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Optics and electronics for tanks
Written by Sakhal
Primitive tanks were almost blind. Vision was effectuated through narrow peepholes protected by crystal blocks and rudimentary periscopes whose mirrors were polished metal plates, for
crystal mirrors crashed very easily. Technology allowed to remedy these shortcomings by incorporating optics first and electronics later.
Vision systems
These systems can be classified within simple optics combined with stadias and optical rangefinders, and within electronic optics such as sights combined with laser rangefinders, starlight enhancers, low-light television cameras and thermographic cameras.
Interior of the turret of an M1A2 Abrams (of the early generation, circa 1992), considered one of the most advanced main battle tanks in the world.
Going back on History, the Mother Mark I cannons were aimed by means of a simple ironsight attached to the barrel. Later they incorporated the idea from Sir Perry Scott, taken in turn from Bradley Fiske of the United States Navy, who in 1892 installed a telescopic sight on a naval cannon. This sight magnified the view but was little used because it was directly attached to the barrel and recoiled with it, hurting the gunner if he did not move the head aside on time. In 1898, Sir Perry installed the telescope on a sleeve so it would not recoil with the cannon. The distance to the target was roughly estimated and then inputted on a drum actuating on the lines of a reticle, and the cannon was directed to set the crosshair upon the target. Gunners used ironsights more often than telescopes, in such an optimistic Army which in the Manual of 1938 stipulated that "fire on movement should be considered the common combat form".
One of the problems of tanks was - and is - to see what happens around, either for driving, watching or shooting. Without proper sighting devices, these actions would be very precarious. It was like this until the Second World War, when it became imperative to equip tanks with such. Until then a telescope was installed for the gunner while the other crew members had to pop their heads out, putting themselves "half way to heaven", in the words of an old tanker, or look through peepholes protected with crystal blocks which almost always broke on the first impact, leaving the user without vision. As sights were perfected it was learnt that vision requirements were different for each of the crew members, and thus each of them received a different vision element in accordance with the nature of his tasks inside the tank, giving preference to the gunner. The driver and the loader had a simple episcope with no magnification, albeit the former was often supplied with infrared sights. Things changed when technical advances allowed the commander to personally intervene in combat during critical situations. Electronic optics began to put at his disposal the advantages that the gunner enjoyed, and sometimes much more sophisticated elements.
The Israeli company Astronautics CA produces diverse systems for armored vehicles and tanks. From left to right we can see: fire control system for the Centurion, display and control panel for the driver of the Merkava, fire control system for the M48, computer and control units for the Centurion and M60, sensor and meteorological mast, and video display unit for weapon systems installed in vehicles.
The Knight is a fire control system intended for the modernization of tanks. It is a joint development between the Israeli companies Elbit (computers and controls) and EL-OP (optical sights, laser rangefinders and meteorological mast).
Commander's sights
The development of tanks and the experience gained by their crews highlighted the necessity of more complete vision elements. A simple persicope or a crown of episcopes no longer was considered sufficient. A fixed periscope provided vision on the line of sight of the cannon, but did not allow surveillance around the tank. A crown of episcopes provided 360-degree vision, but lacking magnification it was effective only at short distances. This type of episcopes was used for the first time in the Panzer II, with many variants regarding size and shape. A notable example is the Mark 2 produced by Helio Mirror Company for the commander's cupola of the Chieftain, which has a 140-degree field of view. Nine of them were installed, granting an excellent panoramic view around the tank, but they were very large and susceptible to sustain damage due to the large area of exposed crystal. They had an anti-reflective design to prevent reflections from the sun or from the lights of other vehicles from revealing the presence of the tank.
The crown of episcopes was complemented with an orientable periscope installed in the same cupola, to get around the shortcoming of episcopes lacking magnification. Many have been the solutions adopted in diverse tanks, from orientable cupolas to fixed cupolas with orientable periscopes, which could be coupled to the gunner's sight and linked by rods to the cannon, to allow the commander to open fire. Some of these solutions were used in tanks with many years of service, such as the Chieftain or the AMX-30. In some cases there is a fixed periscope attached to the top of the turret, which facilitates to link it to the cannon by means of rods, as in the Luchs, which has two PERI Z-11 sights manufactured by Keller & Knappich Augsburg, with x2 and x6 magnification. With this system the head of the periscope can be stabilized, but since it is slave of the cannon the stabilization is not good.
Electronic marvels did not fascinate every army, and the German preferred to wait before overloading their tanks with devices whose performance was unknown, adopting only those of undoubtful necessity. Their pragmatism led them to install in the Leopard 2 an optical commander's periscope, combined with the EMES 15 gunner's sight, which integrates a laser rangfinder and a thermographic camera, linked to the fire control calculator.
Gunner's sights
As aforementioned, the simplest aiming system is a telescope fixedly attached to the cannon, through which the gunner could watch and aim the weapon. Thanks to its simplicity, this system remained as an auxiliary sight so when electronics failed the gunner could resort to the old and simple telescope. This system, however, presents diverse inconveniences. The union with the cannon is never as rigid as intended, for looseness is always present and atmospheric conditions alter the length of the rods, causing distortion on the homogenization; in the Chieftain the rods were hollow and contained a liquid to limit the distortion. The reticle is quite far away from its theoretical axis of rotation in elevation (the trunnions' axis) and this forces to effectuate the homogenization on a preset distance, introducing an error that varies with distance and cannot be compensated. Primitive telescopes which were fixedly attached and had no magnification forced the gunner to follow the zenithal movement of the mounting, raising his head on low depression angles and crouching on high elevation angles.
Some of these inconveniences can be avoided with an articulated telescope in which the reticle is fixed to the ceiling of the turret and some magnification is added. On the sight are drawn distance scales for the weapons and in some a simple stadia, being the most typical case the Soviet tanks until the T-62. The stadiametric system uses a graduated reticle based in the apparent size of a known object, in this case a tank. The target is framed within two marks and the distance calculated is taken to a graduated scale superimposing it to the target. The precision of this method is rather poor, for it requires that the target has the same dimensions than those of the object used for the calculation of the stadia. This is to say, the measurements have a certain degree of precision only when used against the target which was used as reference for the stadiametric device and when that one presents itself in the same attitude, either front or profile, that was used for the calculations. It is influential as well the dexterity of the gunner when aligning the stadiametric marks with the target and this implies that the measurements have to be effectuated while the tank is stationary, for while on movement the alignment will be almost impossible regardless of how skilled the gunner is.
All of these aiming systems were considered little effective because they lost precision as distance increased (which actually happens with any measurement device used). The tension of the trajectory, whose rise is lower than the height of any tank at normal combat distances, would allow to take an acceptable advantage of the method. It is statistically proven that encounters between tanks, with few exceptions, happened at distances between 300 and 800 meters, being the errors incurred on rough estimation relatively small up to 1500 meters; by aiming slightly above the calculated trajectory, the probability of accurate impact against stationary tanks is not much lesser than that achieved with a fire control system. The true problem of these aiming devices is to fire while on movement, for in this case their precision is almost null.
To limit this problem technicians resorted to a nautical trick: the stabilizator. From the beginning it was attempted to stabilize the cannon in two axes (elevation and azimuth), but the Americans had to give up on this and install a single gyroscope on the M4 Sherman and M24 Chaffee. This was not yet proper electronics, for the movements were effectuated by means of metadynes and electrohydraulic means, the reliability was poor and tankers used to turn them off, being on the end a useless expense. Despite the inconveniences, the British persisted in the development of two- axis stabilizers from the Centurion. Basically, a stabilizator involves two systems, one for the zenithal movement of the cannon and another for the azimuthal movement of the turret. Each has a gyroscope which measures the angular speed of the cannon on both movements; the difference between these two speeds and those ordered by the gunner through his manual controls is used to actuate on the motors (zenithal of the cannon and azimuthal of the turret) in the corresponding direction to nullify the difference, thus stabilizing the cannon. This is to say, if the gunner keeps his controls untouched the two-axis stabilization mechanism will keep the position of the cannon regardless of the roll, pitch or yaw caused by the movement of the tank.
In the second generation it was discovered that the stability of vision can be increased by unlinking the sights from the cannon and stabilizing them independently, for the inertia of the head of the sights is lesser than that of the cannon. This does not mean that stabilization is greater, but that the gunner can track the target with increased precision, for the cannon is now actuated by servomechanisms directed by the sight's stabilization system instead of their own. Priorly, the stabilization system directed the movements of the cannon and in turn this one directed the gunner's sight which was attached to it. By stabilizing the sight the opposite happens, as it is the sight which transmits its movement to the cannon. This system was tested in the MBT-70 and the experience gained used in the Leopard 2 and the M1 Abrams, albeit this latter, due to budget reasons, had the sight stabilized in elevation only, while for azimuth it was used the ordinary method. The French used a different system based in their firing doctrine, which gave preference to the commander, so it was of his sight of which the movement of the cannon and turret depended. The COSTAC system installed on the AMX-30B2 used this method which allowed the commander to fire while on movement and indirectly stabilized the gunner's sight linked to the cannon.
Gunner of an M1A2 Abrams looking through a thermal imaging sight.
Optical rangefinders
The obsession about the mistakes made on the estimation of distances and the logical desire of an accurate hit on the first shot led to the adoption of coincidence or stereoscopic rangefinders. Both required a large base which could not be installed with ease in a tank because of its width. The coincidence type suffers from diverse luminic distortions and must be regularly adjusted, while the stereoscopic type demands certain physical conditions from the operator and, for achieving a certain accuracy, a rather long training period. Besides, stereoscopic rangefinders often deteriorated the vision of gunners.
Laser rangefinders
The first laser rangefinder was manufactured for an American tank in 1961. It used ruby as emitting element, having a wavelength of 0.69 microns and the important inconvenience of being the beam visible under certain conditions. It was replaced around 1970 by another type which uses a rod of crystal doped with neodymium, having a wavelength of 1.06 microns. Both the ruby and neodymium-YAG types are hazardous for the human eye and precaution must be taken during the training. Besides they are easily detectable by special devices that alert the crews. Between 1964 and 1967 experimental laser rangefinders were produced for the Leopard 1 and Panzer 61. In 1966 the Belgian created the COBELDA fire control system which incorporated a laser rangefinder for their Leopard tanks, which was later adopted for the Australian Leopard. Three years later France began to develop its own neodymium-YAG laser. In 1970 Sweden created the BOFORS fire control system which incorporated a neodymium-YAG laser rangefinder for the Stridsvagn 103. In 1973 the British incorporated a laser rangefinder to a periscope, interchangeable with the original one of the Centurion and other tanks.
The neodymium-YAG type had to be replaced by the carbon dioxide type, which has a wavelength of 10.6 microns and improved performance in adverse conditions, for the absortion and dispersion caused by water steam or dust in suspension have lesser effect in radiations of longer wavelength, being as well less hazardous for the human eye. On the other hand, laser rangefinders emitted relatively wide beams with a divergence of around 1 milliradian, which gave as result echoes not only from the target but also from other objects, being necessary to discern between the true and the false ones. The abundancy of false echoes caused a loss of trust in the effectiveness, demonstrated in the Leopard 2, whose EMES 12 gunner's sight combined a laser rangefinder with a stereoscopic rangefinder with a base of 1.72 meters in width for checking the measurements; however this device was later removed because of being unnecessary.
The neodymium-YAG laser is superior to the ruby type. It is more efficient, demands less energy, and operates at higher pulse repetition frequencies. But it still pose problems. Itswavelength of 1.06 microns, rather close to the visible spectrum, is dangerous for its radiation is focused by the human eye and transmitted to the retina, being able to blind an unprotected observer at short distances. Its wavelength renders it compatible with ordinary crystal optics but incompatible with that of germanium of the thermographic cameras which operate with wavelengths of 8 to 10 microns. These problems led the investigations toward the carbon dioxide type, whose wavelength of 10.6 microns is compatible with that of thermographic cameras and is not focused by the human eye toward the retina. But this type has shortcomings as well, for its wavelength coincides with a deep atmospheric absorption which degrades its performance in humid conditions. With targets that are wet or covered with snow, the reflection of the beam is weaker and strongly dependant on the geometry of the target. The worst problems are the complexity and lesser reliability, besides the high manufacture costs.
Because of this, there exists a certain reluctance to the adoption of carbon dioxide lasers. In United States, taking as base the neodymium-YAG type, a new rangefinder was developed which used the Raman dispersion effect. Initially it had only a scientific interest and in the late 1970s it was adopted as a safe-for-the-eye laser which was simpler and less expensive than the carbon dioxide type. Due to the better atmospheric penetration of its 1.45-micron wavelength, the Raman-effect laser can exceed the performance of neodymium-YAG and carbon dioxide types, operating in similar output levels. EGLE (Erbium Glass Laser-Eye-Safe) is the denomination of a family of laser rangefinders whose internal equipment differs little from those of 1.06-micron wavelength, but which uses an erbium-glass emitter which radiates on a 1.54-micron wavelength and does not require additional equipment to protect the human eye. The output is of similar nature than that of the Raman-effect type, and the materials smaller and less expensive.
Panoramical sights
The inconveniences posed by telescopes and periscopes led to the adoption of other solutions, among which stand out the fixed periscopes of rotating head, known as panoramical periscopes, for they facilitate 360-degree vision. They offer a better solution due to their easier manipulation by the operator, who does not have to follow with his body the rotation of the periscope, as only its head rotates while the rest of the device remains static. These devices are not new, as they were produced in Germany by Goerz already before the First World War, to be used in artillery sights. In tanks they were used around 1930, in the Leichttraktor and Grosstraktor, this time manufactured by Zeiss, with x2 magnification. Inexplicably, these devices fell into oblivion until the arrival of the Leopard 1, being manufactured in this occasion by Steinheil Lear Siegler, with zoom optics that allowed to sequentially increase magnification from x2 to x20. The rotating head was linked to the cannon (from which it received elevation data), allowing the commander to use the periscope to surveil around and to align the cannon with it when required to indicate a target to the gunner. It also enabled the commander to open fire by means of a prioritary control, removing so the necessity of long spoken explanations in a moment when seconds can be crucial.
The utilization of panoramical periscopes during long lasting combats, with closed hatches, in an environment contaminated by nuclear radiation or chemical agents, will likely cause disorientation. In the case of the commander this risk is minimized thanks to the crown of episcopes on the cupola. Even the most modern devices have a diversity of detractors, but it is fair to say that the inconveniences are outweighted by the numerous advantages that they offer. A controversy remains regarding whether they should be monocular or binocular. The former do not provide stereoscopic panoramic vision, which causes deficient depth perception, while the latter require that the vision differences of an eye in respect of the other could be compensated through the regulation controls of the eyepieces, and sometimes extra training for the operators. Since the measurement of distances is given to the gunner by a laser rangefinder, the necessity of stereoscopic vision is relative, being convenient but not imprescindible.
Zoom optics were discontinued in the PERI R-12 periscope of the Leopard 1A4, which had its line of sight stabilized and an environmental light enhancer, because its display was linked to the gunner's thermal imaging sight. Both sights had x2 and x8 magnification in the diurnal channel. The size of these devices was kept unobtrusive for long time, but as new complexities were added, due to the incorporation of electronics, they grew in size for each new element (laser, stabilizator, thermal vision or low-light television, among others) had to be integrated into the imprescindible elementary sight, which in turn forced to integrate microprocessing to control such a complex set of elements. The PERI RTW 90 day/night stabilized monocular periscope, which measures 825x230x390 millimeters, is among those of most compact dimensions. An uncommon case is the Merkava Mark 3, which received two equal periscopes for the commander and the gunner, manufactured by EL-OP and Elbit, and which use the same channel for both day and night vision.
The tremendous complexity of modern optronics is well represented by the family of electronic sights developed by French company SAGEM, which created the VTI (Viseur Tireur Integre) system for the AMX Leclerc and the SAVAN family which is based on it. The VTI, as the SAVAN 20, has three subsystems: stabilized head, target tracking device and digital electronic unit. The head has two windows, one for the diurnal channel and another for the thermal imaging camera, incorporating a large mirror made of light alloy based on beryllium, of low inertia and great mechanical stability, providing good stabilization of the image in both the visible and the low-high infrared spectrums. Mounted in a universal joint, it is stabilized in two axes by two gyroscopes and an accelerometer, while the movement is transmitted by a transmission belt. The system allows to stabilize the line of sight with a precision of about 50 milliradian.
A second gyroscope and another two accelerometers serve as an optional system for vertical and navigational reference, without having to check the magnetic North. For navigation, the inertial platform can be supplemented with an odometer connected to the propulsion plant for measuring the distance covered. Successive readjustments of the points stored in memory, at the beginning of a mission, are made with a precision of around 1 percent. This function allows each tank to be controlled from a centralized command post, such as the SIR (Sisteme Informatique Regimentaire). The mounting for target tracking consists of several elements, according to user choice. In the case of the HL-60 gunner's sight it can include: eyepiece for the gunner, television displays, infrared and CCD (Charge Coupled Device) cameras, a neodymium-YAG laser and a symbol generator. The diurnal channel serves simultaneously to the eyepiece, the television camera and the laser with a separating prism walled between them.
The digital electronic unit is one of modular design. The card interfaces provide a link for the DIGIBUS (in the HL-60) or the DATABUS 1553 (of American origin). The processors (three of 68000 type in the HL-60, programmed in Pascal language) control the relation between the weapon systems and the components of the sight. The computerized unit determines the angular position relative to the target, the absolute angular speed and the speed, with a precision of 0.1 milliradian or less. Since the cannon is linked to the gunner's sight by the aiming controls and the stabilization of the line of sight, the gunner's sight continuously delivers data of the turret and a vertical reference to the cannon. The HL-60 of the AMX Leclerc is installed to the right of the cannon and has three optical channels: diurnal channel with CCD television camera, nocturnal channel with thermal imaging camera and laser rangefinder channel. All the three use the same mirror, which is stabilized in both elevation and azimuth. An interface allows to transfer to the presentation display of the tank's commander the images taken by the HL-60, which curiously has as well a stadiametric scale.
The SAVAN 10, built for exportation, differs from the HL-60 and SAVAN 20 in that it lacks a thermal imaging camera. The size of the head is smaller (135x300x300 millimeters), it has a single window and a 70-millimeter lens for diurnal vision, through which the laser beam passes by. Other than that, it has the same functions of the other systems including the vertical and navigational reference, the performance characteristics regarding stabilization of the line of sight, navigation, aiming precision and speed (1 radian/second) and aiming acceleration (15 radians/second squared).
Upper picture: gyro-stabilized aiming system developed by SAGEM for the gunner of the AMX Leclerc. Lower picture: also developed by SAGEM is the gyro-stabilized modular periscope VIGY 40, for observation and aiming, with incorporated laser rangefinder, being possible the installation of either a thermal imaging camera or a starlight intensifier.
For the Challenger 2 it was chosen a derivative of the SAVAN 10, denominated SAMS (Stabilized Aiming Mirror System). The program is part of a Franco-British agreement on which the French deliver the head of the sight including the mirror, the stabilizator and the electronic unit, while the British produce the laser rangefinder and the optics. The 68000-type processors in Pascal language were linked to a DATABUS 1553 of American origin. The TOGS thermographic camera (Thermal Observation and Gunnery Sight), manufactured by Barr & Stroud, was not integrated into the sight, but installed above the cannon and stabilized in a very elemental way, being the fine stabilization carried out by the head of the sight. The field of view ranges from -20 to +60 degrees in elevation (-10 to +20 in the HL-60 and SAVAN 10/20) and from -7 to +7 degrees in azimuth.
SFIM Industries manufactured the VS 580 commander's sight, its derivatives and the HL-15. It has a panoramic stabilized head, an intermediate mounting and an electronic unit. The head, identical in every model, has a x2, x3.2 and x10.5 magnification system, plus a mirror mounted in a universal joint, stabilized by a miniaturized two-axis gyroscopic tuner, coupled to an analyzer sensor that provides angular data. The intermediate mounting, according to user choice, integrates several elements, a laser rangefinder and a starlight intensifier. The telescopic mounting comprises lenses, eyepiece with adjustable dioptrias, fixed prism for image correction (with five reflections to shorten the optical path), photo-optical retractable filter and protective filter for laser emissions and brightness control.
While moving in rugged terrain, the VS 580 provides a stabilized line of sight with a precision of 0.2 milliradian, allowing to spot targets at 4600 meters and to identify them at 2600 meters. It is believed that this sight grants a 90 percent probability of accurate impact at the first shot with APFSDS ammunition against a target at 1500 meters, with the probability falling to 30 percent at 2500 meters. These results are comparable to those achieved by the gunner's sight. On the AMX Leclerc the commander had at his disposal the HL-15 sight (manufactured by SFIM Industries as well) which provided diurnal channel with x2, x2.5 and x10 magnification, nocturnal channel with a third-generation starlight intensifier and laser rangefinder channel.
A derivative of the VS 580 is the CASIMIR gyro-stabilized thermographic panoramical periscope, installed in the Osorio. It comprises the upper ensemble of the VS 580 and a Castor thermal imaging camera. SFIM Industries manufactured the stabilized panoramical head along with its interface and the electronic and control unit, while TRT produced the mid-section ensemble, the thermographic camera and the electronic and control unit. It weighs 90 kilograms and has a field of view of 5x3.3 degrees in wide mode and 2.5x1.6 degrees in narrow mode, with an elevation arc ranging from -35 to +35 degrees. Another derivative, developed by Galileo for the Ariete, was modified to cover a vertical arc between -10 and +60 degrees, to allow aerial surveillance. The processing unit was not conceived in the usual form of a ballistic computer, but rather like a nautical onboard computer, in the same way than in the AMX Leclerc or the M1A2 Abrams.
The UTAAS aiming system developed by American company Kollsman, fitted with thermal imaging system, was selected for the Swedish CV 90 infantry combat vehicle of the Hagglunds and Bofors series.
Conclusion
To this point, the obstacles faced since the incorporation of electronics, the laser and the ballistic computer (around the 1960s) were many due to the difficulty of integrating diverse elements that were incompatible with each other. The sights of the next generation should be even more impressive, allowing to automatize the search, localization, tracking and destruction of several targets. The American company Hughes has been developing a sight which incorporates all the elements (and the electronics to manage them) necessary to localize and destroy multiple targets in an almost automatic way. It is a panoramical stabilized sight that provides imaging for tracking moving targets, laser rangefinder, television camera for diurnal vision, thermographic camera in the spectrum of 8 to 12 microns, with possibility of both wide and narrow field of view for the search and localization of targets in any weather condition, millimetric wave radar that allows the localization and identification of targets in two ways, two ordinary sensors and an acoustic sensor for the detection of helicopters which ignores the own noise of the tank. The signals are processed and mixed, which requires computers of great power. It will possibly make use of neuronal networks for the identification and automatic tracking of targets.
Modern crews have access to electro-optic assistance for the search and localization of targets, but their capacity is diminished by the masking, decoy and concealment means used in the modern battlefield. They have to pay attention to diverse displays, panels, lights, dials and other elements that limit their performance, spending an important part of their time in watching the precious electronic devices. Because of this it is imperative to adopt systems that automatize many of the tasks, specially the surveillance and identification of threats, to allow the crews to focus on their most important labor: to make decisions.
Vision systems
These systems can be classified within simple optics combined with stadias and optical rangefinders, and within electronic optics such as sights combined with laser rangefinders, starlight enhancers, low-light television cameras and thermographic cameras.
Interior of the turret of an M1A2 Abrams (of the early generation, circa 1992), considered one of the most advanced main battle tanks in the world.
Going back on History, the Mother Mark I cannons were aimed by means of a simple ironsight attached to the barrel. Later they incorporated the idea from Sir Perry Scott, taken in turn from Bradley Fiske of the United States Navy, who in 1892 installed a telescopic sight on a naval cannon. This sight magnified the view but was little used because it was directly attached to the barrel and recoiled with it, hurting the gunner if he did not move the head aside on time. In 1898, Sir Perry installed the telescope on a sleeve so it would not recoil with the cannon. The distance to the target was roughly estimated and then inputted on a drum actuating on the lines of a reticle, and the cannon was directed to set the crosshair upon the target. Gunners used ironsights more often than telescopes, in such an optimistic Army which in the Manual of 1938 stipulated that "fire on movement should be considered the common combat form".
One of the problems of tanks was - and is - to see what happens around, either for driving, watching or shooting. Without proper sighting devices, these actions would be very precarious. It was like this until the Second World War, when it became imperative to equip tanks with such. Until then a telescope was installed for the gunner while the other crew members had to pop their heads out, putting themselves "half way to heaven", in the words of an old tanker, or look through peepholes protected with crystal blocks which almost always broke on the first impact, leaving the user without vision. As sights were perfected it was learnt that vision requirements were different for each of the crew members, and thus each of them received a different vision element in accordance with the nature of his tasks inside the tank, giving preference to the gunner. The driver and the loader had a simple episcope with no magnification, albeit the former was often supplied with infrared sights. Things changed when technical advances allowed the commander to personally intervene in combat during critical situations. Electronic optics began to put at his disposal the advantages that the gunner enjoyed, and sometimes much more sophisticated elements.
The Israeli company Astronautics CA produces diverse systems for armored vehicles and tanks. From left to right we can see: fire control system for the Centurion, display and control panel for the driver of the Merkava, fire control system for the M48, computer and control units for the Centurion and M60, sensor and meteorological mast, and video display unit for weapon systems installed in vehicles.
The Knight is a fire control system intended for the modernization of tanks. It is a joint development between the Israeli companies Elbit (computers and controls) and EL-OP (optical sights, laser rangefinders and meteorological mast).
Commander's sights
The development of tanks and the experience gained by their crews highlighted the necessity of more complete vision elements. A simple persicope or a crown of episcopes no longer was considered sufficient. A fixed periscope provided vision on the line of sight of the cannon, but did not allow surveillance around the tank. A crown of episcopes provided 360-degree vision, but lacking magnification it was effective only at short distances. This type of episcopes was used for the first time in the Panzer II, with many variants regarding size and shape. A notable example is the Mark 2 produced by Helio Mirror Company for the commander's cupola of the Chieftain, which has a 140-degree field of view. Nine of them were installed, granting an excellent panoramic view around the tank, but they were very large and susceptible to sustain damage due to the large area of exposed crystal. They had an anti-reflective design to prevent reflections from the sun or from the lights of other vehicles from revealing the presence of the tank.
The crown of episcopes was complemented with an orientable periscope installed in the same cupola, to get around the shortcoming of episcopes lacking magnification. Many have been the solutions adopted in diverse tanks, from orientable cupolas to fixed cupolas with orientable periscopes, which could be coupled to the gunner's sight and linked by rods to the cannon, to allow the commander to open fire. Some of these solutions were used in tanks with many years of service, such as the Chieftain or the AMX-30. In some cases there is a fixed periscope attached to the top of the turret, which facilitates to link it to the cannon by means of rods, as in the Luchs, which has two PERI Z-11 sights manufactured by Keller & Knappich Augsburg, with x2 and x6 magnification. With this system the head of the periscope can be stabilized, but since it is slave of the cannon the stabilization is not good.
Electronic marvels did not fascinate every army, and the German preferred to wait before overloading their tanks with devices whose performance was unknown, adopting only those of undoubtful necessity. Their pragmatism led them to install in the Leopard 2 an optical commander's periscope, combined with the EMES 15 gunner's sight, which integrates a laser rangfinder and a thermographic camera, linked to the fire control calculator.
Gunner's sights
As aforementioned, the simplest aiming system is a telescope fixedly attached to the cannon, through which the gunner could watch and aim the weapon. Thanks to its simplicity, this system remained as an auxiliary sight so when electronics failed the gunner could resort to the old and simple telescope. This system, however, presents diverse inconveniences. The union with the cannon is never as rigid as intended, for looseness is always present and atmospheric conditions alter the length of the rods, causing distortion on the homogenization; in the Chieftain the rods were hollow and contained a liquid to limit the distortion. The reticle is quite far away from its theoretical axis of rotation in elevation (the trunnions' axis) and this forces to effectuate the homogenization on a preset distance, introducing an error that varies with distance and cannot be compensated. Primitive telescopes which were fixedly attached and had no magnification forced the gunner to follow the zenithal movement of the mounting, raising his head on low depression angles and crouching on high elevation angles.
Some of these inconveniences can be avoided with an articulated telescope in which the reticle is fixed to the ceiling of the turret and some magnification is added. On the sight are drawn distance scales for the weapons and in some a simple stadia, being the most typical case the Soviet tanks until the T-62. The stadiametric system uses a graduated reticle based in the apparent size of a known object, in this case a tank. The target is framed within two marks and the distance calculated is taken to a graduated scale superimposing it to the target. The precision of this method is rather poor, for it requires that the target has the same dimensions than those of the object used for the calculation of the stadia. This is to say, the measurements have a certain degree of precision only when used against the target which was used as reference for the stadiametric device and when that one presents itself in the same attitude, either front or profile, that was used for the calculations. It is influential as well the dexterity of the gunner when aligning the stadiametric marks with the target and this implies that the measurements have to be effectuated while the tank is stationary, for while on movement the alignment will be almost impossible regardless of how skilled the gunner is.
All of these aiming systems were considered little effective because they lost precision as distance increased (which actually happens with any measurement device used). The tension of the trajectory, whose rise is lower than the height of any tank at normal combat distances, would allow to take an acceptable advantage of the method. It is statistically proven that encounters between tanks, with few exceptions, happened at distances between 300 and 800 meters, being the errors incurred on rough estimation relatively small up to 1500 meters; by aiming slightly above the calculated trajectory, the probability of accurate impact against stationary tanks is not much lesser than that achieved with a fire control system. The true problem of these aiming devices is to fire while on movement, for in this case their precision is almost null.
To limit this problem technicians resorted to a nautical trick: the stabilizator. From the beginning it was attempted to stabilize the cannon in two axes (elevation and azimuth), but the Americans had to give up on this and install a single gyroscope on the M4 Sherman and M24 Chaffee. This was not yet proper electronics, for the movements were effectuated by means of metadynes and electrohydraulic means, the reliability was poor and tankers used to turn them off, being on the end a useless expense. Despite the inconveniences, the British persisted in the development of two- axis stabilizers from the Centurion. Basically, a stabilizator involves two systems, one for the zenithal movement of the cannon and another for the azimuthal movement of the turret. Each has a gyroscope which measures the angular speed of the cannon on both movements; the difference between these two speeds and those ordered by the gunner through his manual controls is used to actuate on the motors (zenithal of the cannon and azimuthal of the turret) in the corresponding direction to nullify the difference, thus stabilizing the cannon. This is to say, if the gunner keeps his controls untouched the two-axis stabilization mechanism will keep the position of the cannon regardless of the roll, pitch or yaw caused by the movement of the tank.
In the second generation it was discovered that the stability of vision can be increased by unlinking the sights from the cannon and stabilizing them independently, for the inertia of the head of the sights is lesser than that of the cannon. This does not mean that stabilization is greater, but that the gunner can track the target with increased precision, for the cannon is now actuated by servomechanisms directed by the sight's stabilization system instead of their own. Priorly, the stabilization system directed the movements of the cannon and in turn this one directed the gunner's sight which was attached to it. By stabilizing the sight the opposite happens, as it is the sight which transmits its movement to the cannon. This system was tested in the MBT-70 and the experience gained used in the Leopard 2 and the M1 Abrams, albeit this latter, due to budget reasons, had the sight stabilized in elevation only, while for azimuth it was used the ordinary method. The French used a different system based in their firing doctrine, which gave preference to the commander, so it was of his sight of which the movement of the cannon and turret depended. The COSTAC system installed on the AMX-30B2 used this method which allowed the commander to fire while on movement and indirectly stabilized the gunner's sight linked to the cannon.
Gunner of an M1A2 Abrams looking through a thermal imaging sight.
Optical rangefinders
The obsession about the mistakes made on the estimation of distances and the logical desire of an accurate hit on the first shot led to the adoption of coincidence or stereoscopic rangefinders. Both required a large base which could not be installed with ease in a tank because of its width. The coincidence type suffers from diverse luminic distortions and must be regularly adjusted, while the stereoscopic type demands certain physical conditions from the operator and, for achieving a certain accuracy, a rather long training period. Besides, stereoscopic rangefinders often deteriorated the vision of gunners.
Laser rangefinders
The first laser rangefinder was manufactured for an American tank in 1961. It used ruby as emitting element, having a wavelength of 0.69 microns and the important inconvenience of being the beam visible under certain conditions. It was replaced around 1970 by another type which uses a rod of crystal doped with neodymium, having a wavelength of 1.06 microns. Both the ruby and neodymium-YAG types are hazardous for the human eye and precaution must be taken during the training. Besides they are easily detectable by special devices that alert the crews. Between 1964 and 1967 experimental laser rangefinders were produced for the Leopard 1 and Panzer 61. In 1966 the Belgian created the COBELDA fire control system which incorporated a laser rangefinder for their Leopard tanks, which was later adopted for the Australian Leopard. Three years later France began to develop its own neodymium-YAG laser. In 1970 Sweden created the BOFORS fire control system which incorporated a neodymium-YAG laser rangefinder for the Stridsvagn 103. In 1973 the British incorporated a laser rangefinder to a periscope, interchangeable with the original one of the Centurion and other tanks.
The neodymium-YAG type had to be replaced by the carbon dioxide type, which has a wavelength of 10.6 microns and improved performance in adverse conditions, for the absortion and dispersion caused by water steam or dust in suspension have lesser effect in radiations of longer wavelength, being as well less hazardous for the human eye. On the other hand, laser rangefinders emitted relatively wide beams with a divergence of around 1 milliradian, which gave as result echoes not only from the target but also from other objects, being necessary to discern between the true and the false ones. The abundancy of false echoes caused a loss of trust in the effectiveness, demonstrated in the Leopard 2, whose EMES 12 gunner's sight combined a laser rangefinder with a stereoscopic rangefinder with a base of 1.72 meters in width for checking the measurements; however this device was later removed because of being unnecessary.
The neodymium-YAG laser is superior to the ruby type. It is more efficient, demands less energy, and operates at higher pulse repetition frequencies. But it still pose problems. Itswavelength of 1.06 microns, rather close to the visible spectrum, is dangerous for its radiation is focused by the human eye and transmitted to the retina, being able to blind an unprotected observer at short distances. Its wavelength renders it compatible with ordinary crystal optics but incompatible with that of germanium of the thermographic cameras which operate with wavelengths of 8 to 10 microns. These problems led the investigations toward the carbon dioxide type, whose wavelength of 10.6 microns is compatible with that of thermographic cameras and is not focused by the human eye toward the retina. But this type has shortcomings as well, for its wavelength coincides with a deep atmospheric absorption which degrades its performance in humid conditions. With targets that are wet or covered with snow, the reflection of the beam is weaker and strongly dependant on the geometry of the target. The worst problems are the complexity and lesser reliability, besides the high manufacture costs.
Because of this, there exists a certain reluctance to the adoption of carbon dioxide lasers. In United States, taking as base the neodymium-YAG type, a new rangefinder was developed which used the Raman dispersion effect. Initially it had only a scientific interest and in the late 1970s it was adopted as a safe-for-the-eye laser which was simpler and less expensive than the carbon dioxide type. Due to the better atmospheric penetration of its 1.45-micron wavelength, the Raman-effect laser can exceed the performance of neodymium-YAG and carbon dioxide types, operating in similar output levels. EGLE (Erbium Glass Laser-Eye-Safe) is the denomination of a family of laser rangefinders whose internal equipment differs little from those of 1.06-micron wavelength, but which uses an erbium-glass emitter which radiates on a 1.54-micron wavelength and does not require additional equipment to protect the human eye. The output is of similar nature than that of the Raman-effect type, and the materials smaller and less expensive.
Panoramical sights
The inconveniences posed by telescopes and periscopes led to the adoption of other solutions, among which stand out the fixed periscopes of rotating head, known as panoramical periscopes, for they facilitate 360-degree vision. They offer a better solution due to their easier manipulation by the operator, who does not have to follow with his body the rotation of the periscope, as only its head rotates while the rest of the device remains static. These devices are not new, as they were produced in Germany by Goerz already before the First World War, to be used in artillery sights. In tanks they were used around 1930, in the Leichttraktor and Grosstraktor, this time manufactured by Zeiss, with x2 magnification. Inexplicably, these devices fell into oblivion until the arrival of the Leopard 1, being manufactured in this occasion by Steinheil Lear Siegler, with zoom optics that allowed to sequentially increase magnification from x2 to x20. The rotating head was linked to the cannon (from which it received elevation data), allowing the commander to use the periscope to surveil around and to align the cannon with it when required to indicate a target to the gunner. It also enabled the commander to open fire by means of a prioritary control, removing so the necessity of long spoken explanations in a moment when seconds can be crucial.
The utilization of panoramical periscopes during long lasting combats, with closed hatches, in an environment contaminated by nuclear radiation or chemical agents, will likely cause disorientation. In the case of the commander this risk is minimized thanks to the crown of episcopes on the cupola. Even the most modern devices have a diversity of detractors, but it is fair to say that the inconveniences are outweighted by the numerous advantages that they offer. A controversy remains regarding whether they should be monocular or binocular. The former do not provide stereoscopic panoramic vision, which causes deficient depth perception, while the latter require that the vision differences of an eye in respect of the other could be compensated through the regulation controls of the eyepieces, and sometimes extra training for the operators. Since the measurement of distances is given to the gunner by a laser rangefinder, the necessity of stereoscopic vision is relative, being convenient but not imprescindible.
Zoom optics were discontinued in the PERI R-12 periscope of the Leopard 1A4, which had its line of sight stabilized and an environmental light enhancer, because its display was linked to the gunner's thermal imaging sight. Both sights had x2 and x8 magnification in the diurnal channel. The size of these devices was kept unobtrusive for long time, but as new complexities were added, due to the incorporation of electronics, they grew in size for each new element (laser, stabilizator, thermal vision or low-light television, among others) had to be integrated into the imprescindible elementary sight, which in turn forced to integrate microprocessing to control such a complex set of elements. The PERI RTW 90 day/night stabilized monocular periscope, which measures 825x230x390 millimeters, is among those of most compact dimensions. An uncommon case is the Merkava Mark 3, which received two equal periscopes for the commander and the gunner, manufactured by EL-OP and Elbit, and which use the same channel for both day and night vision.
The tremendous complexity of modern optronics is well represented by the family of electronic sights developed by French company SAGEM, which created the VTI (Viseur Tireur Integre) system for the AMX Leclerc and the SAVAN family which is based on it. The VTI, as the SAVAN 20, has three subsystems: stabilized head, target tracking device and digital electronic unit. The head has two windows, one for the diurnal channel and another for the thermal imaging camera, incorporating a large mirror made of light alloy based on beryllium, of low inertia and great mechanical stability, providing good stabilization of the image in both the visible and the low-high infrared spectrums. Mounted in a universal joint, it is stabilized in two axes by two gyroscopes and an accelerometer, while the movement is transmitted by a transmission belt. The system allows to stabilize the line of sight with a precision of about 50 milliradian.
A second gyroscope and another two accelerometers serve as an optional system for vertical and navigational reference, without having to check the magnetic North. For navigation, the inertial platform can be supplemented with an odometer connected to the propulsion plant for measuring the distance covered. Successive readjustments of the points stored in memory, at the beginning of a mission, are made with a precision of around 1 percent. This function allows each tank to be controlled from a centralized command post, such as the SIR (Sisteme Informatique Regimentaire). The mounting for target tracking consists of several elements, according to user choice. In the case of the HL-60 gunner's sight it can include: eyepiece for the gunner, television displays, infrared and CCD (Charge Coupled Device) cameras, a neodymium-YAG laser and a symbol generator. The diurnal channel serves simultaneously to the eyepiece, the television camera and the laser with a separating prism walled between them.
The digital electronic unit is one of modular design. The card interfaces provide a link for the DIGIBUS (in the HL-60) or the DATABUS 1553 (of American origin). The processors (three of 68000 type in the HL-60, programmed in Pascal language) control the relation between the weapon systems and the components of the sight. The computerized unit determines the angular position relative to the target, the absolute angular speed and the speed, with a precision of 0.1 milliradian or less. Since the cannon is linked to the gunner's sight by the aiming controls and the stabilization of the line of sight, the gunner's sight continuously delivers data of the turret and a vertical reference to the cannon. The HL-60 of the AMX Leclerc is installed to the right of the cannon and has three optical channels: diurnal channel with CCD television camera, nocturnal channel with thermal imaging camera and laser rangefinder channel. All the three use the same mirror, which is stabilized in both elevation and azimuth. An interface allows to transfer to the presentation display of the tank's commander the images taken by the HL-60, which curiously has as well a stadiametric scale.
The SAVAN 10, built for exportation, differs from the HL-60 and SAVAN 20 in that it lacks a thermal imaging camera. The size of the head is smaller (135x300x300 millimeters), it has a single window and a 70-millimeter lens for diurnal vision, through which the laser beam passes by. Other than that, it has the same functions of the other systems including the vertical and navigational reference, the performance characteristics regarding stabilization of the line of sight, navigation, aiming precision and speed (1 radian/second) and aiming acceleration (15 radians/second squared).
Upper picture: gyro-stabilized aiming system developed by SAGEM for the gunner of the AMX Leclerc. Lower picture: also developed by SAGEM is the gyro-stabilized modular periscope VIGY 40, for observation and aiming, with incorporated laser rangefinder, being possible the installation of either a thermal imaging camera or a starlight intensifier.
For the Challenger 2 it was chosen a derivative of the SAVAN 10, denominated SAMS (Stabilized Aiming Mirror System). The program is part of a Franco-British agreement on which the French deliver the head of the sight including the mirror, the stabilizator and the electronic unit, while the British produce the laser rangefinder and the optics. The 68000-type processors in Pascal language were linked to a DATABUS 1553 of American origin. The TOGS thermographic camera (Thermal Observation and Gunnery Sight), manufactured by Barr & Stroud, was not integrated into the sight, but installed above the cannon and stabilized in a very elemental way, being the fine stabilization carried out by the head of the sight. The field of view ranges from -20 to +60 degrees in elevation (-10 to +20 in the HL-60 and SAVAN 10/20) and from -7 to +7 degrees in azimuth.
SFIM Industries manufactured the VS 580 commander's sight, its derivatives and the HL-15. It has a panoramic stabilized head, an intermediate mounting and an electronic unit. The head, identical in every model, has a x2, x3.2 and x10.5 magnification system, plus a mirror mounted in a universal joint, stabilized by a miniaturized two-axis gyroscopic tuner, coupled to an analyzer sensor that provides angular data. The intermediate mounting, according to user choice, integrates several elements, a laser rangefinder and a starlight intensifier. The telescopic mounting comprises lenses, eyepiece with adjustable dioptrias, fixed prism for image correction (with five reflections to shorten the optical path), photo-optical retractable filter and protective filter for laser emissions and brightness control.
While moving in rugged terrain, the VS 580 provides a stabilized line of sight with a precision of 0.2 milliradian, allowing to spot targets at 4600 meters and to identify them at 2600 meters. It is believed that this sight grants a 90 percent probability of accurate impact at the first shot with APFSDS ammunition against a target at 1500 meters, with the probability falling to 30 percent at 2500 meters. These results are comparable to those achieved by the gunner's sight. On the AMX Leclerc the commander had at his disposal the HL-15 sight (manufactured by SFIM Industries as well) which provided diurnal channel with x2, x2.5 and x10 magnification, nocturnal channel with a third-generation starlight intensifier and laser rangefinder channel.
A derivative of the VS 580 is the CASIMIR gyro-stabilized thermographic panoramical periscope, installed in the Osorio. It comprises the upper ensemble of the VS 580 and a Castor thermal imaging camera. SFIM Industries manufactured the stabilized panoramical head along with its interface and the electronic and control unit, while TRT produced the mid-section ensemble, the thermographic camera and the electronic and control unit. It weighs 90 kilograms and has a field of view of 5x3.3 degrees in wide mode and 2.5x1.6 degrees in narrow mode, with an elevation arc ranging from -35 to +35 degrees. Another derivative, developed by Galileo for the Ariete, was modified to cover a vertical arc between -10 and +60 degrees, to allow aerial surveillance. The processing unit was not conceived in the usual form of a ballistic computer, but rather like a nautical onboard computer, in the same way than in the AMX Leclerc or the M1A2 Abrams.
The UTAAS aiming system developed by American company Kollsman, fitted with thermal imaging system, was selected for the Swedish CV 90 infantry combat vehicle of the Hagglunds and Bofors series.
Conclusion
To this point, the obstacles faced since the incorporation of electronics, the laser and the ballistic computer (around the 1960s) were many due to the difficulty of integrating diverse elements that were incompatible with each other. The sights of the next generation should be even more impressive, allowing to automatize the search, localization, tracking and destruction of several targets. The American company Hughes has been developing a sight which incorporates all the elements (and the electronics to manage them) necessary to localize and destroy multiple targets in an almost automatic way. It is a panoramical stabilized sight that provides imaging for tracking moving targets, laser rangefinder, television camera for diurnal vision, thermographic camera in the spectrum of 8 to 12 microns, with possibility of both wide and narrow field of view for the search and localization of targets in any weather condition, millimetric wave radar that allows the localization and identification of targets in two ways, two ordinary sensors and an acoustic sensor for the detection of helicopters which ignores the own noise of the tank. The signals are processed and mixed, which requires computers of great power. It will possibly make use of neuronal networks for the identification and automatic tracking of targets.
Modern crews have access to electro-optic assistance for the search and localization of targets, but their capacity is diminished by the masking, decoy and concealment means used in the modern battlefield. They have to pay attention to diverse displays, panels, lights, dials and other elements that limit their performance, spending an important part of their time in watching the precious electronic devices. Because of this it is imperative to adopt systems that automatize many of the tasks, specially the surveillance and identification of threats, to allow the crews to focus on their most important labor: to make decisions.
Categories:
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[General]
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Website: Military History
Article submitted: 2018-03-18
E-mail:
Website: Military History
Article submitted: 2018-03-18
