GPS/INS (GPS/INS) combined guidance technology is currently the most advanced, all-weather, autonomous guidance technology with broad application prospects. It is a key technology widely used in fourth-generation medium/long-range precision-guided air-to-ground weapons, especially fourth-generation precision-guided bombs, which are under development abroad. The earliest airborne precision-guided weapon to adopt GPS/INS combined guidance technology was the AGM-84E SLAM air-to-ship missile used by the US Navy's A-7E carrier-based attack aircraft. This missile uses GPS/INS combined guidance for mid-course guidance and infrared imaging plus video data link remote control for terminal guidance. In the Gulf War, which broke out in early 1991, it achieved remarkable results with its high accuracy. After the Gulf War, the improved versions of this missile—the SLAM-ER AGM-84H and the Grand SLAM air-to-ship missile—both use GPS/INS combined guidance for mid-course guidance. Currently, the next generation of airborne precision-guided air-to-ground weapons that have adopted GPS/INS combined guidance technology include: the US AGM-86C air-launched cruise missile, AGM-130 air-to-ground missile, AGM-142 air-to-ground missile, CBU-97/B sensor-detonated (SFW) cluster bomb, and GBU-29/31 JDAM guided bomb. The JDAM, carried by B-2A stealth strategic bombers, was first used extensively in the 78-day bombing campaign against Yugoslavia from March 24 to June 10, 1999, including the brutal bombing of the Chinese embassy in Yugoslavia on May 8. Planned airborne precision-guided weapons to be equipped with this combined guidance system include: the AGM-154 JSOW Joint Standoff Weapon, the JASSM Joint Standoff Air-to-Surface Missile, and JDAM Stage 2 and 3 guided bombs.
I. Global Positioning System (GPS) Technology
The Global Positioning System (GPS), established by the United States in 1993, is a dual-use (military and civilian) space-based radio navigation system managed by the U.S. Department of Defense. It consists of a navigation constellation, ground control stations, and user positioning receivers. The navigation constellation currently comprises 24 satellites: 21 operational and 3 spares. These satellites are positioned in six elliptical orbital planes at an altitude of approximately 20,183 kilometers, with an inclination of 55°. Four satellites are evenly distributed, and the satellites orbit every 12 hours. The coverage area of the three satellites exceeds the entire globe, allowing users worldwide to simultaneously receive navigation signals from at least six satellites and up to eleven. Ground control stations measure and predict satellite orbits and monitor the operational status of onboard equipment, providing user receivers with satellite position data relative to the ground. User positioning receivers utilize the information transmitted by satellites whose spatial positions are accurately determined from their ephemeris data. This information travels at the speed of light; the receivers measure the propagation time of this information and calculate their relative position (distance) with the satellite. Using the distance triangle measurement principle, a user's GPS receiver can simultaneously receive signals from three satellites to calculate its three-dimensional spatial position. Simultaneously, by differentiating the distance obtained within the measurement time, and based on the relationship between linear velocity and Doppler frequency, the user's GPS receiver can measure the Doppler frequency of the satellites, thereby calculating its own velocity. Because the user's receiver's clock reference has an error relative to the GPS atomic clock reference, the actual measured distance is called the "pseudorange," and the velocity measurement value obtained by differentiating this pseudorange within the actual measurement time interval is called the "delta pseudorange," also known as the "pseudorange rate." To determine the user's GPS receiver's three-dimensional position and calibrate its clock error, it is necessary to simultaneously track and receive signals from at least four satellites in the GPS navigation constellation to complete the navigation calculation task. Simultaneously tracking and receiving signals from more than four satellites in the GPS navigation constellation will result in higher accuracy in navigation calculations when using the same inertial navigation system.
Each satellite in the GPS navigation constellation is equipped with a cesium (Cs133) atomic clock for synchronization, serving as the time standard for the measurement system, and a carrier transmitter for transmitting positioning signals. The carrier signals operate at two frequencies in the L-band: L1 at 1575.42 MHz and L2 at 1227.6 MHz. The L1 carrier signal is modulated using a 1.023 MHz pseudo-random noise code with a bandwidth of 1 MHz, with a repetition interval of 1024 bits or 1 millisecond. This modulation code is called the "coarse intercept (C/A) code" and provides "Standard Positioning Service" (SPS) to civilian users worldwide. The L2 carrier signal is modulated using a 10.23 MHz pseudo-random noise code with a bandwidth of 10 MHz, with a repetition interval of 7 days. This modulation code is called the "precise (P) code" and provides "Precise Positioning Service" (PPS) to military users in the United States and its allies. The satellite-broadcast navigation signal has a transmission rate of 50 bits per second. In addition to navigation information, the L1 and L2 carrier signals also contain data bits describing satellite orbit, clock calibration, and other system parameters. To prevent civilian C/A code GPS receivers from being used for military purposes, the U.S. Department of Defense introduced Selective Availability (SA) technology. This involves adding an artificial error of approximately 0.2 milliseconds jitter noise to the clock signal, increasing the received positioning signal deviation by 0.46 meters per second, reducing its positioning accuracy to approximately 100 meters. To prevent enemy interference, the P code is also encrypted, enabling it to operate in Anti-Electronic Spoofing (AS) mode. The cipher file is updated periodically. The encrypted P code is officially called the Y code, but is commonly referred to as the P(Y) code. Only military GPS receivers equipped with a secure AS module can properly receive P(Y) code signals, achieving a positioning accuracy of approximately 20 meters. Because the repetition modulation intervals of P(Y) code and C/A code are 7 days and 1 millisecond, respectively, and the encoding of P(Y) code is much longer than that of C/A code, C/A code is easily intercepted, while P(Y) code is almost impossible to intercept without auxiliary measures. Therefore, military GPS receivers must first receive the C/A code signal to obtain the "handover word" (HOW) information required to quickly intercept the P(Y) code signal before switching to receiving the P(Y) code signal. Simultaneously, because the modulation amplitude of the L1 carrier C/A code is greater than that of the L2 carrier P(Y) code, military GPS receivers can eliminate positioning errors caused by the ionosphere and troposphere from measurements at two frequencies, improving their positioning accuracy to approximately 18 meters. The main error sources affecting the positioning accuracy of GPS receivers are in the space segment, system segment, and user segment, primarily including ionospheric propagation delay and tropospheric propagation delay, the latter including satellite clocks, satellite ephemeris tables, and the receiver itself. Currently, the main methods for reducing satellite clock and ephemeris errors include Wide Area GPS Enhancement (WAGE), Differential GPS (DGPS), and Relative GPS (RGPS).
II. Inertial Navigation System (INS) Technology
An Inertial Navigation System (INS) is an autonomous space reference system composed of an inertial measurement unit (IMU), a control and display device, a status selection device, a navigation computer, and a power supply. The IMU includes three accelerometers and three gyroscopes. The accelerometers measure the acceleration of the vehicle's three translational motions, indicating the direction of the local vertical; the gyroscopes measure the angular displacement of the vehicle's three rotational motions, indicating the direction of the Earth's rotation axis. By integrating the measured accelerations twice, the vehicle's position in the selected navigation reference coordinate system can be calculated. Based on the installation method of the IMU on the vehicle, inertial navigation systems can be divided into two types: platform-type and strapdown-type. Platform-type inertial navigation systems mount the accelerometers and gyroscopes on an inertial navigation platform. Depending on the coordinate system established, they can be further divided into space-stabilized and local-level inertial navigation systems. The former's inertial navigation platform is relatively stable in inertial space, while the latter's platform can track the local horizontal plane, but its orientation relative to the Earth can be fixed or free-floating. Because the platform can isolate the vibration of the carrier, the inertial instruments have better working conditions, reducing measurement errors and improving navigation accuracy. However, they are complex in structure, large in size, and expensive. Strapdown inertial navigation systems mount accelerometers and gyroscopes on the carrier, using computer software to create a mathematical platform that replaces the mechanical inertial platform. This results in a simpler structure, smaller size, lighter weight, and lower cost. However, the inertial instruments operate under poorer conditions, leading to increased measurement errors and decreased navigation accuracy. Therefore, the requirements for gyroscopes are very high; they must be resistant to shock and vibration, and have a large angular velocity measurement range. Using new types of gyroscopes such as electrostatic gyroscopes, laser gyroscopes, and fiber optic gyroscopes is ideal. The earliest weapon to use an inertial navigation system was the V-2 surface-to-surface ballistic missile of Nazi Germany during World War II. Most long-range missiles developed after the war use inertial navigation systems for mid-course or full-range guidance; various short-range tactical missiles widely use strapdown inertial navigation systems as their guidance system. The main advantages of inertial navigation systems are: they can navigate independently without relying on any external system support; they can continuously provide all navigation and guidance parameters, including attitude references; and they have good short-term accuracy and stability after alignment. Their main disadvantages are: complex structure, high cost, navigation errors increasing over time, and long heating and alignment times. Therefore, they cannot meet the requirements of long-distance or long-duration navigation and high-precision navigation or guidance. To improve navigation and positioning accuracy, various integrated navigation methods have emerged, which combine different types of navigation systems with distinct characteristics to complement each other and form a superior new type of navigation system—an integrated navigation system. Examples include inertial navigation combined with Doppler, inertial navigation combined with direction finding/range finding (VOR/DME), inertial navigation combined with Loran, DECCA, Omega, Consol, TRN, or terrain feature matching (TCM), and inertial navigation combined with Global Positioning System (INS/GPS). Of the aforementioned integrated navigation systems, the latter is the most advanced and the most widely used.
III. Satellite Positioning/Inertial Navigation (GPS/INS) Integrated Guidance Technology
Inertial navigation and satellite positioning (INS/GPS) integrated navigation systems are used for weapon guidance. They fully leverage the advantages of both systems, compensating for each other's weaknesses. GPS's long-term stability and moderate accuracy compensate for the propagation or increase of INS errors over time. INS' short-term high accuracy compensates for GPS receiver errors increasing under interference or signal loss due to obstruction. This further highlights the advantages of strapdown inertial navigation systems: simple structure, high reliability, small size, light weight, and low cost. Furthermore, by utilizing the attitude and angular velocity information from the inertial navigation system, the directional control performance of the GPS receiver antenna is improved, enabling rapid acquisition or reacquisition of GPS satellite signals. Simultaneously, by using the continuous high-precision position and velocity information provided by GPS, the position, velocity, and other error parameters of the inertial navigation system are estimated and corrected, achieving airborne alignment and calibration. This relaxes the accuracy requirements, optimizing the entire integrated guidance system and resulting in a high cost-effectiveness ratio. The key component in the GPS/INS integration is the Kalman filter, which serves as the interface between the two systems and performs data fusion. Kalman filtering, proposed by R.C. Kalman and R.S. Busy in the early 1960s, is a new linear filtering model and method developed to meet the computational requirements of high-speed digital computers for artificial Earth satellite orbit and navigation. Using a Kalman filter, the errors of the inertial navigation system (INS), the random drift of the gyroscope, and the errors of the accelerometer are treated as state variables and discretized into state equations. This establishes a statistical mathematical model describing the system. These state equations, along with the measurement equations, describe the dynamic characteristics of the GPS/INS integrated system. Through data processing, the filtering equations provide optimal estimates of the system's state variables. The controller then uses these optimal estimates to correct and synthesize the INS system, minimizing the navigation and positioning error of the integrated system. Because the Kalman filter is an unbiased recursive linear minimum variance estimator (mean or expected value of its estimation error is zero), the GPS/INS integrated system is the optimal integrated guidance system. GPS/INS integration methods, based on the configuration of the Kalman filters, are divided into two types: ① loose coupling (also known as serial coupling), where each component has one Kalman filter. ② Tight coupling, where both systems share a single Kalman filter. Both combination methods have their advantages and disadvantages; from an accuracy perspective, tight coupling is generally preferred. The key technologies for GPS/INS integrated guidance are: ① GPS receiver technology, primarily high-efficiency, low-cost device technology. ② INS technology, including various new inertial sensor technologies such as laser gyroscopes, fiber optic gyroscopes, hemispherical resonator gyroscopes, and various microelectromechanical systems (MEMS) manufacturing technologies. ③ GPS/INS coupling technology, including Kalman filter configuration and error estimation techniques. ④ Airborne/missile-borne integrated GPS/INS technology, including airborne/missile-borne GPS/INS transfer and alignment, GPS/INS conversion, relative GPS aiming and attack, GPS/INS modeling and accuracy analysis, etc. ⑤ GPS jamming and anti-jamming technology, including GPS receiver jamming and anti-jamming technology, encryption and decryption technology, and accuracy compensation technology. Edited by: He Shiping
