Antenna automatic tracking system for receiving satellite TV live broadcast programs on board

During the "Tenth Five-Year Plan" period, China will become the world's largest bus production base. However, none of the current luxury buses are equipped with an automatic antenna tracking system that can receive satellite TV programs and can only watch DVDs. At large international sports games, athletes want to watch real-time TV broadcasts while riding, so it is an urgent project to realize the direct reception of satellite TV programs on moving cars.
Trains, ships, and airplanes are also a big market, but they use different working environments. Ships sway left and right, the front and rear elevation angles are large, trains have to cross caves, and planes need to be balanced. Problems need to establish a stable platform.
To meet these market demands, it is necessary to transplant high-tech in attitude measurement, make full use of missile guidance and satellite attitude measurement technology, and develop an automatic tracking system for receiving satellite TV antennas in vehicles. Because in order to receive satellite live TV programs clearly on the move, the antenna gain requirement is high, so that the antenna beam is very narrow. At this time, if you want to improve accuracy, track the satellite in real time, so that the center axis of the antenna is always aligned with the satellite. Attitude measurement technology must be used.

The composition and working principle of the antenna automatic tracking system The antenna automatic tracking system mainly consists of: gyro sensor, electronic compass, single-chip microcomputer with 16bit CPU (because the gyro sensor is a digital 16bit serial output, so the single-chip microcomputer also chooses 16bit CPU, so the data can be Read once from the data bus), servo system (including servo driver and servo motor), 1.2-meter horizontal offset feed satellite TV receiving antenna, satellite TV receiver, voltage / frequency (V / F) conversion module, GPS receiving The machine module and GPS antenna are composed as shown in the figure on the right.
1. The gyro sensor is mainly used to measure the heading angle of the car. Considering the performance-price ratio, the all-solid-state fiber optic gyro sensor produced in Russia is generally used, or the inertial gyro sensor of the microelectronic mechanical system (MEMS) produced in the United States. Small size, light weight, low power consumption, fast start-up, etc .; small random drift (<1 ° / h), dynamic range of ± 60 ° / s, scale factor error <2%; car output by gyro sensor By integrating the angular rate of the change in heading angle, the amount of change in heading angle during vehicle travel can be obtained Δθ. You can also use the methods given in its manual and manual to program the data it collects and send it to the CPU in the microcontroller for processing.
2. The electronic compass is mainly used to measure the pitch and roll angle changes of the satellite TV antenna. You can use the electronic compass produced by Honeywell in the United States. It is in the NMEA format and provides the pitch angle and roll through the RS232 serial port. The output of angle and yaw variation. It has fast response, up to 20Hz, response time of 0.1s, heading accuracy of ± 0.5 °, resolution of 0.1 °, tilt angle (pitch, roll) of ± 40 °, accuracy of ± 0.1 °, available in Work long hours in harsh environments.
3. The servo system includes a servo drive and a servo motor. Considering the long-term maintenance-free work under on-board conditions, the micro-AC brushless servo motor and servo drive system of the British CT company was selected. Its drive is controlled by a microprocessor. There are ASIC chips that produce adjustable pulse width PWM output for inverters. The output of ASIC is used to control IGBT inverters. They are all application modules that are packaged together and can perform single-axis or multi-axis control at the same time. Full digitalization can eliminate the drift of the analog control, and can also withstand the impact of the load at startup. If you choose a DC servo drive and a DC servo motor, although you can provide good synchronization between linear control, angular speed control, and stretching rate, the brushes of the DC servo motor (even long-life brushes) must be replaced frequently to provide after-sales service. , Maintenance and backup bring a lot of inconvenience.
4. The GPS receiver module is used to measure the current latitude and longitude positioning coordinates of the car. Because the gyro sensor will work for a long time, there will be zero drift and random drift, which will generate cumulative errors, which will affect the tracking accuracy of the antenna, so every other one or two After hours, the current latitude and longitude positioning coordinates of the car given by GPS and the automatic gain control (AGC) signal output by the satellite TV will be used to correct the position of the primary antenna, and its error will be corrected to re-align the satellite. However, when the car is driving in tunnels, high-rise buildings, underpasses, and dense forests, GPS positioning will be invalid due to occlusion, and a larger error will be introduced. At this time, dead reckoning will be adopted for autonomous navigation. With the current latitude and longitude positioning coordinates of the car given by GPS, and then using the coordinates of the synchronous satellite orbit, the heading angle and pitch angle that the satellite TV antenna should maintain can be derived through the calculation formula.

When waiting for a section, GPS positioning will be invalid due to occlusion, and a larger error will be introduced. At this time, dead reckoning will be adopted for autonomous navigation. With the current latitude and longitude positioning coordinates of the car given by GPS, and then using the coordinates of the synchronous satellite orbit, the heading angle and pitch angle that the satellite TV antenna should maintain can be derived through the calculation formula.
Heading
Pitch angle

Where B is the current latitude of the car, L is the current longitude of the car, and δ is the longitude of the satellite.
The resulting heading angle range is 0-360 °, and the pitch angle range is 0-90 °. The detailed calculation method is not repeated here.
5. The vehicle satellite TV antenna chooses a single offset parabolic antenna, because the feed and support of this antenna have the least obstruction, and have a larger focal length diameter ratio f / D, which makes the longitudinal size of the antenna larger and the short diameter decreases , Can also reduce the antenna side lobe level and improve the polarization radiation level of the feed. If it is installed horizontally on the roof of the car to reduce the overall height, the foldable design can reduce the resistance and facilitate the car to drive.
The initialization condition of the entire system is to interrupt the output of the electronic compass data at the highest level, and select the highest response frequency of 20Hz to serially output the changes in pitch angle and roll angle to maintain synchronization with the changes in heading angle measured by the gyro sensor. After being processed by the CPU in the single-chip microcomputer, the servo system is driven at a frequency of 20 Hz, and the rotation direction of the antenna is controlled so that the central axis of the antenna is always aligned with the satellite to realize real-time tracking.
The AGC analog signal output from the satellite TV receiver is converted into frequency by the V / F module and sent to the single-chip microcomputer. The single-chip microcomputer interrupted at a time of 50ms starts to count the frequency of the V / F output. When the signal is maximum, it indicates that the satellite has been captured again.

Conclusion The vehicle antenna automatic tracking system designed with attitude measurement technology can be applied to different carriers by changing the tracking working platform of its antenna. It is not only suitable for receiving satellite TV, but also can be applied to satellite communication and multimedia services. The market prospect is very broad.