1. Passive Bistatic Radar
John W. Franklin

2. "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)

3. Presentation Flowchart
Bistatic Radar
Passive Bistatic Radar
Objective:
Explore the use of ATSC (HDTV) as a Passive Illuminator via Simulation
ATSC (HDTV) Signals
Practical Passive Radar Systems

4. 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

5. 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

6. Properties of Bistatic Radar

7. 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)

8. Monostatic and Bistatic Geometry
Monostatic Radar Geometry
Bistatic Radar Geometry
β<20 degrees
20<β<145 degrees

9. Forward/Fence Geometry
Forward/Fence Radar Geometry (limiting case)
145<β<180 degrees

10. 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
sb is the target bistatic RCS
r2 is the target-to-receiver range
Gr is the receive antenna gain
l is the radar wavelength [ [
Transmitted Power
Fraction of reflected power that is intercepted by receiving antenna
(Bistatic Radar Equation)
Using:
then:

11. 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

12. 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:

13. Properties of Passive Bistatic Radar (PBR)

14. 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

15. 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

16. Idea of a Passive Bistatic/Multistatic Radar

17. Why Passive Radar? Advantages Disadvantages
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

18. 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

19. 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

20. 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

21. 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.

22. 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

23. 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 100 km)
Existing commercial FM transmitters provide low-to-medium altitude coverage
The ambiguity performance of FM transmissions will depend on the instantaneous modulation

24. FM Range Resolution Variance
Variance is due to instantaneous modulation
Four types of VHF FM radio modulation over a two-second interval

25. Analog FM Ambiguity Diagram
Analog FM – Speech Ambiguity Plot

26. 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 100 km)

27. Digital Audio Ambiguity Diagram
DAB-T Ambiguity Plot with speech content

28. Power Density Characteristics
Some transmitters that have been considered for PBR operation

29. 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)

30. 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

31. 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)

32. Practical System Examples

33. 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

34. Silent Sentry 3

35. 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)

36. Experimental Results
Boeing 747 at relative range 7km and Doppler 100Hz
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
20 sec. later at range 12km and Doppler 150Hz

37. High Definition Television Signals

38. 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

39. 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)

40. Major Standards Uses a 6Mhz Bandwidth Channel (Same as NTSC)
MPEG-2 transport stream at a data rate of 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

41. HTDV Receiver Signals Real Captured Data Cornell Bard Project Receiver
I-Q diagram
Station: CBS, 545 MHz, 800 kW,
Sampling Rate: 50Mhz,
Noise Floor 40 dB
Antenna Type: Yagi
Demodulated Signal
Receiver

42. Research Objective

43. 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