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Introduction and Classification

Dongzhongtong is the abbreviation for "Mobile Satellite Ground Station Communication System". Through the dynamic communication system, mobile carriers such as vehicles, ships, and airplanes can track satellite platforms in real time during their movement, continuously transmitting multimedia information such as voice, data, and images, which can meet the needs of various military and civilian emergency communication and multimedia communication under mobile conditions. The dynamic communication system has effectively solved the difficulty of various mobile carriers such as vehicles and ships transmitting real-time multimedia information such as voice, data, high-definition dynamic video images, and fax through geostationary satellites during movement. It is a major breakthrough in the field of communication and a rapidly developing application area in the current satellite communication field. It has extremely broad development prospects in both military and civilian fields.

Composition of the "Dynamic Communication" System

'Dynamic Communication' consists of two parts: a satellite automatic tracking system and a satellite communication system.

Satellite automatic tracking system

The satellite automatic tracking system is used to ensure the accurate pointing of the satellite transmitting antenna to the satellite during the movement of the vehicle body. Its main equipment includes:

(1) The antenna pedestal adopts unloading and power storage methods to reduce the load inertia during antenna transmission.

(2) Servo, using position loop or speed loop control method, using analog hardware to improve circuit response speed and reduce dynamic hysteresis error of servo tracking system.

(3) Data processing, using a dedicated mathematical solution platform to process error signals and dynamic signals of the carrier, and calculate the control signal of the antenna.

(4) Carrier measurement, using a combination of strapdown inertial navigation measurement and other measurement methods to measure the changes in the carrier and reflect them in antenna tracking. Among them, fiber optic gyroscope is a new type of navigation instrument developed on the basis of optical interference principle, which has become an ideal main component of the new generation of strapdown inertial navigation system, used for precise direction finding of envisioned objects. The quartz flexible pendulum accelerometer is a sensitive element made of fused silica. The flexible pendulum structure is equipped with a feedback amplifier and a temperature sensor to measure the linear acceleration along an axis of the carrier. The fiber optic gyroscope three-axis inertial measurement combination consists of three fiber optic gyroscopes and three quartz flexible pendulum accelerometers, which can output real-time data such as angular velocity, linear acceleration, and linear velocity of the carrier. It has multiple working modes such as alignment, navigation, and heading attitude reference, and is used for combined navigation and positioning of mobile carriers, while providing accurate data for the mechanical control device of the follow-up antenna. For the dynamic communication system, its main performance requirements are: adding a meter with an accuracy of 1 × 10-4g; Fiber optic gyroscope accuracy (bias stability) ≤ 1 °/h; The linearity of the scaling factor is ≤ 5 × 10-4.

satellite communication system

The function of a satellite communication system is to transmit television signals upstream to the satellite, and then transmit them downstream to the ground satellite receiving device through a repeater. Its main equipment includes: encoder/decoder, modulator/demodulator, up/down converter, high-power amplifier, duplexer, and low-noise amplifier.

Working principle of the "Dynamic Communication" system

During the movement of the carrier, changes in its attitude and geographical location can cause the original aligned satellite antenna to deviate from the satellite, resulting in communication interruption. Therefore, it is necessary to isolate these changes in the carrier so that the antenna is not affected and always aligned with the satellite. This is the main problem that the antenna stabilization system needs to solve, and it is also the prerequisite for uninterrupted satellite communication on mobile carriers.

The "Dynamic Communication" tracking system measures the heading angle, longitude and latitude of the carrier's location, and the initial angle relative to the horizontal plane using GPS, theodolite, and strapdown inertial navigation system under initial static conditions. Then, based on its attitude, geographic position, and satellite longitude, it automatically determines the antenna elevation angle with the water plane as the reference, rotates the azimuth while maintaining the elevation angle constant with respect to the horizontal plane, and automatically aligns with the satellite in the form of signal maximum. During the movement of the carrier, the changes in the carrier's attitude are measured and transformed into the error angle of the antenna through mathematical platform operations. The azimuth, elevation, and polarization angles of the antenna are adjusted by a servo mechanism to ensure that the antenna stays within the specified range during the carrier's movement, allowing the satellite's transmitting antenna to track the geostationary satellite in real-time during the carrier's movement. There are two types of system tracking methods: self tracking and inertial tracking. Self tracking relies on satellite beacons for antenna closed-loop servo tracking; Inertial tracking is the use of gyroscope inertial navigation combined with sensitive carrier changes for antenna tracking, which is sometimes referred to as guidance. These two types of tracking can automatically switch according to the on-site situation. After the system completes star tracking and switches to automatic tracking, it works in self tracking mode; At the same time, the inertial navigation system also enters the working state and continuously outputs data such as antenna polarization, azimuth, and elevation. When the antenna beacon signal is interrupted due to obstruction or other reasons, the system automatically switches to inertial tracking mode.

Tracking of Dynamic Communication

During the movement of the carrier, continuous tracking of satellite signals or satellite beacon signals is required, and different tracking methods can be adopted according to the needs of different systems. According to the tracking principle, automatic tracking can be divided into three systems: step tracking, cone scanning tracking, and single pulse tracking.

Step tracking

Steppe tracking, also known as extremum tracking, is a step-by-step control of the antenna to rotate in a small step like manner within the azimuth and elevation planes, gradually aligning the antenna with the satellite. The system does not enter a rest state until the received signal reaches its maximum value, and after a period of time, it begins to enter a tracking state, repeating the process. The principle diagram of step tracking is shown in Figure 1.

Figure 1 Schematic diagram of the stepper tracking system

In this approach, the precession of the antenna is divided into two types: search step and adjustment step. After the search step, the entire tracking system starts working, including sampling signal data, field strength memory, comparison, etc. After several searches and determining the direction in which the antenna should rotate, the antenna returns to its original position and then rotates one step towards the satellite direction. This final step is called the adjustment step. So, the main difference between the adjustment step and the search step is that the antenna will not return to its original position after the adjustment step, while the search step is different. No matter how many times the search step is performed, as long as the specified number of times is completed, the antenna will return to its original position and then rotate one adjustment step. In practical systems, they can be separate or in the same step.

Single pulse tracking

The characteristic of the single pulse tracking method is that the antenna beam is fixed, and the size and direction of the antenna beam deviation from the satellite can be determined within the interval of one pulse. The error signals of the antenna azimuth and elevation axis movements are obtained to drive the servo system and quickly align the antenna with the satellite. Single pulse tracking has multiple horn modes and high-order mode modes, with multiple horn modes typically having two types: amplitude comparison and phase comparison. For parabolic antennas, multiple speakers are usually configured to generate error signals for tracking through amplitude comparison. The high-order mode method extracts the high-order modes generated in the feed waveguide as position error signals for tracking when the antenna symmetry axis deviates from the satellite direction. The tracking speed and accuracy of the single pulse tracking system are several orders of magnitude higher than those of the cone scanning tracking system and the step tracking system, but its equipment is complex and the cost is relatively high.

Cone scanning tracking

Cone scanning tracking refers to the conical motion of the feed horn around the axis of symmetry of the antenna, or the tilting and rotating of the antenna secondary surface, so that the antenna beam rotates in a conical shape. When the antenna axis is aligned with the satellite, the beacon level will be amplitude modulated by a very low frequency signal. The modulation frequency is the same as the beam rotation frequency, and the modulation depth is related to the distance of the beam from the satellite. A large deviation results in a large modulation depth; Small deviation and shallow modulation depth; No deviation, modulation depth is equal to zero. The phase of modulation is related to the direction of beam deviation, so the pointing error of the antenna beam can be detected by the amplitude and phase of the modulation signal, and the direction and magnitude of the motor driven antenna rotation can be determined based on the pointing error.

Comparison of Three Tracking Systems

From the working mode of the three tracking principles, they are essentially the same, which is to compare and determine the direction and amplitude of motor rotation after obtaining multiple AGC signals. Step tracking is the process of sampling in both azimuth and elevation planes, and then driving a motor to drive the antenna to rotate in both directions; Cone scanning tracking is the process where the feed source undergoes conical motion driven by a motor and compares the signal levels to align with the satellite during the process; Single pulse tracking can determine the rotation direction and amplitude of the motor within one pulse time.

The accuracy and speed of single pulse tracking are relatively high, but the system is complex and expensive, generally suitable for places with high accuracy requirements; Although the accuracy of cone scanning can meet the requirements of general systems, its structure is relatively complex, causing significant signal loss and slow tracking speed; The speed and accuracy of step tracking are both between the two, and the entire system is relatively simple and easy to implement, thus it has been widely used.

Stable technology

For a dynamic communication system, if only tracking is used, the antenna is prone to losing signal and causing communication interruption under large fluctuations in the carrier. Therefore, there must be an antenna stabilization system to isolate the interference caused by the movement of the carrier. There are two implementation methods for the stability system of car mounted antennas: 1. Physical platform stability; 2. Strapdown stability.

(1) The physical platform is stable and isolated between the vehicle antenna and the vehicle body through a stable platform, and the movement of the vehicle body is isolated outside the vehicle antenna system by the stable platform. This is equivalent to fixing the car antenna on the ground plane, which makes it easy to achieve precise satellite alignment and eliminates the need to adjust the antenna's direction after aligning with the satellite. This method divides the vehicle antenna stabilization system into a stabilization platform system and an antenna servo system. The stability of the antenna depends on a stable platform, with poor autonomy, complex systems, and relatively high costs.

(2) Strapdown stabilization utilizes information provided by gyroscopes, GPS, etc. on the carrier, establishes a mathematical platform through coordinate transformation, and calculates the correction amount of the servo system. The system has been widely used due to its simple structure, good reliability, small size, low cost, and easy maintenance.

The "Dynamic Communication" vehicle can transmit digital television, broadcasting, and data signals in both directions via satellite at a speed of 20-100km/h, ensuring uninterrupted broadband multimedia satellite communication of the moving carrier.

Characteristics of the "Dynamic Communication" System

Dongzhongtong has the following advantages in live streaming:

(1) During the broadcasting process, autonomous tracking is used to track satellites, fully utilizing the characteristics of satellite communication coverage area, strong anti-interference ability, and stable line, which can achieve point-to-point, point to multipoint, and point to main station mobile satellite communication;

(2) The "Dynamic Communication" vehicle has flexible and mobile broadcasting characteristics, which can ensure fast and real-time static and dynamic live broadcasts;

(3) The automatic re capture time is short, and communication can be quickly restored after exiting the communication blind spot;

(4) Compared with OFDM "directionless" mobile microwave equipment, "in motion communication" vehicles do not require operators of receiving and transmitting equipment to work in harsh environmental conditions, saving manpower and material resources, and reducing electromagnetic radiation pollution;

(5) The reduction of nodes in the signal transmission process has improved the quality and reliability of broadcasting;

(6) It can reduce the operating costs of large-scale and complex live streaming processes.

However, due to the limitations of current technology, there are still some shortcomings in "dynamic communication", mainly including:

(1) In complex broadcasting environments such as high and numerous buildings, bridges, and mountainous areas, signal interruption may occur;

(2) Using two "dynamic communication" vehicles to transmit different TV image signals makes it difficult to achieve a no flash point connection during image broadcasting (when both vehicles encounter flash points at the same time);

(3) The signal transmission between the "dynamic communication" vehicle and the mobile signal acquisition vehicle is not easy (the direction and position of the two vehicles are constantly changing).

Advantages of using AgileLight series fiber optic gyroscope in motion communication

The AgileLight series fiber optic gyroscope produced by Wuxi Huilian Information Technology Co., Ltd. has excellent performance, can resist impact and various harsh environments, and has a significant price advantage. It can be used as a main component in low-cost fiber optic gyroscope inertial navigation systems, as well as in servo systems, making it an ideal choice for dynamic communication systems.