2. Brief history
Before beginning the review of the present research and discussing future prospects, it is crucial to understand the history of UAV technology. The following sub-sections concisely discuss the advent of UAVs and their introduction to civilian research.
2.1. Synonyms and definitions
UAVs are popularly called by several synonymous terms such as unmanned aircraft systems (UASs), drones, remotely piloted aircrafts and aerial robots. The term “UAS”, however, refers to an entire system consisting of a UAV as the platform, the payloads and the ground segment controlling the UAV. While the United States Department of Defence (DOD) and the Civil Aviation Authority (CAA) of the United Kingdomprefer the term“UAS” (Colomina&Molina, 2014), the International Civil Aviation Organization (ICAO) uses Remotely Piloted Aircraft System (RPAS) to refer to a UAV (ICAO, 2011). ICAO (2011) defines RPAS as “a set of configurable elements consisting of a remotely piloted aircraft, its associated remote pilot station(s), the required command and control links and any other system elements as may be required, at any point during flight operation”. This definitionmay sound complicated, but it is one of themost holistic definitions of a UAS, as it includes
both aerial and ground segments of UAS. An earlier definition by Peter van Blyenburgh (1999) described UAVs as remotely controlled, uninhabited, reusable, motorized aerial vehicles capable of carrying various
types of payloads for remote sensing missions. Eisenbeiss (2008) originally introduced the term “UAV photogrammetry” as a platform for photogrammetric acquisitions,which can be semi-autonomously or autonomously remotely controlled. Ifwe combine these definitions, as per our understanding, a UAS in remote sensing and photogrammetric terms may be concisely defined as follows:
A system comprised of a ground segment that remotely controls an aerial segment consisting of an unmanned aerial vehicle carrying various remote sensors for capturing high resolution data of the terrain, further enabling the possibility of photogrammetric applications of the gathered data In the present review, we exclusively focus on the UAVs as remote sensing platform and do not discuss the payloads and ground segment of a UAS. Therefore, in rest of the paper, the term “UAV” is preferred.