rus
Issue #6/2021
A. V. Medvedev, A. V. Grinkevich, S. N. Knyazeva
Specific Features of Solar Blind UV-Range Devices
Specific Features of Solar Blind UV-Range Devices
DOI: 10.22184/1993-7296.FRos.2021.15.6.502.524
This article reviews a specific variety of optico-electronic devices operating in the spectral range from 0,25 to 0.3 µm, which is commonly referred to as Solar Blind Ultra Violet (SBUV) spectral range as the solar radiation of the said wavelengths does not virtually reach the earth surface. The devices of this category are operable even in the viewing conditions “against the sun”, which is not possible with any other system. Apart from special features of the SBUV devices, their designs for various areas of application are reviewed.
This article reviews a specific variety of optico-electronic devices operating in the spectral range from 0,25 to 0.3 µm, which is commonly referred to as Solar Blind Ultra Violet (SBUV) spectral range as the solar radiation of the said wavelengths does not virtually reach the earth surface. The devices of this category are operable even in the viewing conditions “against the sun”, which is not possible with any other system. Apart from special features of the SBUV devices, their designs for various areas of application are reviewed.
Теги: flash sensor solar blind uv direction finder uv range uv targeting systems датчик вспышки солнечно-слепой уф-диапазон уф-пеленгатор уф-системы наведения
Specific Features of Solar Blind UV-Range Devices
A. V. Medvedev, A. V. Grinkevich, S. N. Knyazeva
ROMZ OJSC, Rostov, Yaroslavl region, Russia
EVS CJSC, Moscow, Russia
EDB ROMZ OJSC, Rostov, Yaroslavl region, Russia
This article reviews a specific variety of optico-electronic devices operating in the spectral range from 0,25 to 0.3 µm, which is commonly referred to as Solar Blind Ultra Violet (SBUV) spectral range as the solar radiation of the said wavelengths does not virtually reach the earth surface. The devices of this category are operable even in the viewing conditions “against the sun”, which is not possible with any other system. Apart from special features of the SBUV devices, their designs for various areas of application are reviewed.
Keywords: Solar Blind, UV range, UV direction finder, UV targeting systems, flash sensor
Received on: 14.08.2021
Accepted on: 10.09.2021
The Earth’s atmosphere in the stratosphere at an altitude of 25–30 km has an ozone absorption band in the ultraviolet (UV) spectrum of 0.25–0.3 μm. Therefore, UV radiation from the Sun does not reach the Earth’s surface, forming in the lower atmosphere, namely in the troposphere, the so-called “solar blind” zone. In this zone, the solar radiation of the indicated wavelengths practically does not reach the earth’s surface, and the transparency of the optical paths differs from zero.
Optoelectronic systems, which have a solar blind UV working spectral range, function even when they work “against the sun”, which is inaccessible to any other system. For example, in optical-electronic systems of the visible and infrared (IR) spectral ranges, when working “against the sun”, intense solar radiation hits the photosensitive area of the photodetector and creates strong interference or disables the entire device.
Let’s consider the features of solar blind UV devices for different applications.
1. UV direction finders
Devices of the solar blind UV spectrum in the form of UV direction finders have found the most widespread use in the systems of countering terrorist attacks as part of the airborne defense systems of not only military, but also civilian aircraft [1].
With their help, crews can receive warnings about the approach of missiles by detecting traces of their rocket engines. Providing speeds from 600 to 1400 m / s, rocket engines emit a jet of exhaust gases heated to high temperatures, which are high-temperature sources that ionize the air to form UV radiation.
One of the variants of the UV direction finder was developed at the ROMZ PJSC enterprise. The design solution with the traditional placement of functional units in a single body is shown in Fig.
1.
The device contains a narrow-band light filter, a UV lens, an UFK‑2 UV radiation sensor, additional sensors, which include a GPS / GLONASS receiver, a compass, an altimeter, and electronic units for processing and issuing signal information.
In this configuration, the UV direction finder as part of the onboard complex measures the three-dimensional coordinates of UV radiation sources, namely: determination of latitude, longitude and altitude above the Earth’s surface.
The UV radiation sensor is a domestically produced photomultiplier UFK‑2, which converts electromagnetic radiation in the optical range with wavelengths from 210 to 350 nm into an electrical signal. It has a semitransparent photocathode based on heteroepitaxial nanostructures of GaN / AlN compounds grown on a sapphire substrate, a multiplication system consisting of two microchannel plates, and a four-sector anode with four separate leads, which makes it possible to determine the center of an electron avalanche formed by two microchannel plates even in the case of only one photon hits the photocathode. It is the center of the avalanche that is considered the coordinate of the photon hitting the photocathode.
The coordinate is determined by the ratio of voltages equivalent to charges at each anode, according to the following formulas:
X = (U1 + U4 – U2 – U3) / (U1 + U2 + U3 + U4 + UФЭ),
Y = (U1 + U2 – U3 – U4) / (U1 + U2 + U3 + U4 + UФЭ),
where: Ui are the voltages taken from the i-th quadrant of the collector.
The functional diagram of the UV direction finder option is shown in Fig. 2. The functioning of the system is as follows. Current signals from the UFK‑2 anodes are fed to transimpedance amplifiers that convert current into voltage. From the outputs of the amplifiers, the signals are fed to differential amplifiers that generate paraphase signals necessary to match the levels of three differential analog-to-digital converters (ADC) of the STM32H743IGT6 microcontroller with multiplexing of the input analog channels. The use of differential channels of a 16‑bit ADC can reduce the conversion noise from the digital ground of the microcontroller.
In the 14‑bit ADC mode, the maximum conversion frequency is 4 MHz, and when the input multiplex channels are allocated to three channels for each ADC, the maximum conversion frequency for all channels is 4 / 3 MHz.
Please note that the microcontroller has a maximum clock frequency of 480 MHz, which is sufficient to run software without an operating system (OS). For communication with external systems, two interfaces are used: CAN or Ethernet 10 / 100. The microcontroller is connected with the receiver of the GLONASS / GPS / GALILEO / QZSS / SBAS space navigation system by the GeoS‑5M module using two serial USART (RS232) interfaces.
In civil applications, a solar blind UV direction finder can be effective as part of an unmanned complex that solves the problem of fixing defective places of high-voltage power lines upon the occurrence of corona discharges, the emission spectrum of which is shown in Fig. 3.
The calculation of the UV direction finder objective is based on the spectral characteristics of the corona discharge and the final version of the scheme is implemented using the following line of materials – calcium fluoride, quartz glass and sapphire.
A narrow-band optical UV filter blocks the solar radiation spectrum and emits a solar blind portion of the UV spectrum with Δλ = (0.24–0.28) microns.
Main technical characteristics of UV direction finder:
The four-anode multiplier UFK‑2 has high parameters. Thus, an amplification of the order of 106 and more times opens up wide opportunities for detecting weak signals, and the declared accuracy of determining the angular coordinates of ±0.5° is quite sufficient for many practical applications.
To carry out specialized measurements requiring a radical increase in the accuracy of determining the coordinates, the UralAlmazInvest Production and Technical Center proposed a new, structurally simplified scheme of a position-sensitive photodetector based on diamond, with smaller overall dimensions.
Fig. 5 shows a diagram of the square arrangement of the contacts of the receiving element of such a four-contact position-sensitive photodetector with the designation of the “binding” of the coordinates of its points and the illumination spot. The receiver is a fully analog device and instantly responds to ambient light. The resolution of the device depends on the resolution of the optics and the uniformity of the properties of the photosensitive material over the area.
Receiving a signal, digitizing it and determining coordinates takes a minimum amount of time and does not require large computing power, since, unlike a matrix of 1 000 × 1 000 elements, only one, known in advance, the only needed image point is processed.
A characteristic feature of the direction-finding channel using a PMT is the ability to accurately determine the coordinates of only one dangerous target. If there are several targets, their simultaneous position is not determined, and signal processing becomes impossible.
In this regard, the development of a UV photodetector of an EOC architecture based on a new design of a solid-state spectral converter of a UV image in the range of 20–270 nm into an image of the range (738 ± 10) nm and into a visible one, carried out by the Moscow Institute of Electronic Technology and MELZ FEU LLC, is of undoubted interest. (Fig. 6).
Fig. 6a shows the design of a vacuum emission detector: 1 – a vacuum-tight metal-ceramic body, 2 – an entrance window made on the basis of a diamond plate saturated from the outer side of the SiV with centers at least to a thickness of ~α–1 (α is the light absorption coefficient in the working spectral range), 3 – photocathode, sensitive in the spectral range of ~730–740 nm and formed on the back side of the diamond entrance window plate, 4 – microchannel plate (MCP), 5 – cathodoluminescent screen (CLS), 6 – fiber-optic glass (FOG).
Fig. 6b shows a photo of a grid-type UV photocathode based on a diamond film, Fig. 6c shows a photo of a UV EOC in a 2+ generation housing with a diamond photocathode, and Figure 6d shows a visible image of converted UV radiation from a wide-aperture beam from a DDS‑30 (0.18–0.28 μm).
The object image is projected onto the input window 2 and absorbed in the volume of the sensor-transforming layer of the input window (a diamond film with SiV centers on any material of the input window of the EOC, transparent for radiation of 730–740 nm), where nonequilibrium electrons are generated in the plane of the diamond plate in proportion to the illumination and holes. The latter, being captured by SiV centers, radiatively recombine with the production of light quanta with a wavelength of ~738 nm. Their number is proportional to the intensity of the distribution of the input UV image in the plane of the input window of the EOC. The result of the interaction is a direct proportional spectral transformation of the UV image pattern into an optical image at a wavelength of 738 nm.
The resulting image of the object falls on the photocathode 3, deposited on the back side of the entrance window 2 and having a high quantum efficiency with a maximum sensitivity in the range of 730–740 nm, which proportionally transforms it into a two-dimensional image in photoelectrons. Photoelectrons of a two-dimensional image are accelerated by the field, their energy and number are proportionally converted by MCP 4 into secondary electrons, then the secondary electrons of the image pattern are accelerated by the field in the MCP channels, experiencing secondary multiplication of their number, and, leaving the MCP channels, are accelerated and directed to the CLS 5, which converts an enhanced picture of images in secondary electrons into an optical image in the visible part of the range, output from the UV EOC through FOG 6.
The minimum resolved value of such a UV EOC can be fractions of a micron, and the direction finding accuracy with its help increases many times over. In this case, it becomes possible to determine the coordinates of not one, but many dangerous targets.
The principle of registration of a radiation source in the solar blind UV range can be estimated from the characteristic frames of special video filming (Fig. 7).
The figure shows the results of a TV camera that is sensitive to UV and visible parts of the spectrum. The upper image demonstrates the operation of the solar blind channel, which does not react to solar radiation, since the frame was taken in the dark and only that area with a corona discharge is directly observed. The lower image shows the image of the same area under sun exposure. Considering that the TV camera also perceives the visible spectrum, this also gives an idea of the environment in the form in which it is perceived during visual observation.
Direction finders operating in the UV part of the spectrum are much cheaper than infrared direction finders, do not require cooling and, as mentioned above, are less susceptible to interference from solar radiation, since it is absorbed by the ozone layer in the upper atmosphere (Gartley band).
The emergence of aviation UV direction finders led the developers to the idea of the possibility of introducing UV direction finders of attacks into the tank defense systems, and in 2000–2006, attempts were made to develop such devices in the west. During the tests, it was possible to register the triggering impulse of the launching accelerator of the anti-tank guided missiles (ATGM) “Міlаn”, but there was no stable tracking of the propulsion engine.
This is due to the fact that, unlike aircraft missiles, ATGMs have speeds of ~500 m / s, and the older types in service are less than 300 m / s. Their energy is almost an order of magnitude weaker than that of anti-aircraft and aircraft missiles. The peak of the torch radiation is shifted to a longer wavelength region, moreover, the UV radiation present at the nozzle exit is screened by the elements of the body of the rocket flying directly to the tank.
Registration of only the launch impulse is not informative enough, since it does not allow determining the direction of the missile’s flight and assessing the degree of threat. In addition, the complex jamming environment of ground combat (dust, smoke, shots, explosions) reduces the reliability of the direction finder and leads to false alarms. The results of field tests carried out in Germany have shown the lack of sensitivity and noise immunity of UV direction finders. The testers came to the conclusion that it is advisable to switch to more stable channels for collecting information, for example, to a radar channel [2].
Since 2007, due to insufficient efficiency, UV direction finders have not been used to equip serial objects of armored vehicles and were excluded from the proposed composition of the promising German infantry fighting vehicle, Puma.
Domestic studies also allow us to conclude that the UV attack direction finder cannot be the main sensor in the on-board defense system of a ground combat vehicle, but it is useful as an additional source of information collection, the cost of false triggering of which is low and can be checked by other devices [3].
2. UV targeting systems for guided weapons
The features of the solar blind UV range of the spectrum open up the possibility of modernizing ATGM systems in terms of improving missile flight control systems using an IR tracer to correct the flight trajectory.
It is no secret that the use of an IR tracer is susceptible to interference due to reflections of solar radiation from the earth’s surface and terrestrial objects. Their re-radiation becomes the cause of failures of the IR sensors of the targeting system. However, with direct observation of the nozzle cut of the flying ATGM engine from the operator’s side in the absence of interference from the Sun, it is possible to effectively use its own UV radiation from the ATGM engine as a tracer for controlling the rocket flight. This simplifies the design of the ATGM complex, since the IR tracer is completely excluded from the rocket.
The required narrow spectral range is determined based on the design features of the engines of the missiles used, which leads to a number of common features. Let’s dwell on some of them.
The operation of a small-sized engine of orientation on a fuel pair “methane-oxygen” in the visible spectral range is shown in Fig. 8. With the naked eye, the flame front has a blue-green glow. The diagram of the emission spectrum of the flame during gas combustion is shown in Fig. 9.
The graph clearly shows a “burst” of radiation in the ultraviolet region of the spectrum, which occurs just in the zone of the solar blind range of 0.2÷0.3 microns. It is the most expedient to use in systems for correcting the direction of flight of missiles when observed from the side of the engine nozzle.
The range in the solar blind UV band calls in question. This main feature of the UV range was investigated superficially, and the existing engineering method for calculating the transparency of the atmosphere in the range 0.22–14 μm was developed by the S. I. Vavilov State Optical Institute, in the interests of working out the information path of the space echelon of the missile attack warning system, the scope of its application extended to the conditions of observation from space [4], therefore, it did not require special detailing of the observation conditions in the surface layer.
In 2008, the employees of the State Institution “High-Mountain Geophysical Institute” of Russian Meteorological Service conducted experiments to determine the characteristics of the propagation of UV radiation in the mountains, as well as through clouds, which gave unique results. In the clouds, at an altitude of 2 km, with rain and snow, a UV signal was detected from a distance of 1 km at a meteorological visibility range (MVR) of less than 80 m, while it was possible not only to ensure the reception of energy signals in the UV range, but also to find with an accuracy of 1 degree, the position of the source. This confirms the fact that there is no significant absorption of UV radiation by water vapor and makes it possible to talk about conducting work in this spectral range in conditions of fog and cloudiness at ranges of up to several km.
This range is quite enough to control the ATGM flight in a ground environment in the absence of background radiation and, therefore, in the absence of interfering reflections from the earth’s surface and ground objects that interfere with IR sensors and radar systems.
3. UV systems for the implementation of remote detonation of projectiles
In recent years, great importance has been attached to work on the creation of high-precision artillery ammunition, which ensures, in a short time, the defeat in the tactical zone (to a depth of 2–3 km) of small and sheltered targets. The work is based on the technology of remote control of the detonation of ammunition at a given range and at a certain height above the target.
Among the first developers of such projectiles was the Swiss company Oerlikon Contraves AG, which became part of the German arms concern Rheinmetall-DeTec AG in 2000. This company created AHEAD (Advanced Hit Efficiency And Destruction) air bursting ammunition.
Here, the difference between the Western design school and the Russian one was most clearly manifested, which is as follows. On the muzzle of the western guns, special coils are installed, which, when the projectile leaves, feeds it with an impulse with a time of deceleration to rupture. The essence of the method is illustrated in Fig. 10.
The process of entering data on the time of the projectile detonation is carried out according to the following algorithm. The characteristics of the target’s movement are determined by a laser rangefinder and are transmitted to the computer of the fire control system (Fire Control Unit) for calculating the range to the target. Target data is sent to the fuse installer electronics unit (ABM Electronics), where the measured muzzle velocity of the projectile is also transmitted. Muzzle velocity is determined using two induction coils spaced 10 cm apart. When the first coil passes, the timer starts, when the second coil passes, the timer stops. Knowing the distance between the coils and the time of flight of the projectile between them, the actual velocity of the projectile is calculated. This data is fed to the fire control computer. He calculates the time of the meeting of the projectile with the target and, using the programmer, transfers it to the projectile.
The programmer contains a third inductance coil, to which coded pulses of fuse detonation time are fed.
To receive data on the detonation time in the tail of the projectile, there is a fourth coil, a receiver. At a muzzle velocity of the projectile of about 1 050 m / s, the entire process of measuring the muzzle velocity, calculating and programming the projectile takes less than 0.002 seconds, after which the data from the receiving coil inside the projectile is transmitted to a programmable electronic fuse containing a high-precision electronic timer.
A view of the muzzle device of the MK 30–2 / ABM cannon with initial velocity solenoid sensors and an ABM-type projectile fuse programmer as part of the Puma BMP armament complex of the first series (2015) is shown in the photo fig. eleven.
Thus, the Western approach to the implementation of remote detonation is based on a change in the design of the main armament and for its full use, the automatic cannon needs improvements. Furthermore, the sensors and data transmission cable are located outside and are open to any mechanical impact, which is especially critical during combat or direct fire contact. The question of the combat survivability of such a system always remains open.
The problem solved by domestic designers has a different focus and consists in using artillery systems without changing their design, that is, all remote bursting control systems must be built into optoelectronic devices and a projectile with minimal modifications to serial products.
ROMZ PJSC proposed the option of fixing the flame of the shot by the so-called flash sensor built into the sight as a sensor for the exit of the projectile from the barrel. When fired, the flame is formed due to the afterburning in air of combustible gases (carbon monoxide, hydrogen, methane) escaping from the barrel bore and the glow of incandescent gases and solid particles [5]. It appears at some distance from the muzzle in the front part of the gas cloud escaping from the bore (Fig. 12).
By burning gunpowder, the projectile is thrown in the direction of least resistance, while a muzzle glow occurs. This is reddish glow that is visible before the projectile leaves the barrel. The glow is created by superheated gases that seeped past the projectile and emerged from the barrel ahead of time.
Generally speaking, a muzzle flash can be divided into five distinct components. The primary flash is caused by superheated propellant gases escaping from the firearm behind the projectile, which emit their energy into the environment in part as visible light. The brightness of the primary flash is the highest, but its heat dissipates very quickly and therefore is usually not visible to the eye.
Intermediate flash is caused by the shock waves generated by the high velocity of the exiting gases and the projectile and appears as a reddish disc in front of the muzzle.
A secondary flash appears farthest from the muzzle as a large white or yellow flame and results from a re-flash of ignition – an oxidative reaction of an incompletely burnt outburst when mixed with abundant oxygen in the surrounding atmosphere.
After the muzzle flash dissipates, partially unburned gunpowder or other heated materials may be ejected from the muzzle and appear as residual sparks. Muzzle flashes create a clear picture, signatures, that can be detected using infrared imaging, since there are high flash, combustion and explosion temperatures, some values of which [5] for different explosives are given in Table 1.
For each temperature, the maximum radiation corresponds to a certain wavelength, which, according to Wien’s law, obtained by differentiating Planck’s law [6], increases with decreasing temperature.
Using Wien’s law of displacement: λmax = 2898 / T [μm], where T is the temperature of the emitting body [K], and converting the temperature values from Celsius (TC) to degrees Kelvin (TK): TK = TC + 273.16, we define the interval wavelengths with maximum emission for various explosives. For example, for nitrocellulose gunpowder with a flash point of ~200 °C and a combustion temperature of ~2 000 °C (see Table 1), the wavelength interval with a maximum radiation will be ~∆λmax ≈ (1.2–6.1) μm (2898 / 2273.16 and 2898 / 473.16).
With such a spectral range, an FE723-3 type photodetector manufactured by the Girikond Research Institute can be used as a flash sensor, the spectral range of which covers the most effective part of the primary flash radiation. The parameters of the FE723-3 photodetector are given in Table 2.
A significant advantage is that in such a shot sensor, due to the large receiver aperture and short working distances, an additional optical system is not required. Thus, the design of the observation device-sight for remote detonation of projectiles can be performed by a simple modernization of serial sights.
As an example, we can cite the modernization of the standard version of the serial sight of the BTR‑82A combat vehicle, consisting of a head part and two channels: a single optical channel and a multiple optoelectronic channel [7].
It is advisable to install the photodetector of the shot flash sensor in the head of the sight, therefore, in the considered variant of the layout, it is located directly under the head reflective mirror, kinematically connected with the main armament.
The flash sensor registers the flux of infrared radiation in the spectral range of 2.6–4.3 microns, which appears at the time of the primary flash of the shot. The radiation from the primary flash passes through the instrument protective glasses, is reflected from the head mirror and is fixed by the receiving area of the photodetector 1, which forms the corresponding electrical signal (Fig. 13).
The sight is additionally equipped with two modules: an optoelectronic module 2, 3 of a laser programmer-emitter unit (LPE) of a channel for remote control of the projectile detonation time (CRC PDT) and an emitting channel of a pulsed laser rangefinder 4.
Each of the channels has its own receiver. The receiving device of the channel of the pulsed laser rangefinder is built into one of the optical channels of the sight, and the receiving device of the CRC PDT LPE in the form of an independent unit is placed directly on the projectile developed at the NPO Pribor enterprise.
A protective optical window made of leucosapphire with an interference filter for the remotely controlled photodetector fuse (RCF) module is built into the rear of the projectile, which provides the necessary RCF noise immunity.
Thus, the Russian project is based on layout solutions that do not repeat foreign ones.
In the western project, the armament of the object requires the installation of new control systems and an electromagnetic programmer. The installation of the latter can be associated with significant layout and design difficulties, while the domestic project provides for the use of a simple and cheap laser control system, which makes it possible to do with minimal alterations of the combat vehicle.
The advantages of this architecture are obvious. It allows you to give technology new opportunities with minimal time and material costs. It should be noted that a remote-controlled projectile is significantly cheaper than a product with a Western-type programmable fuse.
In August 2014, the first full-scale tests in real operating conditions of a variant of the TKN‑4GA‑02 sight as part of a mock-up stand with the installation of a 30‑mm cannon were carried out at the proving ground of NPO Pribor OJSC [8].
Shooting tests at specified ranges of projectile detonation in various weather conditions were recognized as successful, since the projectile detonation efficiency was more than 75%, which is quite satisfactory for the first prototypes of the sight and projectiles (Fig. 14).
Undoubtedly, the domestic development was a new step in the development of technologies for programming air bursting.
The ballistic computer, made using the 1886BE2U microcontroller, processes the data on the target range obtained from the laser rangefinder, as well as data on the firing conditions (ambient temperature, charge temperature, etc.), and calculates the value of the flight time in the tracking mode. At the moment of firing, the shot flash sensor generates a short impulse, which transfers the ballistic computer to the mode of formation of the LPE control impulses. The ballistic computer provides the input of the range to the target, forming, according to an agreed algorithm, code messages of control pulses that enter the radiation unit. In the radiation unit, pulsed optical radiation is formed, directed along the trajectory of the projectile and ensuring the installation of the projectile fuse for detonation at a given time.
The shot sensor is located at a distance of up to 2 m from the barrel cut, registering the flash from the shot and generating a pulse signal for the ballistic computer. The aperture angle of the photocell equal to ~30° provides reliable actuation of the shot sensor without additional alignment in the direction.
However, for a shot sensor that is sensitive in the infrared range of the spectrum, there is always a problem of external interference, especially when working “against the sun”, the powerful radiation of which literally “clogs” the photodetector (Fig. 15). It is in this situation that the advantages of solar blind UV photodetectors can be fully realized.
Only solar blind photodetectors are capable of registering a muzzle flash in a strong solar background, since they practically do not react to this background, while other receivers of the visible and IR ranges simply “go blind” in such difficult conditions.
Analyzing powder charges, it can be noted that the oldest explosive, black powder, is a mixture of two combustible substances (coal and sulfur) with an oxidizer (potassium nitrate) in the following percentage composition: potassium nitrate – 75%, charcoal – 15% and sulfur 10%. Saltpeter is an oxidizing agent that easily releases oxygen when heated, coal is a combustible substance, and sulfur serves as a cementing agent that binds nitrate with coal, and at the same time a combustible substance that facilitates the ignition of gunpowder.
When sulfur burns in air, a lot of UV radiation is generated. In the diagram of the emission spectrum of a sulfur flame [9], shown in Fig. 16, it can be seen that the radiation of the IR and visible ranges in the spectrum of the flame is almost absent, so the sulfur flame seems to us pale blue. If UV rays could be seen, the sulfur flame would appear bright. When sulfur is burned in air, the presence of a significant fraction of energy in the UV range from 250 to 300 nm makes it possible to use a sun-blind photo-receiving device as a shot sensor and use it in a version of the CRC PDT system with increased noise immunity.
It is also possible to introduce another channel into the sight, a channel for fixing a projectile rupture, consisting of a narrow-angle lens and a corresponding photodetector, which allows you to fully control and evaluate the results of firing with controlled bursting.
When an explosive charge of a fragmentation projectile explodes under the action of expanding gaseous detonation products, the body of the projectile is crushed into fragments, which scatter in different directions, hitting manpower and vulnerable parts of military equipment on its way. For the equipment of ammunition, RDX is usually used, which is, in comparison, for example, with TNT, a more powerful explosive, therefore the explosion is accompanied by very high values of temperature, velocity and pressure.
The explosion temperature reaches 3500K for TNT and 4000K for RDX (see Table 1) and, in accordance with formula (3), in this case, it would be optimal to use a photodetector sensitive in the spectral range ∆λ ≈ 0.7–0.8 μm.
However, in this range there is a lot of interference radiation, both natural and artificial.
However, the gases formed during the explosion, heated to such high temperatures, ionize the air, in which UV radiation will also be formed. This makes it possible to fix the burst of the projectile with the most optimal option – using a multi-element two-spectral diamond photodetector – an advanced development implemented at the UralAlmazInvest Production and Technology Center [10].
On the manufactured prototypes, spectral ranges of sensitivity for the UV channel – 0.19–0.23 μm and for the IR channel – 0.8–3.3 μm were provided, as well as the sensitivity thresholds for the UV channel – 9 ∙ 10–12 W / Hz1 / 2 and for the IR channel – 6 ∙ 10–10 W / Hz1 / 2.
The basic structure of a bispectral photodetector is based on the use of two photodetectors formed on opposite sides of a diamond plate. The diamond-based photodetector is absolutely transparent in the visible and infrared regions of the spectrum, and any imaging matrix can be placed behind it. The UV point from the target can then be superimposed on the visible or infrared image and viewed in one frame.
The technology and design of bispectral multi-element photodetectors and photodetectors based on diamond materials presupposes the achievement of a photodetector format of 240 × 240 elements with a pixel size of ~50 × 50 μm and simultaneous operation in two ranges:
The effectiveness of the use of UV systems is clearly demonstrated by the complexed image obtained in the IR and UV regions of the spectrum. Fig. 17 shows a survey of a forest area on the shore of a lake, where a fire is burning.
On the left is the image in the IR spectrum, in the center – the image of the campfire in the UV spectrum (on the right side of the image there is a bright spot). On the right is a summary of the IR and UV images of this area.
The multi-element photodetector allows not only recording an event, but also performing coordinate measurements necessary for the practical implementation of the channel for fixing the rupture of a projectile built into the main sight.
4. Other applications of solar blind UV devices
Based on the experience of using optoelectronic devices, it should be noted that there is no universal device that solves absolutely all problems with a high degree of probability.
Devices of the solar blind UV range are an important addition to the channels of the visible, IR and radar ranges, and in some cases, they are the only option for solving the problem.
There are many applications where, in addition to one UV targeting or direction finding channel, channels of other spectral ranges are simply not required. These include, for example, devices for orientation by the Sun and stars in outer space, systems for detecting and identifying “friend or foe”, control systems for high-temperature production technologies, fire monitoring systems, aircraft detection systems based on UV radiation from jet engines, detection systems flames of various origins, etc.
For such applications, only one UV channel can be used, consisting of optics, UV photodetector and electronic signal processing units.
Other applications are of no less interest, such as means of covert interference-proof UV optical communication, laser systems for locating and seeing in the UV range, which are structurally more complex devices, since they must contain sufficiently powerful laser sources of UV radiation. In this regard, it will be useful to note that in the solar blind UV spectral region of 0.2–0.3 μm, the use of the radiation of the fourth harmonic (0.266 μm) of a laser based on neodymium glass and on yttrium-aluminum garnet with neodymium will be quite effective [11].
The applied capabilities of such devices will largely be determined by the potentially achievable technical characteristics of laser systems, the main of which are the energy in the laser pulse and the sensitivity of the UV receiver.
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AUTHORS
Medvedev Alexander Vladimirovich, design@romz.ru, General Designer, Rostov Optical and Mechanical Plant OJSC (ROMZ OJSC), Rostov Veliky, Yaroslavl Region, Russia.
Grinkevich Alexander Vasilievich, lyu1455@yandex.ru, ZAO «EVS», Moscow, Russia.
Knyazeva Svetlana Nikolaevna, ksn 61@yandex.ru, Design Engineer, Design Bureau of OJSC «Rostov Optical and Mechanical Plant, (OJSC» ROMZ «), Rostov the Great, Yaroslavl Region, Russia.
Contribution by the members
of the team of authors
The article was prepared on the basis of work by all members of the team of authors.
Conflict of interest
The authors claim that they have no conflict of interest. All authors took part in writing the article and supplemented the manuscript in part of their work.
A. V. Medvedev, A. V. Grinkevich, S. N. Knyazeva
ROMZ OJSC, Rostov, Yaroslavl region, Russia
EVS CJSC, Moscow, Russia
EDB ROMZ OJSC, Rostov, Yaroslavl region, Russia
This article reviews a specific variety of optico-electronic devices operating in the spectral range from 0,25 to 0.3 µm, which is commonly referred to as Solar Blind Ultra Violet (SBUV) spectral range as the solar radiation of the said wavelengths does not virtually reach the earth surface. The devices of this category are operable even in the viewing conditions “against the sun”, which is not possible with any other system. Apart from special features of the SBUV devices, their designs for various areas of application are reviewed.
Keywords: Solar Blind, UV range, UV direction finder, UV targeting systems, flash sensor
Received on: 14.08.2021
Accepted on: 10.09.2021
The Earth’s atmosphere in the stratosphere at an altitude of 25–30 km has an ozone absorption band in the ultraviolet (UV) spectrum of 0.25–0.3 μm. Therefore, UV radiation from the Sun does not reach the Earth’s surface, forming in the lower atmosphere, namely in the troposphere, the so-called “solar blind” zone. In this zone, the solar radiation of the indicated wavelengths practically does not reach the earth’s surface, and the transparency of the optical paths differs from zero.
Optoelectronic systems, which have a solar blind UV working spectral range, function even when they work “against the sun”, which is inaccessible to any other system. For example, in optical-electronic systems of the visible and infrared (IR) spectral ranges, when working “against the sun”, intense solar radiation hits the photosensitive area of the photodetector and creates strong interference or disables the entire device.
Let’s consider the features of solar blind UV devices for different applications.
1. UV direction finders
Devices of the solar blind UV spectrum in the form of UV direction finders have found the most widespread use in the systems of countering terrorist attacks as part of the airborne defense systems of not only military, but also civilian aircraft [1].
With their help, crews can receive warnings about the approach of missiles by detecting traces of their rocket engines. Providing speeds from 600 to 1400 m / s, rocket engines emit a jet of exhaust gases heated to high temperatures, which are high-temperature sources that ionize the air to form UV radiation.
One of the variants of the UV direction finder was developed at the ROMZ PJSC enterprise. The design solution with the traditional placement of functional units in a single body is shown in Fig.
1.
The device contains a narrow-band light filter, a UV lens, an UFK‑2 UV radiation sensor, additional sensors, which include a GPS / GLONASS receiver, a compass, an altimeter, and electronic units for processing and issuing signal information.
In this configuration, the UV direction finder as part of the onboard complex measures the three-dimensional coordinates of UV radiation sources, namely: determination of latitude, longitude and altitude above the Earth’s surface.
The UV radiation sensor is a domestically produced photomultiplier UFK‑2, which converts electromagnetic radiation in the optical range with wavelengths from 210 to 350 nm into an electrical signal. It has a semitransparent photocathode based on heteroepitaxial nanostructures of GaN / AlN compounds grown on a sapphire substrate, a multiplication system consisting of two microchannel plates, and a four-sector anode with four separate leads, which makes it possible to determine the center of an electron avalanche formed by two microchannel plates even in the case of only one photon hits the photocathode. It is the center of the avalanche that is considered the coordinate of the photon hitting the photocathode.
The coordinate is determined by the ratio of voltages equivalent to charges at each anode, according to the following formulas:
X = (U1 + U4 – U2 – U3) / (U1 + U2 + U3 + U4 + UФЭ),
Y = (U1 + U2 – U3 – U4) / (U1 + U2 + U3 + U4 + UФЭ),
where: Ui are the voltages taken from the i-th quadrant of the collector.
The functional diagram of the UV direction finder option is shown in Fig. 2. The functioning of the system is as follows. Current signals from the UFK‑2 anodes are fed to transimpedance amplifiers that convert current into voltage. From the outputs of the amplifiers, the signals are fed to differential amplifiers that generate paraphase signals necessary to match the levels of three differential analog-to-digital converters (ADC) of the STM32H743IGT6 microcontroller with multiplexing of the input analog channels. The use of differential channels of a 16‑bit ADC can reduce the conversion noise from the digital ground of the microcontroller.
In the 14‑bit ADC mode, the maximum conversion frequency is 4 MHz, and when the input multiplex channels are allocated to three channels for each ADC, the maximum conversion frequency for all channels is 4 / 3 MHz.
Please note that the microcontroller has a maximum clock frequency of 480 MHz, which is sufficient to run software without an operating system (OS). For communication with external systems, two interfaces are used: CAN or Ethernet 10 / 100. The microcontroller is connected with the receiver of the GLONASS / GPS / GALILEO / QZSS / SBAS space navigation system by the GeoS‑5M module using two serial USART (RS232) interfaces.
In civil applications, a solar blind UV direction finder can be effective as part of an unmanned complex that solves the problem of fixing defective places of high-voltage power lines upon the occurrence of corona discharges, the emission spectrum of which is shown in Fig. 3.
The calculation of the UV direction finder objective is based on the spectral characteristics of the corona discharge and the final version of the scheme is implemented using the following line of materials – calcium fluoride, quartz glass and sapphire.
A narrow-band optical UV filter blocks the solar radiation spectrum and emits a solar blind portion of the UV spectrum with Δλ = (0.24–0.28) microns.
Main technical characteristics of UV direction finder:
- spectral working range is from 240 to 280 nm;
- detection range of corona discharges is up to 15 km;
- angle of view is 30°;
- the accuracy of determining the angular coordinates is no more than ±0.5°;
- accuracy of determination of three-dimensional coordinates is no more than ±0.5 m;
- communication interface with the bot computer: CAN, Ethernet 10 / 100;
- overall dimensions is no more than 120 × 120 × 250 mm;
- supply voltage is 22.2 V (or another one at the customer’s choice);
- consumption current at a supply voltage of 22.2 V is up to 3 A;
- weight is no more than 2 kg;
- operating temperature range is from –40 to +55 °C.
The four-anode multiplier UFK‑2 has high parameters. Thus, an amplification of the order of 106 and more times opens up wide opportunities for detecting weak signals, and the declared accuracy of determining the angular coordinates of ±0.5° is quite sufficient for many practical applications.
To carry out specialized measurements requiring a radical increase in the accuracy of determining the coordinates, the UralAlmazInvest Production and Technical Center proposed a new, structurally simplified scheme of a position-sensitive photodetector based on diamond, with smaller overall dimensions.
Fig. 5 shows a diagram of the square arrangement of the contacts of the receiving element of such a four-contact position-sensitive photodetector with the designation of the “binding” of the coordinates of its points and the illumination spot. The receiver is a fully analog device and instantly responds to ambient light. The resolution of the device depends on the resolution of the optics and the uniformity of the properties of the photosensitive material over the area.
Receiving a signal, digitizing it and determining coordinates takes a minimum amount of time and does not require large computing power, since, unlike a matrix of 1 000 × 1 000 elements, only one, known in advance, the only needed image point is processed.
A characteristic feature of the direction-finding channel using a PMT is the ability to accurately determine the coordinates of only one dangerous target. If there are several targets, their simultaneous position is not determined, and signal processing becomes impossible.
In this regard, the development of a UV photodetector of an EOC architecture based on a new design of a solid-state spectral converter of a UV image in the range of 20–270 nm into an image of the range (738 ± 10) nm and into a visible one, carried out by the Moscow Institute of Electronic Technology and MELZ FEU LLC, is of undoubted interest. (Fig. 6).
Fig. 6a shows the design of a vacuum emission detector: 1 – a vacuum-tight metal-ceramic body, 2 – an entrance window made on the basis of a diamond plate saturated from the outer side of the SiV with centers at least to a thickness of ~α–1 (α is the light absorption coefficient in the working spectral range), 3 – photocathode, sensitive in the spectral range of ~730–740 nm and formed on the back side of the diamond entrance window plate, 4 – microchannel plate (MCP), 5 – cathodoluminescent screen (CLS), 6 – fiber-optic glass (FOG).
Fig. 6b shows a photo of a grid-type UV photocathode based on a diamond film, Fig. 6c shows a photo of a UV EOC in a 2+ generation housing with a diamond photocathode, and Figure 6d shows a visible image of converted UV radiation from a wide-aperture beam from a DDS‑30 (0.18–0.28 μm).
The object image is projected onto the input window 2 and absorbed in the volume of the sensor-transforming layer of the input window (a diamond film with SiV centers on any material of the input window of the EOC, transparent for radiation of 730–740 nm), where nonequilibrium electrons are generated in the plane of the diamond plate in proportion to the illumination and holes. The latter, being captured by SiV centers, radiatively recombine with the production of light quanta with a wavelength of ~738 nm. Their number is proportional to the intensity of the distribution of the input UV image in the plane of the input window of the EOC. The result of the interaction is a direct proportional spectral transformation of the UV image pattern into an optical image at a wavelength of 738 nm.
The resulting image of the object falls on the photocathode 3, deposited on the back side of the entrance window 2 and having a high quantum efficiency with a maximum sensitivity in the range of 730–740 nm, which proportionally transforms it into a two-dimensional image in photoelectrons. Photoelectrons of a two-dimensional image are accelerated by the field, their energy and number are proportionally converted by MCP 4 into secondary electrons, then the secondary electrons of the image pattern are accelerated by the field in the MCP channels, experiencing secondary multiplication of their number, and, leaving the MCP channels, are accelerated and directed to the CLS 5, which converts an enhanced picture of images in secondary electrons into an optical image in the visible part of the range, output from the UV EOC through FOG 6.
The minimum resolved value of such a UV EOC can be fractions of a micron, and the direction finding accuracy with its help increases many times over. In this case, it becomes possible to determine the coordinates of not one, but many dangerous targets.
The principle of registration of a radiation source in the solar blind UV range can be estimated from the characteristic frames of special video filming (Fig. 7).
The figure shows the results of a TV camera that is sensitive to UV and visible parts of the spectrum. The upper image demonstrates the operation of the solar blind channel, which does not react to solar radiation, since the frame was taken in the dark and only that area with a corona discharge is directly observed. The lower image shows the image of the same area under sun exposure. Considering that the TV camera also perceives the visible spectrum, this also gives an idea of the environment in the form in which it is perceived during visual observation.
Direction finders operating in the UV part of the spectrum are much cheaper than infrared direction finders, do not require cooling and, as mentioned above, are less susceptible to interference from solar radiation, since it is absorbed by the ozone layer in the upper atmosphere (Gartley band).
The emergence of aviation UV direction finders led the developers to the idea of the possibility of introducing UV direction finders of attacks into the tank defense systems, and in 2000–2006, attempts were made to develop such devices in the west. During the tests, it was possible to register the triggering impulse of the launching accelerator of the anti-tank guided missiles (ATGM) “Міlаn”, but there was no stable tracking of the propulsion engine.
This is due to the fact that, unlike aircraft missiles, ATGMs have speeds of ~500 m / s, and the older types in service are less than 300 m / s. Their energy is almost an order of magnitude weaker than that of anti-aircraft and aircraft missiles. The peak of the torch radiation is shifted to a longer wavelength region, moreover, the UV radiation present at the nozzle exit is screened by the elements of the body of the rocket flying directly to the tank.
Registration of only the launch impulse is not informative enough, since it does not allow determining the direction of the missile’s flight and assessing the degree of threat. In addition, the complex jamming environment of ground combat (dust, smoke, shots, explosions) reduces the reliability of the direction finder and leads to false alarms. The results of field tests carried out in Germany have shown the lack of sensitivity and noise immunity of UV direction finders. The testers came to the conclusion that it is advisable to switch to more stable channels for collecting information, for example, to a radar channel [2].
Since 2007, due to insufficient efficiency, UV direction finders have not been used to equip serial objects of armored vehicles and were excluded from the proposed composition of the promising German infantry fighting vehicle, Puma.
Domestic studies also allow us to conclude that the UV attack direction finder cannot be the main sensor in the on-board defense system of a ground combat vehicle, but it is useful as an additional source of information collection, the cost of false triggering of which is low and can be checked by other devices [3].
2. UV targeting systems for guided weapons
The features of the solar blind UV range of the spectrum open up the possibility of modernizing ATGM systems in terms of improving missile flight control systems using an IR tracer to correct the flight trajectory.
It is no secret that the use of an IR tracer is susceptible to interference due to reflections of solar radiation from the earth’s surface and terrestrial objects. Their re-radiation becomes the cause of failures of the IR sensors of the targeting system. However, with direct observation of the nozzle cut of the flying ATGM engine from the operator’s side in the absence of interference from the Sun, it is possible to effectively use its own UV radiation from the ATGM engine as a tracer for controlling the rocket flight. This simplifies the design of the ATGM complex, since the IR tracer is completely excluded from the rocket.
The required narrow spectral range is determined based on the design features of the engines of the missiles used, which leads to a number of common features. Let’s dwell on some of them.
The operation of a small-sized engine of orientation on a fuel pair “methane-oxygen” in the visible spectral range is shown in Fig. 8. With the naked eye, the flame front has a blue-green glow. The diagram of the emission spectrum of the flame during gas combustion is shown in Fig. 9.
The graph clearly shows a “burst” of radiation in the ultraviolet region of the spectrum, which occurs just in the zone of the solar blind range of 0.2÷0.3 microns. It is the most expedient to use in systems for correcting the direction of flight of missiles when observed from the side of the engine nozzle.
The range in the solar blind UV band calls in question. This main feature of the UV range was investigated superficially, and the existing engineering method for calculating the transparency of the atmosphere in the range 0.22–14 μm was developed by the S. I. Vavilov State Optical Institute, in the interests of working out the information path of the space echelon of the missile attack warning system, the scope of its application extended to the conditions of observation from space [4], therefore, it did not require special detailing of the observation conditions in the surface layer.
In 2008, the employees of the State Institution “High-Mountain Geophysical Institute” of Russian Meteorological Service conducted experiments to determine the characteristics of the propagation of UV radiation in the mountains, as well as through clouds, which gave unique results. In the clouds, at an altitude of 2 km, with rain and snow, a UV signal was detected from a distance of 1 km at a meteorological visibility range (MVR) of less than 80 m, while it was possible not only to ensure the reception of energy signals in the UV range, but also to find with an accuracy of 1 degree, the position of the source. This confirms the fact that there is no significant absorption of UV radiation by water vapor and makes it possible to talk about conducting work in this spectral range in conditions of fog and cloudiness at ranges of up to several km.
This range is quite enough to control the ATGM flight in a ground environment in the absence of background radiation and, therefore, in the absence of interfering reflections from the earth’s surface and ground objects that interfere with IR sensors and radar systems.
3. UV systems for the implementation of remote detonation of projectiles
In recent years, great importance has been attached to work on the creation of high-precision artillery ammunition, which ensures, in a short time, the defeat in the tactical zone (to a depth of 2–3 km) of small and sheltered targets. The work is based on the technology of remote control of the detonation of ammunition at a given range and at a certain height above the target.
Among the first developers of such projectiles was the Swiss company Oerlikon Contraves AG, which became part of the German arms concern Rheinmetall-DeTec AG in 2000. This company created AHEAD (Advanced Hit Efficiency And Destruction) air bursting ammunition.
Here, the difference between the Western design school and the Russian one was most clearly manifested, which is as follows. On the muzzle of the western guns, special coils are installed, which, when the projectile leaves, feeds it with an impulse with a time of deceleration to rupture. The essence of the method is illustrated in Fig. 10.
The process of entering data on the time of the projectile detonation is carried out according to the following algorithm. The characteristics of the target’s movement are determined by a laser rangefinder and are transmitted to the computer of the fire control system (Fire Control Unit) for calculating the range to the target. Target data is sent to the fuse installer electronics unit (ABM Electronics), where the measured muzzle velocity of the projectile is also transmitted. Muzzle velocity is determined using two induction coils spaced 10 cm apart. When the first coil passes, the timer starts, when the second coil passes, the timer stops. Knowing the distance between the coils and the time of flight of the projectile between them, the actual velocity of the projectile is calculated. This data is fed to the fire control computer. He calculates the time of the meeting of the projectile with the target and, using the programmer, transfers it to the projectile.
The programmer contains a third inductance coil, to which coded pulses of fuse detonation time are fed.
To receive data on the detonation time in the tail of the projectile, there is a fourth coil, a receiver. At a muzzle velocity of the projectile of about 1 050 m / s, the entire process of measuring the muzzle velocity, calculating and programming the projectile takes less than 0.002 seconds, after which the data from the receiving coil inside the projectile is transmitted to a programmable electronic fuse containing a high-precision electronic timer.
A view of the muzzle device of the MK 30–2 / ABM cannon with initial velocity solenoid sensors and an ABM-type projectile fuse programmer as part of the Puma BMP armament complex of the first series (2015) is shown in the photo fig. eleven.
Thus, the Western approach to the implementation of remote detonation is based on a change in the design of the main armament and for its full use, the automatic cannon needs improvements. Furthermore, the sensors and data transmission cable are located outside and are open to any mechanical impact, which is especially critical during combat or direct fire contact. The question of the combat survivability of such a system always remains open.
The problem solved by domestic designers has a different focus and consists in using artillery systems without changing their design, that is, all remote bursting control systems must be built into optoelectronic devices and a projectile with minimal modifications to serial products.
ROMZ PJSC proposed the option of fixing the flame of the shot by the so-called flash sensor built into the sight as a sensor for the exit of the projectile from the barrel. When fired, the flame is formed due to the afterburning in air of combustible gases (carbon monoxide, hydrogen, methane) escaping from the barrel bore and the glow of incandescent gases and solid particles [5]. It appears at some distance from the muzzle in the front part of the gas cloud escaping from the bore (Fig. 12).
By burning gunpowder, the projectile is thrown in the direction of least resistance, while a muzzle glow occurs. This is reddish glow that is visible before the projectile leaves the barrel. The glow is created by superheated gases that seeped past the projectile and emerged from the barrel ahead of time.
Generally speaking, a muzzle flash can be divided into five distinct components. The primary flash is caused by superheated propellant gases escaping from the firearm behind the projectile, which emit their energy into the environment in part as visible light. The brightness of the primary flash is the highest, but its heat dissipates very quickly and therefore is usually not visible to the eye.
Intermediate flash is caused by the shock waves generated by the high velocity of the exiting gases and the projectile and appears as a reddish disc in front of the muzzle.
A secondary flash appears farthest from the muzzle as a large white or yellow flame and results from a re-flash of ignition – an oxidative reaction of an incompletely burnt outburst when mixed with abundant oxygen in the surrounding atmosphere.
After the muzzle flash dissipates, partially unburned gunpowder or other heated materials may be ejected from the muzzle and appear as residual sparks. Muzzle flashes create a clear picture, signatures, that can be detected using infrared imaging, since there are high flash, combustion and explosion temperatures, some values of which [5] for different explosives are given in Table 1.
For each temperature, the maximum radiation corresponds to a certain wavelength, which, according to Wien’s law, obtained by differentiating Planck’s law [6], increases with decreasing temperature.
Using Wien’s law of displacement: λmax = 2898 / T [μm], where T is the temperature of the emitting body [K], and converting the temperature values from Celsius (TC) to degrees Kelvin (TK): TK = TC + 273.16, we define the interval wavelengths with maximum emission for various explosives. For example, for nitrocellulose gunpowder with a flash point of ~200 °C and a combustion temperature of ~2 000 °C (see Table 1), the wavelength interval with a maximum radiation will be ~∆λmax ≈ (1.2–6.1) μm (2898 / 2273.16 and 2898 / 473.16).
With such a spectral range, an FE723-3 type photodetector manufactured by the Girikond Research Institute can be used as a flash sensor, the spectral range of which covers the most effective part of the primary flash radiation. The parameters of the FE723-3 photodetector are given in Table 2.
A significant advantage is that in such a shot sensor, due to the large receiver aperture and short working distances, an additional optical system is not required. Thus, the design of the observation device-sight for remote detonation of projectiles can be performed by a simple modernization of serial sights.
As an example, we can cite the modernization of the standard version of the serial sight of the BTR‑82A combat vehicle, consisting of a head part and two channels: a single optical channel and a multiple optoelectronic channel [7].
It is advisable to install the photodetector of the shot flash sensor in the head of the sight, therefore, in the considered variant of the layout, it is located directly under the head reflective mirror, kinematically connected with the main armament.
The flash sensor registers the flux of infrared radiation in the spectral range of 2.6–4.3 microns, which appears at the time of the primary flash of the shot. The radiation from the primary flash passes through the instrument protective glasses, is reflected from the head mirror and is fixed by the receiving area of the photodetector 1, which forms the corresponding electrical signal (Fig. 13).
The sight is additionally equipped with two modules: an optoelectronic module 2, 3 of a laser programmer-emitter unit (LPE) of a channel for remote control of the projectile detonation time (CRC PDT) and an emitting channel of a pulsed laser rangefinder 4.
Each of the channels has its own receiver. The receiving device of the channel of the pulsed laser rangefinder is built into one of the optical channels of the sight, and the receiving device of the CRC PDT LPE in the form of an independent unit is placed directly on the projectile developed at the NPO Pribor enterprise.
A protective optical window made of leucosapphire with an interference filter for the remotely controlled photodetector fuse (RCF) module is built into the rear of the projectile, which provides the necessary RCF noise immunity.
Thus, the Russian project is based on layout solutions that do not repeat foreign ones.
In the western project, the armament of the object requires the installation of new control systems and an electromagnetic programmer. The installation of the latter can be associated with significant layout and design difficulties, while the domestic project provides for the use of a simple and cheap laser control system, which makes it possible to do with minimal alterations of the combat vehicle.
The advantages of this architecture are obvious. It allows you to give technology new opportunities with minimal time and material costs. It should be noted that a remote-controlled projectile is significantly cheaper than a product with a Western-type programmable fuse.
In August 2014, the first full-scale tests in real operating conditions of a variant of the TKN‑4GA‑02 sight as part of a mock-up stand with the installation of a 30‑mm cannon were carried out at the proving ground of NPO Pribor OJSC [8].
Shooting tests at specified ranges of projectile detonation in various weather conditions were recognized as successful, since the projectile detonation efficiency was more than 75%, which is quite satisfactory for the first prototypes of the sight and projectiles (Fig. 14).
Undoubtedly, the domestic development was a new step in the development of technologies for programming air bursting.
The ballistic computer, made using the 1886BE2U microcontroller, processes the data on the target range obtained from the laser rangefinder, as well as data on the firing conditions (ambient temperature, charge temperature, etc.), and calculates the value of the flight time in the tracking mode. At the moment of firing, the shot flash sensor generates a short impulse, which transfers the ballistic computer to the mode of formation of the LPE control impulses. The ballistic computer provides the input of the range to the target, forming, according to an agreed algorithm, code messages of control pulses that enter the radiation unit. In the radiation unit, pulsed optical radiation is formed, directed along the trajectory of the projectile and ensuring the installation of the projectile fuse for detonation at a given time.
The shot sensor is located at a distance of up to 2 m from the barrel cut, registering the flash from the shot and generating a pulse signal for the ballistic computer. The aperture angle of the photocell equal to ~30° provides reliable actuation of the shot sensor without additional alignment in the direction.
However, for a shot sensor that is sensitive in the infrared range of the spectrum, there is always a problem of external interference, especially when working “against the sun”, the powerful radiation of which literally “clogs” the photodetector (Fig. 15). It is in this situation that the advantages of solar blind UV photodetectors can be fully realized.
Only solar blind photodetectors are capable of registering a muzzle flash in a strong solar background, since they practically do not react to this background, while other receivers of the visible and IR ranges simply “go blind” in such difficult conditions.
Analyzing powder charges, it can be noted that the oldest explosive, black powder, is a mixture of two combustible substances (coal and sulfur) with an oxidizer (potassium nitrate) in the following percentage composition: potassium nitrate – 75%, charcoal – 15% and sulfur 10%. Saltpeter is an oxidizing agent that easily releases oxygen when heated, coal is a combustible substance, and sulfur serves as a cementing agent that binds nitrate with coal, and at the same time a combustible substance that facilitates the ignition of gunpowder.
When sulfur burns in air, a lot of UV radiation is generated. In the diagram of the emission spectrum of a sulfur flame [9], shown in Fig. 16, it can be seen that the radiation of the IR and visible ranges in the spectrum of the flame is almost absent, so the sulfur flame seems to us pale blue. If UV rays could be seen, the sulfur flame would appear bright. When sulfur is burned in air, the presence of a significant fraction of energy in the UV range from 250 to 300 nm makes it possible to use a sun-blind photo-receiving device as a shot sensor and use it in a version of the CRC PDT system with increased noise immunity.
It is also possible to introduce another channel into the sight, a channel for fixing a projectile rupture, consisting of a narrow-angle lens and a corresponding photodetector, which allows you to fully control and evaluate the results of firing with controlled bursting.
When an explosive charge of a fragmentation projectile explodes under the action of expanding gaseous detonation products, the body of the projectile is crushed into fragments, which scatter in different directions, hitting manpower and vulnerable parts of military equipment on its way. For the equipment of ammunition, RDX is usually used, which is, in comparison, for example, with TNT, a more powerful explosive, therefore the explosion is accompanied by very high values of temperature, velocity and pressure.
The explosion temperature reaches 3500K for TNT and 4000K for RDX (see Table 1) and, in accordance with formula (3), in this case, it would be optimal to use a photodetector sensitive in the spectral range ∆λ ≈ 0.7–0.8 μm.
However, in this range there is a lot of interference radiation, both natural and artificial.
However, the gases formed during the explosion, heated to such high temperatures, ionize the air, in which UV radiation will also be formed. This makes it possible to fix the burst of the projectile with the most optimal option – using a multi-element two-spectral diamond photodetector – an advanced development implemented at the UralAlmazInvest Production and Technology Center [10].
On the manufactured prototypes, spectral ranges of sensitivity for the UV channel – 0.19–0.23 μm and for the IR channel – 0.8–3.3 μm were provided, as well as the sensitivity thresholds for the UV channel – 9 ∙ 10–12 W / Hz1 / 2 and for the IR channel – 6 ∙ 10–10 W / Hz1 / 2.
The basic structure of a bispectral photodetector is based on the use of two photodetectors formed on opposite sides of a diamond plate. The diamond-based photodetector is absolutely transparent in the visible and infrared regions of the spectrum, and any imaging matrix can be placed behind it. The UV point from the target can then be superimposed on the visible or infrared image and viewed in one frame.
The technology and design of bispectral multi-element photodetectors and photodetectors based on diamond materials presupposes the achievement of a photodetector format of 240 × 240 elements with a pixel size of ~50 × 50 μm and simultaneous operation in two ranges:
- spectral range (UV): 0.12–0.28 microns;
- spectral range (IR): 0.7–3.6 microns.
The effectiveness of the use of UV systems is clearly demonstrated by the complexed image obtained in the IR and UV regions of the spectrum. Fig. 17 shows a survey of a forest area on the shore of a lake, where a fire is burning.
On the left is the image in the IR spectrum, in the center – the image of the campfire in the UV spectrum (on the right side of the image there is a bright spot). On the right is a summary of the IR and UV images of this area.
The multi-element photodetector allows not only recording an event, but also performing coordinate measurements necessary for the practical implementation of the channel for fixing the rupture of a projectile built into the main sight.
4. Other applications of solar blind UV devices
Based on the experience of using optoelectronic devices, it should be noted that there is no universal device that solves absolutely all problems with a high degree of probability.
Devices of the solar blind UV range are an important addition to the channels of the visible, IR and radar ranges, and in some cases, they are the only option for solving the problem.
There are many applications where, in addition to one UV targeting or direction finding channel, channels of other spectral ranges are simply not required. These include, for example, devices for orientation by the Sun and stars in outer space, systems for detecting and identifying “friend or foe”, control systems for high-temperature production technologies, fire monitoring systems, aircraft detection systems based on UV radiation from jet engines, detection systems flames of various origins, etc.
For such applications, only one UV channel can be used, consisting of optics, UV photodetector and electronic signal processing units.
Other applications are of no less interest, such as means of covert interference-proof UV optical communication, laser systems for locating and seeing in the UV range, which are structurally more complex devices, since they must contain sufficiently powerful laser sources of UV radiation. In this regard, it will be useful to note that in the solar blind UV spectral region of 0.2–0.3 μm, the use of the radiation of the fourth harmonic (0.266 μm) of a laser based on neodymium glass and on yttrium-aluminum garnet with neodymium will be quite effective [11].
The applied capabilities of such devices will largely be determined by the potentially achievable technical characteristics of laser systems, the main of which are the energy in the laser pulse and the sensitivity of the UV receiver.
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AUTHORS
Medvedev Alexander Vladimirovich, design@romz.ru, General Designer, Rostov Optical and Mechanical Plant OJSC (ROMZ OJSC), Rostov Veliky, Yaroslavl Region, Russia.
Grinkevich Alexander Vasilievich, lyu1455@yandex.ru, ZAO «EVS», Moscow, Russia.
Knyazeva Svetlana Nikolaevna, ksn 61@yandex.ru, Design Engineer, Design Bureau of OJSC «Rostov Optical and Mechanical Plant, (OJSC» ROMZ «), Rostov the Great, Yaroslavl Region, Russia.
Contribution by the members
of the team of authors
The article was prepared on the basis of work by all members of the team of authors.
Conflict of interest
The authors claim that they have no conflict of interest. All authors took part in writing the article and supplemented the manuscript in part of their work.
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