Design of land and sea clutter test control platform

Sea clutter is the radar echo reflected by the ground plane and the sea surface. Due to the influence of various natural factors such as wind and environmental humidity, the sea clutter signal changes complexly and the signal is strong. Its existence seriously interferes with the radar's detection performance of ground and sea targets. In order to improve the detection performance of the radar, a sea clutter test control platform should be used to detect sea clutter. In this way, the sea clutter signal can be effectively detected, so as to grasp the distribution of sea clutter under various conditions, so as to eliminate or reduce the influence of clutter. The test platform consists of three parts: antenna, antenna base and servo control system.

1 Design of elliptical beam offset parabola antenna

1.1 Antenna parameter design

The antenna part in this system uses an offset parabola antenna. An offset parabola antenna is an antenna that uses a part of the half space above (or below) the focal axis of a conventional parabola antenna as the main reflector of the antenna, as shown in Figure 1. In Figure 1: V is the vertex of the parabola, F is the focus of the parabola, V to F is the focal length of the parabola, represented by f, h is the static distance, θh is the static distance angle, θo=θh+θα is the offset angle, θα is the semi-irradiation angle of the feed source to the offset parabola, the XOZ plane is the symmetry plane of the offset parabola, and the YOZ plane is its asymmetry plane.

According to the actual situation of ground and sea clutter signals, the antenna uses a frequency bandwidth of 14.93%, and the irradiation angle of the feed source to the offset parabola is 79.6°. The feed sources that can be used are corrugated horns and multi-mode horns. Since the L-band frequency is low and the wavelength is 224mm, if a corrugated horn is used, the size and weight are too large, so a multi-mode horn is used. The dual-polarization feed length of the multi-mode horn is 200 mm, the multi-mode transmission section is 11993 mm long, and the total length is 1400 mm. The antenna azimuth half-power angle is 6.3°; the elevation half-power angle is 7.8°; the gain G=27.4 dB.

1.2 Antenna interference factors

There is interference during the operation of the antenna. The receiving and transmitting antennas are placed side by side and close together. Radiated coupling exists not only between the two antennas, but also in the housing of the equipment, the holes in the housing, the transmission lines and between the components. In summary, there are three different interference pathways: 1) Radiated interference between the receiving and transmitting antennas; 2) Radiated interference between components or housings; 3) Radiated interference between transmission lines. In addition, interference from surrounding reflectors will also affect the performance of the antenna. However, radiated interference between antennas is the main interference pathway. The calculation of near-field coupling between antennas is a relatively complex electromagnetic field problem, so this article only makes a qualitative analysis. Assuming that the two antennas are located in the far field, that is, the antenna spacing, the maximum aperture of the two antennas is 2.5 m, and the center frequency wavelength is 0.224 m; the antenna spacing is 58 m through calculation. In actual engineering, the distance between the two antennas is impossible to reach 58 m. Since the rotation radius of the antenna is 3 m and the center distance of the two antennas is 6 m, that is, the two antennas are in the near field, the feed of the antenna will have a significant effect on the interference of the other antenna. The most effective way to isolate this interference is to add a conductive edge around the antenna. When the height of the edge is 5.3 times the wavelength, the far side lobe of the antenna can be reduced by 13 dB, and the isolation of the two antennas can reach about 80 dB. The actual height of the edge is 1.2 m.

2 Antenna base design

2.1 Antenna base structure design

The antenna seat is designed with an azimuth and elevation turntable structure. It consists of an azimuth seat, an elevation box drive system, an axis angle device, a limit protection device, a leveling device, a counterweight, etc., as shown in Figure 3. The azimuth part consists of a base, a turntable, a turntable bearing, etc. The turntable has good rigidity and stability. The turntable bearing is directly equipped with a worm gear to ensure the rigidity of the azimuth drive. The base and the turntable are both steel plate welded parts. In order to ensure the -13.5° elevation angle, a support is added to the azimuth to increase the height of the elevation axis. The elevation consists of the left and right bearings of the elevation box, the elevation axis, and the left and right arms. The elevation box is a casting. The left and right arms rotate synchronously with the elevation axis. The upper end is connected to the antenna, and a counterweight is placed at the rear end to balance the weight of the antenna. Adding transition pieces at the upper ends of the left and right arms can connect with other antennas. The screw drive method is widely used in engineering for the drive system, but it is not considered in the design of the antenna base of this system. There are two main reasons: 1) Due to the harsh working environment, the speed uniformity of the antenna is required to be relatively high, and the speed of the screw drive is uneven within the working range. The test platform requires the antenna to rotate at a speed of about 6 (°)/s, which is difficult to achieve with the screw drive method; 2) The screw rotation efficiency is low, and the motor power is relatively large when working under conditions requiring a large wind speed. In view of the actual situation, the drive system chooses to use a worm gear plus a planetary reducer. In order to reduce the volume and weight, the worm gear adopts an envelope type, which has a large load capacity. The planetary reducer has the advantages of small volume, light weight, and high efficiency. The turbine drive method is adopted, and the motor power is about 550 W.

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The shaft angle sensor uses a resolver and a pitch resolver 1:1 to connect with the pitch axis, and the azimuth is connected through gears. To eliminate the backlash, the transmission gear uses a double-piece backlash elimination. The antenna base is installed with a limit switch and a mechanical limit block to ensure the safety of the equipment. The limit switch uses a contactless proximity switch to adapt to the harsh seaside environment. The system uses a worm gear drive with self-locking properties, so the upward locking can utilize its own self-locking properties.

3 Servo control system design

3.1 Servo control principle

The ground and sea clutter test control platform consists of two antennas, antenna bases, and servo control systems. The two antennas use independent antenna bases and are placed side by side on the left and right sides. The rotation of the two antennas can be controlled independently. The servo control principle block diagram is shown in Figure 4. The drive chain uses a first-stage turbine worm gear transmission to give it a self-locking function to prevent the antenna from rotating due to wind load. The drive mode is a stepper motor drive (matching driver), a rotary transformer shaft angle sensor; a 12-bit RDC module shaft angle encoding; a liquid crystal display is used to display the shaft angle and state parameters, and a dedicated control computer PC-104 and matching cards. The computer collects the shaft angle data and sends it to the display screen, and the rotation control of the antenna azimuth and pitch is realized through the keyboard. The servo control system receives the remote control information of the central computer and transmits the relevant parameter information to the central computer, and receives the limit information to realize the limit protection of the antenna. The limit protection adopts an electric limit mode, and the upper and lower limit switches are installed on the pitch, and the left and right switches are installed on the azimuth to realize the limit protection of the antenna.

3.2 Servo control system implementation

The central control chip uses a single-chip microcomputer C8051F020, which has the advantages of high integration and multiple functions and is widely used in control circuits. Figure 5 shows the serial interface schematic diagram between the single-chip microcomputer and PC104. The serial port of PC104 is connected to the level converter interface, and the signal input and output are connected to the single-chip microcomputer serial port through the level converter chip. In order to improve the anti-interference performance of the system, a high-speed optical coupler element 6N137 is used between PC 104 and the single-chip microcomputer to isolate the CPU from the communication interface to prevent serial interference.

The RDC module in this system uses AD2S80, which can achieve 4 different resolutions of 10, 12, 14, and 16 bits, which can be selected through the SC1 and SC2 pins. Considering the conversion accuracy and tracking rate, a 12-bit resolution is selected in this system. When working, the sine and cosine signals generated by the resolver are connected to the corresponding sine input and cosine input pins respectively, and the converted 12-bit digital information is directly transmitted to the central control chip. AD2S8OA and its peripheral circuits are shown in Figure 6.

4 Performance indicators and working principles

4.1 Performance indicators

The antenna rotation range is -13°～90° in elevation and ±170° in azimuth. The antenna rotation mode is automatic and electronically controlled. The beam pointing accuracy is ±0.1° in elevation and ±0.1° in azimuth. The limit protection has the limit protection function. The interface is used for the servo control computer to transmit relevant parameter information, and the interface adopts RS232 serial port.

4.2 Working Principle

According to the requirements of the measurement range, the change of the antenna pointing is determined. The main task of the servo control system is to ensure the stability of the antenna pointing, and at the same time detect the clutter signal in real time by changing the pitch and elevation angles of the antenna. Due to the uncertainty of the clutter signal, it is very important to accurately and stably determine the antenna pointing, so there are two closed loops in the control system: the pitch control loop and the azimuth control loop.

In the pitch control loop, a horizontal reference is introduced. The horizontal reference is very sensitive to the tilt angle in the pitch direction, thereby ensuring a perpendicular relationship with the pitch axis, further improving the accuracy of pitch control and achieving directional control of the pitch command angle.

In the azimuth control loop, an azimuth reference is introduced. The azimuth reference is very sensitive to the azimuth of the antenna platform, thereby realizing the directional control of the azimuth command angle. According to the requirements of this system, the servo control device must have the function of automatically locating the target range, and this task is completed by computer control. The pitch angle and azimuth angle are input into the computer, and the computer determines the direction of the antenna based on the input parameters.

5 Pointing accuracy analysis

During the system testing process, it was found that due to the manufacturing process, the inherent errors of the servo system caused deviations in pointing accuracy. The main error sources and values ​​were obtained through testing, as shown in Table 1.

From Table 1, we can know the total pointing error (RMS value):

The beam pointing accuracy is obtained by calculating errors such as azimuth and elevation .

Through the testing and calculation of relevant parameters such as total pointing error and beam pointing accuracy, the error value is within the controllable error range and the system meets the use requirements.

6 Conclusion

In this paper, a ground and sea clutter test control platform is designed by using an offset parabolic antenna and a servo control system, which realizes the detection of ground and sea clutter in complex situations, and provides a basis for eliminating or reducing the impact of clutter and improving the anti-interference ability of radar. This platform improves the timeliness of work, and its promotion and application are of great significance.