!0;1 !1;1 !2;1 !(N 1)2;1
!0;2 !1;2 !2;2 !(N 1)2;2
... ... ...
!0;(N 1)2 !1;(N 1)2 !2;(N 1)2 !(N 1)2;(N 1)2
3
77
77
77
77
77
5
= A 1
2
66
66
66
66
66
4
1 1 1 1
1 ej ej2 ej(N 1)
1 ej2 ej4 ej2(N 1)
... ... ...
1 ej(N 1) ej2(N 1) ej2(N 1)(N 1)
3
77
77
77
77
77
5
(3.11)
As discussed in the analysis of the two-dimensional case, once the complex co-
e cients have been calculated, all N2 steered patterns can be synthesized at once.
Because the complex coe cients can be calculated prior to scanning, the performance
of the radar system will not be dependent upon the computation time required to
nd these values.
30
Figure 3.1: 2-Dimensional quadratic phase shift values
3.2 Calculation of Phase Shift Values
Before the theoretical equations developed in the previous section can be tested,
phase shift values must be chosen to create the low gain basis patterns. As seen
in Figure 2.2, the author of [22] chose a quadratic phase shift applied across the
array. This served to defocus the beam and reduce the gain of the array. For the
two-dimensional array, the quadratic pattern of the phase shift was reused, only
transformed into a two dimensional pattern, as shown in Figure 3.1. First, one quarter
of a two-dimensional quadratic was created with the equation
a = scale ( 3N2)2 (2 t)2 (3.12)
31
Figure 3.2: Alpha values used to create basis patterns for the two-dimensional array
which was then expanded to form the full quadratic. This pattern was then used to
create an array pattern of the following form
f( n; m) =
NX
n=0
MX
m=0
ej n;mejn nejm m (3.13)
and the maximum gain of the pattern was calculated. A simulated annealing algo-
rithm was then used to minimize this gain by manipulating the scalar scale of the
equation. Once the optimal scale was found, the simulated annealing algorithm was
again used to manipulate the individual values in order to reduce the gain as much
as possible. A tolerance of 1 for each point was used to reduce computation time.
An example of the optimized phase shift values found is shown in Figure 3.2.
32
Figure 3.3: Fundamental basis pattern for an 8x8 array pattern
The resulting phase shift values created a low-gain, "spoiled" beam, as shown in
Figure 3.3 for an 8x8 antenna array. The maximum gain of this spoiled pattern is ap-
proximately 4.7 dB, while the gain of the main lobe of the pattern without the applied
phase shift is approximately 18 dB. Thus, it can be veri ed that by adding a certain
series of phase shifts to an array pattern, such as the two-dimensional quadratic used
above, the gain of the main beam of a pattern can be reduced signi cantly.
33
Chapter 4
Simulation Results
The MATLAB programming environment was used for all the following simu-
lations, due to its advantages in handling the large matrix calculations needed in
the discussed beamforming technique. As discussed in the development of the the-
ory behind the technique, the bulk of the computational resources are needed only
in calculating the complex coe cient weights used in the combination of the basis
patterns. Once these coe cients are calculated, the weights can be applied to the
returns of each individual basis function to form an equivalent high-gain beam.
4.1 Simulation Procedure
As discussed in Chapter 3, the eld of view taken into consideration is de ned by
the range 90 90 and 90 90 . The step size of and control the
precision of the simulated far- eld patterns. From the previous chapters, it is apparent
that the calculation of the complex weights is not dependent upon and ; therefore,
the step size is only relevant for the simulation results. Because the ranges of and
are constant, the values of n = dsin( ) and m = dsin( ) remain constant. Two
arrays representing n and m were created and used throughout the simulations. For
these simulations, the spacing between elements, d, in both the rows and the columns
of the array was set to =2 so that n = sin( ) and m = sin( ). Because the
arrays representing n and m remain constant throughout the development of the
complex weights, the element spacing, as well as the frequency of the transmitted
signal, can be changed without a ecting the complexity of the computations.
34
The plots shown in the following sections are the general normalized power pat-
terns of the array, expressed as [23]
P( ; ) =jF( ; )j2
whereFi( ; ) has been de ned in previous chapters asf0;f1;etc... This power pattern
is further divided by the number of elements, N in the case of the one-dimensional
array and N2 in the case of the two-dimensional array, in order to normalize the
patterns with regards to the number of elements in the array.
35
Figure 4.1: Fundamental array pattern for an 8x8 element array
4.2 Simulation Results for an 8x8 Element Array
The simulations were rst run for an 8x8 element antenna array. As expected,
the fundamental array pattern f0, shown in Figure 4.1, demonstrates a high gain main
lobe directed broadside to the array, as well as reduced sidelobes along the x and y
axes. Figure 4.2 shows the pattern in the XZ plane, allowing clearer distinction of
the main beam and side lobes. The fundamental basis pattern g0 for the 8x8 array
can be seen in Figure 4.3. As desired, this "spoiled" pattern exhibits lower, more
uniform gain than the fundamental pattern. The peak gain of f0 is approximately 18
dB, while the peak gain of g0 is approximately 5 dB. This gain di erence of 13 dB
corresponds to an approximately 95% reduction in the intercept area.
Following the procedures developed in Chapter 3, a set of 8 8 = 64 spoiled
patterns were created, all possessing low, semi-uniform gain similar to Figure 4.3.
Using these patterns, a set of complex coe cients were calculated to allow assembly of
the scanned array patterns from a weighted summation of the 64 basis patterns. The
recreated fundamental array, with the values = 0 and = 0 , is shown in Figure 4.4.
36
Figure 4.2: Fundamental array pattern for an 8x8 element array - XZ Plane
Figure 4.3: Fundamental basis pattern for an 8x8 element array - XZ Plane
37
Figure 4.4: Recreated fundamental array pattern for an 8x8 element array
The recreated pattern appears to be virtually identical to the original fundamental
pattern, and further analysis in MATLAB shows that the average di erence between
the two patterns is approximately 8 10 8 dB.
Similarly, each of the remaining 64 scanned array patterns can be recreated
by the appropriate weighted summation of the basis patterns. After the complex
coe cient values have been calculated, all of the scanned patterns can be constructed
simultaneously. An example of one of the scanned patterns, with the main beam
pointed at = 15 and = 15 can be seen in Figure 4.5. Each of the reconstructed
patterns has the same beamwidth and gain of this pattern, the only di erence being
the pointing angle of the main beam.
38
Figure 4.5: Recreated array pattern for an 8x8 element array with = 15 and
= 15
4.3 Simulation Results for a 32x32 Element Array
Next, the simulations were run for a 32x32 element array. The fundamental
array pattern of this array, when = 0 and = 0 , is shown in Figures 4.6 and
4.7. Again, this pattern demonstrates a high gain main lobe directed broadside to
the array and sidelobes along the x and y axes. As before, a set of 32 32 = 1024
spoiled basis patterns were created. The fundamental basis pattern, when = 0 and
= 0 , is shown in Figure 4.8. The peak gain of the fundamental array pattern f0
is approximately 30 dB, while the peak gain of the fundamental spoiled pattern is
approximately 16 dB. This gain di erence of 14 dB corresponds to an approximately
96% reduction in the intercept area.
Following the theory developed in Chapter 3, a set of complex coe cients were
calculated to allow creation of the scanned array patterns from a weighted summation
of these spoiled beams. The recreated fundamental array, once again with = 0 and
= 0 , is shown in Figure 4.9. Again, the recreated pattern appears to be virtually
39
Figure 4.6: Fundamental array pattern for a 32x32 element array
Figure 4.7: Fundamental array pattern for a 32x32 element array - XZ Plane
40
Figure 4.8: Fundamental basis pattern for a 32x32 element array
identical to the original fundamental pattern, with an average di erence between the
two patterns of approximately 5 10 15 dB. After the complex weights have been
calculated, any scanned pattern can be recreated from the spoiled patterns. Figures
4.10, 4.11, and 4.12 show a recreated beam with scan angles of = 26 and = 44 .
As in the case of the 8x8 array in the previous section, all of the reconstructed patterns
have the same bandwidth and gain as the original pattern.
41
Figure 4.9: Recreated fundamental array pattern for a 32x32 element array
Figure 4.10: Recreated array pattern for a 32x32 element array with = 26 and
= 44
42
Figure 4.11: Recreated array pattern for a 32x32 element array with = 26 and
= 44 - XZ Plane
Figure 4.12: Recreated array pattern for a 32x32 element array with = 26 and
= 44 - YZ Plane
43
Chapter 5
Implementation
5.1 Implementation Into an Existing Radar System
It has been shown that the high gain resulting from a traditional scanned main
beam can be greatly reduced by using the method developed in Chapters 2 and 3.
This technique involves replacing the high-gain beam resulting from a linear array
with a weighted summation of a set of low gain, spoiled beams. In Chapter 4, it
was shown that the beam patterns constructed from the superposition of these low
gain basis patterns result in a far eld pattern of similar shape and gain to that
of the original high-gain pattern. As noted in the original development of the one-
dimensional array in [22], this technique e ectively trades transient peak power with
sustained low power on the target over the search region, resulting in the same amount
of total energy on the target.
In order to implement this theoretical approach into a practical radar system,
the new beamforming technique must be integrated with the existing radar wave-
form. Many di erent waveforms can be used for di erent radar systems; however,
the integration of this technique can most easily be observed with a standard pulsed
waveform.
First, consider operation of a traditional waveform with a high-gain main lobe
that is scanned across the search region. The beam is scanned by applying a linear
phase progression across the elements of the array. If the phase-shifter settings of the
system are designed so that the phase progression is increased with each pulse, then
each pulse corresponds to a particular location of the scanned main beam. Likewise,
if the phase shift values designed to form the low-gain patterns are applied across the
44
array, then each pulse corresponds to a particular low-gain basis pattern, i.e. pulse
#1 for g0, pulse #2 for g1, etc... The returns of each pulse can then be processed
through a matched lter and stored in memory. After all N pulses, and thus all N
basis patterns, have been transmitted and received, the precalculated complex weights
can be applied across the samples and summed. Each set of weights will result in
the equivalent range return of a single high-gain main beam with the same phase
progression. Once returns from all N basis patterns have been stored, any of the
equivalent high-gain patterns can be formed simultaneously, requiring no additional
scanning time when compared to the traditional method.
5.2 Two-way Analysis
In the theory developed in this paper, it is assumed that separate antenna arrays
are used to transmit and receive, i.e., the pattern created from the antenna array is
only a ected on transmit. This is referred to as one-way synthesis. In developing the
pattern theory for a one-dimensional array in [22], the author also considers the case
where the same antenna array is used for both transmit and receive. In this scenario,
the one-way pattern developed in Chapter 2 is not su cient to fully describe the
pattern seen by the receiver, as the target return must now be scaled by the square
of the pattern. The synthesis of the resulting patterns is more complex than with
separate antennas, and is accomplished by a linear combination of two-way basis
patterns.
The analysis of the two-way pattern synthesis follows the same procedure as
that for the one-way synthesis shown in Chapter 2; however, in each step the squared
version of the patterns must be used. For example, for the case of the one-dimensional
antenna array, instead of developing the expressions for fn( ) and gn( ), the analysis
must now develop expressions for f2n( ) and g2n( ), respectively. Following this logic,
45
the weighted summation of the two-way basis patterns can now be written as
f2n( ) = !n;0g20( ) +!n;1g21( ) + +!n;2N 2g22N 2( ) (5.1)
Because the squared versions of the patterns are used, 2N 1 scanned patterns will
be created, as opposed to the N scanned patterns created for one-way synthesis. It
is shown in [22] that, in the context of the developed technique, two-way synthesis
has comparable results to one-way synthesis. That is, each of the 2N 1 scanned
patterns can be recreated from a weighted summation of low gain spoiled patterns.
When considering the two-way synthesis for a two-dimensional antenna array,
the complexity introduced by the squaring of the patterns increases the computa-
tional requirements greatly. When analyzing an array with NxN elements, one-way
synthesis will result in N2 basis patterns, requiring N4 complex coe cients for the
superposition of the spoiled patterns. Two way synthesis of this array would result
in 2N2 1 basis patterns, requiring 2N4 1 complex coe cients. The mathematical
complexity required to develop the matrix equations needed to solve for the complex
coe cients is increased substantially. However, as in the case of one-way synthesis, all
of these calculations can be completed prior to scanning. As a result, two-way synthe-
sis would require no additional scanning time when compared to one-way synthesis,
or to the traditional scanning method.
5.3 Computational Limitations
The computational complexity required to develop the needed set of spoiled
basis functions and complex coe cients can vary greatly. With larger array sizes or
smaller step sizes comes a greater required processing power. From the theoretical
development shown in Chapter 3, it is evident that the creation of the set of spoiled
patterns, as well as the calculation of the complex coe cients, is entirely dependent
46
upon the dimensions of the array and step size. Thus, all of the complex coe cient
weights needed to accurately form an equivalent scanned pattern from the set of basis
patterns can be calculated independently of the actual operation of the radar. This
means that the radar itself is not responsible for any of the computationally intensive
matrix calculations, but only for applying the previously calculated weights to the
stored return information.
5.4 Hardware Requirements
As mentioned before, the low-gain basis patterns that need to be transmitted
are simply the high-gain patterns of a traditional system spoiled by a certain phase
shift. As a result, no additional hardware would have to be added to an existing
array in order to transmit these beams; another phase scan would simply be added to
each element before transmitting. The only other hardware needed to implement this
technique would be a means of storing the returns of each of the N transmitted basis
patterns, as well as the hardware necessary to carry out the weighted summation.
5.5 Doppler
It should be noted that, due to the importance of the phase relationship between
basis patterns, a target must remain coherent over the scan time of the radar. If the
target does not remain coherent, as would be likely in the case of long scan times,
motion compensation may be required to allow for the target dynamics. This extra
processing is a factor that must be considered when integrating this LPI technique
with an existing radar system.
47
5.6 Areas for Future Research
There are several areas in which this research could be continued and expanded.
First, it would be bene cial to work through the calculations to determine the matrix
equations required to fully analyze the two-way synthesis pattern, as this is a scenario
that is likely to occur in practical radar system. Second, it is possible that the gain of
the basis beams could be reduced even further with continued research into nding the
optimal phase shift values used to spoil the beams. Third, it would also be bene cial
to explore integrating this method with the countless other waveforms used in radar
systems for various objectives.
This paper has been focused primarily on the mathematics and theory of this
technique. A great amount of research could be devoted to the integration of this
method into the hardware of an existing radar system. Tests of actual radars imple-
menting this technique need to be performed to verify the theory developed here.
48
Chapter 6
Conclusion
In [22], a method of improving the LPI performance of a linear antenna array was
developed. This method involves replacing the high-gain main beam of a traditional
scanning radar system with a set of low-gain, spoiled beams. These beams, which
are simply the high-gain patterns spoiled by a certain phase shift, can be summed
together to create returns equivalent to that of the traditional system. In this paper,
the method was expanded from the case of a one-dimensional array to that of a two-
dimensional array. This transition increases the complexity of the method, as the
variations in the beam pattern must now be considered in both the x and y planes,
or and , respectively.
After completing the required matrix calculations, simulations were run for both
an 8x8 element array and a 32x32 element array. In the simulations of the 8x8 array,
the peak gain of the main beam for the fundamental array pattern, when = 0 and
= 0 , was found to be approximately 18 dB. The peak gain of the fundamental
basis pattern was found to be approximately 5 dB. This lower gain of the transmitted
signal reduces the detection range of a hostile ESM system by a factor of 1=p20,
which corresponds to a 95% reduction in the intercept area of any potential hostile
ESM systems.
In the simulations of the 32x32 element array, the peak gain of the main beam
for the fundamental array pattern was found to be approximately 30 dB. The peak
gain of the spoiled patterns was found to be approximately 16 dB. Again, this lower
gain of the transmitted signal reduces the detection range of hostile ESM system by
a factor of 1=p25, corresponding to a 96% decrease in the intercept area.
49
In both of these cases, complex coe cients were calculated and and applied across
the basis patterns. It was shown that a weighted summation of the complete set of
spoiled patterns resulted in a return equivalent to that of the unspoiled pattern. When
compared to the original fundamental pattern, the recreated pattern at = 0 and
= 0 was found to di er by a negligible amount. The other weighted combinations
of the basis patterns were also shown to provide returns equivalent to the high-gain
patterns they replaced.
These results verify the claim made in [22]: that the high gain of single scanned
main beam can be reduced by instead transmitting a set of spoiled beams, e ectively
replacing the transient high-power sweep with low power patterns radiated persis-
tently while maintaining the same amount of energy on the target. The results for
both the 8x8 and 32x32 element arrays show a signi cant decrease in intercept range,
an advantage that could provide an existing system with obvious LPI performance
increases. Although these improvements come at the cost of increased memory re-
quirements and extra processing power, the technique has been shown to o er a
promising method to reduce the visibility, and thus the probability of detection, of a
radar attempting to avoid hostile ESM systems.
50
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52
Appendices
53
Appendix A
MATLAB Code
A.1 Optimize phase scan 2D.m
1 % Optimizes the phase shift values used to spoil
2 % the array pattern
3
4 clear all
5 close all
6 clc
7
8 %% Optimize scale
9
10 MinimizeFunction = @minimizeGain 2D scale;
11
12 scale = 0.5;
13
14 tic
15 [optimizedScale,gain1,exitFlag1,output] = ...
simulannealbnd(MinimizeFunction, scale, 0, 1);
16 toc
17
18 %% Optimize individual values
19
20 MinimizeFunction = @minimizeGain 2D;
21
22 M = 32;
23 N = 32;
24 t = [N/2: 1:1]' * [N/2: 1:1];
25
26 x0 = optimizedScale * (3/N?2)?2 * (2*pi*t).?2;
27
28 for row = 1:length(x0)
29 for col = 1:length(x0)
30 lb(row,col) = x0(row,col) 2;
31 ub(row,col) = x0(row,col) + 2;
32 end
33 end
34
35 [x,gain2,exitFlag2] = simulannealbnd(MinimizeFunction, x0, lb, ub);
36
37 alphas = x;
38 alphas = [alphas fliplr(alphas)];
39 alphas = [alphas; flipud(alphas)];
54
A.2 minimizeGain 2D scale.m
1 % Minimizes gain of spoiled pattern by optimizing scale
2
3 function G = minimizeGain 2D scale(scale)
4
5 M = 8;
6 N = 8;
7 t = [N/2: 1:1]' * [M/2: 1:1];
8
9 a = scale * (3/N?2)?2 * (2*pi*t).?2;
10
11 a = [a fliplr(a)];
12 a = [a; flipud(a)];
13
14 tN = 0:.1:pi;
15 tM = 0:.1:pi;
16 psiN = zeros(length(tN),length(tM));
17 psiM = zeros(length(tN),length(tM));
18
19 for timeIndexN = 1:length(tN)
20 for timeIndexM = 1:length(tM)
21 psiN(timeIndexN,timeIndexM) = pi*cos(tN(timeIndexN));
22 psiM(timeIndexN,timeIndexM) = pi*cos(tM(timeIndexM));
23 end
24 end
25
26 f = 0;
27 for indN = 1:N
28 for indM = 1:M
29 f = f + exp(1i*(a(indM,indN) + ((indN 1)*psiN + ...
(indM 1)*psiM)));
30 end
31 end
32
33 G = max(max(abs(f)))?2/(N?2);
55
A.3 minimizeGain 2D.m
1 % Minimizes gain of spoiled pattern by optimizing
2 % individual alpha values
3
4 function G = minimizeGain 2D(a0)
5
6 a = [a0 fliplr(a0)];
7 a = [a; flipud(a)];
8
9 N = length(a);
10 M = N;
11
12 tN = 0:.1:pi;
13 tM = 0:.1:pi;
14
15 psiN = zeros(length(tN),length(tM));
16 psiM = zeros(length(tN),length(tM));
17
18 for timeIndexN = 1:length(tN)
19 for timeIndexM = 1:length(tM)
20 psiN(timeIndexN,timeIndexM) = pi*cos(tN(timeIndexN));
21 psiM(timeIndexN,timeIndexM) = pi*cos(tM(timeIndexM));
22 end
23 end
24
25 f = 0;
26 for indN = 1:N
27 for indM = 1:M
28 f = f + exp(1i*(a(indM,indN) + ((indN 1)*psiN + ...
(indM 1)*psiM)));
29 end
30 end
31
32 G = max(max(abs(f)))?2/(N?2);
56
A.4 Beamer 2D.m
1 clear all
2 close all
3 clc
4
5 %% Setup
6
7 load('8x8 alphas.mat','alphas');
8
9 N = size(alphas,1);
10 M = size(alphas,2);
11
12 tN = 0:.01:pi;
13 tM = 0:.01:pi;
14
15 r2d = 180 / pi;
16
17 %% Create plots of the fundamental patterns
18
19 psiN = zeros(length(tN),length(tM));
20 psiM = zeros(length(tN),length(tM));
21
22 for timeIndexN = 1:length(tN)
23 for timeIndexM = 1:length(tM)
24 psiN(timeIndexN,timeIndexM) = pi*cos(tN(timeIndexN));
25 psiM(timeIndexN,timeIndexM) = pi*cos(tM(timeIndexM));
26 end
27 end
28
29 f0 = 0;
30 g0 = 0;
31 for indN = 1:N
32 for indM = 1:M
33 f0 = f0 + exp(1i*((indN 1)*psiN + (indM 1)*psiM));
34 g0 = g0 + exp(1i*(alphas(indN,indM) + (indN 1)*psiN + ...
(indM 1)*psiM));
35 end
36 end
37
38 f0 max = 10*log10(max(max(abs(f0)))?2/(N?2));
39 g0 max = 10*log10(max(max(abs(g0)))?2/(N?2));
40
41 g0 plot = 10*log10(abs(g0).?2/(N?2));
42 g0 plot(g0 plot 25) = 25;
43
44 f0 plot = 10*log10(abs(f0).?2/N?2);
45 f0 plot(f0 plot 25) = 25;
46
47 mesh(r2d*tN 90,r2d*tM 90,f0 plot);
48 axis([ 90 90 90 90 25 35]);
49 zlabel('Gain (dB)'); xlabel('ntheta'); ylabel('nphi');
57
50 set(gca,'Xtick',[ 90 60 30 0 30 60 90],'Ytick',[ 90 60 30 0 30 ...
60 90])
51
52 figure
53 mesh(r2d*tN 90,r2d*tM 90,g0 plot);
54 axis([ 90 90 90 90 25 35]);
55 zlabel('Gain (dB)'); xlabel('ntheta'); ylabel('nphi');
56 set(gca,'Xtick',[ 90 60 30 0 30 60 90],'Ytick',[ 90 60 30 0 30 ...
60 90])
57
58 %% Create all basis patterns
59
60 pscan = 2*pi/N;
61
62 g = cell(1,N?2);
63 for index = 1:N?2
64 gfindexg = zeros(size(psiN));
65 end
66
67 for a = 0:N 1
68 for b = 0:M 1
69 index = N*a + b + 1;
70 for n = 0:N 1
71 for m = 0:M 1
72 gfindexg = gfindexg + exp(1i*(alphas(n+1,m+1) + ...
n*(psiN + a*pscan) + m*(psiM + b*pscan)));
73 end
74 end
75 end
76 end
77
78 %% Calculate complex coefficent weights
79
80 A = zeros(N?2);
81 B = zeros(N?2);
82
83 for n = 0:N 1
84 for m = 0:M 1
85 row = N*n + m + 1;
86 for a = 0:N 1
87 for b = 0:M 1
88 col = N*a + b + 1;
89 A(row,col) = exp(1i*(alphas(n+1,m+1) + n*a*pscan + ...
m*b*pscan));
90 B(row,col) = exp(1i*(n*a*pscan + m*b*pscan));
91 end
92 end
93 end
94 end
95
96 coefficients = AnB;
97
98 %% Create f0
99
58
100 f0 new = zeros(size(psiN));
101 for index = 1:N?2
102 f0 new = f0 new + coefficients(index,1)*gfindexg;
103 end
104
105 f0 new max = 10*log10(max(max(abs(f0 new)))?2/(N?2));
106
107 difference = f0 new f0;
108 ave diff = mean(mean(abs(difference)));
109
110 f0new plot = 10*log10(abs(f0).?2/N?2);
111 f0new plot(f0new plot 25) = 25;
112
113 figure
114 mesh(r2d*tN 90,r2d*tM 90,f0new plot);
115 axis([ 90 90 90 90 25 35]);
116 zlabel('Gain (dB)'); xlabel('ntheta'); ylabel('nphi');
117 set(gca,'Xtick',[ 90 60 30 0 30 60 90],'Ytick',[ 90 60 30 0 30 ...
60 90])
118
119 %% Create all fundamental array patterns
120
121 f = cell(1,N?2);
122 for index = 1:N?2
123 ffindexg = zeros(size(psiN));
124 end
125
126 for indexF = 1:N?2
127 for indexG = 1:N?2
128 ffindexFg = ffindexFg + coefficients(indexG,indexF)*gfindexGg;
129 end
130 end
131
132 for k=1:length(f)
133 f plotfkg = 10*log10(abs(ffkg).?2/N?2);
134 f plotfkg(f plotfkg 25) = 25;
135 end
136
137 figure
138 mesh(r2d*tN 90,r2d*tM 90,f plot);
139 axis([ 90 90 90 90 25 35]);
140 zlabel('Gain (dB)'); xlabel('ntheta'); ylabel('nphi');
141 set(gca,'Xtick',[ 90 60 30 0 30 60 90],'Ytick',[ 90 60 30 0 30 ...
60 90])
59