GPS/INS Global Positioning System+Inertial Navigation System

1. Necessity of GPS/INS combination

GPS is the most widely used satellite navigation and positioning system at present, with convenient use and low cost, and its latest actual positioning accuracy has reached less than 5 meters. However, there are still some shortcomings in the military application of GPS system, such as easy interference, poor reliability in dynamic environment and low data output frequency.

INS system uses inertial measurement devices (such as accelerometers and gyroscopes) installed on the carrier to sense the movement of the carrier and output the attitude and position information of the carrier. INS system is completely autonomous, confidential, flexible and has multi-function parameter output, but there is a problem that errors accumulate rapidly with time, and the navigation accuracy diverges with time, so it cannot work alone for a long time, so it must be calibrated continuously.

combining GPS and INS can make the two navigation systems learn from each other and form an organic whole. The advantages of GPS/INS integrated guidance are as follows:

1. GPS/INS combination improves the system accuracy

High-precision GPS information can be used to correct INS and control its error accumulation with time. Using GPS information, the error parameters of INS and the clock error of GPS receiver can be estimated. On the other hand, it can provide auxiliary information for GPS by using the characteristics of high positioning accuracy and high data sampling rate in a short time. Using these auxiliary information, the GPS receiver can keep a low tracking bandwidth, which can improve the system's ability to recapture satellite signals.

2. The combination of GPS/INS enhances the anti-jamming ability of the system

When the GPS signal is interfered by high intensity, or when the receiver of the satellite system fails, the INS system can independently navigate and locate. When the GPS signal conditions are significantly improved to allow tracking, the INS system provides the GPS receiver with information about the initial position, speed and so on, so that it can be used when quickly regaining the GPS code and carrier. INS system signal can also be used to assist the antenna of GPS receiver to aim at GPS satellite, thus reducing the influence of interference on the system.

3. Solve the cycle slip problem

For GPS carrier phase measurement, INS can well solve the cycle slip of GPS and the recalculation of ambiguity parameters after the signal is out of lock, and also reduce the requirement of visibility of at least four satellites.

4. Solve the problem of low sampling frequency in GPS dynamic application

In some dynamic application fields, high-frequency INS data can accurately interpolate the position of the event between GPS positioning results (such as the position determination of aerial camera exposure moment).

5. More widely used

The p>GPS/INS integrated system is a complementary and mutually improved integration of GPS and INS, not a simple combination of them. The combined system has stronger performance and wider application fields.

It is precisely because these two systems are extremely complementary that they can not only provide global accurate navigation at low cost, but also meet the requirements of confidentiality in military applications.

second, the wide application of GPS/INS integrated guidance technology in modern warfare

1. GPS/INS integrated guidance has become a widely used full-range guidance and midcourse guidance technology

At present, the midcourse guidance mode of ground attack missiles represented by American Tomahawk cruise missiles is still inertial navigation+auxiliary navigation system. Because of the high accuracy and convenient use of American military GPS, the United States and some other western countries use GPS as the auxiliary navigation system of inertial navigation in the midcourse guidance section instead of terrain matching. In addition, many new guided weapons, such as JASSM developed by Loma Company and JDAM made by Boeing Company, all rely on GPS/INS for high-precision guidance.

Take JDAM as an example. It is an all-weather guided ammunition by adding the tail components of GPS/INS guidance to the existing ordinary bombs, and its inertial navigation part adopts a small laser gyroscope. JDAM is constantly revised by the avionics system of the aircraft before it is put into use. Once released, the bomb's GPS/INS system will take over the avionics system of the aircraft and guide the bomb to C4 reconnaissance. Huang fluorene? What's wrong with you? Hey? The close cooperation between PS component and a three-axis INS component is realized. The guidance control unit provides precise guidance in both GPS-assisted INS operation mode and INS single operation mode.

These weapons are closer to jammers than airplanes, and the jamming intensity they face is much more serious than that of airplanes that launch missiles. The GPS/INS integrated guidance system can identify the existence of jamming signals and conduct precise guidance with small guidance error in a short time.

Integrated GPS/INS integrated guidance not only improves the reliability of weapon system, but also has high accuracy. Usually, the circular probability error is between 11 and 13 meters, while the accuracy of GPS guidance alone is about 15 meters.

2. GPS/INS integrated guidance system provides navigation and positioning services for aircraft and other weapon platforms

At present, most of the main battle aircraft of the air force of the United States and other NATO countries have been replaced with the second-generation standard inertial navigation instrument with laser gyro as the core. The key point of its modification plan is to embed a strong and anti-interference GPS receiver (OEMB board) in the black box of inertial system based on optical gyro. This embedded configuration does not need to set an additional security bus between inertial navigation and a separate GPS receiver, so that the pseudorange/pseudorange rate data of GPS will not be disturbed by threat signals. This deep coupling system of INS and GPS is called "GPS embedded in inertial navigation system", which is abbreviated as EG1. Its positioning accuracy is 1.8 nautical mile/hour (circular probability error), the preparation time is also reduced from the past 15 minutes to 5-8 minutes, and the system reliability is improved from the original hundreds of hours to 2,111-4,111 hours.

3. GPS/INS integrated guidance system provides high-precision positioning signals for military reconnaissance

The purpose of reconnaissance is to find the target, determine its position and evaluate the strike effect of weapons. The hit rate to the target depends on the accuracy of weapon guidance, the ability to find the target and the accuracy of target positioning. At present, many countries are using high-altitude imaging technology to establish global geographic information databases. The high-altitude imaging system is mainly composed of high-altitude reconnaissance aircraft, low-orbit satellites and medium-orbit satellites. This system uses the GPS/INS integrated guidance system, and the real-time position of the unmanned reconnaissance aircraft and the real-time position of the reconnaissance parachute released by the shells will be sent to the base together with the images, so as to determine the position of the target.

Third, the development trend of GPS/INS integrated guidance technology

1. Improve the anti-jamming performance of GPS system, thus improving the reliability of GPS/INS integrated guidance

The United States plans to improve the autonomous working ability of satellites by enhancing the power of signals released by satellites, enhancing the on-board processing capacity, and improving the on-board atomic clock and ephemeris extrapolation algorithm. Three new signals are added to transmit: first, the high-power spot beam military M code, the signal gain will be much higher than the current gain of GPS transmitter, and it has stronger security than P code; Second, the C/ A code is loaded on the L2 carrier, and the C/ A code originally loaded on the L1 carrier continues to be retained; The third is L5 code, which is used as a life safety signal for civilian use only. Future GPS satellites can release two military navigation codes in two frequency bands, which can form four working modes in actual combat, thus greatly improving the anti-jamming ability. At the same time, the satellite can run autonomously for 1.21 days in a short time. In addition, according to the long-term plan for 2125 announced by the US Air Force, the United States also plans to install a backward antenna on GPS satellites for releasing navigation and positioning information to high-orbit space and enabling high-orbit satellites to operate autonomously. At present, the United States military's GPS joint planning office is studying the design scheme of GPS 3 satellite.

in order to further improve the performance, the United States will use more complex GPS receivers on airplanes, ships, ground vehicles and weapons in the future. The length of active C/A code is only 1123 bits, and it takes only 21.5 seconds to search one by one at a speed of 51/s, which is easy to be deciphered by the enemy. The length of P code is about 2. 35×1114 bits, and it takes 267 days to repeat, so it takes a long time to complete a capture and has good security. However, the active military P-code receiver only completes the P-code acquisition through the guidance of C/A code, so it is easily affected by the state of C/A code. To this end, the US military is developing a military receiver that can independently capture P codes. In addition, the US military is still developing anti-jamming military code receivers such as space diversity receiver, null-setting receiver and beamforming receiver, in order to improve the anti-jamming ability of the receiver by improving its performance.

at present, the two most important technologies of GPS receiver in the United States are GPS receiver application module (GRAM) and selective availability anti-fraud module (SAASM). GRAM is a standard electronic plug-in, which can be added to future aircraft, ships, missiles and various weapons to ensure safety and interoperability. All GRAM will adopt an open system structure, which can flexibly add, replace or cancel some components in the system. SAASM is the second generation product safety module of GPS technology, which is used to protect the confidential GPS algorithm, data and calibration. It will be integrated into the receiver application module, which can improve the security of GPS system, make the GPS receiver easier to maintain and reduce its cost.

2. Develop a new INS system, so as to improve the accuracy of GPS/INS integrated guidance

At present, flexible inertial navigation, optical fiber inertial navigation, laser inertial navigation, micro-solid inertial instruments and other inertial navigation systems have been developed. Gyroscopes using laser as azimuth direction finder will gradually replace traditional mechanical gyroscopes. Laser gyro inertial navigation system has high positioning accuracy, small random drift, and can quickly enter the combat state. It was successfully applied to the navigation of aircraft and ground vehicles and naval guns in the early 1981s, and later applied to missiles and launch vehicles. However, the resonant cavity of ring laser gyro must be strictly sealed, and the concentration of He-Ne mixed gas in it must be constant. The mirror coating process requires high requirements, the manufacturing cost is high, and there will be problems such as "locking phenomenon", so it needs to be improved. At present, many scientific research units are working on the research of solid-state ring laser gyroscope.

The basic working principle of fiber optic gyro is similar to that of ring laser gyro. In addition to all the advantages of laser gyro, fiber optic gyro does not need precision machining, strictly sealed optical resonator and high-quality reflector, so it reduces complexity, reduces cost and has stronger market competitiveness. Japan took the lead in using fiber optic gyroscopes on TR1 and M5 rockets. The fiber optic gyro developed in the United States has been applied to the inertial measurement system of aircraft dive, roll and heading reference. However, the current fiber optic gyro will have some defects, such as random angular walk and zero bias instability, and its performance needs to be improved.

with the rapid development of modern micro-electro-mechanical systems (MEMS), the development of silicon micro-gyroscope (commonly known as chip gyroscope) and silicon accelerometer has made rapid progress in recent years. It is reported that the zero bias stability of this new solid-state gyro can reach 1 degrees/hour (under the condition of temperature control). At present, the United States has begun to produce miniature inertial measurement devices composed of silicon micro-gyroscopes and silicon accelerometers in small batches. Its characteristics of low cost, low power consumption, small size and light weight are very suitable for tactical applications. The first applications in aviation will be tactical missiles and unmanned aerial vehicles.

high-precision inertial navigation device needs advanced precision machining technology as the foundation. With the breakthrough of key theories and technologies, many types of inertial gyroscopes will be applied in the military field and play an increasingly significant role.

3. Data fusion technology will further improve the performance of GPS/INS integrated guidance.

The key device of GPS/INS combination is the Kalman filter, which serves as the interface between them and plays the role of data fusion. In order to improve the navigation accuracy, Kalman filter technology is widely used to optimally combine the information of each navigation system, estimate the error state of the navigation system, and then use the optimal estimated value of the error state to correct the system. However, the state equation of the system is time-varying, and the state transition matrix contains navigation information and measured values of inertial components, which make the filter model inaccurate. In addition, it is difficult to accurately estimate or measure the system noise and observation noise, so the conventional Kalman filter often diverges. In order to solve this problem, researchers are studying new data fusion technology. For example, using adaptive filtering technology, while filtering, the model parameters, noise statistical characteristics and state gain matrix are constantly estimated and corrected online by using the information brought by observation data, so as to improve the filtering accuracy and get the optimal estimated value of the object state.

In addition, how to introduce neural network artificial intelligence, wavelet transform and other information processing methods into information fusion technology with GPS/INS integrated guidance as the core is attracting great attention. Once these new technologies are successfully developed, the comprehensive performance of GPS/INS integrated guidance will be further improved.