LOGO
John W. Franklin
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"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. DavisDefense Advanced Projects Research Agency (DARPA)(2007)
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Presentation Flowchart
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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
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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
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LOGO
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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)
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Monostatic and Bistatic Geometry
Monostatic Radar Geometry Bistatic Radar Geometry
β<20 degrees 20<β<145 degrees
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Forward/Fence Geometry
Forward/Fence Radar Geometry (limiting case)
145<β<180 degrees
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Bistatic Radar Range Equation
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21 44 r
AG
rPP e
tB
tr
[
[
Fraction of transmitted power that is reflected to receiver
Fraction of reflected power that is intercepted by receiving antenna
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3
2
)4( rr
GGPP Brttr
(Bistatic Radar Equation)
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
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2r
e
GA Using: then:
Transmitted Power
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Bistatic Doppler
Given the target velocity V and the transmitter and receiver velocities being stationary (VR = VT = 0), the doppler frequency shift is:
The change in the received frequency relative to the transmitted frequency is called the Doppler frequency, denoted by fD
Doppler shift is proportional to the target velocity
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Doppler lets you separate things that are moving from things that aren’t
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 rangeThe angular width of the scattered signal horizontal or vertical plane:
Target cross-sectional area A gives a radar cross-section of:
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LOGO
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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
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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
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Idea of a Passive Bistatic/Multistatic Radar
Bistatic Multistatic
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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
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Applications
Detection of Low Probability of Intercept (LPI) Radar signals
Detection of Stealth TargetsLow Cost Air Traffic Control (ATC)
SystemsLaw Enforcement (Traffic Monitoring)Border Crossing/Intrusion DetectionLocal Metrological MonitoringPlanetary Mapping
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Performance Evaluation
What Type of Waveforms should we use in a PBR System
Modulation Type (Analog/Digital) of the exploited signal Analyze using the Ambiguity Function
We Need to Know
What Type of Power do we need Signal Power Density of the exploited signal at Target Analyze using the Bistatic Range Equation
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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 its2-D correlation function:
The 3-D plot of the ambiguity function versus frequency and time delay is called the radar ambiguity diagram
Wher
e: - is the complex envelope of the transmitted signal
- is the time delay
- is the Doppler frequency shift
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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.
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.
Where:B - bandwidthT - pulse widthfd - doppler delaytd - time delay
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Doppler
Delay
Radar Ambiguity Diagram
The first null occurs at
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
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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
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FM Range Resolution Variance
Variance is due to instantaneous modulation
Four types of VHF FM radio modulation over a two-second interval
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Analog FM Ambiguity Diagram
Analog FM – Speech Ambiguity Plot
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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)
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Digital Audio Ambiguity Diagram
DAB-T Ambiguity Plot with speech content
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Power Density Characteristics
Some transmitters that have been considered for PBR operation
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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)
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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
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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 transmitter-receiver 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)
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LOGO
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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
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Silent Sentry 3
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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)
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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
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
Boeing 747 at relative range 7km and Doppler 100Hz
20 sec. later at range 12km and Doppler 150Hz
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LOGO
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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
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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)
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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
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HTDV Receiver Signals
Real Captured Data Cornell Bard Project
Station: CBS, 545 MHz, 800 kW, Antenna Type: Yagi
Noise Floor 40 dB Sampling Rate: 50Mhz,
Demodulated Signal
I-Q diagram
Receiver
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LOGO
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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
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