LOGO
John W. Franklin
1
"Bistatic radars have fascinated surveillance and tracking researcher for
decades. Despite evolution from the early Chain Home radars in Britain to
today's coherent multimode monostatic radars, there remains a rich research in
bistatic and multistatic applications. The promise of quite receivers, aspect
angle diversity, and improved target tracking accuracy are what fuel this
interest.“
Mark E. Davis
Defense Advanced Projects Research Agency (DARPA)
(2007)
2
Presentation Flowchart
Bistatic
Radar
Passive
Bistatic Radar
Objective:
Practical
Passive Radar
Systems
ATSC (HDTV)
Signals
3
Explore the use of ATSC
(HDTV) as a Passive
Illuminator via
Simulation
Outline
 Overview
 Properties of Bistatic Radar




Geometry
Range Equation
Doppler
Cross Section
 Properties of Passive Bistatic Radar





The Concept and How it Works
Why Passive Radar?
Applications
Performance Evaluation
Signal Processing
 Practical System Examples


FM
Digital Video Broadcast
 High Definition Television Signals

ATSC Terrestrial Transmission Standard
 Research Objective
4
Overview-Bistatic Radar Concepts
 Bistatic radar may be defined as a radar in which the transmitter and
receiver are at separate locations as opposed to conventional
monostatic radar where they are collocated.
 The very first radars were bistatic, until pulsed waveforms and T/R
switches were developed
 Bistatic radars can operate with their own dedicated transmitters or
with transmitters of opportunity
 Radars that use more than one transmitter or receiver or both are
referred to as multistatic
5
LOGO
6
Geometry
 Geometry of a Bistatic Radar is Important - it determines
many of the operating characteristics




Radar Range Equation
Doppler Velocity Equation
Radar Cross Section
Coverage area
 Bistatic Angle: Angle between the illumination path and
echo path
 Bistatic Angle vs. Radar Mode



β<20 degrees – (Monostatic)
20<β<145 degrees – (Bistatic)
145<β<180 degrees – (Forward/Fence)
7
Monostatic and Bistatic Geometry
Monostatic Radar Geometry
Bistatic Radar Geometry
β<20 degrees
20<β<145 degrees
8
Forward/Fence Geometry
Forward/Fence Radar Geometry (limiting case)
145<β<180 degrees
9
Bistatic Radar Range Equation
Fraction of transmitted power
that is reflected to receiver
[
where
Pr is the received signal power
Pt is the transmit power
Gt is the transmit antenna gain
r1 is the transmitter-to-target range
b is the target bistatic RCS
r2 is the target-to-receiver range
Gr is the receive antenna gain
 is the radar wavelength
A

P

P

G

4

r
4

r
r
e
2
B
2 t
t
2
[
1
Transmitted Power
Fraction of reflected power that is
intercepted by receiving antenna
Gr
(Bistatic Radar Equation)
P
G
GB
t t r
P

r
(4
)3r12r22
2
2
Using:
Ae 
4
then:
10
Bistatic Doppler
The change in the received frequency relative
to the transmitted frequency is called the
Doppler frequency, denoted by fD
Given the target velocity V and the transmitter and
receiver velocities being stationary (VR = VT = 0), the
doppler frequency shift is:
Doppler shift is
proportional to the
target velocity
Doppler lets you separate things that are moving from things that aren’t
11
Bistatic Radar Cross Section





Function of target size, shape, material, angle and carrier frequency
Usually, a bistatic RCS is lower than the monostatic RCS
At some target angles a high bistatic RCS is achieved (forward scatter)
Bistatic measurements are essential to understanding the stealth characteristics of vehicles
Almost no data has appeared in the open literature, open research topic
-Low frequencies are more favorable for the
exploitation of forward scatter
-Target detection may be achieved over an
adequately wide angular range
The angular width of the scattered signal
horizontal or vertical plane:
Target cross-sectional area A gives a radar
cross-section of:
12
LOGO
13
Concepts
 A Subtype of Bistatic Radar (all bistatic/multistatic analysis apply)
Geometry, Doppler, RCS
 A Passive Bistatic Radar is a Bistatic Radar that does not emit any Radio
Frequency (RF) of its own to detect targets
 It utilizes the already existing RF energy in the atmosphere
 Examples of such sources of RF energy are Broadcast FM stations, Global
Positioning Satellites, Cellular Telephones, and Commercial Television.
 When the transmitter of opportunity is another radar transmission, the
term such as: hitchhiker, or parasitic radar are often used
 When the transmitter of opportunity is from a non-radar transmission,
such as broadcast communications, terms such as: passive radar, passive
coherent location, or passive bistatic radar are used
14
How does it Work?
 By exploiting common RF energy such as Commercial FM Broadcasts,
as an “Illuminators of Opportunity”, scattered by a target
 The scattered RF energy is received by one antenna and this signal is
then compared to a reference signal from second antenna.
 By using Digital Signal Processing (DSP) techniques, target
parameters such as range, range-rate, and angle of arrival may be
determined
 We are extracting typical radar information from a communication
signal
15
Idea of a Passive Bistatic/Multistatic Radar
Bistatic
Multistatic
16
Why Passive Radar?
Advantages






Lower cost, no dedicated transmitter
No need for frequency allocations
Covert (receiver), Difficulty of Jamming
Virtually immune to Anti-Radiation Missiles
Fast updates
Potential ability to detect stealth targets
Disadvantages



More Complicated Geometry
No direct control of transmitting signal
Technology is immature
17
Applications
Detection of Low Probability of Intercept (LPI)
Radar signals
Detection of Stealth Targets
Low Cost Air Traffic Control (ATC) Systems
Law Enforcement (Traffic Monitoring)
Border Crossing/Intrusion Detection
Local Metrological Monitoring
Planetary Mapping
18
Performance Evaluation
 We Need to Know
 What Type of Waveforms should we use in a PBR System


Modulation Type (Analog/Digital) of the exploited signal
Analyze using the Ambiguity Function
 What Type of Power do we need


Signal Power Density of the exploited signal at Target
Analyze using the Bistatic Range Equation
19
Ambiguity Function
 What is it used for?


As a means of studying different waveforms
To determine the range and Doppler resolutions for a specific transmission
waveform
The radar ambiguity function for a signal is defined as the modulus squared of its
2-D correlation function:
Where:
- is the complex envelope of the transmitted signal
- is the time delay
- is the Doppler frequency shift
The 3-D plot of the ambiguity function versus frequency and time delay is called the radar
ambiguity diagram
20
Radar Ambiguity Diagram
The thumbtack ambiguity function is common to noiselike or pseudonoise
waveforms. By increasing the bandwidth or pulse duration the width of
the spike narrow along the time or the frequency axis, respectively.
Where:
B - bandwidth
T - pulse width
fd - doppler delay
td - time delay
This shows that as we increase the bandwidth B, we have better range
resolution. Conversely if we increase the pulse width T, we increase the
doppler resolution.
21
Radar Ambiguity Diagram
The first null occurs at
Doppler
Delay
The main peak of the ambiguity function corresponds to the resolution of the system in
terms of range and Doppler.
The additional peaks correspond to potential ambiguities, resulting in confusion at
choosing the correct range of the target and its velocity
22
Analog FM Waveforms
 FM analysis has been performed extensively in the U.S. and in Europe
(England/Germany)







FM radio transmissions 88–108 MHz VHF band
The modulation bandwidth typically 50 kHz
Highest power transmitters are 250 kW EIRP
Range resolution c/2B = 3000 m (monostatic)
Power density = –57 dBW/m2 (target range @ 100 km)
Existing commercial FM transmitters provide low-to-medium altitude
coverage
The ambiguity performance of FM transmissions will depend on the
instantaneous modulation
23
FM Range Resolution Variance
 Variance is due to instantaneous modulation
Four types of VHF FM radio modulation over a two-second interval
24
Analog FM Ambiguity Diagram
Analog FM – Speech Ambiguity Plot
25
Digital Audio Waveforms
 Much of the digital waveform analysis in open literature has been done in Europe
(England/Germany) using both Digital Audio Broadcast (DAB-T) and Digital
Video Broadcast (DVB-T)










Uses coded orthogonal frequency division multiplexing (CODFM)
CODFM is the European standard for both Digital Audio and Digital Video in Europe
In COFDM the information is carried by a large number of equally spaced sub-carriers
The sub-carriers (sinusoids) are transmitted simultaneously.
These equidistant sub-carriers constitute a ‘white’ spectrum with a frequency step
inversely proportional to the symbol duration.
CODFM is more noise-like and does not have the dependence on program content as FM
radio does
Modulation bandwidth typically 220 kHz
Highest power transmitters are 10 kW EIRP
Range resolution c/2B = 680 m (monostatic)
Power density = –71 dBW/m2 (target range @ 100 km)
26
Digital Audio Ambiguity Diagram
DAB-T Ambiguity Plot with speech content
27
Power Density Characteristics
Some transmitters that have been considered for PBR operation
28
Processing of PRB Signals
 Two major areas that are of specific signal processing
interest

Suppression of Unwanted Signals




Direct Signal
Multipath
Interference
Target Location and Tracking Measurements



Bistatic Range
Doppler
Angle of Arrival (AoA)
29
Suppression of Unwanted Signals
 The Direct Signal Problem



Greatest system performance limitation
The direct signal received can be several orders of magnitude greater than the
received echo
If not adequately suppressed/cancelled, it will bury the received echo
 Possible Solutions


Physical shielding of reference receiver and echo receiver by topography,
buildings
Spatial cancellation using beamforming with an antenna array to null out direct
signal at echo receiver
30
Target Location and Tracking
 Measurements of Bistatic range, Doppler, and AoA
 (Approach 1)
 Bistatic range from the delay difference between the direct signal and
the targets echo
 Location using multilateration where the bistatic range transmitterreceiver pair will locate the target on an ellipse
 (Approach 2)
 Acquire measurements for a target state vector to give the best
estimates of the vector components (e.g. Kalman Filter)
31
LOGO
32
FM Radio




Lockheed Martin’s Silent Sentry
Uses Analog FM radio transmissions (latest version can also exploit TV signals)
Demonstrated real-time tracking of multiple aircraft targets over a wide area
Real-time tracking of Space Shuttle launches
33
Silent Sentry 3
34
Digital Audio Broadcast
 Experimental PASSIVE RADAR SYSTEM for use with
Digital Audio Broadcast (DAB)


The University of Adelaide, Adelaide Australia
The University of Bath, Bath UK
 A typical digital audio broadcast (DAB) in the UK






Systems run at frequencies of just over 200MHz
Bandwidth of just over 1.5MHz
Signals are close to ideal thumbtack nature
Expected to have good range resolution
Transmitter has an output power of the order of 10kW ERP
Arranged as a network that transmits virtually identical signals
(Single frequency Network)
35
Experimental Results
The radar test bed consists of a four channel digital
receiver, a computer, three Yagi antennas , and a fixed
array of Yagi antennas
Boeing 747 at relative range 7km and Doppler 100Hz
Test bed was located at the University of Bath in the
UK and the antennas pointed towards Bristol airport
in order to observe planes arriving and departing
20 sec. later at range 12km and Doppler 150Hz
36
LOGO
37
Why HDTV Signals?
 No published papers on using HDTV as an Illuminator
 One presentation given at the Association of Old Crows
(AOC) conference in 2005 (not published)
 Some presentation results have been referenced in papers
 Results show that HDTV is an excellent choice for passive
radar applications
 HDTV broadcast signals in U.S. went nationwide in Summer
2009
 Substantial interest expressed in exploiting HDTV signals for
passive radar
38
ATSC Terrestrial Transmission Standard
 U.S. Digital TV is referred to as the ATSC ,DTV or HDTV System
The standard addresses required subsystems for:





Originating
Encoding
Transporting
Transmitting
Receiving
Video, Audio, and Data Transmission


over-the-air broadcast (8-VSB)
cable systems (64-QAM)
39
Major Standards




Uses a 6Mhz Bandwidth Channel (Same as NTSC)
MPEG-2 transport stream at a data rate of 19.29 Mb/s
Modulation is eight-level vestigial sideband signal (8 VSB for broadcast)
Six major functions performed in the channel coder






Data randomizing – assure spectrum is uniform
Reed–Solomon coding - forward error correction
Data interleaving - additional error correction
Trellis coding – more error correction to improve the signal-to-noise ratio
Sync insertion
Pilot signal insertion
Transmitter
40
HTDV Receiver Signals
 Real Captured Data

I-Q diagram
Cornell Bard Project
Station: CBS, 545 MHz, 800 kW, Sampling Rate: 50Mhz, Noise Floor 40 dB
Antenna Type: Yagi
Demodulated Signal
Receiver
41
LOGO
42
Current Plan
 Goals






To give the history and background to bistatic radars, and to give
some examples of their uses in the past
To determine the advantages and disadvantages of the system and
their uses
To describe the geometry of a bistatic radar system, and the theory
behind such a system
To develop software to simulate the bistatic radar system using HDTV
signals as an illuminator of opportunity
To analyze and process the recorded and simulated data
To draw conclusions and make recommendations about the research
43