## 1 Introduction  
Unmanned Aircraft Systems (UAS), also referred to as drones, unmanned aerial vehicles (UAVs), and remotely piloted aircraft (RPA), have a lengthy military pedigree that reflects their long-recognized potential in supporting warfare efforts. Although their highly publicized missions may convey the impression that UAS are a recent technological innovation, the use of unmanned flight pre-dates human-piloted flight. Aerial bombardment of Venice in 1849 was undertaken (albeit ineffectively) by unmanned hot-air balloons [1], and similar balloons were used in the American Civil War [2,3]. Military reconnaissance missions with unmanned aerial platforms occurred as soon as suitable photographic apparatus were developed; Corporal [PERSON] of the US Army used remotely-triggered cameras aboard kites in the 1898 Spanish-American War [4]. Throughout the history of military aviation, development of unmanned aircraft has continued apace, with developments in human-piloted flight often inspiring (or being inspired by) new unmanned aircraft technology. The early focus on UAS supporting the so-called \"three Ds\" (_i.e._, dull, dirty, or dangerous missions in which human pilot operations would be at a disadvantage or at high risk) highlighted the natural niche for UAS. Improvements in reconnaissance and guidance capabilities during the Cold War spurred interest among the scientific community in utilizing UAS for science missions in which pilotless aircraft provided similar advantages and risk mitigation. The US National Aeronautics and Space Administration (NASA) developed unmanned aircraft for high-altitude atmospheric sampling during the \"Mini-Sniffer\" program of the 1970s-1980s, but with limited success, and little follow-on activities. NASA's Environmental Research Aircraft and Sensor Technology (ERAST) program in the 1990s marked the first major steps towards developing the protocols and capabilities for employment of UAS supporting scientific research. However, the military pedigree of most unmanned aircraft systems yielded a dichotomy in the nature of most systems: UAS capable of carrying powerful and accurate sensors tended to be large and very expensive, while small platforms lacked payloads to deliver precision data. The latter group of UAS was better suited to budgets or logistical demands of many research organizations, but had been designed for situational awareness as opposed to survey-grade data delivery. One of the major findings of the ERAST program and subsequent UAS science community workshops was the need for sensor miniaturization to allow the use of smaller-class (and affordable) UAS platforms. Inspired by NASA efforts and improvements in miniaturization, the late 1990s saw numerous efforts among smaller organizations to develop or modify UAS tailored to their own research needs. Applications as diverse as atmospheric sampling [5] and rangeland vegetation monitoring [6] employed hobbyist-grade radio-control (RC) models, with varying degrees of success. The \"age of the 'do-it-yourself' (DIY) drone\" evolved quickly as researchers found low-cost solutions based on modified radio-controlled models to be less than adequate for many needs. Additionally, the recognition that UAS offer tremendous potential for many areas of scientific research and monitoring has spurred significant demand in the public sector for systems that combine the sensor or sampling quality typically found in large aircraft with the portability and cost advantages provided by smaller systems. The user community, relying on RC aircraft to support data gathering also found a niche in the FAA regulations that allow such platform utility within certain flight constraints and conditions. Smaller-class UAS were not considered as RC-type aircraft by the FAA and therefore were excluded from those more lenient use allowances. Today, a large array of options is available to UAS users, and those without long-standing involvement in this field may face a steep learning curve, which can be a barrier to entry.