利用微波光子学技术识别虚假雷达信号

“欺骗性电子干扰“是现代战争中重要的干扰手段之一。例如,通过制造并发射假的雷达回波信号,使雷达获得虚假目标信息从而做出错误判断。为实现反干扰,雷达接收机一般通过测量接收信号的参数进行真假目标识别,如通过频率差别来区分它们。然而,电子干扰设备已能够通过数字射频存储技术制造出与真实雷达信号同频的虚假信号,所以仅通过频率测量已很难准确识别真假目标。

相位噪声是表征微波频率稳定度的重要参数,在对回波频率进行测量的同时监测其相位噪声特性,能显著提升对虚假目标的识别概率。

考虑到雷达的工作频率覆盖范围不断扩大,对宽频带范围的微波频率和相位噪声进行一体化测量是提升雷达对抗欺骗性干扰能力的关键。

近几年,微波光子学的快速发展为微波频率和相位噪声的宽频带测量提供了新思路。相比于电子学测量方法,微波光子学方法具有可测信号频率高、频带宽的优势。然而,微波光子学相位噪声和微波频率的测量方法一直是相互独立发展的,尚无综合一体化实现微波频率和相位噪声测量的报道。

南京航空航天大学微波光子学实验室张方正教授、潘时龙教授等在Chinese Optics Letters第18卷第9期(J. Z. Shi, et al., Photonic-assisted single system for microwave frequency and phase noise measurement)介绍了一种光子学辅助的宽频带微波频率与相位噪声一体化测量系统。该系统使用微波光子正交(I/Q)混频器来获取待测信号与其延时信号之间的相位差,其中延时通过一卷长光纤外加可调光延时线获得。系统在可调光延时匀速变化时将获得待测信号的相位信息,进而通过频率-相位斜率映射方法获得待测信号的频率。当可调光延时固定时,根据I/Q混频器探测相位的功率谱密度可以测量待测信号的相位噪声。作者在实验中成功演示了5至50 GHz频率范围内微波信号的频率和相位噪声测量,频率测量误差控制在150 MHz之内,10 kHz频偏处的相位噪声测量误差控制在3 dB。

张方正教授认为,该成果能为反电子干扰提供重要技术支撑。考虑到目前的实验演示系统仅针对单频信号的测量,未来的工作将专注于线性调频信号等复杂信号的参数测量。

(a)微波光子宽频带相位噪声和频率一体化测量系统;(b)5-50 GHz频带内的频率测量误差;(c)5-50 GHz频带内的相位噪声测量结果。

Photonics-assisted single system for microwave frequency and phase noise measurement

Deceptive jamming is one of the important electronic interference methods in modern warfare. By producing false or misleading target echoes, the radar will obtain false target information and make wrong judgments. In order to resist the deceptive jamming, radar receivers need to distinguish the real and false targets by measuring the parameters of the received signals, like frequency. Unfortunately, the electronic jamming equipment, empowered by the current digital radio frequency memory (DRFM) technology, is capable to produce false signals with the same carrier frequency of the real target echoes. As a result, it is difficult to identify real and false targets by simply monitoring the frequency.

Phase noise is an important parameter that characterizes the stability of a microwave signal. The probability of correctly identifying false targets can be significantly improved by simultaneously monitoring the frequency and the phase noise of the received signal. Considering the continuous expansion of the radar spectral coverage, the measurement of microwave frequency and phase noise in a wide spectral range is the key to improve the anti-deceptive jamming capability.

In recent years, the rapid development of microwave photonics has provided new solutions for broadband microwave frequency and phase noise measurements. Compared with the electrical methods, microwave photonic methods have the advantages such as high frequency and large bandwidth. However, the previous photonics-assisted microwave frequency measurement and phase noise measurement are physically isolated from each other.

In Chinese Optics Letters, Volume 18, Issue 9, 2020 (J. Z. Shi, et al., Photonic-assisted single system for microwave frequency and phase noise measurement), Prof. Fangzheng Zhang, Prof. Shilong Pan and their colleagues from the Microwave Photonics Laboratory in Nanjing University of Aeronautics and Astronautics report a photonics-assisted single system for measuring the microwave frequency and phase noise simultaneously in a wide spectral range. The system uses a photonics-assisted broadband in-phase and quadrature mixer to acquire the phase difference between the signal under test and its replica delay by a span of optical fiber together with a variable optical delay line. By adjusting the variable optical delay line, the frequency of the signal under test can be estimated through frequency-to-phase-slope mapping, and corresponding phase noise can be calculated from the power spectral density of the phase fluctuation under a specific time delay. In the experiment, frequency and phase noise measurement of microwave signals from 5 to 50 GHz is successfully demonstrated, of which the frequency measurement errors are controlled within ±150 MHz and the phase noise measurement errors at 10-kHz offset frequency are kept within ±3 dB.

Prof. Fangzheng Zhang from Nanjing University of Aeronautics and Astronautics believes that this work can provide important technical support for electronic anti-interference. Considering that the current experimental demonstration system only measures single-frequency signals, future work will focus on the parameter measurement of complicated signals such as linearly chirped microwave signals.

(a) Photonics-assisted single system for microwave frequency and phase noise; (b) frequency measurement errors versus frequencies from 5 to 50 GHz; (c) phase noise measurement results of signals tuned from 5 to 50 GHz.