In view of the increasingly serious threat of UAV, the research focuses on anti UAV electronic countermeasure reconnaissance, navigation countermeasure and suppression jamming. A set of experimental platform is constructed, including the anti UAV electronic countermeasure experimental subsystem, anti UAV GPS interference experimental subsystem, which can be used to carry out UAV detection and orientation, UAV GPS The scheme is based on software radio platform and has good open source characteristics, which can be used as the basis of teaching demonstration and product prototype development.
The electronic reconnaissance subsystem of anti UAV is mainly used to carry out telemetry link signal reconnaissance experiment such as UAV data transmission or image transmission, realize telemetry signal reconnaissance and analysis, and adopt dual channel phase interferometer system to implement direction finding of target, so as to provide target indication for subsequent effective anti UAV operation. The subsystem mainly consists of electronic countermeasure reconnaissance equipment and UAV telemetry signal reconnaissance control software. Among them, EW reconnaissance equipment is mainly equipped by self-assembly, including reconnaissance and direction finding antenna, USRP software radio platform and industrial control computer. It is mainly responsible for reconnaissance and interception of UAV telemetry signal and analysis of UAV telemetry signal to realize direction finding.
According to the requirement that the construction target does not need to measure the elevation angle, and the flight altitude of the UAV is low (several hundred meters) and the detection distance is relatively long (several kilometers), the directional jamming antenna beam in the scheme can generally cover the UAV, so the UAV can be interfered without measuring the pitch angle. As shown in the figure, the subsystem consists of two DF antennas, usrpx310 and IPC. The industrial computer controls USRP x310 to sweep or monitor the sensing signal in real time within 2.4GHz and 5.8GHz bands of civil UAV remote control or map transmission signal; if there is signal in 2.4GHz and 5.8GHz frequency band, the remote control and map transmission signal can be identified according to the characteristics of remote control signal and map transmission signal. When the target signal appears, the industrial computer controls the USRP x310 two channels to receive the graph signal, and processes the received data and finds the direction.
There are two central frequencies (2.44GHz, 5.775ghz) in UAV remote control signal, and the bandwidth is about 100MHz. The real-time bandwidth of USRP x310 is more than 100MHz, and the real-time bandwidth of USRP b210 is 56mhz. It can control USRP x310 to monitor sensing signal at 2.44GHz or 5.775ghz (USRP b210 can scan frequency). If a signal appears, USRP is triggered to receive the signal and carry out subsequent identification. The sorting method based on time-frequency analysis can distinguish the above signals. Time frequency analysis method is an effective method to analyze non-stationary signals by analyzing the two-dimensional function of time and frequency. Because different signals have their own unique characteristics, we can distinguish different signals according to these characteristics. The fixed frequency signal appears continuously near a frequency point, and the signal frequency does not change with time, and it is a straight line form on the time-frequency analysis chart; frequency hopping signal refers to the periodic change of signal frequency with time, and the duration of each frequency point is the same, which is shown as a series of short straight lines with the same length on the time-frequency analysis diagram, and they are connected with each other in time Noise signal refers to different channel noise and spatial interference noise, which covers a wide range of time and frequency domain, and its amplitude is generally smaller than the target signal, and it shows a chaotic point on the time-frequency analysis diagram. According to the time-frequency characteristics of different signals, the signals are sorted by setting threshold.
The main UAV map transmission signals are OFDM modulation, the center frequency is about 2476.5mhz or 5722mhz, the bandwidth is 9.5mhz. This scheme mainly aims at the identification of OFDM modulated image transmission signals. According to the center frequency and bandwidth, the graph signal can be almost identified.
When the UAV signal is recognized, the direction finding can be carried out. Since the main UAV image transmission signal is a fixed frequency signal, and the elevation angle is not measured, only the azimuth angle is measured, so the two channel phase interferometer system can be used for direction finding. The system structure is shown in Figure 6. The two DF antennas are connected to two channels of USRP x310, and the industrial computer controls the USRP x310 to receive and process signals. Fig. 7 is the direction finding diagram of dual channel phase interferometer. D represents the distance between two antennas. Select the appropriate distance according to different frequency signals. If the amplitude and phase characteristics of two antennas and two channels are completely consistent, the antenna azimuth can be measured by the phase difference of signal received by the two antennas. The experimental results show that USRP x310, as a two channel phase interferometer, can achieve the required direction finding accuracy.
The electronic countermeasure reconnaissance experiment subsystem of anti UAV is mainly used to carry out telemetry link signal reconnaissance experiment such as UAV data transmission or image transmission, realize telemetry signal reconnaissance and analysis, and adopt dual channel phase interferometer system to implement direction finding of target, so as to provide target indication for subsequent effective anti UAV operation. The subsystem mainly consists of electronic countermeasure reconnaissance equipment and UAV telemetry signal reconnaissance control software. Among them, EW reconnaissance equipment is mainly equipped by self-assembly, including reconnaissance direction finding antenna, usrp-lw x310 (usrp-lw b210) and industrial computer, which is mainly responsible for reconnaissance and interception of UAV telemetry signal; UAV telemetry signal reconnaissance and control software is mainly through outsourcing or acquisition Purchase mode is equipped, including telemetry signal analysis module, direction finding control module and other modules. It is mainly responsible for the analysis of UAV telemetry signal and direction finding. The software is mainly developed based on LabVIEW or MATLAB, with secondary development function.
|Detection frequency band||10MHz-6GHz|
|Detection range||Not less than 1km|
|Adaptability to signals||Direct sequence spread spectrum (DSSS) signal, conventional pulse signal, etc|
|Direction finding accuracy||The horizontal azimuth is 20 ° to 25 °|
Suppression jamming subsystem
GPS suppression jamming experimental equipment. It mainly adopts the way of pre orientation or omni-directional suppression to radiate the noise signal of corresponding frequency band to the surrounding area, which makes the UAV satellite navigation equipment unable to receive the satellite navigation signal effectively, resulting in the increase of position information calculation error or the loss of position information. The working process is shown in the figure below.
GPS suppression signal can use USRP software radio platform to transmit noise signal in GPS signal receiving frequency band. Noise signal is amplified by power amplifier and transmitted to UAV GPS receiver with directional antenna, which makes GPS receiver unable to locate. It can also use USRP to generate false GPS satellite positioning signal, such as recording GPS navigation message in other places, causing GPS The navigation message sent by USRP is the simulation of GPS navigation signal. Based on the broadband characteristics of USRP, the suppression or deception jamming signals can be transmitted in different frequency bands. Since the GPS antenna of UAV is usually installed at the top, and the antenna pattern usually points to the sky, there is a certain degree of isolation for the suppression or jamming signals transmitted from the ground. The operating distance of the suppression signal and deception jamming signal depends on the directional isolation degree or attitude of the antenna, so the operating distance will be affected. In the future, the operating distance can be increased by increasing the output of power amplifier.
When GNSS decoy jamming signal is generated by USRP, official GPS data file can also be loaded through baseband data to generate no less than 4 satellite signals. It is necessary to consider that all four satellites are visible satellite signals under the current sky view, and the satellite numbers are visible satellites. When using USRP to form GNSS decoy jamming signal directly, USRP can be placed in advance, such as GNSS ephemeris and almanac files. The almanac files need to consider the time or weeks of jamming signals, and the two files can be replaced in the future. At the same time, any geographical location of GNSS receiver can be set, which can be manually input to a certain geographical location According to the decoy position, the visible satellites (more than 5 degrees above the ground level) and the satellite level can be calculated automatically. At the same time, the UTC time of decoy can be selected. The maximum number of satellites is 8, which can meet the positioning requirements of GPS receiver. USRP can also use external clock to improve the frequency accuracy of interference signal.
The way of suppressing jamming in remote control link is the same as that of GPS. The cooperation of USRP, power amplifier and directional antenna should be considered. Frequency hopping and spread spectrum technology have been widely used in remote control transmitter, and the frequency hopping parameters can be adaptive and have certain anti-jamming ability. In order to get accurate results, the parameters of frequency hopping and spread spectrum must be known when the interference is small. However, we can still know the general range of interference needed. If there are frequency hopping measures in the remote control signal, and the jammer does not know any parameters of these measures except the frequency range, and can only use noise to cover the whole frequency band violently, then the required power will be increased. In the process of suppression, usrp-lw x310 is used to generate wideband interference signal, which is Gaussian white noise or spread spectrum interference signal and analog WLAN 802.11 standard format signal to realize full coverage of flight control channel. At the same time, with 2.4GHz 100W power amplifier and 2.4GHz log period or Yagi antenna, directional interference suppression is realized.
|Working mode||Support GPS signal of any frequency point|
|Jamming strategy||Full frequency blocking suppression|
|Operating distance||Orientation not less than 1km|
|signal power||Not less than 20W|
|Antenna direction||Orientation: 28゜～50゜|
|Whole machine power consumption||＜300W|
|Secondary development||Support LabView or Matlab for secondary development of control software|
Luowave SDR anti UAV experimental system is based on USRP general software radio platform, which has flexible configuration function, and can further expand its functions on this basis, with good applicability and upgrade ability.
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