Passive Bistatic Radar

Passive Bistatic Radar (PBR) The transmitting and receiving antennas are not co-located. RT RR T’X L RX TX Figure 1. Bistatic and multistatic radar geometry configuration

PBR Applications Air traffic control/detection – important for being able to detect outer-atmospheric phenomena. Detect ionospheric disturbances Remotely sense auroral turbulence, density irregularities in the E and F regions of ionosphere, and meteor trails [1]. Traffic monitoring (law enforcement)

Illuminators of Opportunity FM radio broadcasts - monitor disturbances in the ionoshpere Digital Audio Broadcasts (DAB) – high transmission power ~ 5 kW and wide bandwidth ~1.54 MHz Analog TV broadcasts Global Positioning Satellites Cellular telephones

PBR Performance Example Table 1. Operating parameters the UCL PBR radar [1] Given a minimum signal-to-noise ratio (SNR) of 14.94 dB, determine the maximum distance to the target from the transmitter as well as from the receiver. Parameter Value Transmit ERP (PTGT) 4 kW (CP); 250 kW (Wr) Receive Antenna Gain (GR) 8 dB Wavelength (λ) 3 m Assumed Bistatic RCS (σB) 20 m2 Receiver Bandwidth (B) 200 kHz Receiver Noise Figure (Fn) 6.8 dB Assumed System Losses 10 dB Baseline Length (L) 11.8 km (CP); 37 km (Wr) Integration Time (Tint) 1 s Effective Bandwidth (Beff) 75 kHz Processing Gain (GP) 48.8 dB

Figure 2. Oval of Cassini [2] PBR Example Cont’d To solve this problem, we’ll need to use the bistatic radar equation along with the Cassini range equation. CP: 𝑅 𝑇 =113 𝑘𝑚, 𝑅 𝑅 =107 𝑘𝑚 Figure 2. Oval of Cassini [2] Wr: 𝑅 𝑇 =319 𝑘𝑚, 𝑅 𝑅 =300 𝑘𝑚

Contours of Constant SNR (b) (a) Figure 3. PBR sensitivity plot for the FM radio transmitter at Crystal Palace (a), and Wrotham (b) [1]

PBR Advantages Low cost - no designated transmitter Covert operation Reduced electromagnetic pollution Potential detection of stealth targets

Figure 4. The 20 MHz FM broadcast band (88-108 MHz). [1] PBR Disadvantages Complicated geometry Direct signal interference (DSI) – can mask the signal of interest Time-varying characteristics of received signal – e.g. periods of silence (FM), power outage, leakage from adjacent channels, and soil moisture are all out of our control. Figure 4. The 20 MHz FM broadcast band (88-108 MHz). [1]

Time-Varying Characteristiccs Possible Solutions Quiet spectrum with high transmitted power. FM music station – decreases periods of silence Multiple radio channels – increases robustness/SNR (through integration) however, using MF increase the DSI

Direct Signal Interference Possible Solutions Cross polarization – observe using horizontal polarization if the transmit polarization is vertical Array nulls Shielding by topography – select a location with the weakest DSI

Array Nulls Center the antenna so that the DSI is received by a null and not the main beam. For returns close to grazing angles, the direct and reflected signals will tend to cancel each other due to the imperfect nature of the ground. Targets are at high altitudes where the antenna gain is high.

Shielding by Topography Adelaide system: Built by the University of Adelaide, this system was designed to test the potential of DAB (Digital Audio Broadcasting) for radar applications. Located at the University of Bath, this system monitors air traffic at the Bristol airport. Figure 5. Topographical map showing radar (red X), illuminators (blue +) and airport (red O) posistions[3]

Shielding by Topography Figure 6. Propagation loss from Bath (at 0 km) to Wenvoe (at 64 km). [3] Figure 7. Propagation loss from Bath (at 0 km) to Pur Down (at 20 km). [3]

Shielding by Topography Total DSI power at any given location can be determined through simulations. This allows us to select a location with minimal DSI. Figure 8. DSI contributions from all sources [3]

Shielding by Topography Figure 9. DSI contributions from Naish Hill and Mendip [3] Figure 10. One way loss from low DSI site to airport [3] Targets above 1000 m and at least 20 km away can be detected, assuming 120 dB loss is low enough for passenger jet observations.

Shielding by Topography Figure 11. One way loss at 900 m around a low DSI site [3] Figure 12. One way loss at 900 m around an alternative low DSI site [3] Placing multiple receivers at various low DSI sites could provide a more complete air picture [3].

References [1] C.J. Baker and D.W. O’Hagen, “Passive Bistatic Radar (PBR) Using FM Radio Illuminator of Opportunity,” Dept. Elect. Eng., London Univ., London. [2] Wisstein, Eric W. “Cassini Ovals.” From MathWorld—A Wolfram Web Resource. http://mathworld.wolfram.com/CassiniOvals.html [3] C. Coleman, “Mitigating the Effect of Direct Signal Interference in Passive Bistatic Radar,” Dept. Elec. Eng., Adelaide Univ., Adelaide.