CN107167037B - A kind of anti-UAV system of centralized control based on sound wave attack - Google Patents

Abstract

The invention discloses a kind of anti-UAV system of centralized control based on sound wave attack, including master control and display device, source of early warning group, positioning tracking device group and sound wave attack device cluster, the target position information for detecting prewarning area is sent to master control and display device by source of early warning in real time, the display of movement locus is carried out by it, when target enters tracing area, equipment is attacked for the target selection positioning tracking device and sound wave, and stopping is to the early warning detection of the target, its location information is obtained by positioning tracking device simultaneously, and realize real-time aiming of the sound wave attack equipment to the target；When detecting that the target enters attack region, whether control sound wave attack equipment makes it crash because generating resonance, and destroyed by positioning tracking device detector, if so, control sound wave attack equipment halts attacks to initiating sound wave attack.The implementation of the present invention can carry out intelligent decision to multiple unmanned plane targets simultaneously and carry out physical destruction.

Description

A Centralized Control Anti-UAV System Based on Sonic Attack

technical field

The invention belongs to the technical field of anti-drone, and in particular relates to a centralized control anti-drone system based on sonic attack.

Background technique

In recent years, the civilian UAV market has become increasingly popular. UAVs have been widely used in personal aerial photography, film and television shooting, power inspection, forest fire prevention, police evidence collection, map surveying and mapping, etc. With continuous breakthroughs in technology, drones have become a comprehensive platform with powerful performance and comprehensive functions.

Anti-UAV technology refers to methods and systems for reconnaissance and destruction of illegally flying UAVs.

At present, there are two main types of systems and equipment for UAV countermeasures, which are as follows:

1. The application publication number is: CN105842718A, the patent application titled "A Portable Anti-UAV Rifle". The patent application specifically relates to a portable anti-drone rifle, including a gun body, which is built with a controller, an anti-navigation signal source module and an antenna feeder subsystem connected in sequence, and the controller sends an operation command to the anti-navigation signal The source module outputs an anti-navigation signal and transmits it through the antenna feeder subsystem to achieve the purpose of jamming the drone.

2. The application publication number is: CN105501436A, and the patent application titled "A Portable Anti-UAV Intelligent Interception Network System" adopts the method of carrying intelligent interception nets by drones to a predetermined location for reconnaissance and search , after finding the target, the telescopic control rod will automatically and quickly expand to both ends and release the net gear. Under the action of gravity, the net gear will quickly and vertically expand. Air jamming, interception, capture.

The above anti-drone systems mainly adopt strong interference and physical capture in technology, the process is complicated, the operation is difficult, the technology is difficult, and there are certain conditions and restrictions. With the development of technology, some drones have begun to consider navigation that does not depend on GPS, and the above-mentioned anti-drone rifles have no effect; adopting the method of casting a net to capture is not only very easy to be found by the other party, but also has a low capture rate, especially It is obviously insufficient to consider the complex situation of attacking multiple targets at the same time. The anti-drone system adopted above cannot fundamentally solve the problem of anti-drone. Therefore, it is necessary to adopt physical attacks and hard damage to drones.

Contents of the invention

The purpose of the present invention is to provide a centralized control anti-drone system based on sound wave attack, including a main control and display device, a group of early warning equipment, a group of positioning and tracking equipment and a group of sound wave attack equipment,

Wherein the early warning devices of the early warning equipment group are distributed in different positions of the early warning area, the positioning tracking devices of the positioning tracking device group are distributed in different positions of the tracking area, and the sound wave attacking devices of the sound wave attacking equipment group are distributed in different positions of the attacking area;

The main control and display device periodically send equipment status monitoring signals to each equipment in the early warning equipment group, location tracking equipment group and acoustic attack equipment group, determine the status of each equipment and save it locally, and the equipment status includes idle state, working state and fault state, and at the same time give an alarm to the equipment in the fault state;

The initial equipment state of the early warning equipment is in the working state, and the radio wave is transmitted and received in real time to detect the position information of the airspace target in the early warning area and send the position information to the main control and display device, and the main control and display device display each airspace locally The trajectory of the target; when the airspace target enters the tracking area, the main control and display device will send a cancellation warning command to the early warning equipment that sends the airspace target, and select the positioning tracking equipment and sound wave for the airspace target that the equipment status is idle. The attacking device sends an activation command to the selected positioning tracking device and the acoustic wave attacking device, and sets a synchronous matching table for each positioning tracking device from the selected acoustic wave attacking device and sends it to the corresponding positioning tracking device. The synchronous matching table is: A sonic attack device that is synchronized with the emission direction of the current location tracking device, and the same sonic attack device only exists in one synchronization matching table, that is, the synchronization matching mapping between the location tracking device and the sonic attack device is one-to-one, or one-to-many;

The early warning device that receives the cancel warning command switches the device state to the idle state; the location tracking device and the sonic attack device that receives the activation command switch the device state to the working state; at the same time, the early warning device in the idle state switches the device state to the working state periodically state;

The positioning and tracking equipment in the working state obtains the position information of the airspace target in real time by transmitting and receiving radio waves and sends it to the main control and display device. At the same time, it calculates the aiming position offset based on the position information of the airspace target and sends it to the The sonic attack device, the location tracking device and the sonic attack device respectively adjust the signal emission direction in real time based on the offset of the aiming position, and the initial signal emission directions of all the location tracking devices and the sonic attack device are the same;

The main control and display device judge whether each airspace target enters the attack area, if so, set the airspace target as the attack target, and perform different processing based on the system working mode: if the working mode is manual intervention, prompt the operator to choose whether to attack, if If attack is selected, an attack command is sent to the sonic attack device corresponding to the attack target; if the working mode is no manual intervention, an attack command is sent to the sonic attack device corresponding to the target after waiting for K seconds, where 0<K<10; After the attack command, an attack monitoring command is sent to the location tracking device corresponding to the attack target at the same time;

Based on the current signal emission direction, the sonic attack device that receives the attack command emits a sonic attack signal that resonates the airspace target; the location tracking device that receives the attack monitoring command judges the height of the attack target after time t (preset observation time) Whether it is less than the preset threshold (experience value), if so, generate a destroyed instruction and send it to the main control and display device;

The main control and display device sends a standby command to the location tracking device that sends the destroy command and the sonic attack device in the synchronization matching table of the location tracking device;

After receiving the standby command, the positioning tracking device and the sound wave attacking device will switch the device state to the idle state, and at the same time restore the signal transmission direction to the initial signal transmission.

Owing to adopted above-mentioned technical scheme, the beneficial effect of the present invention is:

1. Advanced early warning mechanism to provide sufficient buffer time. The system has separate early warning equipment, and the early warning distance can reach several kilometers. After passing the warning prompt, the operator has sufficient preparation time to win precious time for the next step to implement specific attack settings.

2. Possess the ability to simultaneously detect and attack multiple targets at one time. The system can not only detect a single target, but also deal with the complex situation of multiple target invasions at the same time. Due to the cluster deployment method, the attacking equipment has the advantage of quantity and can destroy multiple malicious targets at one time.

3. The ability of multiple devices to attack the same target at the same time. At present, most anti-drone platforms are single anti-drone platforms targeting a single target, and the effect is often not good. By default, the present invention adopts the method that several devices in available state located in different places attack a single target at the same time, and its attack efficiency is higher than that of a single platform.

4. With the ability of hard destruction, there is no need to worry about the possibility of being attacked by the same target multiple times. At present, the illegal use of drones is increasing day by day, such as transporting guns and money for prison inmates through drones, and binding explosives on drones to carry out suicide attacks. The destruction of physical hardware not only prevents the occurrence of malicious incidents, but also does not need to worry about the threat of multiple attacks by the same target.

5. More intelligent. The present invention can provide all-weather and all-time autonomous early warning and automatic attack without excessive manual intervention. The default configuration can be used to automatically attack the flying targets that intrude into the security area. At the same time, manual control can also be used to reasonably ignore the allowed flying targets.

6. The invention is simple, practical, highly intelligent, and has strong practical benefits. It has good and significant effects on combating suicide attacks, illegal shooting, violent terror, etc. by using drones, and has a broad application market.

Description of drawings

Fig. 1 centralized control anti-drone system schematic diagram of the present invention;

Fig. 2 is a schematic structural view of the master control and display device of the present invention;

Fig. 3 is a schematic structural view of the early warning device of the present invention;

Fig. 4 is a schematic structural diagram of the location tracking device of the present invention;

Fig. 5 is a schematic structural diagram of the sonic attack device of the present invention.

Detailed ways

In order to make the purpose, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the implementation methods and accompanying drawings.

Based on the location information of the protection target and the performance of the system equipment, the early warning area is set as: a non-intersecting area with the protection target as the center and a radius of H1, that is, a ring-shaped area, and the tracking area is: the protection target as the center of the circle and a radius of H2 Circle, the attack area is: a circle with the protected target as the center and a radius of H3, where H1>H2>H3.

Then, set early warning devices at different positions in the early warning area. The specific number n depends on actual needs, and the early warning devices should first be placed on terrain with good line of sight and no obstruction. The preferred way is to install appropriate physical camouflage, and Matching high and low positions. Set up tracking and positioning devices at different positions in the tracking area. The specific number m depends on actual needs. It is preferable to use terrain with good line-of-sight and unobstructed terrain, and properly camouflage it. At different locations in the attack area, the specific number k of acoustic wave attack equipment is determined according to actual needs.

All the early warning equipment constitutes the early warning equipment subsystem, all the tracking and positioning equipment constitutes the positioning and tracking subsystem, and all the acoustic wave attack equipment constitutes the acoustic wave attack subsystem, and each subsystem is connected with the main control and display device, either wireless or wired, preferably wired Way. In addition, the positioning tracking subsystem and the acoustic wave attack subsystem can communicate with each other to communicate the physical offset of the synchronization signal transmission. See Figure 1 for the system architecture diagram. Among them, the information exchanged between each device of the early warning equipment subsystem and the main control and display device involves: the position information of the airspace target located in the early warning area, the cancel early warning command, the equipment status monitoring command and the corresponding response; the positioning tracking equipment subsystem The information exchanged between each device and the main control and display device involves: activation commands, location information of airspace targets located in the tracking area, synchronization matching tables, attack monitoring commands, standby commands, equipment status monitoring commands and corresponding responses; sound waves The information exchanged between each device of the attacking device subsystem and the main control and display device involves: activation command, attack command, standby command, device status monitoring command and corresponding response.

Among them, the early warning equipment, tracking and positioning equipment and acoustic attack equipment all involve three equipment states, namely idle state (available state), working state and fault state. The initial equipment state of the early warning equipment is in the working state, and the tracking and positioning equipment and the sonic attack equipment are in the idle state. And the main control and display device periodically send equipment status monitoring signals to all early warning equipment, location tracking equipment and sound wave attack equipment, determine the status of each equipment and save it locally, for example, when the test command TC1 is sent, the tested equipment replies to the response signal CT , indicating that it is in the normal idle state; when the test command TC2 is sent, when the device under test returns the response signal CW, it indicates that it is in a busy working state; when the test command TC3 is sent, the device under test returns the response signal CB and there is no response for a long time , indicating a corrupted fault state. When a device in a fault state is found, it will prompt the management personnel in a conspicuous manner in time, thereby ensuring the integrity rate of all devices.

Wherein the main control and display device are preferably set indoors. Referring to Fig. 2, it may specifically include a communication module, a main control module and a display module, wherein the communication module is used for communicating and interacting with each system, and the main control module is used for periodically sending equipment status monitoring signals to each equipment of each system to determine the status of each system. The device status is saved, that is, a device status table T is maintained locally. The device status table T is used to store basic information such as the number, location, and status of early warning devices, tracking and positioning devices, and sound wave attack devices. And select tracking equipment and acoustic wave attack equipment for airspace targets entering the tracking area, and set up synchronization matching tables for each positioning tracking equipment, send activation instructions and standby instructions to positioning tracking equipment and acoustic wave attack equipment, and send cancellation warning instructions to early warning equipment , The human-computer interaction operation is realized through the display module; the display module is used to display the movement trajectory of the airspace target, the human-computer interaction interface, early warning, and fault prompts. The hardware equipment required for the control and display device can be a card computer with a pre-installed Linux system, or a device with control functions purchased from parts and assembled by itself, and the corresponding application control software can be set.

The early warning equipment includes a signal transmitting module, a signal receiving module, a control module and a network communication module, and an information prompt module (optional), see Fig. 3, its network communication module is used for communication and interaction with the main control and display device, and the signal transmitting module It is used to transmit the detection signal, receive the echo signal through the signal receiving module and send it to the control module, and the control module calculates the position information of the airspace target and sends it to the main control and display device through the network communication module. An alert is issued when an airspace target is detected.

The positioning and tracking equipment includes an antenna, a feed network, a transmitter, a receiver, an angle follower unit, a speed measuring unit, a distance measuring unit, a data processing unit and a communication unit, see Figure 4, where the communication unit is used to communicate with the main control and display device , The communication interaction of the sound wave attack equipment, the detection signal is transmitted through the transmitter, the feed network, and the antenna, and the receiver sends the received echo signal to the speed measurement unit and the distance measurement unit respectively, and the speed measurement unit calculates the azimuth speed information and sends it to The data processing unit, the ranging unit calculates the distance information and sends it to the data processing unit, the data processing unit calculates the offset of the aiming position, that is, the physical displacement in the direction of signal transmission, and sends the physical displacement to the angle follower unit , to modulate the signal emission direction; at the same time, the distance information is sent to the main control and display device through the communication unit, and the physical displacement is sent to the sound wave attack device in the synchronization matching table.

The acoustic wave attack equipment includes a signal generator, a power amplifier, a control unit, a matching circuit, a transducer, an angle follower unit and a communication unit, see Figure 5, the communication unit is used for communication with the main control and display device, and positioning and tracking equipment Interaction, the control unit controls the angle follower unit to adjust the signal transmission direction of the transducer in real time according to the received aiming position offset, the signal generator generates an electrical signal based on the signal frequency determined by the control unit, and the electrical signal is processed by the power amplifier Amplified, and then matched and filtered by the matching circuit (for the purpose of protecting the device from damage), the transducer is converted into an attacking sound wave signal and emitted. Wherein the transducer is preferably a transducer array with an energy-gathering effect, such as a plurality of transducers forming an acoustic wave array emission source in the form of a plane vertical array, and a plurality of the transducers forming an acoustic pot emission source in the form of a pot , or other reasonable layouts with energy-gathering effects are acceptable.

The specific workflow of the centralized control anti-drone system of the present invention based on the above-mentioned equipment structure is as follows:

S1: Initialize the initial device state of n early warning devices as working state, and the initial device state of m tracking and positioning devices and k acoustic attack devices as idle state.

S2: The early warning equipment starts to obtain the position information of the airspace target (i.e. the target UAV) located in the early warning field, and sends it to the main control and display device through its network communication module. Display the trajectory of the man-machine in real time, and detect whether it enters the tracking area; if so, execute step S3; otherwise, continue to detect and display the trajectory;

At the same time, the early warning equipment that detects the target drone also sends out an alarm prompt through its information prompt module, which is mainly used to warn the owner of the target drone, so that the drone that strays into the controlled airspace can be withdrawn in time.

S3: The main control module of the main control and display device selects the equipment state as idle state positioning tracking equipment and sound wave attack equipment for the target UAV entering the tracking area. Assume that there are two target UAVs currently entering the tracking area. UAVs a and b indicate that the main control module selects two acoustic wave attack devices and one positioning and tracking device for UAVs a and b respectively from the local device state table T. The drone's nearest device sends an activation command to the selected device. Generate two synchronous matching tables (one positioning and tracking device corresponds to two acoustic attack devices) and send them to the corresponding positioning and tracking devices respectively;

At the same time, the cancellation warning command is sent to the early warning equipment corresponding to UAV a and b respectively;

S4: The device that receives the activation instruction first switches its device state to the working state;

Corresponding to the positioning and tracking equipment of UAV a, obtain its location information in real time and send it to the main control and display device, and at the same time send its physical displacement to the acoustic attack equipment in the synchronization matching table to realize the synchronous follow-up of the emission direction;

Corresponding to the positioning and tracking equipment of UAV b, obtain its location information in real time and send it to the main control and display device, and at the same time send its physical displacement to the rhombic sound wave attack equipment in the synchronization matching table to realize the synchronous follow-up of the launching direction .

The early warning equipment that receives the cancel early warning instruction stops the detection work and switches its equipment status to the idle state;

S5: The main control and display device detects the location information of drones a and b in real time, and once they enter the attack area, mark them as attack targets and execute step S6;

S6: Perform different processing based on the working mode of the system: if the working mode is manual intervention, the operator is prompted to choose whether to attack, if the attack is selected, an attack command is sent to the sonic attack device corresponding to the attack target; if the working mode is no manual intervention, After waiting for 5 seconds, send an attack command to the sonic attack device corresponding to the attack target, so that the operator can interrupt the current attack;

After broadcasting the attack command, the main control unit and the display device simultaneously send an attack monitoring command to the location tracking device corresponding to the attack target;

S7: The sonic attack device that receives the attack command transmits a sonic attack signal that resonates the airspace target based on the current signal emission direction;

The location tracking device that receives the attack monitoring instruction judges whether the height of the attack target after time t is less than the preset threshold, and if so, generates a destroyed instruction and sends it to the main control and display device;

S8: After receiving the destruction command fed back by the positioning tracking device, the main control and display device obtain the corresponding location and tracking device that should stop attacking based on the mapping relationship between the positioning tracking device and the sonic attack device in the synchronous matching table. The sonic attack device sends a standby command to the location tracking device that sends the destroy command and its synchronously matched sonic attack device;

S9: After receiving the standby command, the positioning tracking device and the sound wave attacking device switch the device state to the idle state, and at the same time restore the signal transmission direction to the initial signal transmission.

In the above steps, the early warning equipment in the idle state is switched to the working state in a cycle switcher state. For example, each time it switches to the idle state, the control module of the early warning device records its switching time t1, and based on the preset switching interval period T', when the time t1+T' is reached, the device state is switched to the working state.

This solution realizes the efficient and accurate destruction of malicious UAVs on physical hardware by high-frequency signals, which greatly improves the safety of low-altitude areas, personal and property, and is not only suitable for schools, laboratories and gymnasiums with ordinary needs, but also For prisons and borders with special conditions, it is not only suitable for static defense, such as sports event security, important large-scale conference security, but also suitable for dynamic defense in special occasions, such as special operations, anti-suicide attacks, armed patrols, etc. Compared with the existing anti-UAV technology, the present invention has strong pertinence and high efficiency, and can quickly build an efficient and reliable anti-UAV system according to tasks, environments, and requirements.

The above is only a specific embodiment of the present invention. Any feature disclosed in this specification, unless specifically stated, can be replaced by other equivalent or alternative features with similar purposes; all the disclosed features, or All method or process steps may be combined in any way, except for mutually exclusive features and/or steps.

Claims (7)

1. A centralized control anti-unmanned aerial vehicle system based on sound wave attack, is characterized in that, comprises main control and display device, early warning equipment group, location tracking equipment group and sound wave attack equipment group, Wherein the early warning devices of the early warning equipment group are distributed in different positions of the early warning area, the positioning tracking devices of the positioning tracking device group are distributed in different positions of the tracking area, and the sound wave attacking devices of the sound wave attacking equipment group are distributed in different positions of the attacking area; The main control and display device periodically send equipment status monitoring signals to each equipment in the early warning equipment group, location tracking equipment group and acoustic attack equipment group, determine the status of each equipment and save it locally, and the equipment status includes idle state, working state and fault state, and at the same time give an alarm to the equipment in the fault state; The initial equipment state of the early warning equipment is in the working state, transmitting and receiving radio waves to detect the location information of the airspace targets in the early warning area in real time and sending the location information to the main control and display device, and the main control and display device display each airspace target locally When the airspace target enters the tracking area, the main control and display device will send a cancel warning instruction to the early warning equipment that sends the airspace target, and select the positioning tracking equipment and the sonic attack device whose equipment status is idle for the airspace target The device sends an activation command to the selected positioning tracking device and the acoustic attack device, and sets a synchronization matching table for each positioning tracking device from the selected acoustic attack device and sends it to the corresponding positioning tracking device. The synchronization matching table is: and A sonic attacking device that is currently positioned and tracked in a synchronized emission direction, and the same sonic attacking device only exists in one synchronization matching table; The early warning device that receives the cancel warning command switches the device state to the idle state; the location tracking device and the sonic attack device that receives the activation command switch the device state to the working state; at the same time, the early warning device in the idle state switches the device state to the working state periodically state; The positioning and tracking equipment in the working state obtains the position information of the airspace target in real time by transmitting and receiving radio waves and sends it to the main control and display device. At the same time, it calculates the aiming position offset based on the position information of the airspace target and sends it to the The sonic attack device, the location tracking device and the sonic attack device respectively adjust the signal emission direction in real time based on the offset of the aiming position, and the initial signal emission directions of all the location tracking devices and the sonic attack device are the same; The main control and display device judge whether each airspace target enters the attack area, if so, set the airspace target as the attack target, and perform different processing based on the system working mode: if the working mode is manual intervention, prompt the operator to choose whether to attack, if If attack is selected, an attack command is sent to the sonic attack device corresponding to the attack target; if the working mode is no manual intervention, an attack command is sent to the sonic attack device corresponding to the target after waiting for K seconds, where 0<K<10; After the attack command, an attack monitoring command is sent to the location tracking device corresponding to the attack target at the same time; The sonic attack device that receives the attack command emits a sonic attack signal that resonates the airspace target based on the current signal transmission direction; the location tracking device that receives the attack monitoring command judges whether the height of the attack target after the preset time t is less than the preset threshold, If so, generate a destroyed instruction and send it to the main control and display device; The main control and display device sends a standby command to the positioning tracking device that sent the destroyed command and the acoustic wave attacking device in the synchronization matching table of the positioning tracking device; After receiving the standby command, the positioning tracking device and the sound wave attacking device will switch the device state to the idle state, and at the same time restore the signal transmission direction to the initial signal transmission. 2. The system according to claim 1, wherein the early warning device comprises a signal transmitting module, a signal receiving module, a control module and a network communication module, wherein the network communication module is used for communication interaction with the main control and display device The signal transmitting module is used to transmit the detection signal, the echo signal is received by the signal receiving module and sent to the control module, and the control module calculates the position information of the airspace target and sends it to the main control and display device through the network communication module. 3. The system according to claim 1, wherein the main control and display device includes a communication module, a main control module and a display module, wherein the communication module is used for communication with early warning equipment, location tracking equipment and sound wave attack equipment Communication interaction, the main control module is used to periodically send equipment status monitoring signals to the equipment in the early warning equipment group, positioning tracking equipment group and acoustic attack equipment group, determine and save the status of each equipment, and select positioning and tracking equipment for airspace targets entering the tracking area And the acoustic wave attack equipment, and set the synchronous matching table of each positioning tracking equipment, send activation instructions and standby instructions to the positioning tracking equipment and the acoustic wave attack equipment, and send cancellation warning instructions to the early warning equipment, and realize human-computer interaction through the display module; the display module It is used to display the movement trajectory of airspace targets, human-computer interaction interface, early warning, and fault prompts. 4. The system according to claim 1, wherein the positioning and tracking device comprises an antenna, a feed network, a transmitter, a receiver, an angle follower unit, a velocity measuring unit, a distance measuring unit, a data processing unit and a communication unit unit, in which the communication unit is used for communication and interaction with the main control and display device, and the sound wave attack equipment. The detection signal is transmitted through the transmitter, the feed network, and the antenna, and the receiver sends the received echo signal to the speed measurement unit and the speed measurement unit respectively. The speed measuring unit calculates the azimuth and speed information and sends it to the data processing unit, the distance measuring unit calculates the distance information and sends it to the data processing unit, and the data processing unit calculates the offset of the aiming position, that is, the physical displacement in the direction of signal transmission , sending the physical displacement to the angle follower unit to modulate the signal transmission direction; at the same time, the distance information is sent to the main control and display device through the communication unit, and the physical displacement is sent to the acoustic attack equipment in the synchronization matching table. 5. The system according to claim 1, wherein the acoustic attack device comprises a signal generator, a power amplifier, a control unit, a matching circuit, a transducer, an angle follower unit and a communication unit, wherein the communication unit uses In communication with the main control and display device, positioning and tracking equipment, the control unit controls the angle follower unit to adjust the signal transmission direction of the transducer in real time according to the received aiming position offset, and the signal generator is based on the signal frequency determined by the control unit An electrical signal is generated, amplified by a power amplifier, and then matched and filtered by a matching circuit, converted into an attacking sound wave signal by a transducer and emitted. 6. The system according to claim 5, wherein the transducer of the acoustic attack device is replaced by a transducer array with energy gathering effect. 7. The system according to claim 1, wherein the early warning device, location tracking device and sonic attack device are provided with camouflage.