Design and implementation of a digital tracking and ranging module

0 Introduction

One of the most important functions of radar is to track and measure the distance of the target. Firstly, the basic principle of radar ranging and tracking is explained, and a new method of designing and implementing a pulse radar digital tracking and ranging module is introduced, and the theoretical basis for the development of this module is described. This module uses the delay time of the echo signal relative to the transmitted pulse to measure the target distance. According to the characteristics of the tracking pulse, the target echo delay time count value and the tracking pulse count value are compared. The comparison result is fed back to the tracking controller using the digital signal processing method, realizing the distance tracking of the pulse radar. It has the advantages of high ranging accuracy, stable ranging, strong anti-interference ability and simple circuit. This module can perform manual tracking and automatic tracking and ranging of the target.

1 Theoretical design of this module

Generally, radar digital tracking uses a time discriminator to identify the delay time difference between the echo signal and the tracking pulse, and then uses a clock pulse to count this time difference, which is fed back to the tracking processing unit as a distance error. In essence, it is to convert the output of the analog system into a digital one, that is, it is converted on the basis of the analog quantity, and it is still an analog quantity in essence (as shown in Figure 1). There are inevitable system errors between the conversions. For example, the tracking pulse needs to be timed and counted, and the starting time of the timing count must be synchronized with the radar transmission pulse. It is difficult to achieve complete synchronization, and the error between them is inevitable. For example, in a certain type of radar time discriminator, the time difference between the echo signal and the tracking pulse must be converted into a voltage with polarity (there is an error), and then this voltage must be converted into a counting pulse of the distance counter (there is also an error), so that the distance counter can count up or down to achieve distance tracking of the target.

The module only needs to know the binary distance value of the target echo, and then compare it with the binary value of the tracking pulse to form a distance error as feedback to the distance generator for tracking processing. It does not need to perform special time measurement on the tracking pulse, nor does it need to compare the time difference between the echo signal and the tracking pulse, which simplifies the circuit structure and reduces the system error. When the radar completes the full detection, the counting stop pulse will control the distance counter and latch to zero. The counting stop pulse can be generated by the leading edge of the next transmission pulse, or it can be provided by the fire control computer. Figure 2 is a data flow chart of the tracking and ranging module designed using the above theory. The module can realize automatic tracking of distance data using digital technology. The most basic way to convert the target delay into a digital quantity is to use a counter and control it appropriately. When the radar transmits a pulse signal, the T trigger is triggered, which starts the counter at the same time. Once it is determined that the target echo is received, the T trigger is triggered again to stop the counter counting. In this way, the data output by the counter is read at the moment of echo arrival, that is, the target distance data is obtained. By comparing it with the value output by the manual ranging counter, the distance error value is obtained and sent to the controller for tracking processing. The conversion between manual and automatic tracking and ranging is switched through an AND gate. Obviously, the controller and tracking pulse generator of this module are no longer the controller and tracking pulse generator in the sense shown in Figure 1. At this time, the controller and tracking pulse generator have been integrated (here the traditional three parts are still used to distinguish the entire tracking and ranging module, but the meaning is different). The advantages of this module are simple structure, easy to understand and strong versatility. It can be used not only in the entire radar tracking system, but also in the target echo generator and tracking and ranging trainer of the general radar trainer.

2 Design of manual/automatic distance tracking and ranging module

The digital distance tracking module consists of a time discriminator, a distance generator and a tracking pulse generator.

2.1 Design of the time discriminator

In this module, the time discriminator (distance comparator) consists of an upper latch, a lower latch and a subtraction circuit with a sign bit. The upper latch outputs the actual distance pulse count value of the target echo; the lower latch outputs the distance count value of the tracking pulse; the two distance count values ​​are simultaneously sent to the subtraction circuit with a sign bit, and the distance error value with polarity is output as feedback to the distance generator (i.e., controller) for tracking processing.

2.2 Design of tracking pulse generator

The tracking pulse generator consists of a manual pulse output device, an add/subtract counting unit and a register, so strictly speaking it should be a manual tracking pulse generator, which counts the manual pulses to form tracking pulses, and the data output by the tracking pulse generator is sent to one end of the controller. The add/subtract counting unit is a serial reversible counter composed of 4 74LS191 (single clock 4-bit binary reversible counter). The counting direction and counting mode of the counter are controlled by the working mode control circuit. The manual pulse output device generates two groups of pulses with a phase difference of 90°. When working in manual mode, pulse 1 is used as the clock pulse of the reversible counter, and pulse 2 is converted into a control pulse for controlling the counter to perform addition/subtraction counting through the JK trigger 74HC107. The two groups of manual pulses form the advance and retreat of the tracking pulse through the addition/subtraction counting unit. When the difference between the tracking pulse and the target echo distance value is within a certain range, the target can be automatically tracked and ranging (of course, automatic tracking and ranging can also be omitted at this time). When entering the automatic tracking state, the addition/subtraction counter stops counting, and the target cannot be manually tracked and ranging. The controller performs data tracking processing based on the value already output by the original addition/subtraction counter according to the distance error value sent.

The function of the register is to output data to the controller. It plays the role of selecting the output. When manual tracking is performed, the data of the add/subtract counting unit is selected for output; when automatic tracking is performed, the data output by the output controller is selected and waits for the data output by the time discriminator to be sent to the add/subtract unit for tracking processing.

The distance generator (i.e., controller) has two input terminals, which input the values ​​sent by the tracking pulse generator and the values ​​sent by the distance error register respectively. The distance generator is mainly composed of an addition/subtraction unit and a distance error register. It uses the distance error value within a certain range sent by the distance comparator to correct the tracking pulse generated by the tracking pulse generator, thereby determining whether to track manually or automatically and measuring the target distance. For example: when the distance error value is less than 750 m, the error register is triggered to output the distance error value, that is, automatic tracking and ranging processing is performed; when the distance error value is greater than 750 m, the error register is triggered to output 0, so that only manual tracking and ranging can be performed. The addition/subtraction unit is a 16-bit full carry-lookahead arithmetic operation circuit composed of 4 74HC381 (arithmetic logic unit with 8 binary algorithms) and 1 lookahead generator 74HC182. The use of 74HC182 is to enable the operator to perform parallel operations, which can greatly improve the controller's operating speed. The data generated by the distance generator is output to the time discriminator, fire control and terminal display through the lower latch, so that the radar tracking situation and the measured target distance can be seen on the display terminal.

The distance error register is composed of three 74LS174 unidirectional positive edge triggered 6-bit integrated registers. Its main function is to provide distance error data during automatic tracking and send it to the addition/subtraction unit, which can also determine whether to track manually or automatically. When the clear end of each 74LS174 is set to 0, the distance error register output is 0, and the entire module enters the manual tracking and ranging state; when the clear end is set to 1, it enters the automatic tracking and ranging state, and the distance error register outputs the distance error value.

3 Conclusion

The radar tracking and ranging module is analyzed and designed here, and a method of dual-path distance counting and then distance comparison is proposed (instead of the method of comparing the distance between the tracking pulse and the target echo time difference) to track and measure the distance of the target. This design method does not determine the polarity of the time discriminator output distance error based on the echo leading or lagging the tracking gate, nor does it output the distance error based on the comparison of the echo and the overlapping area of ​​the front and rear gates. It is an innovative design method with relatively large room for exploration in terms of cost saving, circuit simplification and fault repair.