Text of Considerations for choosing a data link for UAV
1. 1 Data link selection for UAV Yaron Benita I.M.C Microwave Industries
2. 1 Data link main components GDT (Ground Data Terminal) (Including RF Box power amplifier, tracking, filters, diplexer, receiver, transmitter) ADT (Air Data Terminal) (Receiver, Transmitter. (Filters, LNA, Diplexer, SPDT, 2 Anetnnas)
3. Data link selection considerations Range and environment factors (link budget, line of sight, precipitation, etc.) Antenna tracking mechanism Analog system vs. Digital system Encryption and anti jam requirements Available frequency and bandwidth General requirements (size, weight, input power, computer interface, etc.) Redundancy requirements So many factors, I am totally lost..
4. Range and Environment Factors Line of sight between the GDT and the ADT. Factors that impact line of sight are: trees, constructions , curvature of earth and topographic structure. Due to curvature of earth the maximal range between GDT and ADT depends on the UAV attitude and the GDT height. Ways to resolve non-line of sight cases: Using a relay GDT on the ground, UAV or Airborne to increase range Use satellite communication as a relay between UAV and Ground station. (Ka, Ku band)
5. Range and Environment factors Link Budget Calculation - all the gains and losses from GDT, through cables, antennas and free space to the ADT. Fade Margin the total gain remaining after adding gains and subtracting losses in the link budget (uplink & downlink). The exact amount of the fade margin required for a UAV system depends on the desired reliability of the link and the environmental conditions.
6. Range and Environment factors Example for Fade Margin Calculation: (Downlink ADT GDT) ADT Antenna gain: 0dBi ADT Video & Data Transmitter power: 50dBm ADT cables loss: 1dB Path loss: 150km (loss in 2.4GHz is: 143dBm) GDT Antenna Gain: 17dBi GDT receiver Sensitivity: -97dBm GDT cables loss: 1dB Fade Margin: 18dB (0dBi+50dB-1dB 143dB + 17dB +97dB - 1dB)
7. Antenna Tracking Mechanism The beam width of a directional antenna is narrow. For long range UAV (> 10km) the GDT should include Azimuth/Elevation tracking. (using Antenna Pedestal) Major Tracking mechanism: Manual Tracking manual antenna transportation based on the input signal level GPS tracking using GDT GPS location and GPS target location. (cant work when GPS signal is jammed) RSSI tracking using software automatic tracking by determination of maximal signal power. (spewing out multiple narrow-beam pulses in different directions and looking for the maximal return) Monopulse tracking - automatic tracking, most accurate and reliable solution. (with a single pulse, the antenna can gather angle information)
8. Analog vs Digital The difference between analog and digital technologies is: Analog technology - information is translated into electric pulses of varying amplitude. Digital technology - information is translated into a binary format (zero or one) where each bit is representative of two distinct amplitudes.
9. Main advantages of a digital system Better resilience to noise and interference Capacity utilization(less bandwidth) Compresses the input video using MPEG4, H264 technics. Mixes multiple video and data inputs in one link Allows data manipulation using software Encryption - better applied to digital data Anti Jamming spread spectrum, frequency hopping Forward Error Correction method (improves receiver sensitivity) Smaller size RF components and data processing is implemented by software. Combines data modulation and data processing in the same unit Easy integration with modern flight computers and GCS.
10. UAV Antenna - Considerations Using Two Antenna solution ( one receiver & one transmitter ) vs. using One Antenna to receive and transmit applying a Diplexer. Interference to the antenna reception UAV structure and movements - might require multiple antennas UAV material type Antenna Switching used with multiple antennas solution Temperature
11. General considerations Size & Weight Input Current/Power low power transmitter on a UAV requires compensation on Antenna size on the ground to achieve long range. Autonomous system that can work when GPS signal is lost. ADT GDT range Azimuth/Elevation Angles (GDT - ADT) UAV Altitude GPS location of GDT (acquire when the system is installed) Interface with the ground station and the flight computer. Use of Half duplex vs. Full duplex RF communication Half duplex Transceiver (TDD) - uplink bit rate is low. Use the available slot of the downlink to transmit uplink data Separate frequency for receiving and transmitting
12. UGV & USV Specific Considerations Multipath The arrival of a transmitted signal at a receive antenna from multiple paths is due to reflections. Reflections - ground, water, trees, constructions, people, or any other obstacle . Diversity, COFDM modulation Tunnel Robots Communication through cable Multiple antennas for relay.
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