# Advances of UAVs toward Future Transportation: The State-of-the-Art, Challenges, and Opportunities
## 1 Introduction  
The UAV is an aerial vehicle that does not have a human operator on board. Commonly referred to as a 'drone,' a UAV can fly autonomously or be piloted remotely. It is either operated remotely by a pilot on the ground or autonomously on a computer using pre-programmed flight plans or more sophisticated dynamic automation systems [1, 2]. According to the degree of remote and/or automatic control, a UAV can be categorized as either expendable or recoverable [3]. Expendable UAVs are usually one-time use UAVs that are relatively more significant in a military context than for civil purposes. An example of expendable UAVs is kamikaze drones, which are designed to attack targets by crashing into them. On the other hand, recoverable UAVs refer to the UAVs that can be recovered and recycled after use. Civilian UAVs are usually considered to be recoverable drones [4]. Due to their unmanned capability, UAVs tend to offer a safer operating environment when compared to manned-aerial vehicles if appropriately regulated. A UAV can be an option for more dangerous missions and tasks without risking any operator's life. From the operational perspective, one major drawback is that UAVs have a relatively short flight, as compared to regular aircraft. Additionally, due to their size, which is often smaller than manned-aerial vehicles, UAVs are easily affected by the weather, and their maneuverability remains a challenge in bad weather conditions. Starting with aerial recordings of world views, UAVs are already being implemented in various applications such as wildfire mapping, agricultural surveys, buildings and bridges inspections, powerlines surveys, pipelines monitoring, and many others. In the near future, the UAV industry will play a pivotal role in global technological advancement. At the time of writing, the emerging UAV-related market is valued at over 127 billion dollars, and this industry is predicted to create more than 100,000 new jobs [5]. Figure 1 highlights the global market size of UAVs in 2018 and the forecasted market size in 2024 for the different regions of the world [6]. UAVs are often used for surveillance with their onboard camera. They can carry equipment other than cameras as well and can even deliver small loads. The military sector has extensively used UAVs for combat and humanitarian aid [7]. About 70% of UAVs available are used by the military sector, 17% by the consumer sector, followed by 13% used by the commercial sector. Although the military sector is the biggest market for UAVs right now, the commercial business sector has the fastest growth opportunity. Currently, the infrastructure sector seems to have the highest growth potential, followed by the agriculture and transportation sector [8]. Due to UAVs' enormous growth potential and market interest combined with the rapid advancement of technology and regulations, research on UAV-related applications and their capabilities is also rapidly increasing [9]. In recent years, the usage of UAVs in commercial industries has escalated, and UAVs are being deployed for various diverse tasks in different sectors. In the agriculture industry, farmers are using UAVs for crop data collection to help improve crop yields. UAVs are proving to be pivotal in environmental conservation efforts by providing detailed forest vegetation and water maps. They are also being used for indoor mapping and inspection by the mining industry, leading to improved worker safety. In the construction industry, UAVs are increasingly being used for mapping and surveying construction sites. This approach has claimed to result in saving both time and cost. Police, firefighters, and other rescue personnel have adopted the use of UAVs in recent years for surveillance, search and rescue, and public safety efforts. The high-resolution aerial photography and videography capabilities of UAVs have been extensively used by the film and entertainment industry.