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. DavisDefense Advanced Projects Research Agency (DARPA)(2007)

2

Presentation Flowchart

3

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

22

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

22

21

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

4

2r

e

GA Using: then:

Transmitted Power

10

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

11

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:

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 TargetsLow Cost Air Traffic Control (ATC)

SystemsLaw Enforcement (Traffic Monitoring)Border Crossing/Intrusion DetectionLocal Metrological MonitoringPlanetary Mapping

18

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

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 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

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.

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

21

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

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 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)

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

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

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 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

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