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I See Airplanes:How to build your own radar system
Eric [email protected]
More fun with GNU Radio...
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What is radar?
● “Radio Detection and Ranging”● Watches the reflection of radio waves
off of objects and figures out:– How far away– Velocity of object– Bearing (direction) to object– Type of object (classification)
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A bit of history
● First radar 1904 Christian Helsmeyer:– Spark gap; 40 – 50 cm; detected ships
● First unambiguous bistatic detection:– Sept 1922, Holt & Young, 50W 60 MHz– Observed reflections from trees and
wooden steamer (boat)● UK 1935 “Daventry experiment”
– Demonstrated aircraft detection● WWII, ...
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Airport surveillance radar
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PAVE PAWS
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Busted!
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Radar configurations
● Monostatic● Bistatic● Multi-static (networked)
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Bistatic radar
● Transmitter & Receiver are at different locations.
● Original motivations:– Avoiding anti-radiation missles– Remote target illumination
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Bistatic triangle
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Bistatic doppler
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Bistatic radar equation
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Passive radar
● A subclass of of Bistatic Radar● Use somebody else's transmitter!
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For example...
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The basic idea
● Use other people's transmitters● Use multiple coherent receivers● One or more Tx and/or Rx locations● Watch reflections● Do a bunch of math● Detemine position and velocity
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Choice of transmitter
● Don't control signal, but know the general characteristics
● Obvious choices:– Broadcast FM (100 kHz wide)– Analog and/or digital TV (6-8 MHz wide)– GSM cellular / UMTS
● Other choices:– High power satellites (DBS)– GPS satellites– Existing radar transmitters
● Primary and/or secondary surveillance
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Existence proofs:
● Lockheed “Silent Sentry”● Manastash Ridge Radar
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Lockheed “Silent Sentry”
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Manastash Ridge radar
● University of Washington– Prof John Sahr & students– Interested in ionospheric phenomenon
● Very simple● Two locations separated by 150 km● Takes advantage of mountains● GPS synced time references● Sees stuff up to 1200 km away!
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What we chose
● FM broadcast– About 100 MHz (3m wave length)– Bandwidth about 100 kHz– Theoretical distance resolution 3 km
● (but see also “super-resolution” techniques)● Why:
– Simplest h/w that could possibly work.– Need to sample multiple antennas
coherently.– Bandpass sampling eliminates
requirement for coherent analog LO
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Universal Software Radio Peripheral (USRP)
● 4 12-bit 64 MS/sec A/Ds● 4 14-bit 128 MS/sec D/As● Altera Cyclone FPGA● USB 2 interface to PC● Pluggable RF daughterboards● See http://ettus.com for info
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USRP block diagram
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Bandpass sampling
● Nyquist sampling criterion:– Need 2x the bandwidth of interest
● USRP samples at 64 MS/s● spectrum “folding” every Fs/2 (32 MHz)● therefore, folds at 96 MHz, middle of
FM band.● Requires bandpass filter to avoid
aliasing. Either:– 87 – 95 MHz or– 97 – 107 MHz
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Experimental setup
● Simplest thing that could possibly work● 2 directional antennas
– 1 pointed at Tx about 45km away– 1 pointed about 120° away (towards
airport approach)● 2 broadband LNA's● 1 USRP with 2 “Basic Rx” d'boards
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Procedure
● Watch for nearby airplanes● Collect the data● Run the analysis software● Plot the range/doppler graph
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Airplanes?
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Hmmm...
● Could be h/w or s/w or both...● Could be RF/Analog
– Filtering– Gain– Antennas– Direct path overwhelming reflection (not
enough dynamic range)● Could be signal processing s/w
– Is it working?
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Simulate!
● Simulate the FM transmitter● Simulate the radar reflections
– Geometry (Tx, Rx, targets: pos & velocity)– Propagation delay– Doppler shift
● Run analysis s/w on reference signal and simulated returns.
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I see (simulated) airplanes!
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Next steps
● Quantitative analysis using simulator:– What RF performance do we require for
s/w to be able to detect targets?– How small (big?) of an object should we
expect to see at a give distance● Design & build low-loss bandpass filters
– Probably helical filters● Antenna ideas:
– Dipoles in front of metal screen– “Corner reflectors”
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And then...
● Determine angle of arrival– Interferometry / phased array– Watch multiple Tx's in different locations– Use multiple Rx's in different locations
● Target tracking (multiple targets)● Nice real-time application with GUI● Try it with digital TV signals
– Theoretical ~50 m resolution
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Resources
● The code is in GNU Radio CVS● http://www.gnu.org/software/gnuradio● Mailing list: [email protected]
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Questions?