Passive Radar Systems: Tracking and Detecting Objects Using Reflections

This report explores passive radar systems and their ability to track and detect objects using reflections. It covers the processing steps used by passive radar systems, WLAN sources, geosynchronous satellites, and bistatic radar. It also provides design recommendations and more.

PASSIVE RADAR SYSTEMS
BY
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Date
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ABSTRACT
In the coming years, passive radars has been viewed to be the missing ingredient to active
radar. Passive radar gives an important operational advantage they are hard or impossible to
locate. This type of radar can never be jammed due to the fact that zero waves are emitted in
its operation. Passive radar mainly employs the services of digital TV and radio frequencies
as the carrier waves thereby avoiding its transmitter.
Passive radar system need maximum amount of power in order to compute and some
software for complicated signal processing. The system uses 20 transmitters with a
combination of digital and VHF frequencies on a high performing on-board computer.
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TABLE OF CONTENTS
Contents
ABSTRACT.............................................................................................................................iii
TABLE OF CONTENTS.........................................................................................................4
LIST OF FIGURES.................................................................................................................5
LIST OF ABBREVIATIONS..................................................................................................6
LIST OF SYMBOLS................................................................................................................7
INTRODUCTION....................................................................................................................8
PASSIVE RADAR SYSTEMS................................................................................................9
Processing Steps used by Passive Radar System...............................................................9
WLAN.....................................................................................................................................12
Standard WLAN sources...................................................................................................12
GEOSYNCHRONOUS SATELLITES................................................................................14
Types of geosynchronous signals around TU...................................................................14
BISTATIC RADAR...............................................................................................................15
RADAR EQUATION IN A SIMULATION MODEL IN MATLAB................................16
Simulating a Bistatic Radar in MATLAB........................................................................16
Bistatic scatter for rain......................................................................................................19
DESIGN RECOMMENDATIONS FOR THE BISTATIC RADAR................................20
CONCLUSION.......................................................................................................................21
REFERENCES.......................................................................................................................21

LIST OF FIGURES
Figure 2: An example of Passive Radar System 9
Figure 2.1: Processing steps used in passive radar system 10
Figure 6.1: MATLAB model showing passive radar with 2 targets 17
Figure 6.2: Simulation of a Bistatic Passive Radar 19
Figure 6.3: Bistatic scattering from rain 20
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LIST OF ABBREVIATIONS
WLAN – Wireless Local Area Network
WiMAX – Worldwide Interoperability for Microwave access
Wi-Fi – Wireless Fidelity
Mbps – Megabits per second
ISP – Internet Service Provider
RF – Radio frequency
DECT – Digital Enhanced Cordless Telecommunication
HIPERLAN – High Performance Radio Local Area Network
DLC – Data Link Control
PAN – Personal Area Network
DVB – Digital Video Broadcasting
TV – Television
GHz - Gigahertz
km – Kilometres
BPR – Bistatic Passive Radar
m/s – metres per second
kW – kilowatts
RCS – Radar Cross Section
IEEE – Institute of Electrical and Electronics Engineers
CFAR – Constant False Alarm Rate
ISM – Industrial, Scientific Medical
ATM – Air Traffic Management
LNB – Low Noise Block
FM – Frequency Modulated
UHF – Ultra High Frequency
VHF – Very High Frequency
GSM – Global System for Mobile
ULA – Uniform Linear Array
GPS – Global Positioning System
MHz – Megahertz
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LIST OF SYMBOLS
Pr – Received power in watts
Pt – Peak transmit power in watts
Gt – Transmit gain
Gr – Receiver gain
λ – Radar operating frequency wavelength in metres
σ – Target’s non-fluctuating radar cross section in square metres
L – General loss factor to account for both system and propagation loss
Rt – Range from transmitter to the target
Rr – Range from receiver to target
K – Boltzmann constants
T – Temperature
Fn – Noise figure
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INTRODUCTION
In majority of the cases, passive radar systems usually do their normal work independent of
broadcasting as well as communication signals. In designing a passive radar, all opportunities
present in the system are there to be exploited. The illumination sources normally have some
properties that dictate the capability of the system in general. This makes the passive radar
become so popular.
It is possible to provide data on weather by using the received radar signals that are reflected
by objects in the atmosphere. It is normal for the passive radar receivers to configure the
signals made available by one or many transmitters.
This report aims at showing the true colours of passive and bistatic radars, and how they can
be applied in real life situations like detecting rain clouds and volume.
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PASSIVE RADAR SYSTEMS
Passive radar systems is a special type of bistatic radar that is used in tracking and detecting
objects with the aid of processed reflections from illumination sources that is in the
surrounding , and may include broadcasts and signals.
Figure 2: An example of Passive Radar System
This system employs receivers that use third party transmitters that are in the environment
hence measuring the difference in time of the signal that is directly coming from the
transmitter and the one coming via an objects’ reflection. Thereby, this allows the
determination of bistatic range as well as an echo’s bistatic Doppler shift. Such information is
important in the calculation of an object’s speed, heading and location.
Processing Steps used by Passive Radar System
 Receiving
The passive radar technique should be able to detect tiny target returns even when there is a
high level of interference. This means that it is critical for the receiver to have maximum
linearity, top dynamic range and minimum noise figure. The system gets an output of a
sampled and digitized signal by employing systems of a digital receiver.
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 Beam forming
Antenna arrays that normally have some elements of an antenna are normally used by passive
radar systems. This enables the calculation of the incoming echoes with the use of
standardized radar beamforming methods that work with the aid of a range of static
overlapping beams.
 Conditioning of signals
It is key for some types of transmitters to do a kind of signal conditioning that is transmitter
specific in order to do cross-sectional processing. Such conditioning may include signal-
Figure 2.1: Processing steps used in passive radar system
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-filtering of the maximum quality analogue band pass thereby improving the radar ambiguity
function.
 Adaptive filtering
Normally the passive radar system has a limited detection range that is caused by the direct
signal that is comes from a transmitter (Simon, 2008). The adaptive filter takes out the direct
signal via the processed termed as active noise control.
 Cross-correlational processing
This is the most important step as it plays the role of the matched filter by providing an
approximation of the bistatic Doppler shift and range of the echo of each target (David B. ,
2008). A lot of signals from digital and analogue broadcasting have some noise effects, and
thereby form a correlation among themselves.
 Detecting targets
Cross-correlational surfaces are used in detecting targets with the aid of an adaptive
threshold. In this step it is to use the constant false alarm rate algorithm (CFAR). It is also
normal that the targets are all the returns on top of this surface.
 Tracking of lines
Line tracking uses the standardized Kalman filter to track target returns from the single
targets, over a specific period of time. At this stage, there is a rejection of majority of the
false alarms.
 Association of tracks and estimating state
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A configuration that uses a single receiver and transmitter can be used to locate the position
of the target, this is by determining the intersection point of bistatic range ellipse and the
bearing.
WLAN
WLAN refers to a wireless method of distribution that involves two or more devices using
radio frequency of high frequency and commonly have the Internet as the point of access.
Standard WLAN sources
 Wi-Fi
This is a protocol (based on 802.11 IEEE network standards) of wireless networking that
enables two or more devices in communicating without the use of internet chords. It basically
operates within a location that is fixed, and is the commonest way of wirelessly
communicating data.
What Wi-Fi requires the most is a device like a phone, router, or computer that can allow the
transmission of wireless signal (Lambert, Radar Networks, 2010). In and around the campus
the popular way in which the Wi-Fi is employed involves the use of Wi-Fi hotspots and ISP.
 WiMAX
This involves a technological standard that offers a wireless network for fixed and mobile
connections and is normally long-ranged. It typically exists in two common ways that is,
receivers and base stations. The technology of WiMAX is fixed on the IEEE 802.16 set of
standards.
It basically offers a platform for telephone access, internet access, as well as video and voice
transfer as the transmitters can span several miles of distance and reach up to 30-40 mbps.
 Bluetooth
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This is a technology of wireless communication that is short-ranged thus allowing for the
connection of electronic devices without cables with the aid of a core system (Richard, 2006).
Its RF transceiver works in the unlicensed ISM band centred at 2.4 gigahertz.
The Bluetooth devices enables an individual to have a phone conversation using a headset, or
even synchronizing data from a mobile phone to a computer using a wireless mouse.
 Home RF
This involves a home networking standard that has combined the DECT and 802.11b into one
system. It mainly employs a frequency hopping method in order to deliver across a distance
reaching 150 feet at speeds of up to 1.6 mbps.
 HIPERLAN
This is a wireless network that provides infrastructure that has low-mobility using data
networks that are cellular based in order to allow a connection to the ATM backbone. It has
various components like the convergence layer, link adaptation, DLC layer as well as
physical layer (Eli, 2004).
It has a data rate reaching 54mbps and achieves the fastest wireless connection.
 Zigbee
This involves a low power and data rate as well as close distance ad hoc wireless network. It
is based on a specification of IEEE 802.15.4 standard protocol that creates PANs that have
low-powered digital radios.
 DVB
This involves a group of standards that describe digital broadcasting with the use of cable,
terrestrial and satellite infrastructures. It was mainly started to start television broadcasting
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via analog signals thereby switching off SECAM/PAL services. The standards set here clarify
the data link and physical layer of the distributing system.
GEOSYNCHRONOUS SATELLITES
This is a satellite that orbits the earth on a daily basis hence returning to the same spot on the
sky after every 24 hours. The satellites are used in communication, forecasting weather and
also broadcasting television signals.
Types of geosynchronous signals around TU
They include;
 Mobile phone signals
This is the signal strength that is normally received from a mobile network using a mobile
phone in terms of dBm. Realistically, mobiles use grounded towers in communication and
can only bounce signals via satellites if one is to make international calls, depending on the
mobile company routes their voice traffics.
 Television signals
The TV signal is normally ferried to an antenna with the use of a wire, and then broadcasted
as electromagnetic waves in the air. Satellite signals take a long route before reaching the TV
screen as a football match or any programming, and must be compressed as it contains high
quality data. This takes out any non-required data before transmitting the signal.
 Radio signals
They are mostly used in carrying radio broadcasts in sending astronauts signals, establishing
connection for Wi-Fi, and also in mobile phone communications. Every satellites uses radio
signals in communicating with the earth. They carry data that is received by the use of dishes
that are put on the roof tops.
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 Internet signals
This involves the wired and wireless internet signals. The wired signals comprises of
broadband services and modems. The wireless signals has the 2G, 3G, as well as 4G mobile
systems.
When one accesses any webpage, there is a request that is sent to the satellite from the
computer. A signal is then transmitted from the satellite to the service hub of the satellite
internet and then it beams back again to the satellite. After which the data is received by the
dish and with the aid of a modem the site loads on the laptop or computer.
BISTATIC RADAR
This is a kind of passive radar that uses multiple antennas that are various locations in order
to transmit and receive signals. Normally, a BPR is able to receive signals with the help of
antennas, and such systems have multiple reasons for having many receiving antennas. BPR’s
can employ available transmitters like FM broadcasting stations to act as an RF wave source
in order to locate its target.
The job of a radar is to process the signals that are received, check it in terms of the
transmitted one, and then show an analysis of the differences in order to get the direction,
location and speed of one or more targets (Eli, 2004).
Passive radars are more like conmen that is because they can be able to use available RF
signals that may be produced by an enemy, neutral or even friendly source. Usually, an
enemy or a neutral source can also be referred to as non-cooperative while a friendly source
can be likened to as being cooperative. An example of a neutral source is a radio or tv station.
The reason for terming third-party sources as “illuminators” is because the RF allows radar to
detect a target as a result of illumination from the RF source.
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In a spectrum, the operation of passive radars is not limited. The current passive radars
employ low frequency broadcasts that have a better detection range and maximum power.
The modulation of the signal schemes rely on the illuminator (Shan, 2015). Examples
include: networks used by cell phones (3G or GSM), HF broadcast, digital TV and audio
broadcast, UHF TV signals, and VHF FM radio signals. Satellite signals like GPS have also
been applied in modern passive radar systems.
The ambiguity function, range and illumination source are normally used to calculate the
work of the radar system. Radar systems that apply the use of satellite signals normally have
a range that is limited as the signals become weak on reaching the ground.
RADAR EQUATION IN A SIMULATION MODEL IN
MATLAB
The radar equation is the relation that exists between the target, radar characteristics and the
signal received. Radar simulation, analysis, and system design is a complicated process due
to the designing spanning RF, analog and digital domains. Such domains usually run all
through the complete signal chain, from the arrays of the antenna, to the algorithms of
processing radar signals, and finally right to control and processing of data.
The complexity level of the system causes the requirement in modelling and simulation of the
whole cycle of development (Alfonso, 2002).
The radar equation is stated as for power at the input of the receiver is:
Pr ¿ Pt Gt Gr λ2 σ
( 4 π )3 Rt
2 Rr
2 L
In order to model the noise term, the formula is calculated as:
P ( f ) =kT
At the output of the receiver the total noise is:
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N= kTFn
τ
Hence the formula for required peak transmit power is calculated as:
Pt = Pr (4 π )3 kT 8 Rt
2 Rr
2 L
Nτ Gt Gr λ2 σ
Modelling and simulation equipment have the ability to raise the bar when it comes to the
workflow of radar system design.
Simulating a Bistatic Radar in MATLAB
This model uses Phased Array System Toolbox Simulink showing a simulation of a bistatic
system that has two targets. In this case, the transmitter and receiver travel along different
paths and are never co-located. The system is commonly divided into the transmitter, receiver
and targets with all propagation channels. The receiver processes the Range-Doppler of the
received echoes in order to create a map.
Figure 6.1 MATLAB model showing passive radar with 2 targets
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When simulating a bistatic radar in estimating the speed and range of targets it is must to
consider the receiver, target and transmitter kinematics.
The operation of the system is at 300 MHz having a linear FM waveform with an
unambiguous range of 48 km. The resolution range is put at 50 m and the product for the time
bandwidth becomes 20.
The transmitters’ maximum power is 2 kW with a gain of 20 dB for both the transmitter and
receiver.
The vertical dipoles are the components of transmit and receive antenna arrays which
normally have at their origins at a constant 4- element ULA (Samuel, 2004).
The same 4- element ULA applies for the receive antenna array that is away from the
transmit antenna, at a location of [20000;1000;100] m, and with a moving speed of
[0;20;0]m/s. This array has a broadside that looks back at the direction of the transmit
antenna.
In space there exists two targets. The number one modelled like a sphere usually is involved
in polarizing the incident signal. It is normally at a location of [15000;1000;500] m and
moves at a speed of [100;100;0] m/s. the number two target is at a location of [35000;-
1000;1000] m and moving at a speed of [-160;0;-50] m/s.
One scattering matrix is the simplest model for a target. It is based on the assumption that
power being distributed between the components H and V is fixed regardless of the reflecting
and incident directions.
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Figure 6.2: Simulation of a Bistatic Passive Radar
A total of 256 received pulses are simulated and beamformed facing the two targets. The
vertical dipole usually misses some targets making the use of circular polarized antenna the
best choice. The circular antenna normally gets all the targets but it does not give maximum
return.
Bistatic scatter for rain
In order to measure the volumes of rain, consider the experiment done on the bistatic scatter
from rain, applying a scatter path of 143 km and frequencies of between 4.5-7.7 GHz. The
ratio of the power that was transmitted to the one received was calculated at a range of 6 –
130 degrees for the scattering angles. At the same time, there were observations on the
weather radar that were made and the frequency turned out to be 1.3 GHz.
The estimated transmission loss for the path of the bistatic scatter were calculated with the
aid of data from the weather radar, the equation for the bistatic radar, and without forgetting a
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cross sectional model for the scattering per unit rain volume (Charles, 2012). This was done
in consideration of the Rayleigh scattering by water spherical ensemble.
Figure 6.3: Bistatic scattering from rain
The values for the transmission loss that were estimated and measured were put side by side
in testing the usage of the model for scattering in estimating the interference. The ratio
average of the measure to calculate loss in transmission for the 4.5 GHz is put at 1.2 in
addition or subtraction of 0.4 dB .
DESIGN RECOMMENDATIONS FOR THE BISTATIC
RADAR
In most cases, there are certain limitations that the rain imposes in designing of the altimeter
for radars. This is mainly caused by the reality that rain clouds normally result in indications
that are false due to the clouds being dense and high transmitter frequencies.
All types of radar altimeters that have a gain pattern, operation heights range, and power that
is transmitted should be designed using the required receiver sensitivity that allows operation
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Terrestrial station
Repeater station
Rain
Receiver station
Common volume
Station separation
Wanted satellite signal
A wR1
R2
Pt Pr
dV

even on a weaker signal power (Merrill, 2008). The altimeters normally show incorrect
height readings when the radar is presented with signal power that is stronger or even “an
elevated earth”. It is a must for the cloud to be large and at the best position considering the
altimeter being carried by the aircraft.
While checking rain clouds, the radar return will be the totals for the backscattering caused
by every droplet in volumes that are massive. In this analysis it is wise to apply the use of
RCS quantity that is the added to the interest of total volume in order to get the clouds’
apparent size as shown in the radar.
It is key for one to have appropriate information on the returning signal from a variety of
water and ground surfaces. In order to get the minimal signal, one must estimate the
maximum height using the mean signal that has been received (Oleg, 2014).
In order to get the density value of rain clouds that is able to run the radar altimeters, the
cloud should be located properly and large enough.
CONCLUSION
It is important to note that passive radars can never be a substitute for active radar. In the
current era, a large percentage of the world is covered by radar illuminators as a result of
availability of digital communication signals (Robert, 2009). An object gives out a variety of
views when the radars are placed in locations that are distant apart. The detection of such an
object is reliable given the streams of data available.
Passive radar is cheaper since there is no need to power and produce any transmission. In
pretty tactical occasions like military missions, targets can be located without the need for
broadcasting any signals.
The future is bright for the system as engineers are using radars in simulating models, the
latest being their use in home security sensors and devices that have access to the internet.
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Passive radar systems are a value adding tool and should be considered by engineers in
solving future problems.
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