john-proposal

Bài báo giới thiệu về hệ thống ra đa bán chủ động

John W Franklin

"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)

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

 Properties of Passive Bistatic Radar

 The Concept and How it Works

 Why Passive Radar?

 Digital Video Broadcast

 High Definition Television Signals

 ATSC Terrestrial Transmission Standard

 Research Objective

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

LOGO

 Geometry of a Bistatic Radar is Important - it determines

many of the operating characteristics

 Radar Range Equation

 Doppler Velocity Equation

 Radar Cross Section

Monostatic and Bistatic Geometry

β<20 degrees 20<β<145 degrees

Forward/Fence Geometry

Forward/Fence Radar Geometry (limiting case)

145<β<180 degrees

Bistatic Radar Range Equation

2 2

2

A G

Fraction of transmitted power

that is reflected to receiver

Fraction of reflected power that is

intercepted by receiving antenna

2 2

2 1 3

2

) 4

G G

P r is the received signal power

P tis the transmit power

G tis the transmit antenna gain

r1 is the transmitter-to-target range

bis the target bistatic RCS

r2 is the target-to-receiver range

G r is the receive antenna gain

is the radar wavelength

 

4

2

r e

G

A 

Transmitted Power

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 f D

Doppler shift is proportional to the target velocity

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 range

The angular width of the scattered signal horizontal or vertical plane:

Target cross-sectional area A gives a radar cross-section of:

LOGO

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

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

Idea of a Passive Bistatic/Multistatic Radar

Why Passive Radar?

Advantages

Disadvantages

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

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

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:

The 3-D plot of the ambiguity function versus frequency and time delay is called the radar ambiguity diagram

Where: - is the complex envelope of the transmitted signal

- is the time delay

- is the Doppler frequency shift

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 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

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

FM Range Resolution Variance

 Variance is due to instantaneous modulation

Analog FM Ambiguity Diagram

Analog FM – Speech Ambiguity Plot

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)

inversely proportional to the symbol duration.

radio does

 Range resolution c/2B = 680 m (monostatic)

 Power density = –71 dBW/m2 (target range @ 100 km)

Digital Audio Ambiguity Diagram

DAB-T Ambiguity Plot with speech content

Power Density Characteristics

Some transmitters that have been considered for PBR operation

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

Target Location and Tracking

 Measurements of Bistatic range, Doppler, and AoA

 Acquire measurements for a target state vector to give the best

estimates of the vector components (e.g Kalman Filter)

LOGO

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

Silent Sentry 3

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)

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

LOGO

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

ATSC Terrestrial Transmission Standard

 U.S Digital TV is referred to as the ATSC ,DTV or HDTV System

The standard addresses required subsystems for:

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

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

LOGO

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