Electronic warfare in the optical band:Main features,examples and selected measurement data

Romn Ostrowski ,Artur Cywiński ,Mrek Strzelec

a Polish Naval Academy,Navigation and Naval Weapons Faculty,69 ′Smidowicza Str.,81-127,Gdynia,Poland

b Military University of Technology,Institute of Optoelectronics,2 Gen.S.Kaliskiego Str.,00-908,Warsaw,Poland

Keywords: Optoelectronics Electronic warfare Optical signature

ABSTRACT The paper presents the possibilities of,and methods for,acquiring,analysing and processing optical signals in order to recognise,identify and counteract threats on the contemporary battleground.The main ways electronic warfare is waged in the optical band of the electromagnetic wave spectrum have been formulated,including the acquisition of optical emitter signatures,as well as ultraviolet (UV) and thermal(IR)signatures.The physical parameters and values describing the emission of laser radiation are discussed,including their importance in terms of creating optical signatures.Moreover,it has been shown that in the transformation of optical signals into signatures,only their spectral and temporal parameters can be applied.This was confirmed in experimental part of the paper,which includes our own measurements of spectral and temporal emission characteristics for three types of binocular laser rangefinders.It has been further shown that through simple registration and quick analysis involving comparison of emission time parameters in the case of UV signatures in“solar-blind”band,various events can be identified quickly and faultlessly.The same is true for IR signatures,where the amplitudes of the recorded signal for several wavelengths are compared.This was confirmed experimentally for UV signatures by registering and then analyzing signals from several events during military exercises at a training ground,namely Rocket Propelled Grenade (RPG) launches and explosions after hitting targets,trinitrotoluene (TNT) explosions,firing armour-piercing,fin-stabilised,discarding sabots (APFSDS) or high explosive (HE) projectiles.The final section describes a proposed model database of emitters,created as a result of analysing and transforming the recorded signals into optical signatures.

1.Introduction

Contemporary military conflicts have shown that threats can appear at any time,and more importantly,in any place and from any direction.Therefore,the dynamics of the battlefield require a very fast reaction to any threat that appears,which in turn entails full automation,also in the process of detection and identification.The received signal should be faultlessly and clearly identified to allow immediate and appropriate counteraction.Therefore,the electronic warfare which currently precedes and accompanies any military activities gains decisive importance and,in the event of victory on the battlefield,will give an almost 100% guarantee of final military success [1-3].

Recognition and counteraction in the optical band of the electromagnetic radiation range is an element of electronic warfare that is becoming increasingly important[4,5].This is due to the fact that modern armies and contemporary battlefields are awash with various optoelectronic technologies.Electronic warfare in the optical band can be realised in three main areas:

-acquisition of signals from optical emitters and their processing in order to select characteristic emission features (optical signatures) that allow for identification of the transmitter and then -the platform;

-collection of data connected to emission in the“solar-blind”UV range (UV signatures) and in the mid-infrared range (thermal signatures);

-blinding of optoelectronic heads (sensors saturation or destruction) in the target’s guidance systems or systems tracking thermal traces.

Greater knowledge of optical signatures will allow for early recognition and identification of events/threats(e.g.missile launch,cannon shot),thereby producing more time to take appropriate countermeasures.The created databases of signatures will enable the development of automated systems for almost faultless threat identification and suitable counteractions to be taken.On the other hand,such knowledge will allow the construction of guidance systems (optoelectronic heads) that are practically resistant to enemy jamming (flares,decoys,etc.).

Aircraft self-defence systems have for many years been equipped with Radar Warning Receivers (RWRs) which allow the detection and recognition of microwave radiation sources in the range of frequencies that corresponds to the working frequencies of on-board radars as well as radars of ground-to-air and water-to-air missile systems.Nowadays,similar devices are also installed on board ships of different classes and duties.RWR versions that allow detection and recognition of irradiation in the optical band,mainly laser irradiation,are also under development [6].Laser Warning Receivers (LWRs),which are much less advanced than RWRs and only allow detection of irradiation and determination of its zone,have for many years been installed on board land platforms,such as tanks,infantry fighting vehicles(IFV),also known as a mechanised infantry combat vehicles(MICV)or troop carriers.Presently,LWRs do not allow recognition of the laser emitter’s type.

One of the most important integral elements of each RWR is a database that guarantees its operation.Without the database,the RWR would be deaf and blind.Its contents,and updating it,are no less important to the effectiveness of the self-defence system than the technical parameters of the countermeasures and of the RWR itself.They materially influence the effectiveness of threat classification and identification.In order for the self-defence system to guarantee an adequate level of safety,it must include a reliable and constantly updated database.On the other hand,RWRs should ensure effective detection and recognition of the entire range of signals emitted by enemy detection and guidance systems,as it is necessary to obtain as much information on potential threats as possible.Such data may also be registered on an ongoing basis,during different missions,and then can be further processed,analysed and finally put into appropriate records in the database.

This study aims to develop a method of acquiring,analysing and processing optical signals for the purposes of identifying and counteracting threats on the contemporary battlefield.Firstly,the article concisely presents the physical bases of electronic warfare in the optical band,and then briefly reviews the optical emitters currently used.Next,different physical parameters and values describing laser radiation emission are discussed,including their importance in terms of creating optical signatures.It has been shown that in such a process only time and spectral parameters of optical signals are applicable.Moreover,it has been further shown that through simple registration and quick analysis involving comparison of emission time parameters in the case of UV signatures in“solar-blind”band,various events can be identified quickly and faultlessly.The same is true for IR signatures,where the amplitudes of the recorded signal for several wavelengths are compared.This is particularly important in today’s battlefield.The terms used to describe IR signatures and UV signatures are defined.A practical description contains the results of measurements and tests of optical signatures,conducted in the course of other research,in particular during training ground tests.The obtained results may be easily converted into a format allowing their introduction to a database of optoelectronic emitters,the structure and arrangement of which are proposed in the final part of the paper.At the current stage of our work,it is difficult to talk about a specific data format,because it depends on how the future database will be organised.Its structure will decide the format of the records it will contain.For now though,we shall focus on the problem of recording and analysing optical signals in order to obtain specific numerical values that can be stored in binary form.Their conversion to the format required by the created database should not pose any difficulties for the IT specialists who will create such a database.In general,all data obtained is saved as variables regarding character type,integer or floating point,normal or double precision.Such variables are recognised by almost all software.Another problem is whether the records (and which records) in the future database are to be compatible with the batch data formats used by LWR devices.

2.Materials and methods

2.1.Materials

The experimental part of the paper includes examples of spectral measurements of three popular rangefinders:

-binocular rangefinder Geovid 8×56 HD-B from Leica [7];

-binocular rangefinders PRLF 10 and Vector IV from Vectronix[8].

The time waveforms of“solar-blind”band emission in the UV spectral range were examined for Rocked Propelled Grenade(RPG)launches and explosions after hitting targets,trinitrotoluene(TNT)explosions,firings of armour-piercing,fin-stabilised,discarding sabots(APFSDS) and high explosive (HE) projectiles.

2.2.Spectral and temporal signatures measurements

The spectral characteristics of the investigated rangefinders were determined using a compact optical spectrometer from Ocean Optics [9] equipped with a linear silicon Charge Coupled Device(CCD) array covering a range of 190 nm to 1.1 μm.The temporal shapes of laser pulses were recorded by means of photodiodes model FPS-1 from Ophir Photonics [10] coupled with a digital oscilloscope model DSO-X-2024A from Agilent Technologies,Inc.

The UV-spectra in the solar blind range were detected using a radiometer designed at the Institute of Optoelectronics,Military University of Technology(IOE MUT),in Warsaw.Its sensitivity was limited to a range of 220-280 nm (maximum at 255 nm) by the addition of a UV filter,specially designed for experiments,described in section 5.3.The radiometer’s specifications are shown in Table 1.

Table 1 Basic parameters of the UV radiometer designed at the IOE MUT.

An example thermal image(Fig.18)was acquired with the use of a model SC 7000 camera with an indium antimonide (InSb) array from FLIR Systems,Inc.

3.Battlefield optical environment and laser transmitters

3.1.Characteristics of the optical band

The various optical ranges that can be distinguished in the spectrum of electromagnetic waves are usually divided into the ranges of infrared (IR),visible (VIS) and ultraviolet (UV) radiation.

Infrared (IR),also called thermal radiation,falls into the wavelength range of 780 nm to 1 mm,situated between visible light and radio waves.Due to the increasing range of applications,the infrared spectrum includes the Terahertz band(formerly called far infrared),situated in the frequency range of 300 GHz -10 THz of the electromagnetic spectrum in free space,which corresponds to a wavelength of 1 mm-30 μm.The radiation in this range is strongly absorbed by water and many chemicals,such as certain drugs and explosives,having their own characteristic spectra.Another important feature of this radiation is that it is completely reflected by metallic surfaces (e.g.guns,knives or hidden military vehicles).These three features provide broad opportunities to use devices operating in the Terahertz band in protection and surveillance systems.

Visible radiation is the part of the electromagnetic spectrum recognised by human sight,in the wavelength range of 400 nm-700 nm[11].This light is absorbed by the atmosphere and water only to a small extent.

UV radiation is electromagnetic radiation with a wavelength shorter than that of visible light,falling in the range of 10 nm-400 nm.Due to its military applications,the most important part of the UV range is the so-called“solar-blind”band(100 nm-280 nm).Solar radiation in this band is strongly absorbed in the atmosphere by molecular oxygen (for wavelengths below 185 nm) and ozone (for wavelengths between 185 and 300 nm).Detection of UV radiation,along with insensitivity to visible radiation and infrared (especially solar radiation),minimises the number of false alarms and allows for a high values of the signal-tonoise ratio [12].

Optical radiation strongly interacts with atmospheric gases,in particular with water vapour,carbon dioxide and ozone.Therefore,a large amount of solar rays is absorbed or reflected by the atmosphere.The same applies to thermal radiation emitted by the Earth’s surface.Ultraviolet rays are largely absorbed by oxygen(especially in the form of ozone in the ozone layer),while infrared is absorbed by greenhouse gases such as methane,nitrous oxide,carbon dioxide or water vapour.Therefore,the atmosphere is mainly transparent to visible light with low absorption by ozone,oxygen and aerosols.Radiation with wavelengths shorter than 0.3 μm in practice does not reach the Earth’s surface due to strong absorption by ozone and atomic oxygen in the upper layers of the atmosphere.The near infrared includes weak absorption bands of oxygen and water vapour.For the latter,the width of the absorption band intensity increases dramatically towards longer waves.

Transmission of optical radiation in the atmosphere is presented in Fig.1.It should be noted that within the ranges of 2.5-5.5 μm,and of 7.5-14 μm,radiation is attenuated to a small extent.These bands are conventionally called mid-wavelength infrared (MWIR)and long-wavelength infrared (LWIR),respectively [13].This has significant practical consequences as the bands overlap with the sensitivity areas of most commonly used infrared detectors made e.g.of indium antimonide (InSb) for the spectral range of 2.5-5.5 μm and mercury cadmium telluride (HgCdTe) for the longer wavelengths in the range of 7.5-14 μm.It should be remembered that the cut-off wavelength for popular silicon detectors is approximately 1 μm [14].

Fig.1.Transmission of solar radiation in the atmosphere.Recreated using vector graphics from:https://commons.wikimedia.org/wiki/File:Atmosfaerisk_spredning.png.

Every object whose temperature exceeds 0 K emits infrared radiation.The amount of energy is determined by the temperature and surface conditions of the object.Bodies with a temperature similar to room temperature emit the most in the infrared range,while bodies with temperatures over 600°C start to emit with a higher intensity in the visible spectrum.However,the most important fact is that the emission maxima for temperatures of 200-400°C fall in the MWIR band.Such temperatures are typical,for example,for the airframes of rocket propelled missiles during flight[15],regardless of whether the sustainer engine is running or not.

3.2.Laser transmitters

Contemporary battlefields are distinguished by constant change,thus quick reaction to emerging threats is essential and basically a prerequisite to survival.This requires full automation,also in the process of detection and recognition of threats.Detected signals must be identified in a faultless and unambiguous manner to allow immediate counteraction,especially since there are an abundance of artificial jamming sources.This (as well as advantages such as their contactless nature,real-time measurement,high accuracy and precision in target location) makes laser techniques very attractive for military equipment and systems.They allow quick and accurate determination of the target’s parameters,and sometimes even detection,when traditional radar methods or visualisations in the visible band,or infrared,fail.Therefore,the number of lasers and associated optoelectronic devices used by armies is constantly increasing.

This growing interest in the use of laser radiation in military technology [16] results from its characteristics,mostly from its straight-line propagation with relatively low divergence and monochromatic features.It is equally important that,in most applications,devices emit beams invisible to the naked eye in the range of near and short-wave infrared.This means that their use can remain secret,and detection of the source is much more difficult.

Laser radiation,reflected from a target object and scattered,may be received,focused and analysed to aid different arms guidance and fire control systems.In general,laser radiation is mainly used for two purposes:acquisition and collection of data on the target object,as well as guiding the missile towards the target.Precise target designation or visualisation as well as secrecy of operation,surprise the enemy,enabling high striking precision and effective reduction of the amount of striking equipment used (one targetone bomb).Furthermore,laser radiation allows a reduction in the effectiveness of passive defence methods,such as camouflage,masking,flares or decoys.

The use of laser radiation on the battleground also implies certain constraints [17].Most of all,the range of different optoelectronic devices is determined by atmospheric attenuation,even if the air exhibits a high transparency (visibility) level.Laser radiation is always partially absorbed and scattered by gas particles composing the atmosphere.These phenomena are greatly intensified by the presence of different aerosols and atmospheric precipitation,and on the battlefield-by smoke and dust.The latter are often totally non-transparent to laser radiation.

The mere method of propagating the laser radiation may create some limitations.Each laser beam is characterised by a certain degree of divergence,as discussed in section 4.Therefore,the size of a laser spot visible on a distant object is a function of the divergence angle and the distance between the laser transmitter and object (target) [18].For instance,a divergence angle of 1 milliradian means a spot of 1 m at a distance of 1000 m.Thus,if the laser spot is larger than the designated target,which is additionally surrounded by other objects,then we are dealing with the so-called spillover effect,presented in Fig.2.

Similarly,straight-line propagation may cause an adverse phenomenon called the podium effect,the idea of which is presented in Fig.3.The effect emerges when the reflected laser radiation is obscured by the target object and does not reach the bullet’s guidance system.Ground irregularities and even the curvature of the Earth may considerably reduce the range of laser equipment and thus its effective use.

The flashlight effect results from uneven propagation of the laser beam relative to the ground and the related extension of the designated area,presented in Fig.4.As a result,the target object reflects only a part of the beam,and the rest is reflected by the ground.This reduces the range of radiation detection.

The next two factors which limit the use of laser equipment and systems are attributable to the receiver.The first is a limited field of view(FOV)featured by receivers included in guidance systems.This requires the tracking system to be guided towards the target with a relatively high precision and at a relatively small distance,so that it may detect,receive and reliably track echo rays reflected by the target.Another factor is the sensitivity of the detector located in the receiving system,which reduces the effective operating range of optoelectronic equipment and systems.

The vast majority of applications of laser equipment and systems on the battlefield involve rays being reflected by the target object.The reflection is almost always diffusive,so,apart from the reflection itself,the laser beam is also scattered at a wide solid angle (often into a hemisphere).This results in the fact that the enemy may easily detect the laser rays and become aware of any pointing and missile guiding processes.On the other hand,the radiation scattering allows for relatively easy detection and recording in order to distinguish its characteristics -so-called optical signatures which enable identification of the emission source[16].Various types of laser equipment and systems are currently owned by armies all over the world,and they exhibit varying levels of technology and complexity,which may be classified on the basis of their application:

Fig.2.Illustration of spillover effect.

Fig.3.Illustration of the podium effect.

Fig.4.Illustration of the flashlight effect.

-illuminator:a laser device used in low visibility conditions,operating in the near infrared range (800-1000 nm),aimed to illuminate the scene in such a way that it is visible for nightvision devices;

-designator:a laser device used to designate the target object for the striking equipment.Generally,high power continuous wave lasers are used;

-dazzler:a laser device used to affect human sight organs to cause temporary or permanent inability to see.Generally,lasers emitting rays in the visible wavelength range are used;

-pointer:a laser device used to support the process of aiming a personal or on-board weapon in low visibility conditions;

-beam rider:a laser system used for active guidance of missiles to the target using a laser beam;

-fire control system:a fire control system includes at least a sight,laser rangefinder,ballistic computer,sensors (of temperature,pressure,speed and direction of wind) and an adjustment system;

-optical data transmission:an alternative solution to radio communication.Its advantages include resistance to tapping and electromagnetic interference.A disadvantage of the optoelectronic link is the impossibility to transmit data in harsh weather conditions,such as a dense fog,heavy rain or snowfall.

The most advanced and complex group of laser devices is currently formed by LIDARs.LIDAR (Light Detection and Ranging)represents a remote and active measurement technique in which an optical beam is used as a test instrument.Contemporary LIDARs are based on lasers ranging from UV to far infrared,generating pulses with a peak power of more than 10 MW and a duration from a few to several hundred nanoseconds.

Classification of LIDARs into different types,presented in Table 2,results from the kind of measured phenomenon which accompanies their interaction between the laser radiation and the tested object.

Table 2 Classification of LIDARs.

LIDARs are useful tools for remote surveying of the environment and detection of hazardous substances.The LIDAR method belongs to the group of active methods of remote laser sensing.LIDARs allow detection and sometimes even preliminary identification ofaerosol particles invisible to the eye and other methods of remote detection.Thus,the use of LIDARs in military applications often combines remote detection of chemical and biological weapons[26],as well as range finding [19].

In addition to the above list of military laser systems,laser spot trackers,laser-guided weapons and Identification,Friend or Foe(IFF) systems,currently under intensive development,must be mentioned.These systems send to the unidentified object a laser beam with appropriately selected spectral and temporal characteristics (encoding),which leads to detecting the echo signal,i.e.the signal reflected exactly backwards.If the sounded object belongs to the same army,its identification module identifies the code as its own signal and responds by initiating a process of modulating the reflected radiation in accordance with a defined time sequence.

4.Characteristics of laser radiation

Laser radiation may be characterised in many ways and using various parameters and physical values which may generally be grouped into energy parameters,spatial parameters,polarisation,spectral parameters and temporal parameters.In terms of optical signatures,the laser radiation characteristics may be classified using the system presented in Fig.5.

In general,lasers may operate in two regimes:pulsed regime and a so-called CW(Continuous Wave)regime.A CW laser is a laser operating with continuous power for 0.25 s or more[11].A pulsed laser is a laser which provides energy in the form of a single pulse or a sequence of pulses with a duration of less than 0.25 s.The radiation of CW lasers may be characterised by (instantaneous)power,mean power and power stability over time.Pulsed lasers parameters include pulse energy,pulse peak power and the stability of the pulse energy in time.For modulated beams,the values mentioned above are supplemented by parameters indicating modulation,such as modulation depth or frequency [27].

Fig.5.Classification of laser radiation characteristics in terms of optical signatures.

Energy parameters are very easy to determine by way of direct measurements.There is a wide range of available meters that measure the power/energy of laser radiation,and which also allow various measurement statistics to be determined,such as average values,dispersions,variances,etc.These parameters,despite being easy measurable,are practically useless in the process of irradiance detection and source identification.The measurement of power or energy at a distant point from the transmitter,with no guarantee that the measuring probe receives the whole laser beam,does not say anything about the actual energy or power of the emitter.The value measured in this case is a complex function of propagation path and atmospheric extinction,not to mention the scattering which accompanies reflection from the target object,the nature of which is unknown in the actual conditions.

In the group of spatial values,there are two main parameters:beam diameter(width)and divergence.Additional ones define the propagation:the position and diameter of the beam waist,Rayleigh range and beam quality factor M2(see Eqs.(6) and (8)).These are complemented by the beam’s polarisation.Although spatial parameters are shown in the group of irrelevant parameters in laser emitter identification(Fig.5),they are included in the NATO Emitter Data Base Electro-Optics (NEDB EO).They are important from the point of view of laser safety on the battlefield.Their main definitions and characteristics are described below.

Even if the actual laser beams are far from the perfect Gaussian beam,equations defining its properties are useful in real-life conditions.The beam pattern of a Gaussian beam in a plane being perpendicular to the propagation axiszis calculated as follows[28]:

wherew(z) is the parameter which,in this case,defines the beam diameter between points 1/e2,as presented in Fig.6.This parameter fulfils the propagation equation:

Fig.6.The intensity distribution of a Gaussian beam and its“natural”diameter 2w(z).

wherew0is the minimum valuew(z),at the position of the socalled beam waistz=z0,presented in Fig.7.The value:

Fig.7.The Gaussian beam diverging from its waist.

determines the Rayleigh range defined as the propagation path from the beam waist to the place where the beam cross-sectional area is twice the area at the waist.The parameterzRillustrates e.g.the partition between the near field (Fresnel diffraction area)and the far field (Fraunhoffer diffraction area).

In accordance with Fig.7,the divergence angle at the far field of a Gaussian beam is defined as:

The spatial parameters of an actual laser beam may be easily connected with the parameters of the ideal Gaussian beam using the beam quality parameterM2[29],determined by the following equation:

whereD0and Θ are the width and the divergence of the actual beam,respectively.By introducing,to the above relationship,the spatial parameters of the perfect beam and grouping the factors,several relations connecting the parameters of actual beams and the Gaussian beam may be formulated.Firstly,if the perfect and actual beams have the same waist widths,the actual beam must diverge faster byM2than the perfect beam:

Secondly,if the perfect and actual beams have the same divergence angles,the actual beam has a waist that is wider byM2.Finally,if the actual beam is wider bythan the perfect beam,its divergence angle must be larger by the same factor:

In accordance with[11],the beam diameter(or width for axially asymmetric beams)duat a given point is the diameter of the smallest circle containingu% of the total laser power (or energy).For the Gaussian beam,the diameterd63corresponding to the point where the irradiance (irradiation) is reduced to 1/e of its central peak value is used.The beam divergence is the cone plane angle determined by the beam diameter in the far field.If the beam’s diameters at two points located at the distancerfrom each other are equal tod63andd’63,the divergence is determined by the following formula:

For the NATO NEDB EO,a definition of beam divergence as per International Standard IEC 60825-1 2007 has been adopted.According to that standard,the beam divergence at the far field is the plane angle of a cone defined by the diameter of the beam which determines the smallest circle containing 63%of the total power or energy of the laser beam.

Similarly to the energy values,the spatial parameters of laser beams are completely irrelevant in the emitter identification.The actual measuring process(to be more precise,the determination of beam width or divergence) is quite complicated and even impossible at a long distance from the source,due to e.g.the divergence and the resulting large laser spot.Furthermore,the process of detection,recording and processing optical signals,in order to characterise the emitters,will almost exclusively record scattered radiation for which the spatial parameters become irrelevant.

In the case of real beams,the presented formulas and conclusions may be difficult to apply.Problems arise from the fact that,in many cases,there are beams with asymmetric spatial profile.For instance,in many rangefinders,semiconductor light sources that are built with strip-shaped emission areas are commonly used.The size of the strips is around several μm in one axis and several dozen or several hundred μm in the other axis.An example of shape of such interfaces is presented in Fig.8 on the left.In this case,the total junction’s emission size is 254×203 μm,and each strip has a size of 2×254 μm.A beam generated by a rangefinder with such a source will approximately present an enlarged image of the junction.Additionally it will be deformed by aberrations introduced by the transmitter’s optical signal and the influence of atmospheric turbulence during spatial propagation.Examples of deformation are shown on the right in Fig.8.

When determining the divergence of such beams,the criterion of energy contentu% or power content in a given area with a diameterDuhas to be used.Normally,the valueu%is equal to 95%or 99%.Beam divergence will be defined as the ratio of the diameterDuand distancerat which the measurement is taken:

Fig.8.The exemplary shape of the laser diode junctions(on the left)and the image of a beam generated by the laser diode rangefinder (on the right).Photo R.Ostrowski.

When analysing the review of the military applications of optoelectronic equipment and systems,included in section 3.2,it is noticeable that they use different lasers which emit in a wide range of the optical band,from UV to long-wave infrared.Table 3 shows the most popular laser types used in military equipment,along with typical operating wavelengths.The possibilities of identifying the emitter based on its emission spectrum can be clearly seen,all the more so since such a measurement is not complex and may be performed for the scattered radiation.The market offers compact spectrometers with a sufficient resolution and sensitivity to conduct such measurements.As a result,basic spectral parameters can be obtained,such as wavelength or width of the emission spectrum,defined as the frequency range measured between the points of the spectrum’s half peak power.This obviously does not provide comprehensive information on the emitter,but it allows for preliminary identification of the type of device or system.

Table 3 Emission wavelengths of lasers applied in military optoelectronic equipment.

The last group of laser radiation characteristics,being very useful for recognition and identification in the optical band,is constituted by temporal parameters.Here,similarly to the spectral parameters,measurements in a simple system using an appropriate photodetector and oscilloscope can be easily performed.The recorded waveform of the optical signal allows determination of all the characteristic temporal parameters.Two subcategories can be distinguished.The first(including the pulse duration τi,rise time τr,fall time τfand pulse asymmetry defined by the ratio τr/τf)concerns the generation of single pulses or a sequence of pulses with a repetition rate offi.The other subcategory relates to the generation of pulse bursts and includes such parameters as pulse burst durationTpor repetition frequencyfp.All the parameters described above are visualised in Fig.9.

Pulse duration is defined as the time interval measured between the points of half the peak power on the rising and falling slope of the pulse [11].Emission duration is the duration of a pulse,sequence or series of pulses,or the period of continuous operation,when people can access the laser radiation when the laser device is operated,maintained or repaired.

For a single pulse,the pulse duration is the period between the point of half the peak power on the rise slope and the corresponding point on the fall slope.For a sequence of pulses (or its section),it is the period between the point of half the peak power ofthe first pulse and the point of half the peak power of the last pulse.

In many real-life cases,the determination of the points of half the peak power on the pulse slopes may be impossible due to the pulse shape.Such a situation is visualised in Fig.10.

The presented pulse has a complex shape.To measure the pulse duration,the energy criterion(energy content ofu%-Eq.(12))can be used.In this case,the gravity center of the pulse should first be determined:

where the integral in the denominator of the above equation is the total energy of the pulseEt.Then we start integrating the pulse to the left from pointt0to pointt1=t0-Δt1

and to the right from pointt0to pointt2=t0+Δt2

so that the total energy in the range fromt1tot2is equal to the previously specified fraction of the total pulse energyEt

Finally,the duration of the pulse is given by:

With appropriate numeric algorithms,the presented method can be used to calculate pulse durations based on energy content.An additional advantage of this method is the possibility to establish the pulse duration as a function of coefficientu,which may be treated as an additional feature of the signal.

5.Selected measurement results

5.1.Spectral signatures

With optical spectrometers,it is possible to obtain information on the emitter’s wavelength and the width of the emission spectrum.Depending on the type of detector and diffraction grating,the spectrometers on the market are distinguished by a measuring range which allows information on the spectrum to be acquired within a range of 190 nm-2500 nm and a resolution of less than 1 nm.This is sufficient to determine the spectral parameters of the optical emitters owned by different military forces.Measurement of spectral parameters is possible with direct exposure or by scattering.In order to cover the entire spectral range,from 190 nm to 2500 nm,two spectrometers are required.One of them must be equipped with a silicon detector covering the spectral range of 190 nm-1100 nm,and the second with an InGaAs detector covering the spectral range of 900 nm-2500 nm [30].

A diagram of the measuring system is presented in Fig.11.The S1 and S2 symbols represent measuring probes.Their shapes,technology and process parameters will depend on the solution assumed by the designer.

Fig.9.Model time waveforms of laser emission.

Fig.10.Example of a pulse with an irregular shape.

Fig.11.Diagram of a measuring system for the acquisition of spectral signatures from optical emitters.

Fig.12 presents on the left side a measured model emission spectrum of a binocular rangefinder Geovid 8×56 HD-B from Leica[7],the source of which is a semiconductor diode emitting radiation in the near infrared spectral range.

Here we can see the distinctive asymmetric shape of the spectrum,its width and maxima.The maximum emission of the Geovid 8×56 HD-B rangefinder is located at a wavelength that is close to 897 nm.The emission spectrum width is approximately 5 nm.Both spectra were recorded by the authors of this paper.

Another example is the spectrum of the binocular rangefinder PRLF 10 from Vectronix [8],presented on the right in Fig.12.Spectral tests of the PLRF 10 rangefinder’s emission have revealed that the shape and width of the spectrum vary from pulse to pulse.Its FWHM width oscillated from approx.6 nm to approx.8 nm.An in-depth analysis of the spectrum reveals that the laser used in the rangefinder generates emission at two lines,around 905 nm and 908 nm.

5.2.Temporal signatures

A diagram of an example system for measuring the temporal parameters of optical emitters is presented in Fig.13.The symbols from D1 to D4 represent the detectors which may be used in any configuration and number,depending on the application.Apart from the detectors,another element necessary in any measuring system is the data acquisition system.In the simplest case,it may be a digital oscilloscope or a dedicated electronic system that ensures time waveforms are acquired with sufficient precision.Moreover,the format used to record temporal changes should allow their further processing.

Fig.12.Emission spectrum of binocular rangefinder Geovid 8 × 56 HD-B (left) and PLRF 10 (right).

Fig.13.Diagram of a system for measuring and acquiring temporal parameters of optical signatures.

The use of several detectors of the same type,with electronic systems capable of different amplification levels,will guarantee the large dynamics required to measure the temporal parameters.Alternatively,single detectors with logarithmic amplifiers can be used.The preferred solution may be chosen after a detailed analysis of the entire electronic path for different types of detectors(bandwidth,signal-to-noise ratio (SNR),sensitivity,background radiation level,etc.),emitters and the conditions in which the signatures will be acquired.

As an example of the acquired temporal parameters of emission that constitute the optical signatures,the characteristics of two laser rangefinders from Vectronix Inc.(PLRF 10 and Vector IV),as well as the binocular rangefinder Geovid 8 × 56 HD-B from Leica,will be presented.

To measure distance,the first rangefinder emits 30 bursts of radiation pulses,the temporal structures of which are presented in Fig.14.The first burst is a probing series with an average power of 5.5 W.If no echo signal appears,the rangefinder sends another 23 bursts with the same average power.If an echo signal is detected,another 29 bursts are emitted with an average power of 1 W.In the latter case,the total duration of the emission is 248 ms.A single burst of laser pulses has a duration varying from 6.83 ms to 7.89 ms,and the burst interval ranges from 0.96 ms to 1.22 ms.Each burst includes 104 to 120 pulses with an interval of 65.5 μs (the pulse repetition frequency is approximately 15.3 kHz).The temporal shape of a single pulse generated by the PLRF 10 rangefinder is presented in Fig.14(on the right).The shape is almost symmetrical,with an average duration of approx.56 ns (FWHM).

To measure distance with the Vector IV rangefinder,61 pulse bursts are emitted.The average power of a laser pulse is 10 W.The total duration of the emission is 980 ms.A single burst of laser pulses has a duration of approx.13.8 ms,and the average burst interval is 2.2 ms.The temporal structure of pulse bursts is presented in the oscillogram on the left in Fig.15.Each burst includes 130 to 133 pulses spaced at 104.0 μs(which gives a pulse repetition frequency of 9.6 kHz).A single laser pulse is asymmetric and irregular,typical for an emission from the semiconductor multijunction laser rangefinder source.The average duration is 102.5 ns(FWHM),and the temporal shape is presented in Fig.15(on the right).

The last of the three analysed devices,the binocular rangefinder Geovid 8×56 HD-B from Leica,emits a series of pulse bursts with a total duration of 228.5 ms.A single burst has a duration of approx.1.16 ms,and the burst interval is 2.44 ms.A single burst includes 62 laser pulses with an average duration of 57 ns and a repetition frequency of 54 kHz.Fig.16 presents the time waveform of a laser radiation emission from the binocular rangefinder Geovid 8 × 56 HD-B.

Table 4presents a synthetic comparison of temporal parameters characterizing the emissions of all three laser rangefinders.It is clear that the recording and time analysis of such emissions allow quick and reliable identification of the transmitter.

Table 4 Comparison of selected binocular laser rangefinders.

5.3.UV signatures

Due to significant atmospheric attenuation of the“solar-blind”signals (<280 nm) and the resulting lack of signals from natural sources and background radiation,each emerging emission means that its source is artificial.Each detection(especially if the emission has some distinctive characteristics)would allow identification of a particular threat,e.g.the launch of an anti-tank grenade.To obtain such a high level of certainty,the characteristics of such an emission-i.e.the optical signatures-have to be analysed.They may be understood,for instance,as time courses of radiation intensity accompanying different events that may occur on the contemporary battlefield,including the launch of smoke grenades,detonation of explosives,outbreak of fires,and most of all the launch of RPGs,missiles or cannon shot.Knowledge of such signatures would make it possible to recognise,almost without errors,the signals of threats and(by eliminating false alarms)to undertake appropriate countermeasures to neutralise the threat or minimise its effects.

Fig.14.Temporal structure of bursts (left) and temporal shape of a single laser pulse (right) in a burst emitted by the PLRF 10 rangefinder.

Fig.15.Temporal structure of pulse bursts (left) and temporal shape of a single laser pulse (right) emitted by the Vector IV rangefinder.

Fig.16.Temporal structure of pulse bursts (left) and temporal shape of a single laser pulse emitted by the Geovid 8x56 HD-B rangefinder.

Fig.17 presents examples of time waveforms of“solar-blind”emission,accompanying different events,recorded with the UV radiometer developed at the Institute of Optoelectronics,Military University of Technology.The temporal shapes of emissions,and above all the related characteristic times that form the optical signatures,allow different events to be identified.For instance,the RPG launch is accompanied by a UV emission with a duration of 2-3 ms (FWHM).In the case of the TNT explosion,the duration is around 1.5 ms (FWHM),and an APFSDS or a HE projectile launch causes an emission lasting approx.37 ms (FWHM).A brief comparison of the basic times typical for the emission accompanying mentioned above events,is presented in Table 5.Table 5 includes also temporal data for an RPG explosion,which are not shown in Fig.17.

Table 5 Comparison of times characteristic for UV emission accompanying different events.

5.4.Thermal signatures

Thermal signatures should be understood as the amount of energy emitted by a heated object in different wavelength ranges.They should not be mistaken for the so-called thermal imaging which involves energy integration in the spectrum.Although temperature distribution on the object is obtained,information on the spectral distribution of energy,which is important for recognition and identification,is lost.Such a distribution is characteristic for each heated object and is difficult to simulate in an artificial manner.Knowledge of such thermal signatures would allow construction of infrared self-guiding heads that were virtually insensitive to enemy countermeasures.In addition,the simplicity of registration and quick analysis algorithms would allow for quick and almost faultless identification of target objects.Fig.18 presents a flame caused by a fuel explosion in the visible band (on the left)and its thermal image (on the right).

Fig.17.“Solar-blind”emission accompanying events that may occur on the battlefield:an APFSDS launch,HE launch,RPG launch and TNT explosion.

Fig.18.Flame image in the visible band (on the left) and thermal image (on the right).Photograph and thermal image recorded by the authors.

The most effective method of acquiring thermal signatures is observing objects in selected,narrow spectral bands using single,fast detectors.This may be achieved in a system with detectors sensitive in the MWIR,coupled with narrow-band interference filters,as presented in Fig.19.By using e.g.a linear or a 2D array of detectors with appropriate filters,it is possible to record the spectrum with a resolution defined by the number and spectral characteristics of the filters.An essential system component is the electronic amplification system and data acquisition module.

Fig.19.Diagram of a thermal signature determination system:F1,F2,F3 and F4 are interference filters,D1,D2,D3 and D4 are detectors.

Fig.20 presents a concept for the acquisition of the MWIR signatures,exemplified by a spectrum of flame caused by a fuel explosion.On the left,normalised solar radiation intensity and the relative intensity of radiation generated by the flame can be seen,with the bands(highlighted in black)where the detectors equipped with interference filters may register signals.

Each of the three detectors records the sunlight radiation intensity,then they generate a useful signal derived from the flame.Analysis of the relations between the signals allows the object or event to be identified.The right-hand part of Fig.20 presents the ratios of the amplitudes of the signals recorded by the detectors:3 μm-3.5 μm,3 μm-4.3 μm and 3.5 μm-4.3 μm.Each object or event has its own characteristic signature which may be used for early recognition and identification/classification.There are many methods of analysing the signals.It is important that the presented method is quick and the measuring system is simple,which consequently allows for real-time analysis of the recorded signatures.This is crucial in military applications.

6.Model of emitter database structure

A model arrangement and structure for an optoelectronic emitter database is presented in Fig.21.The database contains average,maximum and minimum values of the acquired many signal characteristics.It should also include information on different statistics,since the process of identification (comparison of the received signal with the database patterns) involves statistical analysis.Work is already underway to adapt methods of multicriteria analysis [31] for the comparison process,which will allow the probability of incorrect threat identification to be reduced to a minimum,even if the signals are highly disturbed.

Fig.20.Concept for the acquisition of thermal signatures;on the left,normalised intensity of solar and flame radiation as a function of wavelength;on the right,a comparison of signal amplitudes at selected lines (grey:solar radiation,black:flame).

Fig.21.Model of database of optoelectronic emitters.

The database records a lot of different information on the emitter,and due to the complexity only the main types are shown in Fig.21.The emission itself is identified,as is the emitting device or the system it is part of,supplemented by technical information from data sheets or the manufacturer’s specifications,the manufacturer itself,country of origin,as well as any relation to -and interaction with -other devices or systems (e.g.coupling to a microwave radar).The subsequent fields and records should include information on whether a given emitter is a component of a weapon(e.g.guiding system)and in which platform it is installed.The platforms are divided,first of all,into land,sea and air platforms,and then the system provides a detailed specification of the possible object (e.g.a particular type of aircraft or ship).From the perspective of RWRs and security during a mission,it is very important to know the countries where a given emitter is used and where on the Earth it is used most often.

Apart from the values of different signal (laser beam) parameters characteristic of a given emitter,the database should contain encoded information on its operating mode.Here,initial searching,tracking,guiding and scanning(e.g.sector-based,raster-based)are distinguished.Additionally,the database should include operation events without scanning or with an unknown type of scanning,supplemented by scanning parameters such as scanning speed and period.

The fields and records concerning the operating mode also have their respective signal parameters which reveal if we are dealing with a continuous (CW) or pulsed emission.In the first case,the database records parameters relating to the type of modulation -amplitude modulation,frequency modulation and phase modulation-where a continuous change in phase is distinguished from a step change.In the case of pulse modulation,due to various changes in the pulse repetition interval(PRI),there may be:a fixed sequence,a stagger sequence(with 2-8 values)or a jitter sequence.

The database should also contain different parameters characteristic of the signal itself.Here,pulse amplitude (measured signal power or energy),spectrum (measured signal wavelength) and duration can be distinguished.It is recommended to include the possibility to record full information on different disturbance sources and phenomena/events which may occur on the battlefield,i.e.the above-mentioned UV and IR signatures.

7.Conclusions

Recognition and counteraction in the optical band of the electromagnetic radiation range is an important element of electronic warfare.This is due to there being a large number of optoelectronic means used by modern armed forces.

Fast detection of laser irradiance,analysis and identification of its source-i.e.the emitter type(rangefinder,illuminator,etc.)and then the platform where the emitter is located -will allow us to find out if we are dealing with our own forces or with the enemy,as well as what threat the enemy may pose.Such recognition will also allow for effective disturbance of a laser-guided weapon used by our enemy.

Electronic warfare in the optical band should be considered in regards to two aspects.First,it is about the acquisition of signals from optical emitters and their processing in order to select characteristic emission features that allow for identification of the transmitter.Among the numerous parameters of laser radiation,only the spectral (wavelength and emission linewidth) and temporal parameters(duration and repetition frequency of pulses and pulse bursts) are relevant when it comes to acquiring emitter characteristics.The ability to identify an emitter on the basis of the recorded spectral and/or temporal signature of its laser pulse has been experimentally demonstrated using popular binocular laser rangefinders as examples.The maximum emission of the Geovid 8 × 56 HD-B rangefinder (Leica) is located at a wavelength that is close to 897 nm with an emission spectrum width (FWHM) of～5 nm.For comparison,spectral tests of the PLRF 10 rangefinder’s(Vectronix Inc.)emission have revealed that the shape and width of the spectrum varies from pulse to pulse.This rangefinder generates emission at two lines,around 905 nm and 908 nm,with an emission spectrum width(FWHM)in the range of 6-8 nm.A synthetic comparison of the measured temporal parameters of laser pulses(Table 4) has shown more significant differences.Three laser rangefinders were tested:the PLRF 10 and Vector IV from Vectronix Inc.,as well as the Geovid 8×56 HD-B binocular rangefinder from Leica.The total time duration of the emissions varied from 228.5 ms(Geovid 8 × 56 HD-B) to 980 ms (Vector IV).The time duration of the single bursts of laser pulses varied from 1.16 ms(Geovid 8×56 HD-B),through 6.83-7.89 ms(PLRF 10)to 13.8 ms(Vector IV).The duration of single pulses was similar in the case of the PLRF 10 and Geovid 8 × 56 HD-B rangefinders (56 ns and 57 ns,respectively),but almost twice longer(102.5 ns) for the Vector IV rangefinder.

Another equally important aspect of electronic warfare in the optical band is collecting data concerning emissions in the“solarblind”(UV signatures) and mid-infrared (IR signatures) bands.Knowledge of these signatures would,on the one hand,let us develop systems that allow for almost faultless identification of threats and undertake appropriate counteractions,and on the other hand,create guidance systems (optoelectronic heads) that were virtually insensitive to the countermeasures deployed by the enemy (flares,decoys,etc.).This would enable effective aircraft and UASs to defend themselves against the enemy’s striking equipment.Optical signatures will allow early recognition and identification of events/threats(e.g.a missile launch,cannon shot),thereby leaving more time to take appropriate countermeasures.It has been experimentally confirmed that the temporal shapes of emission(and above all the related characteristic times),which form the optical signatures,allow different events to be distinguished.An RPG launch is accompanied by UV emission with a duration of 2-3 ms,but its explosion lasts 32.3 ms.In the case of a TNT explosion,the duration is around 1.5 ms,and an APFSDS or HE projectile launch causes emission lasting approximately 37 ms.The values measured (FWHM) are shown in Table 5.Each object or event also has its own characteristic IR signature which may also be used for early recognition and identification/classification.It has been shown that proper selection of MWIR wavelength measurement(e.g.4.3 μm instead of 3 μm)can increase the signal to noise ratio around 10-fold.

In addition,one more aspect of electronic warfare should be mentioned:blinding the optoelectronic heads (i.e.triggering saturation conditions or the destruction of detectors)used by systems that track the target spot or thermal trace.Even a momentary blinding of such a head causes the striking device to lose its target and become harmless.

Funding

The authors would like to acknowledge the National Center for Research and Development in Poland for grant No.DOB-1-6/1/PS/2014:“Laser Systems for Directed Energy Weapon,Laser Systems for Non-Lethal Weapon”,which provided a proportion of the funds needed to conduct this research.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.