low cost wire detection system

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LOW COST WIRE DETECTION SYSTEM

Dr Petras Avizonis,. T rex Enterprises, San Diego, California

Bruce Barron, Trex Enterprises, Sa n Diego, California

Abstract

Power lines have always been ofprimary concern to helicopter pilots. In thepast laser based systems have been built to alertthe pilot to power lines in the flight path.These systems were heavy, required highpower, suffered from high false alarm rates andwere not considered cost effective even for

military helicopters. Using a simple cw diodelaser with a unique range determination systemand a novel filtering algorithm, we havedeveloped a system that is suitable as a stand-alone system for even the cost sensitive civilmarket.

Introduction

Powerlines, as well as towers, poles andisolated tress have always been a major concernto helicopter pilots. The U.S. Army reports thatthey have lost more helicopters due to hitting

powerlines than they have to combat. A reviewof their Safety Center data shows that amajority of these collisions occur duringdaylight VFR weather. A review of FA A datashows the similar results for civilian aircraft.Military pilots as well as services such aspolice, news services and MedEvac services getinvolved in other aspects of their flights (egother air traffic, events on the ground, etc) thatthey don’t see the wires in time to avoid them.

In the past the military has had anumber of programs to develop systems to givethe pilot’s a display showing all of the wires,etc in the aircraft’s flight path. These systems,using pulsed lasers, tried filling large volumesof space with optical energy to provide a fullimage of what lie ahead. These systems were

very expensive ($120,000 - $250,000),heavy,used significant power (and needed a way todissipate this power) and were quite susceptibleto false alarms. There has also been concern onhow to present all of the data. to the pilotwithout creating an information overload.

Trex has reexamined the issue from adifferent point of view, and . using threesignificant breakthroughs has developed a

warning system that should cost under $15,000in production quantities.

Approach

Past approaches tell the pilot where allhazards are located. Trex has taken a differentapproach. Our system is designed to tell thepilot if he has a clear path ahead. Instead ofusing a power consuming volume filling scan,we use a peripheral scan. We scan along thesurface of a cone and detect hazards as they

cross the edge of this scan.This has a number of favorable features.

Since we are only scanning along a surfacerather than trying to fill a volume of space, wecan use much less laser power. Rather than apulsed system with all of its optical complexity,high cost and high power, our approach lets us‘

get by with a single cw laser diode.

With a volume filling scan, the scanstypically take half to one second to complete.Even with interlacing there is a chance of

missing a target due to aircraft dynamics duringthe scan. In addition, due to this long scantime, every return or noise spike has to beconsidered a possible target resulting in a largefalse alarm rate. Since we are only scanningalong a surface and we are using a cw device,

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we scan much faster; typically 6 millisecondsper scan. At this rate, we are insensitive toaircraft motion. It also permits us to collectmultiple samples before alerting the pilot of apo ten ti a1 hazard.

Breakthroughs

In addition to the change in philosophy,our approach is based on three technologicalbreakthroughs.

Dynamic Parallax

Rather than just tell the pilot that he hasan “All Clear” it is necessary to tell the pilothow far away and in what general directionhazards ore located. This tells him where tolook and how soon he must initiate evasive

action. Pulsed systems determine range totarget by measuring the time delay between atransmitted pulse and the return signal. Thiscan’t be done with a cw system. If the pilotwas only interested in a small range ordistances then a focused optical system couldbe used. Since range information is required,and a number of ranges is required thisapproach cannot be used either. Instead we usewhat we call “dynamic parallax”. Thecollimated laser beam is transmitted at an anglethrough a prism spun at high speed. (See Figure

1) The beam emerges through this prismtraveling along the surface of a cone. Theenergy is reflected off of a target and returnsalong the same path. If everything were static,

the energy would be returned along an axisparallel to the laser diode. However, everythingis not static. During the time of flight of thelaser, the prism has rotated a small amount. Asa result the returned energy path is displacedwith the displacement being a function of howfar the prism has rotated during the photon’stime of flight. By using a series of detectors

and centroiding the received signal, the time offlight and therefore range is determined. Thedirection of the target is determined by whenthe return is received and referenced to theprism rotation orientation (encoder).

Figure 1 - Optics

Multi-phase Background Normalization(MPBN)

One of the problems that has plaguedprevious wire detection systems has been

background radiation from the sun or othersources known as clutter. We have developed ascheme to eliminate this problem. We place anadditional detector in a position equivalent to anegative range which will see the samebackground as the other range detectors justbefore the range detectors. The signal receivedby this detector is appropriately phase shiftedand subtracted from the signals received by allthe of other detector cells. Any signal

Figure 2 - MPBN

remaining after this subtraction is the result of

reflected laser energy (or detector noise) onlywith background energy removed. (Figure 2)

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Event Filter

Because of our short scan times wemaintain a sliding window average of the signalin each range/direction bin. We do not declare atarget unless we detect returned energy more

than a given faction of the time in that bin.Single noise spikes are ignored.

System Performance - Breadboard

A breadboard system was built on anoptical bench (Figure 3) and tested in the field.Figure 4 hows results of one such test. In this

Figure 3 - Breadboard System

figure the light circle shows the scan circle

superimposed on a photograph taken ofpowerlines in front of a hill. The red dots(wires) indicate returns from close objects.

Yellow dots indicate returns fromfurther targets (neared portions of the hill). Theremainder of the hill is beyond the set range ofinterest so isn't shown as a potential problem.

Brassboard

A flightworthy brassboard system iscurrently undergoing laboratory testing and willbe flight tested just after the first of the year.This unit is designed to detect wires as small as,A" diameter at ranges up to 300meters. This isindependent of wire orientation. The systemwill isolate returns into range bins of 50 meters

RcalWire Data - 24 June, 999

Figure 4 - Field Test

and position bins of 5 degrees of sweep alongthe cone.

A simple, low cost display (figure 5 ) isalso planned for this flight test. To minimizeinformation overload this display has been

simplified to display 3 range bins and position

octants. When there are no hazards within theaircraft's flightpath, the green light in thecenter of the display will be illuminated. Whenhazards are detected between 150 meters andthe outside range threshold (typically set to 250meters) an amber LED(s) will light to indicatethe octant where the hazard(s) is (are) located.A momentary aural warning will also be givento alert the pilot to check the display.

When the aircraft closes to 100 to 150

meters of a hazard, the amber LED in themiddle ring of the display will light. When the

aircraft closes to less than 100 meters from ahazard the appropriate red LED(s) will lightand a steady aural warning will be given untilmanually reset.

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Laser

Initial evaluations will be performedusing an 830nm laser. It is intended to use thiswavelength for the production configurationbecause of low cost diodes and detectors

available at this wavelength. At the powerlevels being used the system is operationallyeye safe. We have also evaluated the use of 1.5microns should it become necessary to use thiswavelength for safety approval though this willincrease the cost.

Authors

Production

Every design aspect of this system hasbeen driven by th e requirement for lowestproduction cost. Although there is only onecritical optical alignment none of thealignments are difficult. There are no

Figure 5 - Cockpit Display

mechanical tolerances that cannot easily bemet. A majority of components are available of

the shelf and from multiple sources Our initialestimates place the high volume recurringproduction price well below $15,000 (beforeamortization of certification costs)

Dr P etras A vizonis

Ph.D. Physics 1997, University ofMaryland at College Park, M.S. Physics

1995, University of Maryland at College Park,B.S . Physics 1989, Universityof New Mexico. Staff Scientist at TrexEnterprises, system development andintegration for optical windspeed lidar andaircraft hazard sensing system.Previously employed as an engineer atRockwell Power Systems in Albuquerqueinvolved in coherent imaging with fiber opticsand high power laser cavity design.

Bruce BarronBSEE MIT 1967, Masters of Project

Management Keller 1994. Director of AviationProduct Development for Trex Enterprises.Formerly positions include Team Leader,Flight Controls and Navigation Systems forAH-64D Longbow Apache Helicopter,Technical Director Flight Controls / VehicleManagement Systems for YF-22, F-117, IsraeliLavi. Also responsible for preliminary designof 777 Actuator Control Electronics. Mr Barronalso designed the electronics for the first

Sidewinder class missiles to slave to the pilot’shelmet and a Laser Guided Cannon LaunchedProjectile (ie Tank Shell). Mr Barron also had akey role in the design and flight test of the DC-

10 Automatic Landing System. Mr Barron is apilot and a former aircraft owner.

0-7803-5749-3/99/$10.000 1999 IEEE 3.c.3-4



OPTICAL DRIFT DETECTOR

Bruce Barron, T rex Enterprises, San Diego, California

Dr Petras Avizonis. Trex Enterprises, San Diego, C alifornia

AbstractThe Embedded GPS INS (EGI) has

become the primary navigation sensor foraircraft. The system however does not solve allof the needs of the military helicopter for eitheren route navigation or as a hover hold positionsensor. The ground velocity signals from aDoppler Navigation System (DNS) arecurrently used as inputs to the navigation

Kalman filter to augment the navigationsolution. By replacing the DNS with acontinuously swept cw laser, we can eliminatethe problems inherent with these RF basedsensors.

Introduction

The Embedded GPS INS (EGI) is anexcellent navigation device, however, it doesnot solve all navigational needs especially withregard to military rotorcraft. While EGI is idealfor long range navigation for high flying

aircraft, it does not provide a total solution forthe rotorcraft segment. Rotorcraft often fl ythrough canyons, both natural and manmade,where they do not have line of sight to foursatellites. In addition, some rotorcraft havetight requirements for a drift free hoverhold.For example, the AH-64D Longbow helicopterhas a requirement to drift less than 4 meters in5 minutes. The EGI, even with 4 satellites inview, does not support this requirement. Tosatisfy these requirements it has been necessaryto include a Doppler Navigation Sensor to the

EGI's Kalman filterDNS works by projecting four

microwave beams along the edges of arectangular pyramid. Some of the RF energy isreflected back towards the aircraft with a

Doppler shift as a result of the motion in thedirection of the beam. If a beam hits a specularsurface (such as a smooth surface or calmwater), insufficient energy is reflected backtowards the aircraft to make a validmeasurement. The systems also tend to havedropouts when the aircraft is maneuvering.

Trex Enterprises is developing anoptical system that is less susceptible to theseproblems, is lighter, lower cost and more covert

than the traditional designs.

Rather than aiming a beam in fourdiscrete directions, the Trex design sweeps alaser beam continuously along the surface of acone. The returned energy is detected directlyrather than by heterodyning with thetransmitted signal, and the signal is processedcontinuously using a unique algorithm todetermine the direct-detected Doppler shift.

Hardware

The hardware is shown schematically inFigure 1. The laser is a single diode operatingin a cw mode. The laser energy is collimatedand aimed at a spinning mirror. The lightreflected off this mirror travels along thesurface of a cone. The centerline of this cone isaimed down when the aircraft is in its normalflight attitude. The light is reflected off theground and some of this energy is scatteredback towards the sensor. Scattered energy iscollected by a telescope in the sensor, wherethe light is passed through a sequence of

polarizers and atomic line filters (ALF)'.The ALF consists of a small (1 cm

diameter x 1.5 cm long) optical cell containingcesium vapor in a magnetic field. Thepolarization of light passing through the ALF is

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rotated due to anomalous dispersion. Theamount of polarization rotation is a function ofthe frequency. The wavelength of the laser andthe ALF is tuned (and maintained with a closedloop regulator) such that the laser wavelength isnear the center of a sharply varying portion of

the ALF’s polarization curve. The output ofthe ALF then goes to a polarized beam splitterand the two resulting polarized beams go to apair of photodetectors.

As the wavelength of the returnedenergy changes due to Doppler shifts, thetransmission through the ALF system allowsfor direct measurement of the shift. This isdue to the high sensitivity of the anomalousdispersion in Cesium vapor to frequency andpolarization. The greater the frequency shift,

the more of one polarization passes through theALF and less of the other polarization. Twodetectors sense the orthogonal components ofthe signal. By taking the ratio of the difference

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Figure 1

9’U st4

Optics

between the two detectors and their sum we geta signal that is directly proportional to thefrequency shift (direct Doppler measurement),

independent of the amplitude of the scatteredlight. Comparison of this signal to a referencesignal (the transmitted light passed through thesame optical system) provides an absolutereference to the magnitude of the measuredshift. This is a direct measurement of the

velocity component along the light propagationvector.

Processing

The Doppler shift at any point in time is

a function of the relative velocity vectorparallel to the laser pointing direction.

This system could be designed toemulate existing rf doppler systems by

processing the data at four points during thescan - these four points occurring when thescan mirror is aiming along the same directionsas these systems. This would work but wouldbe susceptible to the same type problemsplaguing the existing systems.

Instead, since our energy source is cwand the scan is continuous, we process thesignal throughout the scan. The detectoroutputs are sampled at a rate only limited byprocessor throughput and available memory.The data for a full scan is recorded in memory.A Fast Fourier Transform is then taken on thisdata. The Fourier coefficient for the “DC” termof the transform is proportional to the velocityparallel to the centerline of the scan cone. Thecoefficients for the two first order terms areproportional to the horizontal velocitycomponents. By properly referencing the“start” of a data frame, these components willbe “along-track” and “cross-track”. The“energy” in the higher order terms is a measureof system noise and can be used as a“confidence” factor as well as an input to thecorrelation matrix of the navigation systems’Kalman filters.

With a conventional 4 beam rf system,the system is forced into a “hold” mode when areturn is lost from one (or two depending on the

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system) of these beams. With our approach aslong a signal return is detected over any portionof the scan, velocity information is available.

Fourier Analysis

Our application of the Fourier transformis a somewhat unconventional approach.Normally a Fourier Transform is used todetermine the frequency content of a signal.Instead, we use the transform to determine therelative magnitudes of known frequencies.

Figure 2 -Scan Definitions

Analysis of the components that make up theDoppler velocity signal illustrate why this

approach is valid.

Vd= Doppler Velocity (Along Beam)

v h = Horizontal Velocity

V, = Along Track Velocity

V, = Cross Track Velocity

V, = Vertical Velocity

Vh = V, sin(Q2)+V, cos(Q)

Vd= V, sin (0)+ vh cos(0)

v d = V, sin(e)+cos (e)(V, sin (Q)+V, cos (Q))

Vd=kl Vz+k2 (v, sin (wt)+V, cos (wt))

Status

We have built a proof of principal testunit by modifying a unit originally designed tomeasure wind speed (Figure 3) at ranges of 5

* L

Figure 3 - Proof of Principal Unit

and 30meters. Using this unit we have verifiedthat both the hardware and processingalgorithms perform their intended functions.

Analysis shows that using a 30 degree

half angle scan cone this system will measurevelocities with an accuracy of 1 knot per axisand work at altitudes from a few feet from thesurface to above 1000 feet in altitude.

We estimate that a production systemusing this approach would cost less than$25,000 and weigh less than 25 pounds.

References

1. J. Menders, K. Benson, S.H. Bloom, C.S.

Liu and Eric Koevaar. Ultranarrow LineFiltering Using a Cs Faraday filter at 852 nm.

Optics Letters /Vol . 16, No.11 /June 1, 1991

0-7803-5749-3/99/$10.000 1999 IEEE 3.c.4-3



Authors

Dr Petras Avizonis

Ph.D. Physics 1997, University ofMaryland at College Park, M.S. Physics

1995, University of Maryland at College Park,B.S. Physics 1989, Universityof New Mexico. Staff Scientist at TrexEnterprises, system development andintegration for optical windspeed lidar andaircraft hazard sensing system.Previously employed as an engineer atRockwell Power Systems in Albuquerqueinvolved in coherent imaging with fiber opticsand high power laser cavity design.

Bruce BarronBSEE MIT 1967, Masters of Project

Management Keller 1994. Director of AviationProduct Development for Trex Enterprises.Formerly positions include Team Leader,Flight Controls and Navigation Systems forAH-64D Longbow Apache Helicopter,Technical Director Flight Controls / VehicleManagement Systems for YF-22, F-117, IsraeliLavi. Also responsible for preliminary designof 777 Actuator Control Electronics. Mr Barronalso designed the electronics for the first

Sidewinder class missiles to slave to the pilot’shelmet and a Laser Guided Cannon LaunchedProjectile (ie Tank Shell). Mr Banon also had akey role in the design and flight test of the DC-

10Automatic Landing System. Mr Barron is apilot and a former aircraft owner.

0-7803-5749-3/99/$10.000 1999 IEEE 3.c.4-4

low cost wire detection system

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