# A Review of Unmanned Aerial Vehicle Based Antenna and Propagation Measurements
## 3 UAV-Based Propagation Measurements
### Accuracy Analysis of UAV-Based Antenna Measurement
In [7], the horizontal deviation between the trajectory followed by the UAV and the trajectory planned in the software was less than 2 m. These deviations were due to environmental conditions such as wind; in any case, these effects resulted in an angular deviation of less than 0.38\({}^{\circ}\) in the UAV measurements. In [101], small deviations in the trajectory followed by the UAV resulted in a deviation of 0.02 dB, and the variations in the relative orientation of the UAV could produce an uncertainty of \(\pm\)2%, producing a variation of 0.005 dB in the UAV measured results. Similarly, in [50], differential GNSS, which can provide an accuracy of only sub-meter level accuracy, was used. Although a centimeter-level accuracy GPS such as D-RTK was not used here, a deviation of 0.03\({}^{\circ}\) was observed in the UAV measurements due to the deviations in the UAV positioning. This is because the measurements were performed at 175 MHz; in this case, such low frequencies do not demand centimeter-level accuracy in UAV positioning. In [99], where UAV measurements were performed at 8-12 GHz, D-RTK was used for UAV positioning, and deviations in the UAV positioning resulted in a variation of \(\pm\)0.01\({}^{\circ}\) in the UAV measurements. From these analyses of the accuracy of UAV-based antenna measurements, it is evident that factors such as GPS positioning, vibrations in UAV, and changes in UAV alignment due to external environmental conditions result in relatively small deviations in the radiation pattern of the antenna. However, in [42; 102], major deviations in UAV measurements and measurements from anechoic chambers were observed. This is due to the ground reflections that account for deviations in the antenna parameters, such as side-lobe level and HPBW, and, in certain scenarios, in the peak gains. Ground reflections create multipath interference, where signals add constructively or destructively before reaching the receiver antenna [104]. Based on the constructive or destructive interference, ground reflections lead to either an increase or decrease in the measured antenna gain. To mitigate the effects of ground reflections, strategies such as those using radiation-absorbent material or diffraction fencing can be employed. Radiation-absorbent material, simply known as an absorber, is used in anechoic chambers, which helps to reduce interferences due to reflection from the ground. However, several absorbers would be required for outdoor measurements to mitigate the ground reflections; this is impractical and costly. Another approach is to use a metallic diffraction fence to block the ground-reflected waves. This approach is applicable and used in elevated slant measurement ranges [105]. To overcome ground reflections, we propose the use of two UAVs, which allows the AUT and receiving antenna mounted on the UAV to maintain higher altitudes from the ground level. In the two proposed UAV antenna measurements, one of the UAVs is configured as a transmitter consisting of a portable signal generator and an AUT. Similarly, the second UAV, which is configured as a receiver, consists of a real-time spectrum analyzer and an antenna to receive signals from the transmitter UAV. The proposed solution enables non-tethered UAV operation, allowing UAVs to maintain a higher altitude from the ground level, thus avoiding the effects of ground reflections. By adopting two UAV antenna measurements and placing the antennas at appropriate locations on the UAV to avoid electromagnetic coupling, antenna measurements with good accuracy can be performed in outdoor environments.