A technology called Sensor Fusion can be used in drones in which drones carry different sensors with software combining the data together for better results. These systems automatically combine input data from several different sensors such as regular RGB camera, thermal camera for the purpose of improving system performance or application. Combining data from multiple sensors reduces and corrects the errors as compared to using individual sensors to calculate the accurate position and orientation information. For e.g. Multi-Spectral sensors in drones can create Digital Elevation Maps (DEMS) of land areas to provide precise data on flowers, shrubs, trees, fauna, and on the health of crops or more.
Drones with Time-of-Flight (ToF) sensors came to the market in 2016. ToF is also called Flash Lidar which can be used on its own or can be combined with RGB and regular LIDAR sensors to provide economical solutions across various sectors. ToF depth ranging camera sensors can be used for 3D photography, tracking objects, measure volumes, object scanning, obstacle avoidance, indoor navigation, gesture recognition, gesture recognition, augmented reality games, and much more. These Flash LIDAR ToF cameras have a huge advantage as it can measure distances to objects within a complete scene in a single shot. In photogrammetry and lidar mapping, the UAV is programmed to fly over an area autonomously, using waypoint navigation [19]. The camera on the drone will take photographs at a continuous interval like every 0.5 or 1 s then these photos will be stitched together using specialized photogrammetry software to create 3D images as shown in Figure 1.8.
DroneDeploy is a widely used 3D mapping software. It’s a mobile app and a live map is being used in various sectors for creating 3D models and maps. Their specific software solution for the agricultural sector and others work with most of the latest drones.
Figure 1.8 Visual representation of a fixed-Wing platform taking photographs at its designated points [39].
Capturing high-resolution images on a drone that is stabilized is important and using top photogrammetry software to process them into real maps and models is just as important. Some of the top drone mapping softwares are as follows:
• Autodesk ReCap Photogrammetry Software.
• Pix4D-Mapper Photo-grammetry Soft.
• Drone-Deploy 3D Mapping Solutions.
1.3.5 UAV Communication Network
The unmanned aircraft system (UAS) or UAV is facing enormous growth from capital as well as the technological point of view. The number of drones in operation is increasing in gigantic number, with several million produced annually in recent years. The scale and mass of drones vary from micro-drones weighing a few grams to heavy gigantic machines as big as the size of almost a commercial aircraft.
According to ICAO (International Civil Aviation Organization), UAS has been classified broadly into two categories:
• Remotely-piloted Aircraft systems: It consists of various elements which include unmanned aircraft, ground control & some necessary command and control links, or any other element which is configurable together and is required for flight operation.
• Autonomous aircraft: It is an unmanned aircraft which don’t require pilot involvement in the controlling or management of the flying aircraft [20].
Communication is a key aspect of the UAS system due to the presence of a human in a remote place. Therefore, the operation of UAS depends upon communication which enables safe UAS operation in non-segregated airspaces.
1.3.5.1 Classification on the Basis of Spectrum Perspective
International Telecommunication Union (ITU) classifies unmanned aircraft broadly in three categories on the spectrum perspective. It is highly dependent on the range, cruise speed, and altitude of the unmanned aircraft. The Table 1.1 depicts the same classification [21]:
Table 1.1 UA categories on the basis of spectrum perspective.
| Category of unmanned aircraft (UA) | Weight (kg) | Maxim height/ altitude (m) | Cruise speed (km/h) | Endurance (h) | Maximum range (km) |
| Small | <25 kg | <300 m | <111 km/h | <5 h | Visual LoS <3 km |
| Medium | 25–2,000 kg | 300–5,500 m | 111–185 km/h | 5–30 h | RF LoS 150–250 km |
| Large | >2,000 kg | >5,500 m | >185 km/h | >30 h | BeyondRF LoS |
1.3.5.2 Various Types of Radiocommunication Links
For the safe operation of unmanned aircraft under Beyond LoS (Line-of-Sight) and Line of sight (LoS), three types of radio communication between UA & ground control are required, which is as follow [22]:
1. Radiocommunication for the air traffic control relay.
2. Radiocommunication for command and control.
3. Radiocommunication for the sense and avoid function.
After this, it is left to the UA system designer to combine the links accordingly as per the needs of these three radiocommunications into a common physical link.
1.3.5.2.1 Radio-Communications for the Air-Traffic Control
In non-segregated airspace, the link between ATC (air traffic control) and UACS via Unmanned Aircraft (UA) is called ATC relay and it will be required to relay ATC and air-to-air communications received and transmitted by the UA. For communication with ATC, the UA uses the same equipment as that of the manned aircraft. There are two types of communication between UACS and UA via ATC i.e. the downlink bringing the ATC information from the UA to the UACS (Unmanned Combat Aircraft System) and the uplink from the UACS to the UA allowing the UACS to communicate with ATC. This type of communication is critical from the point of view of the safety of the controlled airspaces, especially in areas with a high density of aircraft and terminal in approach. In this case, Future ICAO standards are of immense importance for these kinds of communication.
1.3.5.2.2 Radio-Communications for Command and Control
Command & control is the typical link between the Control station and the unmanned aircraft. The following are two ways of communications:
The uplink: For sending telecommands to the aircraft for flight and navigation equipment control.
The