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Passive Bistatic Radar

Passive Bistatic Radar. John W. Franklin.

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Passive Bistatic Radar

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  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

  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 Bistatic Multistatic

  17. 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

  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 • 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

  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 Doppler Delay 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

  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 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

  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 range @ 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 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

  41. HTDV Receiver Signals I-Q diagram • Real Captured Data • Cornell Bard Project 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

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