rus
Issue #6/2024
A. O. Sinelnikov, N. V. Tikhmenev, A. A. Ushanov, V. M. Medvedev
State-of-the-Art and Development Trends of Inertial Navigation Systems Based on the Ring Laser Gyroscopes
State-of-the-Art and Development Trends of Inertial Navigation Systems Based on the Ring Laser Gyroscopes
DOI: 10.22184/1993-7296.FRos.2024.18.6.450.466
The article summarizes and analyzes information on the batch-produced high-end inertial navigation systems. The market dynamics for the high-end inertial systems and gyro pickups over the past 10 years are given. The dominance of inertial systems based on the ring laser gyroscopes for the mid-term and autonomous navigation is shown. The prevailing design and technological solutions of modern ring laser gyroscopes are considered. The similarity of foreign devices due to the optico-physical circuit is noted. The key developers of laser inertial systems are identified. A trend towards the reduced weight and dimensional specifications, decreased cost and energy consumption of the ring laser gyroscopes while maintaining the required accuracy and resistance to the external influences is shown.
The article summarizes and analyzes information on the batch-produced high-end inertial navigation systems. The market dynamics for the high-end inertial systems and gyro pickups over the past 10 years are given. The dominance of inertial systems based on the ring laser gyroscopes for the mid-term and autonomous navigation is shown. The prevailing design and technological solutions of modern ring laser gyroscopes are considered. The similarity of foreign devices due to the optico-physical circuit is noted. The key developers of laser inertial systems are identified. A trend towards the reduced weight and dimensional specifications, decreased cost and energy consumption of the ring laser gyroscopes while maintaining the required accuracy and resistance to the external influences is shown.
Теги: инерциальная навигационная система инерциальный датчик инерциальный измерительный блок кольцевой лазерный гироскоп оптический гироскоп резонансный гироскоп
State-of-the-Art and Development Trends of Inertial Navigation Systems Based on the Ring Laser Gyroscopes
A. O. Sinelnikov 1, 2, N. V. Tikhmenev 1, A. A. Ushanov 1, V. M. Medvedev 1
State Research Institute of Instrument Engineering JSC, Moscow, Russia
Peoples Friendship University of Russia (RUDN University), Moscow, Russia
The article summarizes and analyzes information on the batch-produced high-end inertial navigation systems. The market dynamics for the high-end inertial systems and gyro pickups over the past 10 years are given. The dominance of inertial systems based on the ring laser gyroscopes for the mid-term and autonomous navigation is shown. The prevailing design and technological solutions of modern ring laser gyroscopes are considered. The similarity of foreign devices due to the optico-physical circuit is noted. The key developers of laser inertial systems are identified. A trend towards the reduced weight and dimensional specifications, decreased cost and energy consumption of the ring laser gyroscopes while maintaining the required accuracy and resistance to the external influences is shown.
Keywords: inertial navigation system, inertial measurement unit, inertial sensor, ring laser gyroscope, optical gyroscope, resonant gyroscope
Article received: 25.05.2024
Article accepted: 14.08.2024
Introduction
The high-end gyroscopic sensors and accelerometers are the key components of modern inertial navigation systems (INS), stabilization and positioning systems and many others. At present, the inertial systems have become widely used in various fields, including civil and commercial aviation, aerospace, maritime navigation, military hardware, robotics, industry and agriculture robots [1].
The occurrence of ring laser gyroscopes (RLG) in the 60s of the twentieth century opened up potential for constructing strapdown INS [2]. The laser strapdown INS turned out to be cheaper and easier to manufacture in comparison with the platformINS based on the mechanical gyroscopes [1, 3].
Today, the RLGs has a strong presence in the most popular segment of the inertial sensor market with the bias stability in the range of 0.1–0.001°/h [1]. The main RLG advantages over other types of gyroscopes are the high stability of scale factor and bias, as well as reliability and resistance to the external influences.
Modern scientific research in the field of laser gyroscopy is aimed at reducing the random error drift, as well as improving algorithms and output data processing methods [4, 5]. The high development of RLG production technology contributes to the continuous improvement of the operational specifications of INS based on them.
Following the global technological trends to minimize the SWaP+C criterion (size, weight, power and cost), the manufacturers are releasing new RLG modifications that are more compact, energy efficient and cheaper. This opens up prospects for the use of RLGs in new areas, including the drones, autonomous vehicles and portable devices.
This study provides an analytical overview of the market current status in terms of the high-end INS and development trends of the laser gyroscopy against the background of high competition with other technologies of serial inertial sensors, primarily the fiber optic (FOG), hemispherical resonator (HRG) and micro-machined gyroscopes (MEMS).
Market performance of the high-end INS and gyroscopic sensors
Recently, information has begun to appear in the open data sources not only about the scheduled developments and finished INS and gyroscopic sensors, but also about the statistics on production and sales volumes [6–11]. The most complete information is provided in the analytical reviews of the French company Yole Developpement [6–9]. Based on the available data, we will analyze the market performance of the high-end INS and gyroscopic sensors over the past 10 years.
Figure 1 shows the market evolution for inertial technologies in the monetary terms with due regard to its segmentation by the types of gyroscopic technologies [6, 8]. As can be seen in Fig. 1, in 2011–2019 the total market volume increased 2.5 times from $1.29 billion to $3.24 billion. Other significant changes also occurred during this time period. The volume of INS based on the mechanical gyroscopes (MG) has been reduced to a non-relevant value. The number of INS based on the dynamically tuned gyroscopes (DTG) decreased by 3.8 times. Contrary to the positive forecasts, the INS based on the FOG are gradually losing their positions and have not been able to replace RLG. Their growth is 2.2 times that is below the market average value. The largest increase of 5.2 times is observed in the SWG technology, but the production volume itself is almost 10 times less than the RLG. The leaders in the terms of growth rates are MEMS and RLG. Simultaneously, the volume of laser INS in the monetary terms accounts for more than half of the entire world market of high-quality inertial systems.
The report results [9] noted a decrease of 1% in the total market value of the high-end gyroscopic sensors up to $3.1 billion. This phenomenon is caused by the COVID‑19 epidemic consequences that most strongly affected the civil aviation sector. In particular, it had an influence on the production volumes of aviation laser INS. At the moment, the crisis in this area has been successfully overcome. According to the forecasts, the RLG market volume should exceed $1 billion by 2030 [10, 11] that is clearly shown in Fig. 2.
We will have a look at the main application areas of the high-quality inertial sensors and systems. Depending on the zero offset stability and field of application, the high-quality gyroscopic systems are divided into four categories, as shown in Table 1.
Figure 3 shows the distribution of the monetary volume of the gyroscopic technologies market by the listed categories in 2011–2019 [6, 8]. It is shown in the given diagrams that the RLGs have the recently strengthened dominant position in the most popular classes of applications. The growth of the laser INS influence is especially significant in the segment of high-end autonomous and mid-term navigation systems. In this case, the RLGs have significantly displaced FOGs and other types of gyroscopes. In the fierce competition conditions in the market for high-end inertial systems, this has become possible due to the continuous improvement of technology, optimization of the RLG design and operating mode.
Design and technology features of the up-to-date RLGs
We will take a closer look at the main scientific and technological solutions underlying the RLGs. Despite the achieved technological level, the frequency synchronization of counterpropagating waves in the ring resonator remains a critical factor that determines the appearance and ultimate sensitivity of modern RLGs [4, 12]. The reason for this phenomenon is the radiation scattering and absorption on the reflectors and other selecting elements of the resonator [2]. Therefore, the criterion for dividing inertial laser sensors by the design and technological features is the method to develop initial difference in the frequencies of counterpropagating waves (frequency bias). In the serial RLGs, the following methods are used to remove the operating point from the lock-in zone of gyro output characteristic:
For a long period time, the efforts of RLG developers were aimed at developing a frequency bias using various non-mechanical methods. The American corporation Northrop Grumman has achieved the greatest success in this field. This company produces a four-frequency Zero-Lock (ZLG) type RLG with a non-planar optical cavity and a Faraday non-reciprocal element. ZLG is a development of the famous company Litton (USA) taken over by Northrop Grumman. This tool with a perimeter of 18 cm has a bias stability of 0.003°/h and is used by Northrop Grumman in a LTN‑101 type aviation INS with a positioning error in the autonomous mode of 1.1–1.5 km/h (Fig. 4). By 2013 (over a total of 20 years of production), more than 50,000 ZLG-type lasers and 8,000 systems based on them have been produced [13, 14]. These values are low and do not affect the market of high-end INS. In the review [8], Northrop Grumman is no longer mentioned as the RLG developer.
Another well-known development of Litton is the four-mirror single-axis dither RLG LG‑8028. The batch production of ring lasers with a perimeter of 8–34 cm, built according to this optical-physical circuit, has been mastered by many enterprises around the world (Fig. 5).
At present, the dominant position in the laser INS market is occupied by the international corporation Honeywell that has selected the area of improving RLG technology with the maximum simplicity of resonator design [8–11]. This approach has made it possible to implement the fundamental principle of laser gyroscopy: the lower the light losses in the cavity, the higher the RLG accuracy. According to the practical issues, the complicated laser design necessary for the transition to a non-mechanical bias, leads to an increase in the light losses, decreased accuracy, more comprehensive technology and an increase in the cost of the device.
Despite the noticeable differences in specific design solutions, the devices made by all leading foreign manufacturers have common technical solutions that ensure the RLG implementation:
The listed basic design principles of the RLG are closely related to each other. Any amendments lead to the significant changes in the entire range of accuracy, operational, weight and size characteristics, as well as the technology and cost. The given optical-physical circuit has been generated as a result of the 60‑year development of laser gyroscopy in the conditions of intense competition with other promising scientific and technological solutions [6–8]. The RLGs built according to this circuit are able to solve all navigation tasks for various controlled objects operating under the conditions of harsh mechanical and climatic influences [8–11, 15].
Despite the apparent simplicity of the optical-physical circuit and design of modern RLGs, only the achieved level of a number of technologies makes it possible to simultaneously ensure high accuracy and operational specifications of these devices. Such technologies include the following:
We will briefly consider the features of technological processes for the RLG production.
The mirror sputtering technology is a key one and largely determines the accuracy and performance specifications of RLGs. The reflection coefficient of the best up-to-date mirrors is 99.999% and even 99.9999%, that is, the total losses due to the scattering and absorption are equal to 1–10 million‑1 [4]. Today, the ion beam sputtering technology is used to apply the reflective thin-film coatings. The ion-beam sputtering is specified by the high stability of applying the reflective thin-film coatings and provides a coating structure with a quality that is not inferior to the monolithic material structure. It was the development of the ion-beam deposition technology for the mirrors, together with the metrological support, that made it possible to achieve high accuracy and reliability of modern RLGs.
The polishing technology for super-smooth mirror substrates is critical to obtaining the ultra-low light loss of the mirrors. The smooth surfaces suitable for the mirrors are specified by a root-mean-square roughness that is less than 0.1 nm for the best modern samples [4]. The works to improve the polishing quality are being performed by all manufacturers of laser optic devices. It is already possible to make the mirror substrates with the losses of up to several ppm that ensures bias stability of up to 0.001°/h for the dither RLGs.
The thermal vacuum processing technology for ring cavities has much in common with the processing technology for the gas-filled tubes where a high degassing and reliability degree is ensured by a vacuum heating temperature of up to 450oC. An important point is the reliable sealing of the cavity that is controlled at the vacuum degassing stage, as well as during the subsequent processes training. The tightness control is used as a basis to predict the laser durability indicators [16]. At the moment, new ion-beam technologies have removed the restriction on the vacuum heating temperature, and the multilayer dielectric mirrors obtained using this technology can withstand the temperature of more than 500 °C.
In the up-to-date RLG production technology, the mirrors and other laser parts are connected using the optical contact method. Its quality is determined by its strength and ensuring a vacuum tightness of the connection for a long period of time. For the laser gyroscopes, the shelf life is tens of years [17].
Currently, all RLGs for the mid-term and autonomous navigation are made of glass ceramics with the ultra-low TEC up to 1·10–7 °/C (astrositall SO‑115M, Zerodur, Cervit, ClearCeram, etc.) [18, 19]. Such materials have low gas permeability to He and Ne. This fact ensures the device average operating time (MTBF) from 104 to 105 hours and allows maintaining the high accuracy specifications of the RLG during the period of autonomous operation under severe temperature conditions.
Key manufacturers of the laser INS and their main products
We will consider the products of laser INS manufacturers that have a significant impact on the market for the high-end gyroscopic sensors and systems (Tables 1 and 2).
The key manufacturers of laser INS are concentrated in three large regions: North America, Europe and Asia. According to the foreign analysts [9], the world’s largest manufacturers of RLGs and laser INS are the following companies: Honeywell, Kearfott, EMCORE (USA); Safran, Thales (France); Raytheon Anschütz (Germany); IAI (Israel); CASC China Aerospace, NAVTECH INC, JAE, StarNeto (China); Research Institute “Polyus”, Moscow Institute of Electromechanics and Automation PJSC (Russia), etc.
The industry leader for many years has been Honeywell that accounts for 34% of the market volume of all gyroscopic pickups [9,10]. The Honeywell ‘s flagship devices are as follows:
digital RLG GG1320 with a cavity of 15 cm, a weight of 0.5 kg and bias stability of up to 0.0035°/h;
miniature RLG GG1308 with a cavity of 6 cm, a weight of 65 g and bias of up to 1°/h.
Based on these technologies, Honeywell has manufactured a range of laser INS and IMUs used for the most navigation applications (Fig. 6) [20–25]. The most popular systems are HG 9900 and HG 1700 [22, 23]. The production volume of such INS is at least 80,000 items per year.
In contrast to the high-precision RLG GG1320, the concept of the GG‑1308 device is aimed at obtaining minimum SWaP+C indicators to the detriment of accuracy specifications. Although the optical and physical circuit of these lasers is similar, there are fundamental technological differences. The GG‑1308 devices uses inexpensive optical glass BK‑7 as a construction material. The vacuum joints are made using the glass cement. This makes it possible to obtain a minimal connection area that is significantly smaller than when connecting by an optical contact. In such a small intracavity volume there is no place for a getter and the active medium space is limited, so an additional small-area hollow cathode is introduced into the design, and the vacuum technology has its own features. The most popular system in the tactical accuracy class is the HG1700 INS based on the GG‑1308 gyroscopes. It is well-known that by 2023 more than 450,000 of these systems have been sold. This circumstance has contributed to the development of the HG5700 INS for medium-term navigation based on the RLG technology GG1320 and the HG1700 system concept with the minimum SWaP for this segment [21].
EMCORE specializes in the positioning, guidance and ground navigation systems for the military purposes, such as PNU/UPNU, DRU-H-R etc. (Fig. 7) [26, 27]. These systems are built on the basis of a high-precision RLG RL‑34 by L3 Harris Space & Navigation with a bias stability of 0.001°/h (see Fig. 5).
Using the SWaP+C minimization principles, Kearfott produces a range of compact laser INS of the KN and MILNAV series, as well as the inertial measurement units (IMU) of the KI –4902 and KI‑4921 types based on the three-axial monolithic RLGs of the T16-B and T24-B types for marine, ground and aerospace applications, differing in the accuracy, weight and size characteristics (Fig. 8) [28].
The leader of the European market is Safran that produces a range of high-end laser INS in demand in aviation, maritime navigation and some military applications [29, 30]. The most popular systems are Sigma‑95L and Sigma‑95N (Fig. 9). Sigma 95L is a lightweight and compact INS based on the four-mirror RLG of GLC‑16 type, designed for the airplanes, helicopters, and adapted for the unmanned aerial vehicles (UAVs). Due to the built-in GPS receiver, Sigma 95L can be used either as a stand-alone position control and navigation system, or as an integrated SINS. Sigma‑95N is designed for the most demanding applications requiring the high precision navigation and guidance. All available sensors can be integrated into the system, including GPS, GLONASS, air data system, etc. The Sigma‑95N sensitive elements are the three-mirror RLGs of the GLS‑32 type that are similar in the optical and physical design to the devices made by Honeywell.
Another French company, THALES, produces the integrated IMUs TopAxyz and TOTEM High Land based on the three-axial RLGs in a single monoblock of the PiXYZ‑22 type (Fig. 10) [31] These IMUs provide a navigation accuracy level under external influences for the air, sea, and ground platforms for civil and military purposes.
The Israeli group IAI specializes in the development of compact TRL‑16 m IMUS with the tactical precision class for the stabilization and position control systems of various purposes, autonomous hybrid TMAPS INS and NSF-R gyrocompasses for the ground military equipment. These systems are based on the RLGs with three-mirror resonators and a cavity of 9 cm (Fig. 11) [32].
The German companies, Raytheon Anschütz and iMAR Navigation & Control, produce the high-quality INS based on the GG1320 RLG for defense, air, maritime, underwater, above-water and railway applications (Fig. 12) [33, 32].
At the annual international exhibition “Photonics. World of Lasers and Optics”, the Chinese developers regularly present the optical elements of ring lasers [35]. Thus, in 2023 Xi’an SNP Precision Optics Co., Ltd. presented the resonator housings and optical mirrors. The housings differ in their dimensions and are designed for the flat uniaxial and cavity three-axial RLG monoblock (Fig. 13). For example, the 90 LGC housing (Laser Gyro Cavity) has an overall size of 9 cm on the quadrangle side that includes a cavity with a side of 7 cm and a perimeter of 28 cm.
The Russian manufacturers, such as Moscow Institute of Electromechanics and Automation PJSC and Research Institute “Polyus” have mastered the production of RLGs with a vibration suspension device and a magneto-optical bias based on the Zeeman effect for the IMUs for a wide range of applications. The achieved level of optical technology and production capacity of the domestic developers correspond to the European ones.
Conclusion
The RLG market is experiencing significant growth driven by the increasing demand for the accurate and reliable navigation systems in various applications such as autonomous vehicles, UAVs, aerospace and many others.
Such a demand has increased in the defense and aerospace sectors due to the modernization of military equipment and the need for high-end navigation and guidance systems for aircrafts, missiles and satellites [36].
To solve the vast majority of navigation issues, the most popular sensitive elements are the dither RLGs with linear radiation polarizations.
The up-to-date technological advances have led to the development of compact and lightweight RLGs with the improved performance [37]. Miniaturization and integration have expanded their use in various industries.
ABOUT THE AUTHORS
Sinelnikov Anton Olegovich – Ph.D. in Technical Sciences, Head of the Laboratory, State Research Institute of Instrument Engineering JSC, Moscow; Associate Professor of the Department of Nanotechnology and Microsystem Engineering Peoples Friendship University of Russia (RUDN University), Moscow.
Area of expertise: laser gyroscopy, inertial navigation systems, gas lasers.
ORCID 0000-0002-5579-3509
Tikhmenev Nikolay Vadimovich – Ph.D. in Physical and Mathematical Sciences, Head of the Department of State Research Institute of Instrument Engineering JSC, Moscow.
Area of expertise: laser gyroscopy, vacuum and optical technologies of inertial sensors, technological reliability assurance.
Ushanov Aleksander Aleksandrovich – leading electronics engineer of State Research Institute of Instrument Engineering JSC, Moscow.
Area of expertise: laser gyroscopy, inertial navigation systems, system analysis.
ORCID 0009-0009-3703-9981
Medvedev Vladimir Mikhailovich – Doctor of Technical Sciences, Professor, Director General of State Research Institute of Instrument Engineering JSC, Moscow.
Area of expertise: technical diagnostics, arrangement and technical issues of the product operation management.
Contribution of authors
The article was prepared based on the work of all team members.
Conflict of interest
The authors declare no conflict of interest.
A. O. Sinelnikov 1, 2, N. V. Tikhmenev 1, A. A. Ushanov 1, V. M. Medvedev 1
State Research Institute of Instrument Engineering JSC, Moscow, Russia
Peoples Friendship University of Russia (RUDN University), Moscow, Russia
The article summarizes and analyzes information on the batch-produced high-end inertial navigation systems. The market dynamics for the high-end inertial systems and gyro pickups over the past 10 years are given. The dominance of inertial systems based on the ring laser gyroscopes for the mid-term and autonomous navigation is shown. The prevailing design and technological solutions of modern ring laser gyroscopes are considered. The similarity of foreign devices due to the optico-physical circuit is noted. The key developers of laser inertial systems are identified. A trend towards the reduced weight and dimensional specifications, decreased cost and energy consumption of the ring laser gyroscopes while maintaining the required accuracy and resistance to the external influences is shown.
Keywords: inertial navigation system, inertial measurement unit, inertial sensor, ring laser gyroscope, optical gyroscope, resonant gyroscope
Article received: 25.05.2024
Article accepted: 14.08.2024
Introduction
The high-end gyroscopic sensors and accelerometers are the key components of modern inertial navigation systems (INS), stabilization and positioning systems and many others. At present, the inertial systems have become widely used in various fields, including civil and commercial aviation, aerospace, maritime navigation, military hardware, robotics, industry and agriculture robots [1].
The occurrence of ring laser gyroscopes (RLG) in the 60s of the twentieth century opened up potential for constructing strapdown INS [2]. The laser strapdown INS turned out to be cheaper and easier to manufacture in comparison with the platformINS based on the mechanical gyroscopes [1, 3].
Today, the RLGs has a strong presence in the most popular segment of the inertial sensor market with the bias stability in the range of 0.1–0.001°/h [1]. The main RLG advantages over other types of gyroscopes are the high stability of scale factor and bias, as well as reliability and resistance to the external influences.
Modern scientific research in the field of laser gyroscopy is aimed at reducing the random error drift, as well as improving algorithms and output data processing methods [4, 5]. The high development of RLG production technology contributes to the continuous improvement of the operational specifications of INS based on them.
Following the global technological trends to minimize the SWaP+C criterion (size, weight, power and cost), the manufacturers are releasing new RLG modifications that are more compact, energy efficient and cheaper. This opens up prospects for the use of RLGs in new areas, including the drones, autonomous vehicles and portable devices.
This study provides an analytical overview of the market current status in terms of the high-end INS and development trends of the laser gyroscopy against the background of high competition with other technologies of serial inertial sensors, primarily the fiber optic (FOG), hemispherical resonator (HRG) and micro-machined gyroscopes (MEMS).
Market performance of the high-end INS and gyroscopic sensors
Recently, information has begun to appear in the open data sources not only about the scheduled developments and finished INS and gyroscopic sensors, but also about the statistics on production and sales volumes [6–11]. The most complete information is provided in the analytical reviews of the French company Yole Developpement [6–9]. Based on the available data, we will analyze the market performance of the high-end INS and gyroscopic sensors over the past 10 years.
Figure 1 shows the market evolution for inertial technologies in the monetary terms with due regard to its segmentation by the types of gyroscopic technologies [6, 8]. As can be seen in Fig. 1, in 2011–2019 the total market volume increased 2.5 times from $1.29 billion to $3.24 billion. Other significant changes also occurred during this time period. The volume of INS based on the mechanical gyroscopes (MG) has been reduced to a non-relevant value. The number of INS based on the dynamically tuned gyroscopes (DTG) decreased by 3.8 times. Contrary to the positive forecasts, the INS based on the FOG are gradually losing their positions and have not been able to replace RLG. Their growth is 2.2 times that is below the market average value. The largest increase of 5.2 times is observed in the SWG technology, but the production volume itself is almost 10 times less than the RLG. The leaders in the terms of growth rates are MEMS and RLG. Simultaneously, the volume of laser INS in the monetary terms accounts for more than half of the entire world market of high-quality inertial systems.
The report results [9] noted a decrease of 1% in the total market value of the high-end gyroscopic sensors up to $3.1 billion. This phenomenon is caused by the COVID‑19 epidemic consequences that most strongly affected the civil aviation sector. In particular, it had an influence on the production volumes of aviation laser INS. At the moment, the crisis in this area has been successfully overcome. According to the forecasts, the RLG market volume should exceed $1 billion by 2030 [10, 11] that is clearly shown in Fig. 2.
We will have a look at the main application areas of the high-quality inertial sensors and systems. Depending on the zero offset stability and field of application, the high-quality gyroscopic systems are divided into four categories, as shown in Table 1.
Figure 3 shows the distribution of the monetary volume of the gyroscopic technologies market by the listed categories in 2011–2019 [6, 8]. It is shown in the given diagrams that the RLGs have the recently strengthened dominant position in the most popular classes of applications. The growth of the laser INS influence is especially significant in the segment of high-end autonomous and mid-term navigation systems. In this case, the RLGs have significantly displaced FOGs and other types of gyroscopes. In the fierce competition conditions in the market for high-end inertial systems, this has become possible due to the continuous improvement of technology, optimization of the RLG design and operating mode.
Design and technology features of the up-to-date RLGs
We will take a closer look at the main scientific and technological solutions underlying the RLGs. Despite the achieved technological level, the frequency synchronization of counterpropagating waves in the ring resonator remains a critical factor that determines the appearance and ultimate sensitivity of modern RLGs [4, 12]. The reason for this phenomenon is the radiation scattering and absorption on the reflectors and other selecting elements of the resonator [2]. Therefore, the criterion for dividing inertial laser sensors by the design and technological features is the method to develop initial difference in the frequencies of counterpropagating waves (frequency bias). In the serial RLGs, the following methods are used to remove the operating point from the lock-in zone of gyro output characteristic:
- Mechanically dithered systems RLG cavity, implemented by various devices (dither, torsion bars, etc.);
- magneto-optical frequency bias of counterpropagating waves based on the non-reciprocal Faraday or Zeeman effects.
For a long period time, the efforts of RLG developers were aimed at developing a frequency bias using various non-mechanical methods. The American corporation Northrop Grumman has achieved the greatest success in this field. This company produces a four-frequency Zero-Lock (ZLG) type RLG with a non-planar optical cavity and a Faraday non-reciprocal element. ZLG is a development of the famous company Litton (USA) taken over by Northrop Grumman. This tool with a perimeter of 18 cm has a bias stability of 0.003°/h and is used by Northrop Grumman in a LTN‑101 type aviation INS with a positioning error in the autonomous mode of 1.1–1.5 km/h (Fig. 4). By 2013 (over a total of 20 years of production), more than 50,000 ZLG-type lasers and 8,000 systems based on them have been produced [13, 14]. These values are low and do not affect the market of high-end INS. In the review [8], Northrop Grumman is no longer mentioned as the RLG developer.
Another well-known development of Litton is the four-mirror single-axis dither RLG LG‑8028. The batch production of ring lasers with a perimeter of 8–34 cm, built according to this optical-physical circuit, has been mastered by many enterprises around the world (Fig. 5).
At present, the dominant position in the laser INS market is occupied by the international corporation Honeywell that has selected the area of improving RLG technology with the maximum simplicity of resonator design [8–11]. This approach has made it possible to implement the fundamental principle of laser gyroscopy: the lower the light losses in the cavity, the higher the RLG accuracy. According to the practical issues, the complicated laser design necessary for the transition to a non-mechanical bias, leads to an increase in the light losses, decreased accuracy, more comprehensive technology and an increase in the cost of the device.
Despite the noticeable differences in specific design solutions, the devices made by all leading foreign manufacturers have common technical solutions that ensure the RLG implementation:
- The multilayer dielectric mirrors are used as the cavity reflectors;
- The amplification of the He-Ne gas active medium is provided by a direct current glow discharge;
- The laser radiation is generated at a light wavelength λ = 0.632 μm with the linear polarization;
- Using reverse rotation of the laser cavity around the sensitivity axis to remove the operating point from the dead lock-in zone of RLG output characteristic;
- The RLG cavity is stabilized using a piezoceramic actuators;
- The cavity structural elements are made of optical glass ceramics with an ultra-low thermal expansion coefficient (TEC) and gas permeability.
The listed basic design principles of the RLG are closely related to each other. Any amendments lead to the significant changes in the entire range of accuracy, operational, weight and size characteristics, as well as the technology and cost. The given optical-physical circuit has been generated as a result of the 60‑year development of laser gyroscopy in the conditions of intense competition with other promising scientific and technological solutions [6–8]. The RLGs built according to this circuit are able to solve all navigation tasks for various controlled objects operating under the conditions of harsh mechanical and climatic influences [8–11, 15].
Despite the apparent simplicity of the optical-physical circuit and design of modern RLGs, only the achieved level of a number of technologies makes it possible to simultaneously ensure high accuracy and operational specifications of these devices. Such technologies include the following:
- Sputtering technology for reflective dielectric mirrors;
- Polishing technology for the precision RLG mirror substrates;
- Electrovacuum processing technology for the ring cavities;
- Optical assembly technology;
- Development technology for the glass-ceramic materials with the low TEC.
We will briefly consider the features of technological processes for the RLG production.
The mirror sputtering technology is a key one and largely determines the accuracy and performance specifications of RLGs. The reflection coefficient of the best up-to-date mirrors is 99.999% and even 99.9999%, that is, the total losses due to the scattering and absorption are equal to 1–10 million‑1 [4]. Today, the ion beam sputtering technology is used to apply the reflective thin-film coatings. The ion-beam sputtering is specified by the high stability of applying the reflective thin-film coatings and provides a coating structure with a quality that is not inferior to the monolithic material structure. It was the development of the ion-beam deposition technology for the mirrors, together with the metrological support, that made it possible to achieve high accuracy and reliability of modern RLGs.
The polishing technology for super-smooth mirror substrates is critical to obtaining the ultra-low light loss of the mirrors. The smooth surfaces suitable for the mirrors are specified by a root-mean-square roughness that is less than 0.1 nm for the best modern samples [4]. The works to improve the polishing quality are being performed by all manufacturers of laser optic devices. It is already possible to make the mirror substrates with the losses of up to several ppm that ensures bias stability of up to 0.001°/h for the dither RLGs.
The thermal vacuum processing technology for ring cavities has much in common with the processing technology for the gas-filled tubes where a high degassing and reliability degree is ensured by a vacuum heating temperature of up to 450oC. An important point is the reliable sealing of the cavity that is controlled at the vacuum degassing stage, as well as during the subsequent processes training. The tightness control is used as a basis to predict the laser durability indicators [16]. At the moment, new ion-beam technologies have removed the restriction on the vacuum heating temperature, and the multilayer dielectric mirrors obtained using this technology can withstand the temperature of more than 500 °C.
In the up-to-date RLG production technology, the mirrors and other laser parts are connected using the optical contact method. Its quality is determined by its strength and ensuring a vacuum tightness of the connection for a long period of time. For the laser gyroscopes, the shelf life is tens of years [17].
Currently, all RLGs for the mid-term and autonomous navigation are made of glass ceramics with the ultra-low TEC up to 1·10–7 °/C (astrositall SO‑115M, Zerodur, Cervit, ClearCeram, etc.) [18, 19]. Such materials have low gas permeability to He and Ne. This fact ensures the device average operating time (MTBF) from 104 to 105 hours and allows maintaining the high accuracy specifications of the RLG during the period of autonomous operation under severe temperature conditions.
Key manufacturers of the laser INS and their main products
We will consider the products of laser INS manufacturers that have a significant impact on the market for the high-end gyroscopic sensors and systems (Tables 1 and 2).
The key manufacturers of laser INS are concentrated in three large regions: North America, Europe and Asia. According to the foreign analysts [9], the world’s largest manufacturers of RLGs and laser INS are the following companies: Honeywell, Kearfott, EMCORE (USA); Safran, Thales (France); Raytheon Anschütz (Germany); IAI (Israel); CASC China Aerospace, NAVTECH INC, JAE, StarNeto (China); Research Institute “Polyus”, Moscow Institute of Electromechanics and Automation PJSC (Russia), etc.
The industry leader for many years has been Honeywell that accounts for 34% of the market volume of all gyroscopic pickups [9,10]. The Honeywell ‘s flagship devices are as follows:
digital RLG GG1320 with a cavity of 15 cm, a weight of 0.5 kg and bias stability of up to 0.0035°/h;
miniature RLG GG1308 with a cavity of 6 cm, a weight of 65 g and bias of up to 1°/h.
Based on these technologies, Honeywell has manufactured a range of laser INS and IMUs used for the most navigation applications (Fig. 6) [20–25]. The most popular systems are HG 9900 and HG 1700 [22, 23]. The production volume of such INS is at least 80,000 items per year.
In contrast to the high-precision RLG GG1320, the concept of the GG‑1308 device is aimed at obtaining minimum SWaP+C indicators to the detriment of accuracy specifications. Although the optical and physical circuit of these lasers is similar, there are fundamental technological differences. The GG‑1308 devices uses inexpensive optical glass BK‑7 as a construction material. The vacuum joints are made using the glass cement. This makes it possible to obtain a minimal connection area that is significantly smaller than when connecting by an optical contact. In such a small intracavity volume there is no place for a getter and the active medium space is limited, so an additional small-area hollow cathode is introduced into the design, and the vacuum technology has its own features. The most popular system in the tactical accuracy class is the HG1700 INS based on the GG‑1308 gyroscopes. It is well-known that by 2023 more than 450,000 of these systems have been sold. This circumstance has contributed to the development of the HG5700 INS for medium-term navigation based on the RLG technology GG1320 and the HG1700 system concept with the minimum SWaP for this segment [21].
EMCORE specializes in the positioning, guidance and ground navigation systems for the military purposes, such as PNU/UPNU, DRU-H-R etc. (Fig. 7) [26, 27]. These systems are built on the basis of a high-precision RLG RL‑34 by L3 Harris Space & Navigation with a bias stability of 0.001°/h (see Fig. 5).
Using the SWaP+C minimization principles, Kearfott produces a range of compact laser INS of the KN and MILNAV series, as well as the inertial measurement units (IMU) of the KI –4902 and KI‑4921 types based on the three-axial monolithic RLGs of the T16-B and T24-B types for marine, ground and aerospace applications, differing in the accuracy, weight and size characteristics (Fig. 8) [28].
The leader of the European market is Safran that produces a range of high-end laser INS in demand in aviation, maritime navigation and some military applications [29, 30]. The most popular systems are Sigma‑95L and Sigma‑95N (Fig. 9). Sigma 95L is a lightweight and compact INS based on the four-mirror RLG of GLC‑16 type, designed for the airplanes, helicopters, and adapted for the unmanned aerial vehicles (UAVs). Due to the built-in GPS receiver, Sigma 95L can be used either as a stand-alone position control and navigation system, or as an integrated SINS. Sigma‑95N is designed for the most demanding applications requiring the high precision navigation and guidance. All available sensors can be integrated into the system, including GPS, GLONASS, air data system, etc. The Sigma‑95N sensitive elements are the three-mirror RLGs of the GLS‑32 type that are similar in the optical and physical design to the devices made by Honeywell.
Another French company, THALES, produces the integrated IMUs TopAxyz and TOTEM High Land based on the three-axial RLGs in a single monoblock of the PiXYZ‑22 type (Fig. 10) [31] These IMUs provide a navigation accuracy level under external influences for the air, sea, and ground platforms for civil and military purposes.
The Israeli group IAI specializes in the development of compact TRL‑16 m IMUS with the tactical precision class for the stabilization and position control systems of various purposes, autonomous hybrid TMAPS INS and NSF-R gyrocompasses for the ground military equipment. These systems are based on the RLGs with three-mirror resonators and a cavity of 9 cm (Fig. 11) [32].
The German companies, Raytheon Anschütz and iMAR Navigation & Control, produce the high-quality INS based on the GG1320 RLG for defense, air, maritime, underwater, above-water and railway applications (Fig. 12) [33, 32].
At the annual international exhibition “Photonics. World of Lasers and Optics”, the Chinese developers regularly present the optical elements of ring lasers [35]. Thus, in 2023 Xi’an SNP Precision Optics Co., Ltd. presented the resonator housings and optical mirrors. The housings differ in their dimensions and are designed for the flat uniaxial and cavity three-axial RLG monoblock (Fig. 13). For example, the 90 LGC housing (Laser Gyro Cavity) has an overall size of 9 cm on the quadrangle side that includes a cavity with a side of 7 cm and a perimeter of 28 cm.
The Russian manufacturers, such as Moscow Institute of Electromechanics and Automation PJSC and Research Institute “Polyus” have mastered the production of RLGs with a vibration suspension device and a magneto-optical bias based on the Zeeman effect for the IMUs for a wide range of applications. The achieved level of optical technology and production capacity of the domestic developers correspond to the European ones.
Conclusion
The RLG market is experiencing significant growth driven by the increasing demand for the accurate and reliable navigation systems in various applications such as autonomous vehicles, UAVs, aerospace and many others.
Such a demand has increased in the defense and aerospace sectors due to the modernization of military equipment and the need for high-end navigation and guidance systems for aircrafts, missiles and satellites [36].
To solve the vast majority of navigation issues, the most popular sensitive elements are the dither RLGs with linear radiation polarizations.
The up-to-date technological advances have led to the development of compact and lightweight RLGs with the improved performance [37]. Miniaturization and integration have expanded their use in various industries.
ABOUT THE AUTHORS
Sinelnikov Anton Olegovich – Ph.D. in Technical Sciences, Head of the Laboratory, State Research Institute of Instrument Engineering JSC, Moscow; Associate Professor of the Department of Nanotechnology and Microsystem Engineering Peoples Friendship University of Russia (RUDN University), Moscow.
Area of expertise: laser gyroscopy, inertial navigation systems, gas lasers.
ORCID 0000-0002-5579-3509
Tikhmenev Nikolay Vadimovich – Ph.D. in Physical and Mathematical Sciences, Head of the Department of State Research Institute of Instrument Engineering JSC, Moscow.
Area of expertise: laser gyroscopy, vacuum and optical technologies of inertial sensors, technological reliability assurance.
Ushanov Aleksander Aleksandrovich – leading electronics engineer of State Research Institute of Instrument Engineering JSC, Moscow.
Area of expertise: laser gyroscopy, inertial navigation systems, system analysis.
ORCID 0009-0009-3703-9981
Medvedev Vladimir Mikhailovich – Doctor of Technical Sciences, Professor, Director General of State Research Institute of Instrument Engineering JSC, Moscow.
Area of expertise: technical diagnostics, arrangement and technical issues of the product operation management.
Contribution of authors
The article was prepared based on the work of all team members.
Conflict of interest
The authors declare no conflict of interest.
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