In addition to the inertial navigation system, there are also many other different navigation definitions, such as geomagnetic navigation, celestial navigation, terrain aided navigation, radio navigation system, radar navigation system and satellite navigation system, which can be combined with the inertial navigation system to form an Combined navigation system.
According to the different methods of obtaining navigation data, navigation and positioning techniques can be divided into autonomous & non-autonomous, direct position determination & dead reckoning navigation.
The autonomous navigation system does not rely on external information and does not communicate with the outside world. The non-autonomous navigation system must rely on ground equipments or other external equipments to complete the navigation task.
Direct positioning is to directly calculate the position of the current point in a specific coordinate system by measuring azimuth and distance from a reference point whose position is known, or by comparing features of the current position with known information (feature matching).
Dead reckoning is to calculate the position change (relative position) of the vehicle by measuring the distance and direction of the vehicle. The position of the current point in a particular coordinate system is determined by adding it to the initial position. The distance in dead reckoning can be measured directly. It can also be calculated by measuring the velocity or acceleration by integration.
With the development of geomagnetics, sensors, computers and other technologies, the geomagnetic field can not only be used for orientation, but also can be located according to the difference of the geomagnetic field at various positions on the Earth. This is just geomagnetic navigation.
Modern geomagnetic navigation technology is based on the fact that the geomagnetic field is a vector field. Its strength and direction are functions of position. At the same time, the geomagnetic field has the characteristics such as total strength, vector strength, magnetic dip, magnetic declination and intensity gradient, which provides sufficient matching information for geomagnetic matching. Therefore, the geomagnetic field can be regarded as a natural coordinate system, and the navigation and positioning of the vehicle can be realized by measuring the geomagnetic field.
The magnetic field intensity vector of any point in the near-earth space of the Earth is unique. The magnetic field vector of each point on the Earth corresponds to its latitude, longitude and height from the center of the Earth. As long as the geomagnetic field information of the vehicle’s location can be measured, the vehicle will be located.
The positioning technology based on geomagnetic is to make a reference map of a certain geomagnetic field characteristic value in the pre-selected area and store it in the computer on the vehicle. When the vehicle passes through these areas, the geomagnetic sensor measures the relevant characteristic value of the geomagnetic field in real time and forms a real-time map, that is, the matching point, so as to determine the precise real-time position of the vehicle.
The use of geomagnetic field for navigation has the characteristics of passive, non-radiation, all-day, all-weather, all-area, small size, low power consumption, reliable performance and strong anti-interference in technology.
Celestial navigation uses the characteristic that the stars in the sky have a relatively fixed relationship with the geographical position of the earth at a certain time, and determines the vehicle’s position by observing the stars.
Celestial navigation indirectly determines the orientation of the vehicle by observing two stars with optical or radio telescope and automatically tracking the position of the stars, so that the altitude and azimuth angles of the stars relative to the reference plane of the vehicle can be measured at any time. The position and heading of the vehicle can be obtained by calculation. Usually, the determination of the reference plane of the vehicle is realized by the gyro stable platform.
The celestial navigation system has the characteristics of accuracy not decreasing with the increase of working time (no accumulative error), good concealment, high reliability, no radio interference, strong autonomy and so on. Therefore, celestial navigation, especially the combination of celestial navigation and other navigation methods, has a wide range of applications in the flight, long distance, across the ocean, over the polar and through the desert.
Terrain-aided navigation (TAN) is a technology that uses digital maps to assist inertial navigation. The accuracy of terrain assisted navigation depends on the accuracy of the map and the variation of the terrain.
Terrain aided navigation system has high autonomy, so it has good military application value. The terrain assisted navigation system mainly consists of the following hardware devices.
- Inertial navigation system, which can provide all navigation information.
- Radio altimeter, used to measure the relative altitude of the aircraft to the ground.
- Barometric altimeter or atmospheric data system, used to provide the altitude or absolute altitude of the aircraft alone or in combination with the inertial navigation system.
- Navigation computer and mass storage, which are used to complete navigation calculation and store digital map respectively.
Terrain-aided navigation can be classified into two types, correlation analysis and Kalman filtering for state estimation according to its working principle.
A typical algorithm for terrain height correlation is terrain contour matching (TERCOM).
Another typical terrain aided navigation technology is Sandia inerlia lerrain-aided navigation (SITAN) algorithm,
According to the straight line propagation and constant speed characteristics of radio wave in uniform medium and free space, the navigation parameters such as orientation, distance and speed of the vehicle are determined to realize navigation. The VOR navigation system, LORAN-C navigation system, TACAN navigation system and Omega navigation system are all radio navigation systems.
The radio signal contains four parameters: amplitude, frequency, time, and phase. According to the navigation parameters to be measured, the radio navigation system can be divided into direction-finding (azimuth or elevation Angle), ranging, direction-finding and ranging, ranging difference (or phase difference) and speed measurement types.
It can also be divided into 3 types according to the distance with the ground supporting equipments,
- Short-range navigation system, the action distance is 100~500 km, such as Tacan system.
- Medium range navigation system, the action distance is 500~2000 km, such as Loran-A system.
- Long range navigation system, the action distance is more than 2000 km.
Tacan navigation is a short-range direction finding and ranging system, while Loran-C and Omega navigation are long-range ranging systems.
Doppler radar navigation is a method that uses the frequency difference between the transmitted wave and the reflected wave with the vehicle speed to measure the velocity of the aircraft relative to the ground, and then determines other navigation parameters to complete the navigation task.
This navigation method only needs to install a radar transmitting and receiving device on the plane to measure the ground speed. Then, it is to output the heading with the heading system on the plane, and decompose the ground speed into the speed components along the geographical north and east, and then determine the distance change of the two directions and the longitude and latitude, and finally determine the position of the aircraft.
As shown in the below figure, the transmitter-receiver system on the aircraft moves with the aircraft at the speed \(v\). The transmitter sends electromagnetic waves with the frequency of \( f_1 \) to point B, and part of the energy reflected from point B is received by the plane’s receiver.
Doppler navigation does not need ground equipment, but it still needs to set up a transmitting station and a receiving station on the plane. Since there is no ground station, Doppler navigation is active, with strong anti-interference ability and high accuracy.
However, it is easy to expose itself because it must transmit radio waves when it works. In addition, the working performance is related to the shape and nature of the reflective surface. For example, when flying over water and deserts, the navigation performance will be reduced due to poor reflectivity. The navigation accuracy is also affected by the antenna attitude. When the antenna cannot receive the reflected wave, the system will completely lose its ability to work. In addition, due to the complex shape of the illuminated ground, the synthesized transmitted signal is the sum of countless signals with different amplitude and phase, which limits the accuracy of Doppler.
Satellite navigation is a technology with the artificial satellites running on a predetermined space orbit. By measuring its position relative to the satellite, the vehicle can determine its geographic coordinates and velocity vector.
Satellite navigation system is another important development of navigation technology after inertial navigation. At present, the widely used satellite navigation system is the Global Positioning System (GPS) of the United States, the Russia’s GLONASS , the European Galileo satellite navigation system and the China’s Beidou satellite navigation system. They all use the linear and isovelocity of radio wave propagation to implement time, ranging and positioning, and also use the Doppler frequency shift between the vehicle and the satellite for Doppler velocity measurement.
Satellite navigation consists of three parts. They are navigation satellite, ground station and user equipment. As shown in the figure above. Ground stations are mainly used to track, calculate and send data to satellites. The user equipment includes receiving, processing, and display parts. Because the position of the satellite in the sky is known at any time. It’s like a radio navigation station on the ground moved into space.
Therefore, it is possible to measure the distance from the satellite to the aircraft to achieve positioning. At the same time, the Doppler frequency shift between the radio waves transmitted by the satellite and measured by the receiving equipment on the aircraft can determine the change rate of the distance between the aircraft and the satellite, that is, the vehicle motion velocity.
