1. Overview of Inertial Technology and Inertial Navigation Inertial technology is a collective term for inertial navigation technology, inertial guidance technology, inertial instrument technology, inertial measurement technology, and inertial testing equipment and devices. It has a history of over forty years. Due to its autonomous nature, inertial technology can achieve guidance and navigation without external information. Therefore, it occupies a very important position in national defense science and technology, and is widely used in military fields such as aerospace, aviation, and navigation. With the continuous development of inertial technology and computer technology and the reduction in cost, many countries have expanded its application to modern transportation, marine development, geodesy and exploration, oil drilling, mining, tunnel excavation and breakthrough, robot control, modern medical equipment, photographic technology, and civilian fields such as forest protection, agricultural sowing, and fertilization. An inertial navigation system (INS) is an autonomous dead reckoning system that uses inertial sensing elements, reference directions, and initial position information to determine the azimuth, attitude, and velocity of a vehicle. Inertial navigation systems can be broadly classified into two categories: platform-based inertial navigation systems (INS) and strapdown inertial navigation systems (SINS). Platform-based INS mounts gyroscopes and accelerometers on a stable platform, using the platform's coordinate system as a reference to measure the motion parameters of the vehicle. Strapdown inertial navigation systems (SINS) mount inertial sensing elements (gyroscopes and accelerometers) directly on the vehicle, eliminating the need for a stable platform or constant-mount system. The purpose of navigation is to obtain the real-time position, attitude, and velocity of the vehicle. In engineering applications, many methods exist for measuring the motion parameters of an object: displacement can be measured using odometers, radio positioning technology, astronomical positioning technology, and satellite positioning technology; velocity can be measured using tachometers; angle can be measured using angular position sensors (potentiometers, photoelectric encoders, etc.); and angular velocity can be measured using tachometers, speedometers, etc. However, none of these measurement methods can simultaneously and accurately measure the linear and angular motion of a vehicle in real time. Inertial technology is precisely the ideal means of measuring these motion parameters. Inertial navigation systems not only comprehensively detect almost all motion parameters, but also have a significant advantage—they are a completely autonomous navigation and measurement method. They do not rely on external information such as sound, light, magnetism, or electricity to measure the motion parameters of objects, and their operation is completely unaffected by natural or human interference, giving them extremely important military significance. Therefore, inertial technology cannot be replaced by any other navigation, positioning, or orientation method. Because of its crucial position, inertial technology has received widespread attention from technologically advanced countries worldwide. The United States, Britain, France, Germany, and the former Soviet Union have all invested considerable resources in research on inertial technology and related devices. Modern technological development has promoted the development of inertial navigation technology, and inertial navigation technology has become one of the hallmarks of modern high-tech development. 2. Development of Strapdown Inertial Technology Strapdown has a long history. A strapdown inertial system directly attaches inertial sensing elements (gyroscopes and accelerometers) to the aircraft to achieve guidance. Strapdown inertial navigation technology can be traced back to the 1750s, when the renowned German scientist Johann Gottlob Friedrich von Bohnenberger invented a gyroscope model with a stable platform. A century later, the French experimentalist optics physicist Leon Foucault invented the modern gyroscope and proposed the theory of the gyrocompass. From then until World War II, a large number of prominent scientists made outstanding contributions to inertial technology, including Dr. Hermann Auschutz-Kaempfe, Elmer Ambrose Sperry, Dr. Charles Stark Draper, and Professor Max Schuler. The first truly successful navigation mission was achieved by the famous V-2 rocket invented by Wernher von Braun and his research team at the end of World War II. The navigation system mounted on the V-2 rocket was the most primitive strapdown inertial navigation system. This rocket, flying from Nazi Germany across the English Channel, accurately hit London, shocking the world. Strapdown inertial navigation technology developed rapidly in the United States and the Soviet Union, primarily for military weapon systems. From 1950 onwards, MIT's Draper Laboratory successfully completed test flights of platform inertial navigation systems for aircraft and sea trials for ships. Simultaneously, the strapdown system was explored and matured. In 1969, during Apollo 13's journey to the moon, an explosion in the service module damaged the command module's power supply. In this emergency, it was the low-power backup strapdown inertial navigation system (LM/ASA) from Draper Laboratory that guided the spacecraft back to Earth's orbit, allowing for a safe landing in the Pacific Ocean. Due to the inherent advantages of the strapdown system, and with the emergence of high-speed, high-capacity digital computer technology and high-precision gyroscope technology, strapdown navigation systems gradually replaced platform systems in low-cost, short-term, medium-precision navigation. During this period, the strapdown system transitioned from the test flight phase to the application phase. New solid-state gyroscopes, such as those using lasers and fiber optics, have gradually matured. These new gyroscopes offer advantages such as unrestricted angular velocity measurement, strong overload capacity, accuracy independent of overload, high reliability, and fast startup—precisely what strapdown inertial navigation systems (SINS) aim to achieve. In Europe, most new and improved military aircraft use laser gyroscope inertial navigation systems; in the United States, all military inertial navigation systems were platform-based in 1984, but by 1989, half had switched to strapdown systems, and by 1994, strapdown systems accounted for 90%. Strapdown inertial navigation systems are rapidly developing towards higher altitudes, higher reliability, lower cost, miniaturization, digitalization, and wider application areas. Based on this, combined navigation systems such as SIN/GPS, SIN/DVL, and strapdown/astronomical navigation will become a future development direction. 3. Comparison between Strapdown Inertial Navigation Systems and Platform Inertial Navigation Systems Platform systems use a constant-riding platform on which inertial sensing elements are installed. The platform can isolate the influence of carrier motion on the sensing elements, and angle sensors on the frame axis directly output attitude angles for navigation calculation. While the platform system has reached a high level, its cost and maintenance expenses are very high, and its use of a frame servo system will reduce its reliability. The strapdown system, on the other hand, uses a mathematical attitude conversion platform, directly mounting inertial sensors onto the carrier. The output information of the sensors is directly transmitted to the navigation computer for real-time attitude matrix calculation. The attitude matrix converts the information measured by the accelerometers in the inertial navigation system into the navigation reference coordinate system for navigation integration and attitude angle extraction. A comparison of the working principles of the platform system and the strapdown system is as follows: 1) Strapdown system sensors are easier to install, maintain, and replace; 2) Strapdown system sensors can directly provide all navigation parameters of the ship's coordinate system to the navigation, stability control, and weapon control systems; 3) Strapdown system sensors are easy to repeatedly arrange, achieving redundancy at the inertial sensor level, which is highly beneficial for improving performance and reliability; 4) The strapdown system eliminates the need for a constant-level platform, eliminating various errors in the stabilization process of the stabilization platform while reducing system size. Strapdown systems, by directly fixing the sensing element to the carrier, degrade the working environment of the inertial sensing element, reducing the system's accuracy. Therefore, error compensation measures must be implemented, or new types of optical gyroscopes must be used. Current research on inertial navigation system technology focuses on several aspects, including inertial sensing devices, system accuracy, system size, reliability, system integration, and system correction. Key aspects include correction, the establishment and real-time compensation of inertial element error models, and the updating of the strapdown matrix. (Edited by He Shiping)