Radar was developed for military use by several nations
before and during the period of World War II. It is a system used to detect the
presence, direction, speed of aircrafts, ships and other objects by sending out
pulses of radio waves which reflect off an object and return to the receiver,
providing information such as the speed, movement and location of an object. It
uses a transmitter which produces electromagnetic waves in the form of radio or
microwaves domain, transmitting through an antenna to a receiver and processor
to determine the properties of an object.
Modern usage of radar is applicable for a wide range of
diverse options. It can be used for ocean surveillance systems, locating
landmarks, controlling and guiding of weapon, distinguishing one class of
target from another and assist in navigating the ships. In today’s technology,
a successful and effective radar must be able to perform their function, ‘to see without being seen’.1
Low Probability of Intercept (LPI) radars address the need by making them
difficult to be detected and characterized by intercept receivers. It focuses on the ability
to defeat all the external threats that may lead to a precise identification of
the systems. This can be achieved by using wide operational
bandwidth, frequency hopping, minimum power management and using a
frequency-modulated continuous-wave signal. Pulse compression also reduces the
probability of detection. It maintains the range and resolution of the radar while
reduces the peak transmitted power2.
In this essay, the techniques used by LPI radars, pros and
cons of LPI radars and the future trend of this technology on military systems
will be discussed.
Types of Techniques
used by LPI Radars
There are several LPI radar techniques available in modern
technology. It may come in various combinations depending on the types of
application that is required. The most common design of LPI radar technique is
pulse compression signal processing technique. This can be achieved by
modulating the frequency, phase shift keying and frequency shift keying
Most of the LPI radars uses Frequency Modulation Continuous
Wave (FMCW) Radar which is a frequency modulation, pulse compression technique.
It is the simplest and easiest technique to implement with solid-state
transmitters. Pulse compression produces a propagating waveform converted from
electrical network properties. The pulse is modulated either in phase or in
frequency, providing method to resolve targets which may have overlapping
returns. It amplifies the transmitted impulse power by temporal compression, a
combination of high energy of long pulse width together with high resolution of
a short pulse width. Due to its limited temperature and dielectric strength,
transmitter in solid-state technology are unable to produce such high-power
pulses. Therefore, the transmitting pulse of the radar must be much longer to
radiate the same transmission energy. Each part of the pulse has its unique
frequency, hence the return of pulse can be separated and integrated into a shorter single output
pulse. A special filter is used to compress the echo signal in its pulse
duration. The filter reduces the amount of noise received from the echo signal
so that a narrow or compressed pulse can be produced. To conclude, pulse
compression technique reduces the probability of detection due to the peak
transmitted power is lower while the range and resolution is the same.
Frequency hopping spread spectrum is a method of transmitting
radio signals by switching a carrier among many types of frequency channels,
using a process that appears to be random and have the tendency to reappear
again and again which is known to both transmitter and receiver. Frequency
hopping is considered to be one of the effective method for combating active
jamming in military radio systems.
Wide operational bandwidth reduces the signal detectability
by spreading the signal energy in frequency. By spreading the signal energy, it
reduces the signal-strength-per-information bandwidth. Noise in the receiver is
part of a function in the bandwidth, therefore any receiver attempting to
receive and process the signal will be greatly reduced by signal spreading.
Cons of LPI Radars
The main advantage of LPI radars is to go undetected while
maintaining a strong battlefield awareness against hostile enemy without
alerting them. The LPI radars features prevent the radar from tripping off
alarm systems and prevent passive radar-detection equipment in a target.
Features include reducing the transmitted pulse-power, spreading the radar
pulses over a wide band, using an intra-pulse modulation with an
inconspicuously wave-form and as such to prevent its interception by Electronic
Other advantages of LPI radars includes FMCW, the pulse
compression technique. FMCW radars having large modulation bandwidth which provides
very good range resolution. The deterministic nature of this
waveform provides practical advantages over other modulated CW waveforms
because the form of the return signal can be predicated.
However, there is also a few limitation on LPI radars. For example,
Frequency Hopping (FH) technique it is not a very effective LPI signaling
technique. Disadvantages of FH includes the complex and costly digital
frequency synthesizers are required to be used and also, the bandwidth of FH
system is too large. It can also cause interference with adjacent direct
sequence spread spectrum systems.
Trend of this Technology on Military System
Radar plays a significant role in meeting the needs of
military services and it will continue to have critical applications for
national defense purposes. In military radar systems, it can be further divided
into three main classes, namely the land-based, shipborne and airborne.
LPI radars is an improvise invention of radar design that
allows the radar to search or engage in target tracking without alerting them,
while making it difficult for them to detect by passive radar detection
equipment. LPI technology are used for many purposes. It can assist in target
tracking without alerting the enemy, assist in navigation, detecting the
weather condition, assist in locating the landmark, identification and many
more. In the case of surveillance role, these radars can be used to detect
ground targets due to long integration times. In additional, LPI radars is able
to detect targets at a longer range than an intercept receiver therefore it
increases the capability and effectiveness of modern intercept receivers to
locate and detect radar emitters, which possibly might lead to an electronic
attack or physical destruction of the radar by guided munitions or
In recent years, LPI radars are integrated into modern
platforms and weapons, such as anti-ship missiles and littoral weapons systems
for armed forces to develop new techniques, strategies and equipment.
The invention of Low Probability of Intercept (LPI) radars has
benefited the world in the signal environment at a rapid pace. Researchers have
come out with the different type of techniques to achieve the LPI design
concept, ‘to see without being seen’.