Contents 1. Introduction 2. Detection Techniques 3. Ladar Target Description

LADAR

CONTENTS

1. INTRODUCTION 1

2. DETECTION TECHNIQUES 2

3. LADAR TARGET DESCRIPTION 7

4. LADAR TRANSMISSION LINKS 9

5. LASER SOURCES USED IN LADAR 11

6. LADAR SIGNATURE 12

7. LADAR SYSTEM APPLICATIONS 14

8. ADVANTAGES AND DISADVANTAGES 19

9. LADAR/RADAR COMPARISON 20

10. CONCLUSION 21

11. BIBLIOGRAPHY 22

DEPT. OF ELECTRONICS & COMMUNICATION ENGG.

LADAR

1. INTRODUCTION

The LADAR acronym stands for laser detection and ranging. The term laser is the

expansion of Light Amplification by Stimulated Emission &Radiation.

LADAR operates on the same principle that of RADAR laser radar constitutes a

direct extension of conventional RADAR techniques to a very short wavelength. Because

of they operate at a much shorter wavelength ,laser radar is capable of higher accuracy

and more precise resolutions.

Modern laser radar systems combine the capabilities of Radar and optical systems

to allow simultaneous measurement of range ,reflectivity ,velocity, temperature &

elevation angle. These dimensions of target information can be utilized in fire control &

weapon system applications to allow the target acquisition, tracking .classification &

imaging.

Useful lasers operating at a wavelength in the UV, VISIBLE & INFRA via

Telescope . -The scattered target radiation can be detected and processed in a laser radar

receiver to remotely determine the target characteristics.


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LADAR

II. DETECTION TECHNIQUES

A LADAR receiver convert the photons of light collected by the receiver aperture

& a choice in the design of such a system must be made between two well known

techniques .The first & more sophisticated technique is called HETERODYNE

DETECTION & the second and simpler one is called ENERGY DETECTION.

The heterodyne detection represents a direct extension of lower frequency

heterodyne receiver techniques. Energy detection decisions are based on the result of

energy measurement.

HETERODYNE DETECTON:

The model adopted for heterodyne detection LADAR is illustrated in fig. 1. The

transmitter consists of low power continuous wave source followed by a pulsed laser

amplifier and transmitting optics. By means of a pair of beam splitters a portion of the

CW signal is sent to the receiver ,where it serves as a local oscillator.

The pre -detection portion of the receiver consists of receiving optics ,an optical

filter to reduce background interference & a photo detector, on which the local oscillator

and the target light is present, (fhe photo current consists of a photo mixing component at

a frequency given by the Doppler shift introduced by a medial target motion| The local

oscillator frequency could be offset to any desired amount from subsequent signal

processing hardware considerations , so that photo mixing component would appear at a

frequency given by Some of offset and Doppler shift.

ENERGY DETECTION:

The model adopted for an energy detection LADAR is illustrated in fig.2,

The transmitter consists of pulsed source of coherent optical radiation followed by a

transmitting optics.

The receiver consists of an energy collecting optics, an optical filter, a photo

detector, photo detection processing circuits And finally a decision apparatus.


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LADAR

FIGURE.1 MODEL OF HETERODYNE DETECTION LADER


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SPLITERTRANSMITTER

LADAR

OPTICAL FILTER

The receiver optical filter limits the range of wavelength that passes through it. It

therefore plays the same role as an RF front end filter, with its prime function is being to

reduce the amount of unwanted radiation entering to the photo detector.

The range of wavelength around the laser wavelength allowed by the optical filter is

called optical bandwidth, the optical bandwidth ranges from 100 micrometer(IR) to 0.1

micrometer (VISIBLE).

PHOTODETECTION:

The primary element for receiving an optical field is a focusing lens and a photo

detecting surface. \ In RF system an antenna dish concentrates the imaging field at the

feed point, which electronically converts field at the feed directly to an electronic signal

current.J In optics the focusing lens focuses the field on to photo detector. Since optical

field is at a higher frequency , it cannot be directly detected.

Instead the photo detector response quantum mechanically by using the radiation

to excite its photo-emissive surface and produces a photo electron current flow. The later

depends on the instantaneous field intensity over a detector surface, Hence photo detector

detects instantaneous field power of the focused field on the detector.

The amount of field power focused on to detector is equal to amount of field power

incident over the focusing lens. Hence photo detected power can be

equivalently computed by determining the receiving field power over the lens area instead

of focused field over the detector area,

DETECTOR EFFICIENCY:

Only a portion of the incident power may be detected by a photo emissive surface.

The detector efficiency indicates the fraction of received power that is actually detected.

Detector efficiency is wavelength dependent and depends on the material used in the

photo emissive surface.


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LADAR

III LADAR TARGET DESCRIPTION

Two simplest types of target (in statistical sense) are the specular and diffuse it is

also have a " hybrid " target ,which has both specular and diffuse portion .if there is only

one specular scatter contributing the return , the target will be referred as'a simple hybrid ,

while more completed targets will be referred to as complex hybrid .

TARGET REFLECTANCE:

To ascertain the number of intensity of specular and diffuse Return levels, target

laser signature and target laser cross section (LRCS) calculated , estimated or measured.

The LRCS will depend on the fescr reflectivity of the surface area of the target under

consideration .The reflectance properties of the material have been found to. depend

spectrum and surface roughness. For an opaque object , by KIRCHOFF'S

LAW .absorption , the fraction of the incident flux absorbed is equal to emittance and

Emittance=I-reflectivity

Available data can be given in terms of emittance or reflectance can be the n

utilized to account for the effect on laser radar cross-section of the target surface

description in terms of material , finish and /or roughness,


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LADAR

LADAR CROSS SECTION:

LRCS of a target could have a simple numerical value , which is a function of

static and dynamic properties of the target (size, shape , composition .spin rate etc)in

addition to clearly identifiable properties of the beam(wavelength polarization)

The quantity identified as the LRCS can be effected by range (near field ,far

field), size of the LADAR beam relative to the target, beam profile and curvature

characteristics, coherence and other effects which enormously complicate the

interpretation of the return signal ,LRCS is equal to the area of loss less Lambertain

surface ,which produces same amount of scattered power at the receiver , as docs the tar

get.

For LADAR signature purpose its usually better of the polarization are specified

in terms of the symmetry coordinate of the target this implies that two coordinate systems

arc required one to describe the scattering properties of the target and another define the

orientation of the target relative to the laser transmitter receiver reference frame .


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LADAR

IV. LADAR TRANSMISSION LINKS

Because of the extremely small wavelengths, the disturbances to the propagating

beam be atmospheric particles are much are pronounced at the optical frequency. The

principle effect of the atmosphere in the optical range are (1) attenuation due to the

energy absorption (2) beam spreading due to scattering of light waves (3) beam bending

due to rcfocusing of optical beams (4) beam break up to the loss of field coherence over

the beam front. Beam attenuation loss is computed from atmospheric absorption co-

efficient which depends on the elevation angle , type of atmosphere (clear, hazy,

cloudy ,etc) and wavelength losses will be significantly higher in rain.

Atmospheric transmission is a sensitive function of operating wavelength , with

LADAR applications best suited near wavelength region of maximum transmittance. Of

the three general effects of the atmosphere on laser propagation(absorption, scattering,

and turbulence) absorption can cause the most serious. LADAR limitations, particularly

that attributed to water vapour. Since water vapour is concentrated near ground level (the

first few thousand feets), it is interest to consider the variation of one way transmission as

a function of LADAR platform altitude above sea level,

HIGH POWER PROPAGATION.

Other factors effecting beam propagation include atmospheric turbulance.

Because of random nature of refractive index laser beam spreading distortion and

wandering will had to occur.

Thermal blooming refers to the self induced spreading , distortion and bending of

laser beam that occurs as a result of absorption of a small fraction of a laser power by

molecular and aerosol constituents in the atmosphere,

The absorbed power heats the atmosphere causing density and refractive index

changes, which act as a distributed non linear lens. The resulting non linear propagation

characteristics distorts the laser beam and limits the optical flux density that may be

transmitted to the LADAR target, independent of the available transmitted power and

hence limit the maximum range and LADAR target cross sections examinations

attainable in LADAR systems operating in the atmospheric heating may be less for pulse


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LADAR

than for continuous wave system. One way to minimizing thermal propagation effects

may to use pulse transmission.

PROPAGATION IN WEATHER

The effect of weather can severely degrade laser radar performance . rain ,fog and

haze are condition ,which must be considered in estimating performance. Both the

absorption and back scatter return must be considered in the laser radar two way

propagation path. In general, deterioration in laser transmission characteristics is most

severe in weather conditions for which the scattering particle size are nearly equal to the

wavelength to which sensor response.


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LADAR

V. LASER SOURCES USED IN LADAR

There are different laser sources depending upon the applications .The main

important applications of the LADAR techniques are used in the space environment.

1) Military application need a high energy laser sources, so that use Nd:YAG laser

sources which 1.06 microns &havc range 2000 meter to 10 kilometer. Nd:YAG

laser are durable compact red and reliable lasers.

2) Carbon dioxide laser sources can be used for range finders at 1.06 microns, which

has been penetration through atmosphere , haze, fog and smo'ke. The spectral purity

cajbon dioxide has relative high operating efficiency make an attractive for ground

based, air born and space born environments.

3) Chemical lasers such as Hidrogcn fluride & Duterium fluride which operates at 2.9

micrometer & 3.8 micrometer respectively, have higher efficiencies than Carbon

dioxide laser and they have high output energies.

4) Solid state laser can act as laser sources but they cannot provide a\the necessary

spectral purity to utilize phase processing of LADAR signals. Carbon dioxide,

He-Ne has higher spectral purity can modulate amplitudes of frequency with

bandwidth up to 525 MHZ with Relatively low drive powers..

5) High power laser sources for damaging sensors use gas dynamic Carbon dioxide

and CO, chemicals HF , Iodine lasers and free electron lasers.


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LADAR

VI. LADAR SIGNATURE

Although laser radar cross section values determine the return signal level

LADAR target will usually present a sequence of composite reflective surfaces for

LADAR. In a mono static LADAR system , return signal amplitude is observed which

varies as a function of time during a tracking interval in which target aspect angle with

respect to the LADAR line-of-slight may be changing. The target relative motional effect

both due to it's the vector velocity and local motions, cause and LADAR returns signal

very accordingly. These situations more complex when bi static or mono static LADAR

systems are employed. These observed return from a target, is called LADAR

SIGNATURE and is used for obtaining the information about the target.

One means of determining LADAR signature of a tracked target is to either

compute or measure(or combination of these techniques) the laser radar cross section at

each aspect angle to obtain an amplitude verses angle record. If the Target is net

symmetrical about the rotational axis these date must be used for a series of roll angle as

well. The geometry relating the location of the illuminators and the receiver of the

LADAR system with the target motion will then allow the synthesis of its LADAR

SIGNATURE neglecting the propagating effect s which must be included of the effect the

return signal. A six degree of freedom motional program is used for the target vertical,

lateral, inclined and rotations about these axes of yaw, pilch and roll respectively. The

start range of these variations may be included or neglected , depending upon the

presence or absence of automatic gain control in LADAR receiver other signal processing

circuits.

For the purpose of laser radar cross section , the target can be decomposed into

basic shapes or sub areas and the return from each is determined using existing computer

programs which have been highly developed for bodies with rotational symmetry. Once

the object has been decomposed into a large number of small flat plates, it is relatively

straight forward operation to calculate the total cross section.


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LADAR

VII. LADAR SYSTEM APPLICATIONS

1) Ranging and tracking

2) Automatic air traffic landing systems

3) Communication purposes

4) Railway traffic control

5) Air born terrain profilometcr

6) To monitor the environment to provide air pollution location

7) Measuring the earths movement

LADAR imaging by combining the passive electro optical imaging receivers with

scanning laser illuminators is being used in an increasing variety of applications ranging

and tracking were among first function of LADAR. The use of pulse duration in the tens

of nano seconds range at millimeter wavelength to obtain BOTH HIGH RANGE

RESOLUTIONS AND ACCURACY gave way to short pulse LADAR , primarily

because of cost and reliability factors, in addition to energy /pulse available , a quantity

which is fundamental in ranging capabilities.

High directivity and narrow beam width enables the laser to become prime choice

of ranging .LADAR system has been applied to ranging and tracking of target of interest

form ground based, air born ship born, orbital and space platforms. One of the first

LADAR applications was for satellite tracking.

One way to facilitate LADAR applications in the cooperative tracking of a target is

to increase its reflection . This may be accomplished either by using reflective clothes or

paints which increases reflection by a factor of few hundred or by placing a retro reflector

array , which increases the signal return , by many order of magnitudes.

In addition to emplacement to retro reflectors on the satellite for cooperative

tracking, air craft have may augmented for traffic control. It is interesting to note that

lost pilots have been located by their display of retro reflected target included in their

survival kit to replace it is the older reflection g mirrors. Air craft control is an excellent

area for the application of the LADAR . For automatic air craft landing system, it is

necessary to have extremely narrow transmitting bandwidth to prevent ground


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LADAR

reflections from causing erroneous air craft tracking signal. Microwave Radar for these

applications have used millimeter wavelength and very large diameter antenna to obtain

narrow beam required, but these approach has resulted in the system of great cost and low

reliability. Coded optical transponders becomes can be placed on aircraft for improving

range considerably ascertaining individual aircraft identity and determining landing

runway characteristics. The air traffics control applications for LADAR will provided

reception and date rates which coupled to the digital computers will greatly improve the

airport operations. Unnamed LADAR systems has been used to detect and to monitor to

this approach both air port have use for traffic management purpose. Another air craft

control application is to measure vertices on air port runways in the measurement of

residual density gradients. After one air craft takes off these information provide a go /no

go decision for the next take of landing.

When the vessel traffic in harbour and channel area increases it becomes necessary

a monitor and control vessel traffic. The relatively slower vessels speed and only a two

dimensional environment is involved (since all vessels are on water surfaces.) results in

an easier traffic management for air traffic control.

The LADAR techniques have unique applicability to one way and two way

communications. The extremely wide information bandwidth available at laser operating

wavelength results in an extremely economical for information transmission over line of

sight distance.

For railway traffic control and data transmission, communication line running

roughly parallel to the rail would be possible. Use of coded LADAR reflector a rail could

allow simple LADAR and the dale processing unit to monitor rail traffic flows and lead to

automatic rail car switching system.

By continuously measuring the distance to the ground from air craft in flight, the

LADAR system can function as an air born terrain profllometer, providing the height and

shape of perhaps in accessible hills and valley below. Remotely locate instrumental coded

beacons, placed by helicopter, can interrogated by a LADAR and the coded, beacon

return signal determined by the instrumentation at the sight of remote temperature

humidity, barometric measurement pressure of wind velocity and direction measurement.


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LADAR

LADAR techniques have been developed to monitor the environments to provide

air pollution location and flow patterns in addition to assisting in weather prediction. The

back scatter return from aerosols have been analysed to determine composition and

intensity in meterological investigation.

The moment of earth surface as small as 2 cm can be measured using LADAR

target. It can also serve as calibration target for ground based , air born and space born

system for navigation purpose.

Modem warfare requires surveillance and during nights and adverse weather

conditions, low light or no light conditions. Infra red surveillance provides under such

conditions.

Missiles, bombs and other projectiles can be laser guided for homing onto'' target

using laser designator located either on ground or an air born platform, and a laser seeker

unit located at the projectile. The radiations scattered by the laser designed target get

focused by the seeker on to a quadrant detector which measure the projectiles deviation

from the reference axis and the computer error signals drives a closed loop mechanism to

correct the flight path and guide the projectile on intended target.

Free space laser communication, provided high band width, small size light weight

low power and low cost. Laser communication yet to achieve the last three qualities.

Laser communication being developed for satellite to satellite, satellite to ground , ground

to satellite to deep space vehicle and air to air communication.

A single laser beam with beam expander optics is used to illuminate and scan the

entire target. A gaited CCD sensor with suitable image optics allows the display of laser

image on the target on TV monitor.

In contrast with down the road laser devices , which must be carefully aimed at

target vehicle across the road laser system. Need not beamed at all after it has been set up

along side of a road it employs two infra red laser beams and operates on the time distant

principle


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LADAR

Its laser is mounted on a horizontal bar and emits parallel light bar that is separated

by point for meter. Adjacent to each laser on the bar is a detector, which measure the

amOunt of light reflected back to the instrument . The equipment directs the beam across

the roadway perpendicular to direction of traffic flows.

To eliminate interference effect between the two beams of light ,each beam is

independently pulse modulated .

A beam detects a vehicle by scnsi9ng a change in the intensity of the reflected

light .since the distance between the beam is known , the transit time from the front edge

of the vehicle to pass from beam to beam ids converted into distance of 0.4 meter.

To prevent error due to multiple line of traffic , measures one vehicles speed twice

when vehicle entire the laser system's operational area and when it leaves . if the two

readings do not match with in a very Light tolerance , the reading is rejected .Thus if two

cars in adjacent lines overlap each other no reading will result unless the cars happen to

be moving at almost identical speed.

VIII ADVANTAGES AND DISADVANTAGEAS

OF LADAR

LADAR system synthesis, performance, evaluation and analyses established that

optical radar techniques and many wavelength related advantages and some

disadvantages (yielding large information bandwidth and extremely high angular

resolution possibilities, while low efficiencies and atmospheric propagation limitations

where among the disadvantages.") With the advent of gas lasers, the use of phase in

LADAR signal processing becomes feasible along with the envelope amplitude

processing. The later has been uses alone when solid state LADAR transmitter was

employed.

The target reflecting properties and two way propagation path loss will largely

determine the power requirement of the LADAR transmitter. Path loss penalties for laser

radar operating wavelength when compared to microwave and millimeter wave weight, is

a major comparison is LADAR/RADAR choice for system applications when operations

in the sensible atmosphere is required.


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LADAR

Long range LADAR application has been the ground to space and space to space

environment, while atmospheric operations have been limited to relatively short range.)

In ground to space operation, the propagation path is limited to approximately the

thickness of the atmospheric shell encompassing the earth while in the space top space

operation, propagation path losses re negligible.

IX LADAR/RADAR COMPARISON

Laser radar arc capable of higher accuracy and more precise resolution than

microwave radar but they are subject to the vagaries of the atmosphere and are thus

generally restricted to shorter ranges in the lower atmosphere than micro wave radar,

Because of the extremely narrow transmitting bandwidth to prevent ground

reflections, air traffic control is an excellent area of application of LADAR , where as for

microwave radar for this application have used millimeter wavelength and very large

diameter antennas, have resulted in the system of great cost and low reliability.

One way laser communication must considered as Attractive when compared to

conventional microwave radar ,sincc they arc simple and reliable component and devoice

with nearly complete immunity to natural and man-made electromagnetic interference.

LADAR RADAR

1 )Laser source is used Electronic oscillators and amplifiers, are

used

2}Shorter wavelength Higher wavelength

3)Narrow light beam width. Antenna gain control

4)Lensing system for beam transmission

and focusing

RF antenna is used for transmission

5)Photo detectors for light reception Field current conversion in RF reception

6 )A different tecrmologyfquantum

mechanical principle) used for component

and device development

Electronics used for component and device

development

7)Atmospheric effects arc most severe. Atmospheric effects are less severe


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LADAR

X. CONCLUSION

Due to very short wavelength narrow beam width and more precise output the

application of laser radar is more in space to space environment . Modern laser radar

system combine the capabilities of radar and optical system to allow simultaneous

measurement of range. Reflectivity ,velocity , temperature azimuth and elevation angle.

DSP technique capable of handling the extremely high information rate available from

LADAR operating bandwidth have increased attractiveness for LADAR in many

applications .

Laser cameras and advanced DSP arc poised to help isolate individual offenders

on crowding highways. Ordinary radar is easily confused by loo much traffic, but a laser

can pick out a motor cycle at over 300 meters.)

Technological advantages in the development of highly efficient laser

sources and miniature integrated circuits laser based devices gets more advances, that is,

if laser sources advances LADAR system advances.


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Documents

Contents 1. Introduction 2. Detection Techniques 3. Ladar Target Description