UAV system is mainly divided into three parts: ground station, flight control system and wireless communication link. Let’s learn about UAV wireless communication link system together.
Link System is an important part of UAV system. Its main task is to establish a two-way data transmission channel in open space to complete remote control, telemetry and mission information transmission of UAV by ground control station. Remote control realizes long-distance operation of UAV and mission equipment, and telemetry realizes monitoring of UAV status.
Task information transmission transmits video, image and other information obtained by airborne task sensors to the measurement and control station through downlink wireless channel, which is the key for unmanned aerial vehicles to complete the task, the quality is directly related to the ability to discover and identify targets.
1. China’s regulations on the frequency band of unmanned aerial vehicles
UAV communication links need to use radio resources. At present, the spectrum of UAV in the world is mainly concentrated in UHF, L and C bands, and other frequency bands are scattered. At present, the radio administration bureau of the Ministry of Industry and Information Technology of our country has initially formulated “the frequency usage of unmanned aerial vehicle system”, planning 840.5-845MHz, 1430-1444MHz and 2408-2440MHz frequency bands for unmanned aerial vehicle system. It is stipulated that 1.840.5~845MHz frequency band can be used for Uplink Remote control link of UAV system, among which 841~845MHz can also be used for Uplink Remote control and downlink telemetry information transmission link of UAV system in time division mode. The 2.1430 ~ 1446MHz frequency band can be used in the downlink telemetry and information transmission link of UAV system, of which the 1430 ~ 1434MHz frequency band should give priority to ensuring the use of police UAV and helicopter video transmission, 1434 ~ 1442MHz can also be used for police helicopter video transmission when necessary. When deploying unmanned aerial vehicles in urban areas, the frequency band below 1442MHz should be used. The 3.2408 ~ 1440MHz frequency band can be used for downlink of UAV system. The radio station shall not affect other legitimate radio services or seek radio interference protection.
2. UAV link system composition
The airborne part of UAV link includes airborne data terminal (ADT) and antenna. Airborne Data Terminals include RF receivers, transmitters and modems used to connect receivers and transmitters to the rest of the system. In order to meet the bandwidth limitation of downlink, A processor for compressing data is also provided. The antenna adopts omnidirectional antenna, and sometimes directional antenna with gain is required.
The ground part of the link is also called the ground data terminal (GDT). The terminal includes one or several antennas, RF receiver and transmitter, and modem. If the sensor data is compressed before transmission, the ground data terminal also needs to use a processor to reconstruct the data. The ground data terminal can be divided into several parts, generally including a local data connection connecting the ground antenna and the ground control station and several processors and interfaces in the ground control station.
For long-time unmanned aerial vehicles, relay is a commonly used way to overcome the influence of factors such as terrain barrier, earth curvature and atmospheric absorption, and to extend the working distance of the link. When relay communication is adopted, relay platform and corresponding forwarding equipment are also one of the components of UAV link system. The working distance between UAV and ground station is determined by radio sight distance.
3. UAV link channel frequency band
In the process of UAV ground-to-air data transmission, wireless signals will be affected by terrain, ground objects, atmosphere and other factors, causing reflection, scattering and diffraction of radio waves, forming multi-path propagation, in addition, the channel will be interfered by various noises, resulting in a decrease in data transmission quality.
In measurement and control communication, the influence of wireless transmission channel varies with work frequency band. Therefore, it is necessary to understand the main frequency band used by UAV measurement and control. The carrier frequency range available for UAV TT & C link is very wide. The cost of low frequency segment equipment is low, the number of channels that can be accommodated and the transmission rate are limited, while the cost of high frequency segment equipment is high, which can accommodate more channels and higher data transmission rate.
The main frequency band of UAV link application is microwave (300MHz ~ 3000GHz), because the microwave link has a higher available bandwidth and can transmit video images, the high bandwidth and high gain antenna used by it has good anti-interference performance. Different microwave bands are suitable for different link types.
Generally speaking, the VHF,UHF,L and S bands are more suitable for low-cost sight distance links of short-range unmanned aerial vehicles; The X and Ku bands are suitable for sight distance links and air relay links of medium-range and long-range unmanned aerial vehicles; ku and Ka bands are suitable for medium and long-range satellite relay links.
