The power of regular product remote control transmitters is usually 100mW, while special purpose or self modified remote control transmitters may have higher power. If considering 100mW, the remote control transmitting antenna is equipped with a regular whip antenna with a gain of about 3dB, and the receiving antenna on the drone also has a gain of 3dB. Assuming that the operator is 100 meters away from the drone and uses a frequency of 2450MHz, the maximum power level received by the receiver is: 20+6-32.45+20-68=-54.45dBm.
It can be seen that the strength of remote control signals is much greater than that of GPS signals. However, the main lobe direction of the remote control receiving antenna must face the ground, so it cannot provide isolation from ground interference like GPS antennas.
At present, remote control transmitters have widely adopted frequency hopping and spread spectrum technologies, and the frequency hopping parameters can also be adaptive, with certain anti-interference capabilities. When calculating the required interference level, it is necessary to know the parameters of frequency hopping and spreading in order to obtain accurate results. However, we can still know the approximate range of interference required. The remote control transmitter still follows the above parameters. Assuming that the defender is 100 meters away from the drone and the antenna gain is 3dB, if relevant interference is used, the required interference power is close to the remote control transmission power, which is above 0.1W. If there are frequency hopping measures in the remote control signal, and the interferer does not know any parameters of these measures except for the frequency band range, and can only use noise for full frequency band brute force coverage, then the required power will be increased. In terms of experience, it is usually necessary to increase by 30dB, specifically 100W.
This interference power is much more high than GPS and high cost. At the same time, high interference power may affect other normal wireless communications, while the drone still flies.
As shown in the figure below, if the frequency hopping range of the remote control signal is 2405-2495MHz and the defender does not know the hopping parameters, then they have to use noise to cover the entire frequency band, that is, the yellow area. And when the power of the remote control signal is concentrated, when its total power level is lower than the total power level of the interference, it may still be much higher than the interference level locally, so as not to be affected by interference, such as in the red area. At present, advanced remote controllers can automatically adjust the frequency hopping frequency according to the interference situation, so for remote controllers that use frequency hopping, narrowband strong interference is not effective.
The joint application of frequency hopping and direct spread spectrum can compensate for their respective shortcomings. However, the spread spectrum gain of the remote control is much lower than that of GPS, so the anti narrowband interference ability of the spread spectrum part is poor, usually requiring only 3-6dB of signal-to-noise ratio. Therefore, using a comb spectrum interference source, such as 100 interference peaks spaced 1 MHz apart, has a total interference power 26 dB higher than the useful signal, which can save 3-10 dB of power compared to broadband noise interference.
In addition to broadband interference in the frequency domain, it can also vary in the time domain by using pulse interference sources. If the remote control does not adopt repetitive coding measures, using pulse interference can save average power, or increase pulse power when the average power is constant. But if repeated encoding measures are taken, the pulse interference effect is not good.
At present, there are illegal "extended range" remote control devices in the 430MHz frequency band on the market, with a transmission power of usually 2W. After amplification, they can have higher power, such as 5W or even 50W. Moreover, some products have recently added frequency hopping function on the basis of the high power and low frequency mentioned above, with a frequency hopping range of up to 50MHz. Common devices use modulation methods such as GFSK and spread spectrum, with channel bandwidth in the MHz range and high power density. If not spread spectrum, the bandwidth is only a few tens of KHz. To achieve the same power spectral density through noise interference in the 50MHz range, the required power will be astronomical. However, the anti blocking ability of the receivers of these remote control devices is relatively poor.
This part of interference is not fundamentally different from interference with remote control signals, the difference is that the offensive and defensive situation is more unfavorable to the defender. As the target of interference is the receiver of the operator, generally speaking, the distance between the defender and the operator is greater than or close to the distance between the drone and the operator. In addition, drones have an altitude of at least tens to hundreds of meters, and the signal propagation conditions are much better than those of ground-based defenders. Operators can also use directional antennas to aim at drones, and even use antennas that can be automatically zeroed to isolate interfering signals. The advantage of defenders is that their antenna gain can be higher than that of drones with limited space and weight. But since the operator's position is unknown, we can only find a solution in the vertical plane. Generally speaking, this issue is considered based on the unknown orientation of the operator (receiver), the interference distance being the same as the communication distance of the drone, the isolation provided by the receiving antenna, and the additional loss near the ground, totaling 20dB. To make matters worse, the transmission power of image or telemetry signals for the latest product drones is constantly increasing, with 2W power already available. According to the above conditions, if the spread spectrum gain is 20dB, Cb/N0 is 6dB, and unrelated noise interference is used, the antenna gain should be the same as that of the drone, and the total power level should be higher than 33+34=67dBm to produce the effect, which is equivalent to 5KW! Assuming a horizontal directional antenna (such as a coaxial in-phase array) that is 10dB higher than the drone is used on the ground, a power of 500W is also required.
From the above calculations, it can be understood that if the drone adopts spread spectrum and frequency hopping technology, and the defender does not know the relevant parameters , the required power will be very high.
The ancient image transmission used a fixed frequency, and if the specific frequency could be detected, simple aiming interference could be deployed. If an omnidirectional antenna is still used and assuming a signal-to-noise ratio of 0dB is sufficient for effective interference, the required power will be reduced to 33+20=53dBm, equivalent to 200W. If a high gain antenna that is 10dB higher than a drone is used, only 20W is needed.
Hot News2024-08-15
2024-08-15
2024-08-15
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