Updated 3/20/14

Distributed micro-radar system for detection and tracking of low-profile, low-altitude targets

Ashok Gorwaraa, Pavlo Molchanov*a

aPlanar Monolithics Industries Inc., 7311-F Grove Road, Frederick, MD, USA 21704

ABSTRACT

Proposed airborne surveillance radar system can detect, locate, track, and classify low-profile, low-altitude targets: from traditional fixed and rotary wing aircraft to non-traditional targets like unmanned aircraft systems (drones) and even small projectiles. Distributed micro-radar system is the next step in the development of passive monopulse direction finder proposed by Stephen E. Lipsky in the 80s. To extend high frequency limit and provide high sensitivity over the broadband of frequencies, multiple angularly spaced directional antennas are coupled with front end circuits and separately connected to a direction finder processor by a digital interface. Integration of antennas with front end circuits allows to exclude waveguide lines which limits system bandwidth and creates frequency dependent phase errors. Digitizing of received signals proximate to antennas allows loose distribution of antennas and dramatically decrease phase errors connected with waveguides. Accuracy of direction finding in proposed micro-radar in this case will be determined by time accuracy of digital processor and sampling frequency. Multi-band, multi-functional antennas can be distributed around the perimeter of a Unmanned Aircraft System (UAS) and connected to the processor by digital interface or can be distributed between swarm/formation of mini/micro UAS and connected wirelessly. Expendable micro-radars can be distributed by perimeter of defense object and create multi-static radar network. Low-profile, low-altitude, high speed targets, like small projectiles, create a Doppler shift in a narrow frequency band. This signal can be effectively filtrated and detected with high probability. Proposed micro-radar can work in passive, monostatic or bi-static regime.

Keywords: Monopulse, micro-radar, drone detection, tracking, low-altitude, low-profile, fly eye, distributed antenna array, digital interface

1. INTRODUCTION Drones can be more than just an annoyance. Because they can reach high into the air, on the ground, and in the water, drones literally add a new dimension to eavesdropping and spying on facilities, individuals and infrastructures in a wide variety of environments and industries. They have the power to shrink the realm of public safety, privacy and physical security. Proposed distributed micro-radar system can detect, locate, track, and classify low-profile, low-altitude targets: from traditional fixed and rotary wing aircraft to non-traditional targets like unmanned aircraft systems (drones) and even small projectiles. Proposed system based on nature inspired Fly Eye radar concept [1] provide possibility to detect, recognize and track low-profile low-altitude targets (drones) momentary in wide area of observation. To compensate for its eye’s inability to point at a target, the fly’s eye consists of multiple angularly spaced sensors which give the fly the wide-area visual coverage it needs to detect and avoid the threats around him. Each sensor is coupled with a detector and connected separately to memory. Proposed Fly Eye antenna array consists angularly spaced directional antennas with wide-area coverage. Each directional antenna coupled with front end circuit and digitizer and separately connected to processor by digital interface. Application of Fly Eye antenna array provides high-accuracy amplitude and phase measurement with small size antenna array. Fly Eye antenna array presented in Figure 1 is next step in development of passive monopulse direction finder proposed by Stephen E. Lipsky in 80s [2].

2. MONOPULSE ANTENNA ARRAY

Monopulse is the concept of receiving a signal simultaneously in a pair of antennas covering the same field of view and then comparing the signals by forming them into a ratio (Figure 1-3). Monopulse angle information always appears in the form of a ratio. The value of ratio is independent of the signal and any common noise or modulation present in it [2]. Monopulse microwave radar system can work in passive, monostatic or bi-static regime.

Figure 1. Concept of Fly Eye antenna array. Overlap angular shifted directional antennas cover entire sky. Each antenna integrated with front end circuit and connected to direction finder processor by digital interface.

Azimuth Cover 360 degrees

Overlap Antenna Patterns

A1A2

A3

Directional antennas with

space angle shift

Overlap Antenna Patterns

Elevation Cover >180 degreesPossible 360

degrees

1 mile

0.5

mile

Figure 2. Distributed micro-radar system for detecting and tracking of low-profile low-altitude targets (drones). Overlap angular shifted directional antennas can cover 360 degree by azimuth and elevation area of observation. Radar range approx. 0.5 miles and can be extended by increasing transmitting power. Radar accuracy approx. 1 degree.

2.1 Advantages of Fly Eye antenna array application

Proposed distributed micro-radar system provide entire sky all-weather momentary detection, recognize and tracking capability;

Monopulse multi-beam method provides simultaneous high-accuracy ratio measurement for 360 degree by azimuth and elevation;

Array of angular shifted directional antennas is not phase dependent and can be multi-band and multi-function; Directional antennas may be installed closely or loosely distributed over the perimeter of the carrier platform or

between separate robotic carriers in swarm or distributed around of protected building or border; Receiving of 2-4 orders more signals than regular scanning systems provides 2-3 orders longer radar range; Each directional antenna is integrated with a separate front end circuit consists Analog-to-Digital Converter

(ADC) and connected to direction finding processor by digital interface. Integration of ultra-wideband antennas with front end circuits allows to exclude waveguides, which limiting frequency bandwidth and creating phase/frequency dependence;

Digitizing of signals directly on directional antennas provides wide installation possibility. Antennas can be close positioned in small size aperture and installed on small aircraft or UAS or distributed around perimeter of defense object or border;

Digitizing of signals directly on antennas allows dramatically decrease of phase errors because phase/frequency dependence distance waveguides to processor;

3. RADAR ACCURACY AND RANGE In proposed monopulse array ultra-wideband directional antennas angular shifted and has overlapping antenna patterns. One antenna can be used as reference for high-accuracy amplitude/phase measurement. Each antenna integrated with front end circuit and connected to imaging processor by digital interface. It allows to store amplitude and phase of received signals with high accuracy and in ultra-wide frequency band simultaneously for different targets and directions. Receiving a signal simultaneously in a pair of antennas covering the same field of view and then comparing the signals by forming them into a ratio provides better azimuth accuracy α and elevation accuracy ξ (Figure 3) because monopulse angle information always appears in the form of a ratio.

Figure 3. Azimuth accuracyα and elevation accuracy ξ of measurement increasing because application of signals ratio.

300 180 Bearing, Degrees

Am

plitu

de

RF Signal

A1 A2

Ratio of amplitudes in antennas A1, A2

High-accuracy phase measurementA

B

RF Signal

A1A2

Angular Shift30 degrees

Antenna patterns Difference in

amplitudes

Figure 4. Amplitude monopulse response of two 60 degree angle shifted directional antennas provides high angle of arrival accuracy [2, 3].

The difference in the phase angle as measured in each antenna of and arriving phase front of a signal is ψ. Difference in path length of S = D sin φ due to the antenna aperture displacement (angle shift in space) D. Letting φ be the phase lag due to the difference in the time of arrival of the two signals gives [2]:

(1) Where: φ = the angle of arrival measured from bore sight; λ = the wavelength; If A and B are the RF voltages at each antenna, then

A = M sin ( ωt ) (2) and B = M sin ( ωt + ψ ) = M sin ( ωt - sin φ), (3) where M is a common constant. This shows that the angle of arrival φ is contained in the RF argument or phase difference of the two beams for all signals off the boresight axis. Amplitude comparison direction finding can provide Root Mean Square (RMS) accuracy smaller than 2 degrees in 100 ns after direct wave arrives [3].

𝑅𝑅 = [(𝑃𝑃𝑡𝑡 𝑰𝑰𝒆𝒆𝑴𝑴)𝐺𝐺𝑡𝑡𝐺𝐺𝑟𝑟𝜎𝜎𝜆𝜆2𝐹𝐹𝑡𝑡2𝐹𝐹𝑟𝑟2

(4𝜋𝜋)3𝑃𝑃𝑟𝑟]

14

3.1 Extension of radar range

Extension of antennas Field Of View (FOV) leads to decrease of detecting range. Scanning antenna with narrow beam has large detecting range, but needs long time to scan wide area of observation. The maximum range equation for monostatic scanning radar is given by the following equation [4]:

(4)

Where: R - radar-to-target distance (range); σ - radar target cross section; λ - wavelength; Pr - received-signal power being equal to the receiver minimum detectable signal Smin; Pt - transmitted-signal power (at antenna terminals); Gt, - transmitting antenna power gain; Gr - receiving antenna power gain; Ft - pattern propagation factor for transmitting-antenna-to-target path; Fr - pattern propagation factor for target-to-receiving-antenna path.

Let’s estimate the maximum range for a monopulse radar when the antenna beamwidth is, for example, 10 times wider (beamwidth increased from 3 to 30 degrees). For the same transmitted power, Pt, the energy of the transmitted signal spreads over a 10x10 square and the signal reaching the target (according to the inverse square law) is decreased 100 fold.

Radar with a phase antenna array can scan the entire area of observation and receive 1 target hit pulse every one second. One pulse hits the target per scan. For monopulse radar, pulses with a 1 microsecond width may be transmitted and reflected from the target every 10 microseconds. This means that monopulse radar can transmit to and receive 100,000 pulses per second from any target direction. Integration of the received 100,000 pulses will dramatically increase information about the target. The Maximum range equation for monopulse radar must include the number of integrated pulses: (5) Where: Ie - integrator efficiency;

M- number of transmitted/received pulses per period of integration.

As follows from equation (5), for Ie=1, M=100,000 and Pt 100 times smaller, the maximum radar range will be increased 1000 times!

Continuous target observation and integration of the reflected signals increases radar range and decreases minimum receivable power because of lower Minimum Detectable Signal (MDS). As result the range of detectable target Rr will approach the range of detectable transmitted power Rt in low power monopulse radar as presented in Figures 5,6. If range of detectable target Rr approaches range of detectable transmitted power Rt, then low power monopulse radar will be “invisible” for regular scanning radars with narrow high power beam and RF guided missiles.

Phase and frequency domain processing in monopulse radar, where digital signals from one antenna using as reference provides high-sensitivity and high-accuracy phase/time domain and frequency domain measurements (Figure 5,6). Digitizing of signals directly on antennas allows distribute antennas around perimeter of carrier (aircraft) or perimeter of area of observation. Separate passive sensors can be distributed between swarm/formation and connected wirelessly (Figure 9-12).

𝑅𝑅 = [𝑃𝑃𝑡𝑡 𝐺𝐺𝑡𝑡𝐺𝐺𝑟𝑟𝜎𝜎𝜆𝜆2𝐹𝐹𝑡𝑡2𝐹𝐹𝑟𝑟2

(4𝜋𝜋)3𝑃𝑃𝑟𝑟]

14

Figure 5. High power narrow beam transmitted signal in scanning radar can be detected at long range Rt. For Minimum Detectable Signal (MDS) MDS =-100 dBm; Rt - where transmitted signal detectable, Rr –distance where target can be detected by receiving one reflected from target pulse is much smaller.

(a) (b) Figure 6. Monopulse radar (MHR) can provide one pulse per microsecond (approx. three order faster). As result range of detectable target Rr will approach to range of detectable transmitted power Rt and minimum detectable signal (MDS) will be lowered (a). Multi-frequency and SMART modulation allow for the creation of passive (not transmitting) radar, which receiving reflected from target outside transmitted energy of Broadcast, TV stations or Global System for Mobile Communications (GSM) (b).

4. STATE OF THE ART Passive direction finder systems are known approx. ninety years, look picture from 1927 (Figure 7 a). Passive radar “Kolchuga” is developed in 1987 in the Soviet Union and manufactured in Ukraine (Figure 7 b). It consists parallel receivers allowing the instant discovery and analysis of signals of radio technical equipment (RTE) in the range 100 MHz -18 GHz and continual tracking across the entire band. [Other sources claim from 130MHz to 18GHz, and a 36 channel preset receiver]. VERA passive radiolocator (Figure 7 c) is an electronic support measures (ESM) system that uses measurements of time difference of arrival (TDOA) of pulses at three or four sites to accurately detect and track airborne emitters.

(a) (b) (c) Figure 7. British post office interference finding truck, 1927 (a). Passive radar ‘Kolchuga” from Ukraine (b) and passive radar “Vera” from Chech Republic (c) with omnidirectional antenna for 360 degree observation.

Figure 8. 3D 360 degrees panorama tracking antenna and screenshot from Aaronia Inc. Germany.

Panorama tracking antenna (Aaronia Inc. Germany) presented in Figure 8 provides 9 kHz to 40 GHz 3D tracking with 22.5 degree (for 4-16 sectors) accuracy without rotation. System can provide super-fast tracking speed, up to 1 microsecond. But system large for small aircraft (approx. 10 kg) and not provides 2-3 degree accuracy.

5. DISTRIBUTED MICRO-RADAR SYSTEM `Proposed distributed micro-radar can be installed on UAS or distributed around perimeter of defense object or border (Figures 9-11).

Figure 9. Directional antennas in distributed radar can be installed in small aperture or distributed around UAS, or distributed between small UAS in swarm.

1 mile

2.5 miles Receiver only

Intruder

Transmitter

Transmitter

Transmitter

Figure 11. Distributed radar. Option 1. Separated transmitters and receiver.

1 mile 1 mile

Data LinkData Link

Up to 5 miles

Figure 12. Distributed radar, Option 2. Transmitter/receiver modules connected via wireless network.

Figure 13. Micro-radar transceiver is very small and can be disguised as stone or brick. Small transmitting power and small size allow to disguise radar transceivers (Figure 13).

6. TRACKING OF SMALL PROJECTILE Application of Doppler effect allow apply proposed micro-radar for detection even small projectile. How passive Doppler radar can detect small projectile in noisy battlespace? There are lot of ambient RF/microwave sources: different kinds of communication, radar, navigation and datalink transmitters, and same time lot of moving with different size and speed objects in battlespace. Array of directional antennas in passive Doppler radar will receive direct microwave signals from the ambient sources and reflect from objects part of these microwave signals. The reflected radar signals from moving objects will consist of a Doppler frequency shift [5]. The Doppler frequency is calculated as shown in Equation 6:

( (6) Where: Fd - Doppler frequency; V - Velocity of the target; Ft - Transmit by ambient signal source frequency; c - Speed of light (3 X 108 m/sec); θ - The angle between the object moving direction and the axis of the ambient signal source. For GSM 1900 MHz and projectile velocity 2500-3500 ft/s, Doppler frequency band will be narrow: 9-12 KHz only. For 2500 ft/s velocity and ambient sours frequency band 1-5 GHz, Doppler frequency band will be approx. 5-25 KHz. Application of bandpass filter with narrow bandwidth (4 KHz only for one ambient source) for Doppler signals will allow dramatically increase signal/noise ratio and detect small arms fire with high probability of detection. One can say: size of projectile small relative to microwave signals wavelength, it means microwave energy reflection will be small. Position of ambient microwave signals unknown, and as result reflected signals will be reflected to unknown direction and undetectable. If wavelength of microwave ambient source larger than projectile, projectile will create diffraction of microwave signals as shown in Figure 14. The diffraction phenomenon is described as the interference of waves according to the Huygens–Fresnel principle. These characteristic behaviors are exhibited when a wave encounters an obstacle or a slit that is comparable in size to its wavelength [6,7]. Reflected from projectile microwave signals with Doppler shift will be detected by array of directional antennas. Low frequency narrow band Doppler frequency filter will provide incredible selectivity and will allow to detect and locate small arms fire with high probability and accuracy.

ϕ

A1A2

90 18045 135

45

Amplitude

, V

Projectile

Direct Signal

Source of Ambient RF

Energy

Reflected Signal with Doppler Shift

ProjectileArray of Directional

Antennas

Diffraction of Microwave Signals

Figure 14. Azimuth and range measurement with passive Doppler radar. Phase difference (azimuth of projectile) calculated as ratio of amplitudes in two (few) angle shifted directional antennas with high accuracy and not depends on the base size. Range calculated as ratio of amplitudes or by triangulation of direct and reflected from projectile signals.

7. DESIGN OF ANTENNAS AND FRONT END CIRCUITS

To extend high frequency limit and provide high sensitivity over the broadband of frequencies, Stephen E. Lipsky proposed to integrate detector and mixing elements with antenna [8]. Multiple angularly spaced directional antennas in proposed micro-radar are coupled with front end circuits and separately connected to a direction finder processor by a digital interface (Figure 15). If the lobes are overlap or closely spaced, micro-radar can produce a high degree of pointing accuracy within the beam, adding to the natural accuracy of the conical scanning system. Whereas classical conical scan systems generate pointing accuracy on the order of 0.1 degree, monopulse radars generally improve this by a factor of 10, and advanced tracking radars like the AN/FPS-16 are accurate to 0.006 degrees [9].

PLL

Front End Circuit Integrated with

Antenna and ADC

Digital Interface

Direction Finder

Processor

Dual Polarization Directional Antenna

Figure 15. Block diagram of proposed IDF. Preliminary designed front end circuit integrated with antenna and analog to digital converter presented in Figure 16. Wideband antenna with balun (BAL-0036, 300 kHz – 36 GHz) via limiter, Low Noise Amplifier (LNA) and mixer (ML1-0218SM, 2-18 GHz) connected to Successive Detection Log Video Amplifier (SDVLA); and via buffer amplifier (ADM-0026-59295M, DC - 26,56 GHz) connected to analog to digital converter.

Front End Circuit Integrated with Antenna and ADC

Balun SDVLA AADC

To PLL

Digital Interface

to Direction

Finder Processor

Dual Polarization Directional Antenna

Balun Limiter MixerLNA

Figure 16.Wideband dual polarization directional antenna integrated with front end circuit and analog to digital converter.

Integration of antennas with front end circuits allows to exclude waveguide lines which limiting system bandwidth and creates frequency dependent phase errors. Digitizing of received signals proximate to antennas allows dramatically decrease phase errors connected with waveguides. Accuracy of direction finding in proposed micro-radar in this case will be determined by time accuracy of digital processor and sampling frequency. Result of design and test of two kinds of miniature directional antennas and preliminary designed passive radar receiver presented in Figure 17 [10-18]. Phase detector for measurement range and azimuth presented in Figure 18. Azimuth of signal source was measured on laboratory bench. High accuracy direction measurement made in noisy laboratory environment, no shielding or screening was applied.

Figure 17. Two kinds of miniature directional antennas, preliminary designed passive radar receiver and test results.

Figure 18. Test and accuracy estimation for direction finder with two directional antennas.

Array of dual polarization directional antennas are small enough and can fit small 2’ x 2" aperture opening and installation to aircraft. In Figure 19, 20 presented antenna arrays with helical antennas with circular polarization manufactured by Sarantel Inc. UK, Pulse Electronics Inc. Finland, Cobham Inc.

Figure 19. Array of angle shifted directional antennas with overlap antenna patterns for direction finding.

Angular shift between antennas

Angular shift between antennas

Figure 20. Miniature wide bandwidth antennas and antenna arrays designed by Cobham Inc.

PMI Inc. (http://www.pmi-rf.com) already designed and tested main multibeam radar receiver parts (for example: PTRAN-100M18G-70-MAH, VPX transceiver 100 MHz- 18 GHz) need to implement proposed approach to micro-radar prototype (Figure 21).

Figure 21. Samples of wideband microwave components manufacturing by PMI (http://www.pmi-rf.com ): Low Noise Amplifier PEC-38-30M18G-12-SFF, 30MHz to 18GHz; Small Signal, High Gain Amplifier with Bias-T at Output PE2-30-218-4R0-20-12-SFF-BT, 2-18 GHz; CW Immune, Extended Range DLVA ERDLVA-218-CW-LPD-100, 64 dB dynamic range; PTRAN-100M18G-70-MAH, VPX transceiver 100 MHz- 18 GHz;

REFERENCES

[1] Molchanov P. A., Asmolova O. V., “Fly Eye Radar or Micro-Radar Sensor Technology,” 2014 Defense+Security, Session C31, Sensors, Control, Communications, Technologies for Homeland Security and Homeland Defense XIII, Proc. Of SPIE Vol.9074 907405, May 6, Baltimore, <http://spie.org/Publications/Proceedings/Paper/10.1117/12.2050063> (2014).

[2] Lipsky Stephen E. [Microwave Passive Direction], SciTech Publishing Inc. Raleigh, NC 27613, (2004). [3] Houngsun Yang, Seungheon Kim, Soonyoung Chun, “Bearing accuracy improvement of the amplitude comparison

finding equipment by analyzing the error”. Int. Journal of Communication Networks and Information Security. Vol. 7, No.2, Aug. (2015).

[4] Skolnik M. I. [Radar Handbook] Chapter 1. p.62, McGraw Hill, 19906. [5] Gorwara A., Molchanov P., Asmolova O., “Doppler micro sense and avoid radar”, 9647-6, Security+Defense 2015,

Toulouse, France, <http://pmi-rf.com/documents/DopplerMicroSenseandAvoidRadarPaper.pdf > (21-24 September 2015).

[6] <https://en.wikipedia.org/wiki/Diffraction> [7] European patent application PCT/JP88/00131, 0 371 133 A1. Fujisaka Takahiko, Ohashi Yoshimasa, Kondo

Mithimasa, Mitsubishi Denki Kabashiki Kaisha, JP, Holographic radar. (1988). [8] Lipsky Stephen E., “Integrated Receiver Antenna”, US patent 4,573,212, Feb.25, (1986). [9] Radar Set - Type: AN/FPS-16. US Air Force TM-11-487C-1, Volume 1, MIL-HDBK-162A. (15 December 1965).

[11] Molchanov P. “All Digital Radar Architecture,” Paper 9248-11, Security+Defense Conference, Amsterdam, <http://spie.org/Publications/Proceedings/Paper/10.1117/12.2060249 > (25 September 2014).

[12] Molchanov P., Asmolova O., “Sense and Avoid Radar for Micro-/Nano Robots” (Invited Paper), Security+Defense Conference, Amsterdam, <http://spie.org/Publications/Proceedings/Paper/10.1117/12.2071366 > (24 September 2014).

[13] Molchanov P., “New Protected GPS Antenna Technology,” Joint Navigation Conference 2014, Orlando, FL, Session C2 Antenna Technologies and Interference Mitigation. (17 June 2014).

[14] Molchanov P., Contarino V., “Directional antenna array for communications, control and data link protection,” 2013 Defense +Security. Session 9, Communication, Control, and Enabling Technologies, Paper 8711-31, <http://spie.org/Publications/Proceedings/Paper/10.1117/12.2015607 > (30 April 2013).

[15] Molchanov P., Contarino V., “New Distributed Radar Technology based on UAV or UGV Application,” 2013 Defense +Security. Session 6, MIMO Radar, Paper 8714-27, <http://spie.org/Publications/Proceedings/Paper/10.1117/12.2016379 > (30 April 30, 2013).

[16] Molchanov P., Contarino V., Asmolova O., “Protected GPS Directional Antenna Array,” Colorado Springs, JSDE/ION Joint Navigation Conference 2012, Session C7: Navigation Warfare: GPS in Military Applications II, June, (2012).

[17] Contarino V., Molchanov P., Healing R., Asmolova O., “UAV Radar System of Low observable Targets,” Conf. Unmanned Systems, Canada. Nova Scotia, Canada, K1P1B1, Nov. 7-10, (2011).

[18] U.S. Patent and Trademark Office (USPTO) application titled “Multi-beam Antenna Array for Protecting GPS Receivers from Jamming and Spoofing Signals” (reference USPTO EFS-Web Receipt 13562313 dated 31 <http://www.google.com/patents/US20140035783 > (20 July 2012).